SOM: Iconic architecture as a result of structural solutions: From Sears Tower to Burj Khalifa 9783955532246, 9783955532239

Citation preview

SOM Structural Engineering

∂ engineering

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SOM Structural Engineering

∂ engineering

4

Editor: Christian Schittich Editorial services: Cornelia Hellstern, Sandra Leitte Editorial assistants: Samay Claro, David Heilinger, Nina Müller, Jana Rackwitz, Hartmut Rändchen, Alina Reuschling SOM editorial team: Bill Baker, David Horos, Dmitri Jajich, James Crouch Copy editing: Susanne Hauger, Philadelphia (USA); Raymond D. Peat, Alford, Aberdeenshire (GB); David Wade, Lörrach (D); Stefan Widdess, Berlin (D) Translation into English: Terri White for keiki communication, Berlin (D); David Wade, Lörrach (D) Drawings: Ralph Donhauser, Kwami Tendar Production / DTP: Simone Soesters Reproduction: ludwig:media, Zell am See Printing and binding: Grafisches Centrum Cuno GmbH & Co. KG, Calbe

© 2015, first edition DETAIL – Institut für internationale ArchitekturDokumentation GmbH & Co. KG, Munich www.detail.de ISBN: 978-3-95553-223-9 (Print) ISBN: 978-3-95553-224-6 (E-Book) ISBN: 978-3-95553-225-3 (Bundle)

This work is subject to copyright. All rights reserved. Reproduction of any part of this work in individual cases, too, is only permitted within the limits of the provisions of the valid edition of the copyright law. A charge will be levied. Infringements will be subject to the penalty clauses of the copyright law. Bibliographical information published by the German National Library. The German National Library lists this publication in the Deutsche Nationalbibliografie; detailed bibliographical data are available on the Internet at: http://dnb.d-nb.de

The FSC-certified paper used fot this book is manufactured from fibres proved to originate from environmentally and socially compatibles sources.

Contents

INTRODUCTION

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EFFICIENCY + ECONOMY

Preface Past and future – reaching new heights

7 8

Structural art Constraints spur creativity Optimising design goals The economy of construction Sustainability Integrating discipline and play

84 85 88 95 99 103

RESEARCH + FUTURE

104

SIMPLICITY + CLARITY

16

Architecture and engineering at SOM – in the genetic code Informing design

18 24

SCALE + FORM

34

Scale and proportion Clarity of design – giving things a name Sensory fields, self-reflection and the future Structural design of tall buildings Tall building case study – Burj Khalifa

36

82

Quo vadis – megatalls as the focus of the SOM Research Gang Structural optimisation – developing new design tools Research timeline

111 120

46 52 58

PROJECTS + PEOPLE

122

Catalogue of projects People

124 140

HIERARCHY

62

Picture credits

144

The importance of hierarchy Structure as poetry Exchange House in detail

64 66 76

106

41

INTRODUCTION

6

Preface

“One of the biggest challenges is to get engineers to question the problem they’ve been given … If an architect gives you a sketch, don’t take it as a solution, take it as a statement of the problem and then come up with your own counterproposals.” This is what structural engineer Bill Baker recommends in the introductory interview to this book, thereby underlining an approach that has always been embraced at SOM. In a firm which includes architects, structural engineers and urban and landscape planners at the start of each new project – even before the design stage – representatives of all the various disciplines sit down around a table together to discuss each problem and its possible solutions. This type of collaboration is most evident in the numerous high-rise projects on which the firm has worked. With skyscraper projects in particular, in which an unusual shape necessarily generates considerable additional effort and expense, the link between form and structure is particularly close. In recent decades – particularly in the field of supertall buildings – SOM has repeatedly been responsible for groundbreaking innovations, giving rise to previously unimaginable construction possibilities (and heights). Historical examples include braced tubes and framed tubes, the external structures of which not only provide structural benefits but are also used as design tools to forge a building’s visual identity. These innovations in skyscraper construction reached their zenith a few years ago in Burj Khalifa, Dubai, which scaled new architectural and engineering heights thanks to its buttressed core and an overall shape derived from its structural behaviour and the prevailing wind loads.

With the benefit of hindsight, however, even Burj Khalifa will come to be seen in future as just another stepping stone. SOM’s research group of engineers and architects is already working on the requirements and solutions of tomorrow. One of its principal tasks is to look at ways of optimising structures and find the optimum balance between structure and shape within the framework of ever more complex specifications and conditions outside the constraints of any specific construction project. At SOM, they see structural engineering as a constantly evolving discipline. This publication sets out the philosophy and approach of SOM Engineering, which, as an autonomous unit, works not only for the firm but also, occasionally, as a service provider for outside architects and artists. As its buildings show, SOM Engineering’s fundamental values are simplicity, structural clarity, sustainability, efficiency and economy. Christian Schittich

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INTRODUCTION

Past and future – reaching new heights 1.1 An interview with the four SOM leaders of the structural group: Bill Baker (Chicago), Chuck Besjak (New York), Dave Horos (Chicago) and Mark Sarkisian (San Francisco)

Detail: With more than 1000 employees and around 10,000 completed projects worldwide, Skidmore, Owings & Merrill is one of the biggest architecture and engineering firms today. How would you describe SOM? Bill Baker (BB): I think that, at heart, SOM is a design practice with architects; structural, mechanical, electrical and plumbing engineers; interior designers; urban planners and sustainability specialists – all of them experts and all working together on the same project, with everybody listening to everybody else. SOM is a self-selecting group, made up of engineers who like working with architects and architects who like working with engineers. SOM has basically been an integrated practice since it started in the 1930s, so this integration is part of the firm’s culture. Detail: Could you give us a short history of SOM / SOM Engineering?

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Mark Sarkisian (MS): From the viewpoint of SOM Structural Engineering, in the 1960s and 1970s, the practice rallied around the idea of efficiency – creating structures that could reach new heights while reducing material use. Dr. Fazlur Khan’s genius led to new structural systems that not only managed forces efficiently but also created a dialogue with architecture. Examples include the Hajj Terminal in Saudi Arabia, the Baxter Travenol Building in Deerfield, One and Two Shell Plaza in Houston, the Brunswick Building in Chicago … among many others. These projects express their personality through structure and were designed based on refinements of an original idea. The SOM Structures Studio has benefited from the leadership of Khan as well as from the engineering contributions of those who worked closely with Khan. Hal Iyengar, John Zils, Stan Korista, Navin Amin and Bill Baker continued the development of new structural engineering ideas and passed those ideas on to others. Today there are so many firms that claim to engineer the most efficient structures, but boasting of designs with least material quan-

tities is hardly enough to differentiate one practice from another. Achieving life safety in structures is expected and not a differentiator, nor does it alone typically lead to innovation. BB: Certainly Faz was a very famous engineer who did some very innovative and very highquality work. As the computer was just coming into play at the time, he was able to use it to perform studies, to devise and validate concepts – something that hadn’t been possible previously. But there were many more excellent engineers who worked on these projects together. None of these things is ever just one person; it’s always a team. Apart from John Merrill, Khan was the first engineer to be made a partner within the firm. A lot of their work goes back to the Second Chicago School of architecture, the post-war modernism period with Mies van der Rohe, etc. Detail: With all the different experts and the idea of a self-selecting group, does SOM have a specific architectural style? BB: I always think of our architecture as a philosophy, not a style. We produce buildings that express contemporary technology in a straightforward manner, but at the same time try to be timeless. Detail: And this is the philosophy behind SOM Engineering as well. BB: A few years ago some of the design partners – engineers, urban planners, architects and interior designers – got together and talked about what an “SOM building” is. Our firm has been around for a long time, so what we do can’t just be a chance product. There’s got to be something that makes it hang together from an aesthetic and philosophical point of view. Although the discussion is still in progress, what has emerged so far is this: when you look at one of our buildings, you should be able to distinguish one or all of the following three things – simplicity, structural clarity and/or sustainability.

Past and future – reaching new heights

1.1 1.2 1.3 1.4

Model presentation area, SOM office Chicago Structural components, connection components, nautilus shell Looking across internal atrium at open studio design concept, SOM office Chicago Model presentation area, SOM office London 1.3

Detail: Simplicity and clarity are two of your core values, as well as hierarchy, efficiency and economy – what is their impact on your work? BB: Simplicity and clarity are core values of the firm; this is what separates SOM from other design firms. It is not a question of simplicity and clarity being better than complexity and ambiguity, but this is what we value as a joint practice. Hierarchy helps us organise our thoughts and our designs. It helps us sort out what is highly important and what is not. Efficiency and economy commensurate with the goals of the project are values that inspire engineers to seek better solutions. They also have important implications for society – not to waste resources or wealth. Our projects, I hope, embody the principles of efficiency and economy on several levels. It’s much more challenging to design an efficient structure than an inefficient one. It’s about finding solutions that are not always the least expensive, but hopefully offer the highest value. Detail: To fulfil the idea of teamwork nowadays, how is SOM organised? BB: SOM is a partnership. We don’t answer to shareholders. The partners only answer to each other. So we are able to share resources amongst ourselves. All partners have a great deal of freedom in what they do as long as everything is done in a responsible manner regarding the firm and everyone else. There are currently 20 partners. We are organised into individual offices, but we try to act firm-wide. The three big offices, where the partners currently sit, are New York, Chicago and San Francisco. Other major offices are London, Washington DC, Los Angeles and Shanghai. Dave Horos (DH): SOM generally organises project work around the partners and their clients. This means that it is possible and actually common that each of the offices can be working on projects in the same cities across the world.

As a result, consistency in our design approach and methodology is important. The senior engineering staff from all of the offices meet regularly to share successes and challenges, to discuss and modify the direction of the practice and to make sure we know one another’s strengths and expertise for reference on future projects. The four of us – Bill, Mark, Chuck and myself – also meet frequently to discuss issues related to the group. Another advantage of practicing firm-wide relates to diversity. Because most of our work includes SOM’s architecture practice, we gain diversity by being flexible enough to shift resources between offices and project types in order to respond to the direction, expertise and workload of each of SOM’s offices. Chuck Besjak (CB): Prior to 2008, most of the structural engineering for the New York office was supported by the Chicago structural group. After 2008, an in-house structural group was developed in New York to align with SOM’s overall philosophy as a multi-disciplinary firm in each of the major offices and to expand the global practice as a whole. This has proved to be extremely successful on multiple fronts. It reinforced the proactive, multi-disciplinary design approach used to successfully innovate on all projects, established an everyday platform to connect to the developer-driven markets and was a natural extension of our global practice. Detail: You have individual offices. Does each office act independently? BB: We have no chief executive officer; there is no “uber-partner”. There is an executive committee that tries to keep people from bumping into each other too much, but each office pursues the work it wishes to do, and we talk to each other to avoid competing for the same project. Though sometimes there’ll be projects where different developers going for one project will hire different SOM offices. But it’s understood by everyone that these are separate teams going after one project. 1.4

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INTRODUCTION

1.5

Detail: How does the firm-wide cooperation between the individual SOM offices work? BB: At each of the three main offices, there is a group of partners who work together to run the practice. We also try to break down the office barriers as far as we can, and in the structural group we definitely work firm-wide. We are all one group that happens to sit at multiple locations. Dave Horos, as the director, is responsible for smooth collaboration between the offices, and I think it works pretty well. From a business point of view, the cities are paired: New York City works with Washington DC, Chicago with London and San Francisco with Los Angeles. Shanghai is jointly run by all the offices. Even though there are seven different cities, it’s really like three organisations. And people move between offices quite a bit. I only know of a few partners who haven’t been to another office. So there is a great deal of mixing – we do try to be one firm. DH: My focus within the structural group relates to the firm-wide practice. SOM’s culture over the years has often been office-centric. We realise, however, the benefits for a structural practice of

our size, with the type of large-scale projects we work on, of operating as one practice across the firm. We consider ourselves a relatively small group compared to some of our competitors who work on similar-sized projects, and believe practicing as a close-knit group is necessary to make the most of our resources to continue to create the quality designs to which we aspire. More specifically, we evaluate the success of the group, both financially and professionally, as a combined practice. We share resources between offices and seek to staff project teams with the people best suited for projects, regardless of their office. We review work between the offices both as a checking mechanism and as a way to improve consistency. We regularly have what we call senior engineer reviews to discuss fundamental issues related to projects, and these often include senior engineers from multiple offices. Detail: SOM is perceived as an architectural office with outstanding engineers. How important is SOM Engineering within the organisation? BB: I think we are very well respected within the practice. Our colleagues recognise that we’re pushing boundaries. There used to be a lot more integrated practices than at present. Formerly, almost every office in Chicago had architects and engineers: Bertrand Goldberg, Adler & Sullivan, Frank Lloyd Wright, William Le Baron Jenney. I think part of it is that the engineers there had a tendency to be complacent. As they didn’t push as much, they kind of lost their edge against outside consultants. Our goal is to maintain that edge, and we work at it a lot. CB: Physically having a structural group in New York has helped bridge the gap between architects and engineers on an everyday basis. As a by-product of the growth of the structural engineering group, a broader knowledge of how we work more efficiently in a shared environment between the disciplines was probed. I believe the growth of the structural group has helped us evolve into a more sophisticated group that

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Past and future – reaching new heights

1.5 1.6 1.7

Weather Field No. 1, Santa Monica, California (USA) 2013, artist: Iñigo Manglano-Ovalle Jay Pritzker Pavilion, Millennium Park, Chicago, Illinois (USA) 2002, architects: Frank O. Gehry & Associates World Voices, art installation, entrance hall of Burj Khalifa, Dubai (UAE) 2010, artist: Jaume Plensa 1.7

utilises more strategic expertise from across the firm to facilitate different projects. There is a constant interaction between offices that has become an efficient template not only for the way the structural group but also how the entire firm works. Detail: How do architects and engineers collaborate within SOM? BB: We really push this a lot. The goal – which I’d say we achieve most of the time – is for us all to look at new projects coming into the office together at the same time. Not just in terms of structure and architecture, but also MEP, interiors, planning, sustainability. The project could be a competition or a direct commission. We look at the function of the building, the city, the area it’s in. Maybe it’s a tall building, maybe it’s a short building. There is no design at this point, no architecture, no engineering. It’s just a problem statement. We always develop multiple schemes for every project. The schemes may have a different emphasis – one may be based on a structure, the other on sustainability. And so you have these multiple ideas in play, that you test. Particularly important for taller buildings is the role of architecture and the shape of the building as the single most important structural parameter. This makes a huge difference. For every project we handle, we try to have structural presentations at the very early stages. We were doing this for one project not too long ago, and the first part of the presentation was about the structural engineering, defining the problem. CB: In New York, one of the more fascinating consequences of the rapid growth of the structural group was the innovative, interactive and efficient approach to communicating with the other disciplines in the office. There was a real-time response to the need of working efficiently without loss of quality. New strategies to streamline the modelling of complex geometries quickly and effectively with the architects became a priority. Now these new ideas and

approaches are being shared and communicated across the firm. Detail: Do you collaborate with other firms? BB: We do work with some other firms, but not very many. The ones we work with are firms that we, as an architectural engineering firm, respect, such as David Chipperfield, Tom Phifer and Jeanne Gang. The people we work with tend to have much in common with SOM. Detail: What is the difference between this and in-house cooperation? BB: We don’t like to compete against ourselves. So when we work with those firms, it’s usually a commission, and SOM won’t have an opportunity to do the architecture. Or the project is on a scale where SOM is not very likely to get the commission – a smaller scale, such as houses. There is a phrase: “You can’t be a prophet in your own land”, so that’s what’s nice about this. Sometimes you need to go outside to be validated inside. We also do a lot of work with artists, which is something I particularly enjoy. James Carpenter in New York, for example. We have also worked with James Turrell and Jaume Plensa – and here in Chicago with Iñigo Manglano-Ovalle on installations. Artists – at least the ones I have worked with – are surprisingly open to technical suggestions. They may even change the art; they are very comfortable in their own skin. So this is kind of fun. Detail: SOM is a company with projects all around the world. What does this mean for your processes? BB: We are a worldwide practice and we like working overseas. Part of it is quite simply to do with maintaining business. A presence at multiple locations gives us a lot more potential projects to work on. In this era, any market, even one as big as the USA, is just not diverse enough. When the market crashes, the whole

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INTRODUCTION

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country goes down. In the recent recession, there was work in China that kept us active and busy and able to hire new talent. But we also learn a lot. We’re like little honey bees that pick up an idea in one market and take it to another. We’ll see some innovation and go “Ah, that’s clever, let’s use that over here or somewhere else”. Detail: How does the budget for a project influence your work? BB: I do enjoy working on projects – and this may seem strange – that have tight construction budgets, because this normally means that things have to be very rational. The structure in particular has to be very rational and efficient, as otherwise there is no project. I sometimes think that some of the best architecture is produced by people who are clever at dealing with budget constraints. A project with too much money sometimes turns out the worst architecture. Detail: But tight budgets often mean more time and effort is needed for the design process.

1.9

BB: Yes, it’s more work. Making something simple is a lot of work. You have to be cleverer to produce an efficient building. Generally, your first solution for any project is a collection of ideas that you then refine to their essence. And that’s not easy. Whatever happens, the structure has to be built. If you have a wasteful structure, there is no money left over for the lobby or the glazing. So to protect the quality of the building, you need to have an efficient and economical structure. Detail: What are the differences between building in the USA, Europe, Asia or the Middle East? BB: I have often encountered great similarities between the US and Europe in a variety of European countries. The developers and builders there are pretty market-savvy. However, in other

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parts of the world, I sometimes find they are not as clear-minded with regard to their performance, budget or ideas. And, for much of the time, I’m sure they are not as economically fastidious as, say, a developer in Chicago or a German developer. I see some very wilful structures created in some of these markets, which could only have come about because of an apparent lack of clarity about the costs. Detail: How does SOM differ from firms like Arup or sbp? BB: We think very highly of both these firms and we hope they think highly of us. In fact, we have some pretty good professional friendships with people in both companies. It is basically consultancy versus integrated practice. We occasionally work with outside architects, but it’s a very small part of our business, whereas these people always work with outside architects as consultants. Philosophically, I think we have a lot in common, especially with sbp. I greatly admire their consistency. The work they do really hangs together – there is a philosophy to it. SOM is a much bigger firm with a lot more variety, so I would say we are not as consistent as sbp. Probably we are a bit more of a circus, too, with lots of different architects and engineering ideas. Detail: Does the public image of structural engineers reflect their true role? BB: That’s an issue. There are very few engineers who are well-known. Personally, I like to be respected by the people I respect. For me, it’s irrelevant whether or not I’m known by the general public. Maybe that’s not so good for the engineers; we are not out there in a black cape gathering all the attention. The downside may be that some young talented people don’t go into engineering because they don’t see it as being as sexy as other things that grab more attention. But I think that to some extent this lies in the nature of engineers: they are satisfied with their own science and skill set. And quite frankly,

Past and future – reaching new heights

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a lot of engineers don’t have an ego the size of some of the other design professions. Detail: SOM has made a fundamental impact on the development of tall buildings. The relevant literature mainly draws attention to the achievements of the 1960s and 1970s. MS: Yes. Khan’s work was experimental. He proved advanced mathematical and engineering theories with physical testing, and carefully integrated the ideas into design and construction. Khan conceived of new structural systems and gave them an identity. The DeWitt Chestnut Apartments, I would say, was arguably his most important work because he conceived of a frame system appropriate for tall buildings by incorporating structural engineering principles that were adaptable to further developments where systems for even taller structures could be created. The structure system for DeWitt Chestnut was named the tubular frame, an efficient response to imposed lateral wind loads that was principally achieved by proportioning the spacing and relative stiffness of column and beam within a rectilinear frame. Khan recognised structural system behaviour shortcomings and responded to them. He recognised that shear lag occurred, caused by the inefficiencies of activating axial load-resisting elements around the perimeter frames of a floor plan. He also recognised that, as the floor plan got larger, the problem got worse. He discovered that, if interior perpendicular frames were introduced to create a cellular-type structure, the problem diminished. This discovery led to the bundled tubular frame used in the Sears Tower, now Willis Tower, in Chicago. He also discovered that a wider spacing of perimeter columns combined with a diagonal bracing system could gain efficiency. This finding led to the invention of the braced tubular frame and the John Hancock Center in Chicago. Detail: What were SOM’s main contributions to the development of high-rise buildings after that?

BB: A lot of what we’ve been doing in the innovations of the last few years has been in the form of the building itself, which is not structure in the “beam or column” sense. The focus has been on the shape of an object. As I said, the most important parameter of tall buildings is the architecture. So we look at tall buildings holistically. This is because, although unfavourably shaped buildings – for example buildings that behave badly in the wind – can still be provided with well-performing structures costing lots of money, this entails solving a problem that doesn’t need to exist. So we are trying to change the design process, to involve the engineers in a more fundamental sense, not only in matching the shape of the building with the structure it needs, but also in terms of wind behaviour. We are also working on developing next-generation bracing systems that will change the architecture of buildings like the Hancock did in the past. We are forever innovating new solutions. MS: In the early 2000s, the SOM Structures Studio in San Francisco developed algorithms to calculate the carbon dioxide created by the construction of buildings. Up until this point, the industry was focused only on the carbon generated by the operation of buildings. As we published papers and presented their findings to the engineering community, most other engineers dismissed this effort as not being the appropriate focus for structural engineers, thinking that the industry should only focus on designing structures for life safety. Today, a majority of subjects at technical conferences are focused on the environment. The SOM Studio developed the Environmental Analysis Tool to calculate carbon while promoting the use of enhanced systems that would reduce carbon over a structure’s life by minimising damage and reducing the need for repair and even reconstruction, specifically after a seismic event.

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Al Hamra Tower, Kuwait City (KWT) 2011 Sketches (from Bill Baker’s notebook), conceptual structural system ideas, Bank of Beijing Science and Technology Research and Development Center, Beijing (CN), anticipated completion 2018 Bank of Beijing Science and Technology Research and Development Center Hand calculations for bracing studies from Bill Baker’s notebook John Hancock Center, Chicago, Illinois (USA) 1970 Willis (formerly Sears) Tower, Chicago, Illinois (USA) 1974

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Detail: What do you think makes a good high-rise? 1.13

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INTRODUCTION

GEN 1

GEN 6

GEN 22

GEN 33

GEN 46

GEN 108 1.14

1.14 Shape optimisation: genetic algorithms take the idea of natural evolution based on survival of the fittest, beginning with random shapes and converging to identical solutions 1.15 Skyline of Dubai (UAE) with Burj Khalifa 1.16 Shenzhen Kingkey (CN)

BB: Functionality is very important. If it doesn’t get compromised in the process, the building will work for years. It hopefully inspires the people in the building as well as the people outside the building, while also delivering efficiency benefits. It’s money well spent, smartly spent and not wasteful. Detail: How do you see the future evolution of tall buildings? BB: I think we’re going through a period of irrational exuberance. It was even worse a few years ago – I’m thinking of those ridiculous proposals for tall buildings that were way too big for their area, way too expensive and way too ego-driven. It’s hard to say – in a way all these buildings are ego-driven. As we’ve seen many times over the years, it seems to happen in cycles. I hope there is a swing back to more thoughtful tall buildings with more substance, rather than just wilful forms. The fact that things

impossible in the past can now be done doesn’t mean we should do them. Detail: What does research within SOM look like? What value does it have? BB: The research we do at SOM is in some ways very informal. Although we don’t have a research department, research is conducted all the time. Nobody has to do it, but anyone who wants to do it is welcome. I am particularly inspired by the artist James Carpenter, who is an expert in glass and stainless steel technology. I remember going to his office, and he was doing some pretty innovative stuff with a very small number of people. And I came to the conclusion that he was doing it by just trying to find something new. By just making the effort, you can do great things. But people who do research at SOM actually work on projects, so it is relevant. If you have just a pure research group, I don’t know how

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Past and future – reaching new heights

1.16

you remain anchored to seeing how what you’re doing could be applied.

Detail: What direction will SOM Engineering take in the future?

MS: The engineers in the various offices have invented ideas that controlled behaviour in difficult environments and created greater resiliency in structures. Devices such as pin-fuse seismic systems, passive damping systems and seismic isolation were developed to enhance performance in areas of high seismicity. The ideas were further developed through mathematics and density optimisation principles, in which placement of material is based on positioning of matter rather than beams and columns. Nature has been an important inspiration in using mathematical definitions of growth to design structures. The work of the engineer and mathematician A. G. M. Michell and others has led to new structural engineering concepts for tall buildings and long-span bridges. These new systems focus on resiliency, longevity and longterm value in addition to life safety – products that extend well beyond efficiency. These systems have a high degree of stiffness using the least amount of material, but also include components that can fuse and dissipate energy when needed: stiffness combined with softness. These types of systems are highly adaptable to complex environments, including those subjected to high winds and high seismicity.

BB: I think that we are certain to change. What we’re doing today is not what we’re going to do in the future. What the future holds I can’t tell. Each generation does things a little bit differently from their predecessors. We’ll hopefully build on and learn from our predecessors, but we’ll go in different directions. My approach, which is essentially research-based, is not everybody’s approach. But, as you can observe, the firm is very elastic.

Detail: How do you promote young talent at SOM? BB: Renewal and personal development are key priorities at SOM. The firm has been around for almost 80 years, so we have spent a lot of time considering who will take our place. Because we are a culture, you need to grow up in the firm. We bring in people before they get set in their ways. So we try to do a lot of recruitment at universities. There is a large group of SOM alumni in other influential architecture and engineering firms around the world. We’re somehow an institution, like a grad school with a good stipend. We hire these people right out of school, we teach them how to be engineers, how to be architects. Some of them stay, some move on.

MS: Yes, with the world’s continuously increasing population and development, depletion of natural resources and changes in worldwide climatic conditions, design and construction of the built environment must focus on systems that are resilient, adaptable and reduce carbon emissions. Detail: One final question: what are the biggest challenges for structural engineers in the future? BB: Not being seduced by the computer, this very powerful tool. Not spending your life making things work. But saying, “OK, we can do this, but we are going to try to find a better solution, something that makes more sense.” And I really see this as the path to better architecture. It’s certainly more meaningful architecture if it’s responding to the problem it’s solving. One of the biggest challenges is to get engineers to question the problem they’ve been given to solve. I keep telling the engineers: if an architect gives you a sketch, don’t take it as a solution, take it as a statement of the problem and then come up with your own counterproposals. Then things go back and forth because everyone sees different aspects of it and understands different parts of the problem and different potential solutions. You have to identify the problem the sketch is trying to address and try to find what might be a better solution. And that takes work.

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SIMPLICITY + CLARITY

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Architecture and engineering at SOM – in the genetic code

Nicholas Adams

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Informing design

Nina Rappaport

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SOM strives to design buildings that are simple and straight-forward, with clearly defined structural systems. Many of the firm’s greatest buildings express their function clearly through their structure and allow it to inform the aesthetic. SOM’s approach to structural design embraces this philosophy. In many of their buildings, the “story of the structure” is plainly visible and intuitively obvious to anyone who cares to read it; in others, simplicity is expressed through bold forms or a clear focus on a single idea. SOM endeavours to strip away the visual clutter and obscuring detail that so often hides the structures of buildings in order to reveal the beauty and elegance of the bones beneath. Good design is often a process of reduction and clarification, an “editing” job that refines and improves, leaving behind only what is essential. In avoiding “complex structures and complicated solutions”, SOM adopts a “less is more” approach, striving to express the full essence of the building with the simplest possible solution without sacrificing function or beauty. Whether the structure is quietly or boldly expressed, it helps define the architecture and is reflected by it in turn. It tells a story, and as in any great novel, the telling seems effortless.

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Architecture and engineering at SOM – in the genetic code Nicholas Adams is Mary Conover Mellon Professor in the history of architecture at Vassar College, New York. His books and articles cover topics in architectural history from the Italian Renaissance to contemporary Scandinavia. He published the first independently-authored book on SOM (2007).

Lever House, New York City Though barely tall enough to dominate its site, Lever House (1950 – 1952) was the pre-eminent symbol of the post-war US corporation. It was the first all-glass curtain wall office building and caused a sensation when it opened. SOM used heat-resistant, blue-green glass held lightly by steel mullions over a masonry core. Elastomeric polysulphides, developed in World War II, provided the sealant to close the building. Steel channel mullions over the structural piers provided a track for a window-washing gondola suspended from the roof that moved with spectacular effect around the building.

Neither Louis Skidmore (1897–1962) nor Nathaniel A. Owings (1903 –1984) were particularly inclined towards engineering. Their own first commissions were exhibitions, museum displays and a dozen private houses. Owings summarised that experience succinctly: “At the beginning of the year: 12 friends, no buildings. At the end: 12 houses, no friends.” When they learned that their architect and engineer friend John O. Merrill (1896 –1975) had lost his job (due to the death of his employer), they offered him a partnership; his expertise compensated for their weaknesses. Owings’s vision for the firm was grandiose: a Gothic Builders Guild, anonymous, collaborative, dedicated to the biggest and toughest problems of construction. His goal for the firm was to build architecture of “both economy and aesthetics – with proof that they were the same”. Above all, he wanted to build at scale because “volume meant power”. In 1937, he and Skidmore jumped at the idea of a second office in New York. Owings stayed in

Chicago with Merrill; Skidmore had New York to himself. They briefly added a fourth partner in New York when it looked as though he might bring in substantial business [1]. New York and Chicago Owings and Skidmore ran their offices quite differently. For Gordon Bunshaft (1909 –1990), the brilliant New York-based designer, Skidmore found engineers locally: initially, Weiskopf & Pickworth for structure and Jaros Baum & Bolles for building services. In the post-war period, they provided the expertise needed to realise the new skin-and-bones curtain wall architecture (Figs. 1.1 and 1.2). In Chicago, Owings could not immediately find a counterweight to Bunshaft’s design strengths, and Merrill made a space for engineering within the office. As design talent matured, Chicago became the home to a long list of brilliant SOM engineers who tackled many of the most complex modern architectural problems.

Inland Steel Building, Chicago SOM’s brushed steel-faced skyscraper (1956 –1957) for Inland Steel replaced the company’s previous headquarters, a stolid brick office building in East Chicago. It was the first skyscraper to be built in the Loop since the Great Depression and, like Lever House in New York, its steel and glass exterior set a new symbolic tone in the city centre. The vertical structural beams of the main shaft are pulled to the outside, leaving the internal floor plate clear. A lift and service shaft adjacent to the main tower provide vertical circulation. “We wanted a building we’d be proud of”, an Inland vice-president said at the time of construction, “a building that spelled steel.” 1.1

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When Owings sought “scale” for his new firm, he was, in all likelihood, thinking of extension and area. Stage-managing big projects had been his training ground at the Century of Progress Exhibition (1933) and, together with Merrill, he was soon arranging for the construction of the town of Oak Ridge, Tennessee (1942 –1949), where the USA built the first atom bomb. Hurry-up programming and design, efficient sequencing and super-fast construction were the firm’s early specialities. Only in the post-war period did it take on a clearer stylistic identity [2]. Expressed in an eclectic international-style modernism that owed much to Le Corbusier (Bunshaft’s favourite), the US-American Mies van der Rohe (whose students soon filled the SOM studios in Chicago) and the lightness of Scandinavian modernism (favoured by Owings and a number of SOM’s MIT-trained architects), the new firm soon positioned itself as the company of choice for the US corporation.

designer but as an intrinsic participant in the design process. Buildings like the elegant Dewitt Chestnut Apartments, Chicago (1961; Figs. 1.1 and 1.2, p. 85), undertaken with Bruce Graham and Myron Goldsmith, used a framedtube construction that made the wall both external structure and surface decoration. The Brunswick Building (now Dunne Cook County Administration Building, 1965; Fig. 1.5), developed with Graham and Myron Goldsmith, used the tube-in-tube structural system that brilliantly synthesised structural and design principles. In order to create the open entrance hall underneath the building, an encircling girdle transfers the weight from shaft to base (Figs. 1.3 and 1.4), a literal expression of the containment of the structural forces. At the John Hancock Center (1969) and the Sears Tower (now Willis Tower; 1973), both in Chicago, the symbiotic collaboration of architect and engineer reached its apex.

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Lever House, New York City, New York (USA) 1950 –1952 Inland Steel Building, Chicago, Illinois (USA) 1958 Testing of deep beam transfer girder, Brunswick Building (now Dunne Cook County Administration Building), Chicago, Illinois (USA) 1965 Construction, Brunswick Building Brunswick Building

Chicago’s engineering tradition Though Gordon Bunshaft dominated design in the early 1950s, the Chicago office soon developed its own designers: Walter Netsch (1920 – 2008), Bruce Graham (1925 – 2010) and Myron Goldsmith (1918 –1996). All three, to a greater or lesser extent, looked to Chicago’s own rich tradition of architectural engineering. They collaborated with a sympathetic in-house group of structural engineers. Foremost among these engineers was the Bangladeshi-born Fazlur R. Khan (1929 –1982). Khan came to SOM in August 1955 with a double master’s degree and a PhD from the University of Illinois. Although still forming his vision of the interdependence of architecture and structure, he quickly found a home at SOM. Khan blossomed in Chicago. His greatest strength was the capacity to respond to architecture in its totality [3]. As he said: “I put myself in the place of the whole building, feeling every part … In my mind I visualise the stresses and twisting a building undergoes”. [4] Khan read his role as neither master of nor slave to the 1.5

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Willis (formerly Sears) Tower, Chicago Bruce Graham compared the profile of the Willis Tower (1973) to a newly opened packet of cigarettes in which the cigarettes emerge unequally. 1.6

Structure as design: John Hancock Center At the Hancock Center (Figs. 1.8 –1.10), Graham and Khan adopted a simple strategy to accept the distinctive tapered sides of the truncated obelisk and designated the lower levels as office and the upper levels as residential accommodation. The original structural solution proposed by Khan was based partially on the tube construction techniques, though its adaptation here was troublesome [5]. Corners (where the horizontal and vertical beams meet the diagonal braces) naturally produce different angles at different levels with a concomitant variation between long and short faces. Furthermore, though the tapered shape could accommodate office and residential functions effectively, each had its own standard ceiling height expressed on the outside as different floor heights. The integration of architectural form and structure is nonetheless expressed clearly and effectively on the exterior. Khan’s instinctive sympathy for design was revealed when the developers proposed to eliminate the diagonal bracing elements from the perimeter of the tower at its highest levels. In response, Khan argued that, from a philosophical and a structural-visual point of view, such a decision would be “a tragedy”. (In fact, terminating the building with the diagonal bracing was not a structural requirement.) [6] Failure to terminate the building consistently would, of course, have made the building appear formally incomplete, transforming the forced perspective of the system of diagonals into decorative overlay, rather than declaring its integral structural necessity. Bundled tubes: Willis Tower The 110-storey Willis (formerly Sears) Tower (Fig. 1.7) was, until the construction of the Petronas Towers in Kuala Lumpur (1996), the tallest building in the world, at 527 m including the antenna masts. Here, Graham and Khan used linked cross walls or cross frames between the towers. The final result was a cellular structure with a stiff exterior frame for the nine linked towers. Khan called the system a “bundled 1.7

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tube”. A belt, introduced at four levels on the towers, stiffens them against lateral wind forces to create what is, in effect, a rigid vertical cantilever [7]. Innovative construction processes At both Hancock Center and Sears Tower, SOM developed innovative construction techniques. At Hancock, the architectural coordinator, Richard Lenke, developed an accelerated process of securing tenders from the contractor using “scope documents”. This method entailed blocking out the major design components for each trade and initiating the tendering process with contractors and subcontractors prior to completion of the design and construction documents with finished working drawings and specifications. This allowed the design process to be shortened and better prices secured, a particular benefit in a period of high inflation [8].

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of Technology and worked in the office of Mies van der Rohe (1946 –1953) [10]. He was the key figure in the design of the Farnsworth House, and his work on the 50 ≈ 50 house is at the root of all Mies’s two-way gridded roof structures. Although he took the licensing exam to become a professional engineer in 1943 – as a wartime convenience – his degrees from IIT were in architecture: he considered himself, first and foremost, an architect. His master’s thesis, “The Tall Building: The Effects of Scale” (1953; Fig. 1.11, p. 22), focused on a tall concrete building with an exoskeleton, yet the broader significance of the study lay in its theoretical analysis.

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Typical fabrication unit (“erection tree”), Willis (formerly Sears) Tower, Chicago, Illinois (USA) 1973 1.7 Construction, Willis (formerly Sears) Tower 1.8 Primary and secondary structural elements, John Hancock Center, Chicago, Illinois (USA) 1969 1.9 Brace detail, John Hancock Center, not to scale 1.10 Construction, John Hancock Center

At Sears, SOM utilised a technique for issuing working drawings and specifications known as “fast-track”. This technique was based on the construction schedule developed by the contractor. The construction schedule identified when various trade drawings and specifications were required. Architects and contractors duly agreed on dates for various trade packages and the drawings were released sequentially as required. This fast-track process significantly accelerated the completion date for the building as the contractor did not have to wait for fully finished drawings from all disciplines before starting construction. Both Hancock and Sears demonstrated SOM’s ability to control costs, to build at speed and, ultimately, to deploy the building as an icon to express a fully consistent structural-architectural concept [9]. These buildings represent SOM’s core values of innovation, and of structural and architectural clarity. John Hancock Center, Chicago The distinctive shape of the John Hancock Center (1969) against the skyline provides one of the two iconic symbols of the city. The Hancock Center was instrumental in the development of the Streeterville neighbourhood.

Structural philosopher: Myron Goldsmith Of the three design partners in Chicago (Netsch, Graham, Goldsmith), only Myron Goldsmith was a true child of the city. Born and raised there, he received all his degrees from the Illinois Institute 1.10

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1.11 Study for a tall office building, from Myron Goldsmith’s thesis “The Tall Building: The Effects of Scale” (1953) a Floor plan of conventional construction b Floor plan of proposed construction c Front elevation of proposed construction d Side elevation of proposed construction 1.12 The Republic Newspaper Office and Printing Plant, Columbus, Indiana (USA) 1971 1.13 McMath-Pierce Solar Telescope, Kitt Peak National Observatory, Arizona (USA) 1962 1.14 Hajj Terminal, King Abdulaziz International Airport, Jeddah (KSA) 1981

Using sources from Galileo to Sir D’Arcy Wentworth Thompson, Goldsmith provided a model for thinking about change in structure over time. At the heart of the thesis is a relatively simple proposition: that, as the scale of a structure increases, the structural system must change. Thus, he argued, though there might be natural limits to building height, the structural components would inevitably be rethought or reconceptualised when the time came. It was a pragmatic and thoroughly open-ended American idea. Although Goldsmith worked on many different types of building at SOM (from tall towers to the elevated train stations of the Chicago underground), over time he developed specialisations. (His work includes, for example, four telescopes and a string of sports facilities.) Goldsmith’s approach to design was thoroughly consistent with that of Graham and Khan. “Structure”, he argued, “once determined, contains within itself the promise of commodity and delight.” [11] Although it sounds so simple, for this thoughtful architect, determining structure

was no easy matter; and simplifying it further was equally difficult. His greatest buildings, the McMath-Pierce Solar Telescope at Kitt Peak in Arizona (1962; Fig. 1.13) and the Republic Newspaper Office and Printing Plant in Columbus, Indiana (1971; Fig. 1.12), both magical in their own way, prove that – in the manner of his mentor Mies van der Rohe – a clear and simply expressed structure could become transcendent in the hands of a great architect [12]. Research and development Finally, the collaboration of architecture and engineering at SOM stimulated an active research environment within the firm as a whole that influenced buildings and led to the publication of numerous articles and research papers. This was the forerunner to today’s research programmes. SOM architects and engineers were also active as teachers and dissertation advisers. In the late 1960s and early 1970s, SOM engineers and architects (under Graham and Khan and directed by Douglas Stoker) created original computer programs that helped build the Sears Tower and the airport in Jeddah

The Republic Newspaper Office and Printing Plant, Columbus, Indiana The building, designed by Myron Goldsmith, consists of an aluminium frame enclosed by glass walls. To maximise efficiency, the offices and plant occupy one floor. The press itself, an offset machine painted bright yellow, was encased in an acoustically insulated pavilion. Symbolic of the transparency and visibility of the free press, the modern building was used as a recruiting tool. In 1981, Republic editor Harry McCawley stated that the building “often makes the difference as to whether we get the reporter we want”. 1.12

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McMath-Pierce Solar Telescope, Arizona Goldsmith called the Kitt Peak Observatory “very Miesian, trying to make architecture out of the facts…”. Unlike Mies, however, Goldsmith does not isolate facts as a wideflange steel structural beam, to be taken as his ideal. Having transcended Mies’s idealisation of form, Goldsmith creates a work that is as much a work of sculpture as an earlier observatory: Erich Mendelsohn’s Einstein Tower in Potsdam (D) 1921.

(Fig. 1.14), among many other buildings [13]. In short, in creating a firm that gave equal weight to Engineering and Design (both with capital letters), SOM attracted engineers and designers who believed in the integration of their disciplines in order to give – at its best – an epochal expression of the modernist vision. References [1] For the early history of Skidmore, Owings & Merrill, see Owings, Nathaniel Alexander: The Spaces in Between. An Architect’s Journey. Boston 1973. Also Adams, Nicholas: Three’s Company. The Growth of Skidmore & Owings. In: SOM Journal 4, 2006, pp. 160 –167. For an overview of the history of SOM, see Adams, Nicholas: Skidmore, Owings & Merrill. SOM Since 1936. New York 2007 [2] SOM made its statement of architectural values clear in the exhibition at the Museum of Modern Art in the autumn of 1950 by placing Lever House, then still in model form, in a central position. [3] In describing Fazlur Khan, John Zils, structural engineer and associate partner who worked closely with Khan, comments that what marked out his genius was his ability to look at a building in its totality: “Maybe that doesn’t sound so difficult, but as a structural engineer you immediately want to focus on the structure, but he was really able to address the totality.” Author’s interview with John Zils, 25 January 2005 [4] Interview for the Engineering News-Record in 1972, quoted by Khan, Yasmin Sabina: Engineering Architecture. The Vision of Fazlur R. Khan. New York 2004, p. 90

1.13 [5] These ideas had also been elaborated in an IIT dissertation written under Goldsmith’s direction by Sasaki, Mikio: A Diagonally Braced Tall Office Building. Chicago 1964 [6] see ref. [4], p. 127 [7] ibid., p. 214 [8] The stimulus for the development of SCOPE at the John Hancock Center had to do with inflation, then running at 1 –1.5 % a month. In the run-up to Hancock, SOM first tested the SCOPE technique at 500 North Michigan Avenue, Chicago. Author’s interview with Richard Lenke, 8 March 2005. [9] The team of Graham and Khan (and, after Khan’s death, Hal Iyengar) ensured that structural architecture yielded fruitful results elsewhere. See also One and Two Shell Plaza, Houston (1971), US Bank Center, Milwaukee (1973), Broadgate, London (1982), Onterie Center, Chicago (1986). Khan also collaborated with Gordon Bunshaft and architects from the New York office on the King Abdulaziz International Airport, Jeddah (1981). [10] A year spent in Rome (1953) on a Fulbright enabled him to study with the Italian architectural engineer Pier Luigi Nervi (1891 –1979). [11] Goldsmith, Myron: Structural Architecture. In: Werner Blaser (ed.): Myron Goldsmith: Buildings and Concepts. New York 1987, p. 24. Of this statement he commented: “To fulfil that promise, in structures of our own time, has been my chief purpose as an architect”. [12] Adams, Nicholas; with McElroy, Nicola: Column and Frame. Mies van der Rohe and Skidmore, Owings & Merrill. In: Roberto Gargiani (ed.): La Colonne: nouvelle histoire de la construction. Lausanne, 2008, pp. 484 – 493 [13] Adams, Nicholas: Creating the Future (1964 –1986). How a passionate group of SOM architects and engineers came together to envision their profession through the lens of technology. SOM Journal 8, 2012, pp. 120 –136

Hajj Terminal, Jeddah King Abdulaziz International Airport is located 69 km west of Mecca. For its Hajj Terminal design, SOM utilised the highly identifiable form of the Bedouin tent to create a marvel that was the world’s largest cable-stayed, fabric-roofed structure. Completed in 1981, the terminal serves as a physically welcoming, culturally symbolic and structurally innovative portal for more than one million pilgrims annually. The complex contains facilities for sleeping, food preparation and various support services. 1.14

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Informing design 2.1 Nina Rappaport is an architectural critic and curator as well as an educator and publications director at Yale School of Architecture. She has published a book about her research on the engineer’s role as a designer and written numerous articles on the topic for magazines and journals. Her current work focuses on the intersection of engineering and factory design for the future of urban manufacturing.

Structure expressed as form The Center for Character and Leadership Development of the United States Air Force Academy in Colorado Springs comprises an architecturally exposed diagrid of painted structural steel plates that forms a dramatic 32-m tall inclined skylight aligned with the North Star. Like for the Fishers Island Residence and the Roche Learning Center, the structural components and connections were developed in close collaboration with the architects and express the sharp corners and structural logic of structural steel. In the final form, architecture and structure are indistinguishable.

The idea of communicating a clear and legible structure has many implications for an architecture /engineering firm such as Skidmore, Owings & Merrill. Historically, the company has approached both engineering and architecture with the respective teams collaborating from the outset of a project. The joint expertise of engineers and architects has been applied in developing innovative design solutions that have met numerous challenges with inspiring results. The collaborative dialogue results in a clarity of structure. Projects in which engineers interact not only with architects but also with artists or other designers are evaluated from both the design and structural perspective with a shared language and design vocabulary. The architects’ and engineers’ methods of working together on parallel but separate tracks, with numerous interwoven points along the design path, enable them to reveal elements of a building, bridge or art installation that inform their design. The collaborative process is evident even in the SOM office, where it is hard to tell which is the engineer’s and which is the architect’s workplace, as both display structural drawings and models, building details and design sketches on their walls. The office also has conference tables where the teams sit to work together on a project from the outset of a commission, rather than one taking the lead and calling in the other after the design has been sketched out. A fundamental structural conception is developed in which form relates directly to function, based on a deeper reading of a building. Yet, that does not mean that SOM sees the design as simplistic or obvious. Rather, the design unfolds as each layer is further explored, giving clear purpose to structure. Typical questions arise in engineering structures. For example, what is the engineer’s role in design? Where do points of innovation exist that contribute to a project’s design from an engineering standpoint? And what parts of the problem-solving process are creative? Just as

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architecture has multiple vocabularies, so does engineering have a grammar, the elements and syntax of which can be shifted and manipulated, stretching the norm and making invention possible. Structural typologies can be categorised in a new lexicon of forms, such as new types of truss and frame, cantilevers, tube extrusions, cable-net facades, column and mullion combinations, and exoskeletons. When engineers combine their calculations with intuition and experience, these give form to the structure [1]. These efforts, among others, to refine structure and integrate it into a holistic design are exemplified by the SOM projects featured here. At certain times during the history of architecture, a building’s structure was hidden by stone, metal or other type of cladding or element in line with the prevailing design aesthetic. In the modern era, the structure and its direct exposure have offered architects the means to achieve their design goals. Structure and design are stripped down and their intrinsic value is seen in buildings in the same way as it is in nature – through the bones supporting a shell or skin, or a self-supporting shell. Structure is integrated, not as an afterthought, but seamlessly into the design. In the recent engineering work of SOM, two aspects, which are explored here, continue to provide a valuable focus: that of simplicity and clarity, and that of structural expression. Structural simplicity and clarity Carefully considered functional elements guide and express form with effects that are poetic and sometimes even sublime, as engineers rise to the challenge of simplifying complexity in an efficient way. This simplicity and clarity becomes a driving philosophy of the firm in terms of a theory of structure [2]. The conscious clarity of structural language produces an elegant form that allows articulation of the architecture in an efficient structural design resolution. This is seen in the exacting details of many smaller projects, such as Fishers Island

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Residence, James Turrell’s Skyspace and Roche Labs, as well as those of larger projects, such as San Francisco International Airport and the US Force Academy’s Center for Character and Leadership Development (Figs. 2.1 and 2.2). Lightness – Fishers Island Residence A client commissioned architect Thomas Phifer and Partners to design an open, transparent house connected to the landscape of Fishers Island in the Long Island Sound (Fig. 2.5). SOM’s engineers often work with Phifer and, on this project, collaborated on the design of a minimalistic structural system, deceptively simple, through which an impression of lightness was created through close attention to detail.

Center for Character and Leadership Development, United States Air Force Academy, Colorado Springs, Colorado (USA) 2015 Center for Character and Leadership Development, United States Air Force Academy Finite element model result showing 3D stress contours in casting and branches, Fishers Island Residence, New York (USA) 2007, architects: Thomas Phifer and Partners Detail of ductile iron branch arm, Fishers Island Residence Fishers Island Residence

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Combining both prefabricated elements (transported to the island by ship) and customdetailed connectors (Fig. 2.4), the engineers employed two systems – one for the house and one for the surrounding canopy. The 430-m2, open-plan glass house is supported by an exposed steel modular structure that adds a sense of weightlessness through a series of 56 ≈ 8.8-m steel roof beams. These span an uninterrupted interior space, clad in glass, with steel columns and mullions unified in one piece. To achieve this thinness, the engineers stripped everything down to the minimum supports, eliminating an entire system by making the small 7-cm square column and mullion one and the same. Similarly, for the independent canopy structure

Thomas Phifer and Partners Architect Thomas Phifer founded his firm in 1997. Recent projects include the Corning Museum of Glass North Wing, Corning, New York, and the United States Courthouse, Salt Lake City, Utah. Thomas Phifer and Partners worked with the engineers from SOM on the Fishers Island Residence, the Turrell Skyspace and the Lee Hall III at Clemson University.

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James Turrell The US artist is known for his visceral and atmospheric work, that juxtaposes natural and artificial light with space to exquisite effect. “I make spaces that apprehend light for our perception, and in some ways gather it, or seem to hold it … my work is more about your seeing than it is about my seeing, although it is a product of my seeing”, says James Turrell.

surrounding the house, its physical and visual lightness resulted from the engineers’ use of 50 cold-rolled steel, 8.3-cm columns, from which cantilever four fork-like branches. These in turn support a roof trellis comprising a series of parallel aluminium rods. The freestanding tree canopies are set forward and interlock at the branches. Inventive connecting elements, such as the custom-cast, pin-to-puck connections with bolts concealed within the column shafts, join the forked tree branch to the stem in a pragmatic detail. The resulting canopy protects the house from direct sunlight and, through its delicacy, affords views out into the landscape. This permits the trellis to float above the primary volume, maintaining the desired aesthetic and minimising the structure. While the project is designed to resist harsh winds, its thin steel system conceals its strength. As engineer Dmitri Jajich notes, “It was as much about craft as it was about analysis and engineering, with its elegance from the leanness of materials as craft meets engineering.”

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Floating roof – Skyspace Another method that SOM used to float a roof canopy, minimising structural expression while maximising structural performance, was implemented for artist James Turrell’s Skyspace at Rice University in Houston, with Thomas Phifer and Partners as architect (Fig. 2.7). Turrell designed an installation in which visitors walk through a tunnel under a raised berm and up a pair of staircases where they can view the underside of what seems to be a floating roof, pierced by a 4.3 ≈ 4.3-m oculus. The light projected by the artist on the underside of the roof surface contrasts with the sky tones seen through the opening. The design effect targeted by Turrell was made possible by the structure. Engineer Bill Baker gave the following explanation as to why he particularly enjoyed the challenge of this project: “Artists are more open to suggestions than a lot of architects; there is not as much of a question of authorship when it is an art piece”. Turrell based the geometry of the project on his calculation of sight lines, attempting to minimise the apparent roof depth as it recedes in space, with the aim of concealing the top of the roof from the ground-level perspective. The engineers initially considered stressed skin and fibreglass for the roof. Yet, as this proved too complicated, they opted for a steel structure and aggressively tapered, custom-built beam sections. The finished solution comprises a 30 ≈ 30-m octa-symmetric, tapered, cantilevering frame supported by eight 15 cm-thick cantilevered steel columns. Two concentric squares of steel girders support a series of tapered double cantilevers that project to both the inner and outer roof edges (Figs. 2.6). This framing is fabricated from steel sections, tubes and stiffener plates. Supported by the steel is a secondary system of light-gauge steel studs, held up by a steel beam every 4.6 m. A structural foam and plaster system on top of plywood was then coated with plaster. Tapering the steel and longitudinal wide-flange sections enhanced the final thin dimension at the roof’s perimeter. The perpendicular top

Informing design

Exemplifying simplicity and clarity The new Federal Courthouse in Los Angeles, California, suspends the perimeter columns to create a clear cube that hovers above a plaza. Perimeter hangers in lieu of conventional columns allow thinner members that emphasise verticality and slenderness. Like the Skyspace at Rice University, the project expresses a simple floating geometric volume.

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flange beams were canted at the same angle as the top roof surface slope to maximise the structural depth and fit the structure tightly within the roof envelope. The cantilever projects from the centre several feet beyond the primary steel beams until the upper and lower roof surfaces meet at a point at each edge. The last 60 cm were the most difficult elements to configure due to the necessary reduction from 10 cm to 5 cm over a very limited length. Gradually, the engineers minimised the depth, transitioning from a tapered steel plate to the plate itself as one contiguous surface. The roof is supported by eight slender tube section columns, each 3.7 m tall, that allow the geometry of the sight lines to support the floating surface. As the result of a unique collaboration with the artist, the structure is essential to the desired visual effect. It is simple, though subservient to the surface. The geometry conceals the structure while also being a play on weight and strength. Thinness – Roche Learning Center The Learning and Development Center for Roche Diagnostics Corporation in Indianapolis, Indiana is another surprising SOM project due to its innovative, clear structure – an expression of the firm’s preferred minimalist design (Fig. 2.10). The two-storey, 137 ≈ 34-m building comprises a medical instrument training centre and offices

on the ground floor, with additional offices, repair workshops, a showroom and a marketing office on the first floor. To admit natural light, the architects organised each separate function around a double-height skylit space, with northfacing monitors like those in traditional factory buildings. These are also used by the building’s mechanical systems for energy-efficient environmental control. In contrast to the building’s straightforward beam-and-slab construction, the engineers employed exposed, slender 14 ≈ 14 cm, solid steel bar columns to support the floors and roof without taking on the lateral loads. Most of SOM’s buildings are so large that the required fireproofing and cladding conceals the structure and produces a thick object. Given the small scale of the Roche project, the design team was able to leave the delicate structure exposed. To achieve this effect, they analysed a series of connection vocabularies with shared drawings sent back and forth between the engineers and architects. A detailed mock-up was set up in the office to investigate different possible connection configurations (Fig. 2.9). As Bill Baker notes, they initially wanted cold-rolled steel bars due to the potential of the crisp, sharp corner, though eventually opted for hot-rolled steel. In a minimalist structure, the columns are pulled outside the floor plate and the exposed end of the beam attached to the column to brace it directly

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Structural framing, axonometric geometry diagram, Skyspace, Rice University, Houston, Texas (USA) 2015, artist: James Turrell, architects: Thomas Phifer and Partners 2.7 Skyspace 2.8 Federal Courthouse, Los Angeles, Caliornia (USA), anticipated completion 2016 2.9 Full-scale mock-up of column to beam connection, Learning and Development Center, Roche Diagnostics Corporation, Indianapolis, Indiana (USA) 2015 2.10 Construction, Learning and Development Center

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in one direction while using the beam’s flexural stiffness to brace the column in the transverse direction. The exposed beam-to-column connections complement the glass rails and windows. The column design took into account the fabrication and all structural issues potentially resulting from this thinness. The project is comparable to the 1960 SOM-designed Republic Newspaper Plant in Columbus, Indiana (Fig. 1.12, p. 22) in terms of simplicity, and to the Fishers Island Residence in terms of the slender steelwork. Yet the decision to move the column off the beam’s centre was in keeping with the client’s aesthetic preference for emphasis on structural detail.

2.11 International Terminal, San Francisco International Airport, California (USA) 2000 2.12 Glass installation, Schubert Club Band Shell, Raspberry Island, St. Paul, Minnesota (USA) 2002, artist: James Carpenter 2.13 Full-scale mock-up of tubeand-rod system, Schubert Club Band Shell 2.14 Schubert Club Band Shell

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Floating Terminal – San Francisco International Airport The new International Terminal at San Francisco International Airport, for which SOM won the mandate in a 1995 competition, opened in 2001 (see “Structural poetry”, p. 66ff.). The roof structure ultimately became the most compelling design element, challenging the architects and engineers to invent a structure that could adhere to seismic requirements at the northern end of the San Andreas Fault. Both the seismic constraints and the vast airport terminal unlocked opportunities for an innovative lightweight structure. A key design and structural feature is the economical main truss steel chord geometry using straight, segmented elements. At the same time, the overall form of the main terminal roof profile appears visually to be defined by continuous curved forms (Fig. 2.11). The spatial organisation and structural lightness were achieved through the materiality of the undulating roof membrane and the simple, though precisely calculated exposed steel truss system Structural invention is also seen in the roadside entrance canopy, which projects from the main window wall facade and was originally designed to be supported on columns. Peter Lee, project structural engineer, recalls how the then 77-year-old Myron Goldsmith, acting as design team consultant the year before he died,

arrived at 7.30 a.m. one morning with a sketch of a canopy structure cantilevered directly off the steel, west-facade window wall mullions that eliminated all the kerbside canopy columns. Architect Craig Hartman took one look and said, “That’s it!” The sketch was carefully detailed: here, tapered members following the shape of the structural bending moments, with elegant pinned connections, created a simple, clear and unobtrusive canopy structure. Seismic requirements necessitated isolation of the columns, such that the main terminal superstructure floats above the pile caps on sliding bearings. This reduced lateral seismic forces, minimised the need for cross-brace solution and ultimately produced a lighter structure. These structural elements, which are minimised throughout, enhance the transient quality of the airport terminal, imbuing it with a lightness that is appropriately symbolic of flight. Structure expressed as form Another structural design approach, consistent with the idea of clarity as discussed above, is that in which the structure dominates the design and is revealed and integrated with the building’s form, often directly shaping it, in what can be called “deep” structure [3]. By paring down the building to its structure, the latter is not only revealed but also with its arrays and repetitive patterning, essentially becomes one with the building. What results is a clarity of design and a successful integration of structure and form. SOM’s innovations in projects with exposed structure range from the smaller, object-like forms of the Schubert Club Band Shell and Nebraska Tower, to those of the larger corporate office buildings of Rural Bank and the Poly Beijing. Contiguous lattice – Schubert Club Band Shell In 2002, SOM worked with artist James Carpenter to engineer the Schubert Club Band Shell on Raspberry Island in the Mississippi River between St. Paul and Minneapolis (Fig. 2.14), and to create a shape directly and visibly informed by its structure [4]. The engineers developed a saddle structure using a 7.6 m-wide,

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stainless-steel lattice grid shell with glass panels, which spans 15.2 m between the concrete abutments (pre-cast with precise steel formwork) and shelters a wooden stage. Three below-ground concrete tie beams resist the lateral thrust of the structure. The lattice comprises two layers of 48 mm-thick tubes that cross each other to form a grid: the upper-layer arched tubes spanning between the abutments and the lower-layer tubes crossing below. The structure is stabilised and the latticework stiffened by two large, stainless-steel box shapes at the outer edges. Diagonal stainless-steel tension rods brace the two layers of tubes and are connected to stainless-steel posts and bolts to form an X-shape in each group of four square panels. By simplifying and rationalising geometry generation, they could use the arc of a circle and translate it along another arc that seg-

ments the glass into square panels, which looks more elegant and lowers costs. The form also helped to simplify fabrication. The resulting structural lattice tube-and-rod system (Fig. 2.13) developed with structural hardware company TriPyramid, which often creates integrated glass systems with architects and engineers, was stronger than a welded-steel grid. This allowed the structural elements to be wrapped in square, flat laminated-glass panels (Fig. 2.12), thereby avoiding the use of curved glass, which would have been a much more complex and costly proposition. The saddle form of the band shell was thus the expression of a fully rationalised structure. Mesh array – Tower of Hope In a further collaboration with James Carpenter, SOM engineered the Tower of Hope in Omaha, Nebraska for the University of Nebraska Medical

2.13

James Carpenter The US light artist and designer studied architecture and sculpture at the Rhode Island School of Design. He worked on the development of new glass materials including photoresponsive glasses and various glass ceramics. Since 1978, James Carpenter has been working to develop independent and integrated building structures that have progressively synthesised art and architecture. 2.14

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Exemplifying structure expressed as form Like the Tower of Hope, the form of the Manulife Pedestrian Bridge expresses its structure through pairs of parabolic tension cables and compression arches tuned to share loads equally. The structural connections are expressed with a conscious clarity of the structural language that is at once functional and elegant.

2.15 1 280 ≈ 280 mm steel box, built-up 2 Vertical steel strut, built-up H section 3 Arch: 110 ≈ 210 mm steel box, built-up 4 Cable saddle and clamp 5 Ø 32.1 mm stainless steel spiral cable 6 150 ≈ 20 mm steel flat vertical hanger

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Center (Fig. 2.17). The numerous geometric studies conducted for sway, weight and gravity produced a filigree sculpture for the campus. The 36.5-m tall tube is 3.5 m in diameter and is constructed from stainless-steel bars varying in diameter from 6.4 cm to 2.9 cm at the top. The tower is punctuated by a series of slender 2.5 cm-thick hoops, spaced 1.5 m apart, and braced by steel bars encircling the structure in a triangulate pattern. These are bolted to the rings at 16 interval connection points at the top and bottom of the ring to form a circular diagrid frame. This is both the gravity and lateral system for the structure. Between the bars, shimmering triangular, perforated, titanium-coated stainlesssteel panels are bolted to the tower’s exterior or slightly recessed towards its interior. The tower was constructed from four prefabricated sections, erected on-site. In response to dynamic wind force issues, the engineers worked closely with the artist to increase the perforations near the top in order to mitigate vortex shedding. Carpenter focused his design on light effects and the ways in which the steel reflects sunlight and atmosphere at different times of the day. At night, the tower is lit from the interior and glows like a lantern. The tower is, in its directness, exactly what it appears to be; with no additional elements concealed, it both encompasses a volume of space and captures a sense of transience.

The 150 m-tall office tower, which sits on top of a six-storey basement (Fig. 2.18), is a tube structural system that echoes the exterior diagrid bracing of other SOM buildings, such as the Alcoa Building in San Francisco. The diagrid itself is a stiff system with a reinforced concrete core wall. Hence, the structure both wraps and supports the building, and provides additional built-in sunshading. The interior rain screen, set back from the facade, is shielded by a cable-net glass wall. Given that the bracing of the diagrid is provided by diagonal beams (lateral) from the corner columns to the core and by box beams extending from the floor system to the nodes, there is moment and shear resistance at both

Exoskeleton – Rural Commercial Bank Headquarters A frequently employed mode of structural design uses an exoskeleton, where the exposed structural system becomes the building’s essential characteristic. Structural concepts, such as the “network-pattern” diagonal structural grid (or diagrid) featured here, dominate the interior and exterior of the building and lend it an identity. The structure is both performative and efficient while also informing the architectural design. With the Rural Commercial Bank Headquarters (Fig. 2.19), the diagrid-braced megaframe is not only an expression of the building’s structure but also a symbol of the bank’s rural origins. 2.17

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axes. As the diagrid structure has no horizontal ties between the diagonal members, the slab is not engaged with the perimeter frame system. This allows the provision of column-free open floor spaces that offer greater office layout flexibility, as required by the bank. Yet, to resolve the different hoop tension forces, diaphragm trusses at the corner nodes are engaged on every other floor in the floor system along with the perimeter spandrel beam and the corner diagonal beam. The evenly distributed system thus balances the loads to the outside. With additional co-reinforcing at level five, an embedded steel beam transfers the tension forces from the corner diagonal across the core. The diagrid steel is also encased in concrete for corrosion and fire protection and is designed to meet seismic requirements. The diagrid is both a design element and a structural system. As with the Tower of Hope in Nebraska, the pattern is its structure, though on a much larger scale. By limiting the connecting nodes and placement of supports from the interior tube to that of the exterior structure, the facade also takes on a filigree quality. The structural strength is both revealed and hidden, clear and complex, in a crisp uniform expression.

1,500 mm centres. A single-span cable net would have been impractical for this scale as the required pretension forces would exceed the capacity limits of the cable types most suitable for cable-net facades. The engineers therefore subdivided the facade into three smaller units by introducing diagonal boundary conditions that stiffen the net. These stiffened boundary conditions were provided by 235 – 275 mm diameter, high bridge cables and were architecturally articulated by folding the cable net along these diagonal lines to form a unique faceted surface. The cables strung between these boundary conditions are held together at their crossing points by clamps,

Exposing mechanisms – Poly Corporation Headquarters SOM designed the world’s largest cable-net glass facade for the 110 m-tall office building of the China Poly Group Corporation in Beijing (Fig. 2.21, p. 32), far surpassing those previously built. The facade encloses the L-shaped office plan to form a triangular envelope around a 90 m-tall atrium, which contains a museum suspended within the space. The innovative engineering structure features three key elements: the cable-net facade, the building within the building or “lantern”, and the immense rocker mechanism to hold it all in place. The building’s main cable-net facade is the largest of its type, measuring 90 ≈ 60 m, with 34 mm-thick horizontal cables at 1,333 mm centres and 28 mm-thick vertical cables at

2.15 Manulife Pedestrian Bridge, Calgary, Alberta (CDN), anticipated completion 2017 2.16 Intersection of arch and cable, scale 1:20, Manulife Pedestrian Bridge 2.17 Tower of Hope, University of Nebraska Medical Center, Omaha, Nebraska (USA) 2011, artist: James Carpenter 2.18 Exoskeleton, Rural Commercial Bank Headquarters, Shenzhen (CN), anticipated completion 2016 2.19 Rural Commercial Bank Headquarters 2.19

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which also support the glass panes. Although, as engineer Aaron Mazeika notes, the large diagonal cable elements, forming a V-shape in elevation, fulfilled an important function in subdividing the facade, they actually introduced a significant design challenge, given the tendency of the diagonal cables to attract seismic forces during an earthquake. While the engineers required highly pretensioned cables, as light and small as possible, to eliminate any sag, designing for seismic forces ran counter to that approach. The conceptual problem was how to disconnect the diagonal elements from the overall building system and decouple seismic behaviour from the facade system. With the two diagonals sloping in different directions, any lateral movement of the building would mean that one would go into tension and the other into compression, so they realised there could be a way to offset these two forces. An early solution was to replace the two diagonal cables by a single longer cable that starts at the bottom of the V-shape and travels diagonally to one of the top corners of the facade, across a pulley and horizontally to the other top corner, across another pulley and back down to the bottom of the V-shape. In this way, the cable force would be constant, with the two pulleys simply rotating as the building swayed back and forth, and the complicated pulley mechanisms could be hidden from view at the top of building. Yet, instead of this, the architectural team embraced the dramatic idea of exposing this critical structural element. The engineers thus placed the rocker mechanism at the base of the V-shape, in a prominent position on the accessible roof of the “lantern” (Fig. 2.22). Two straight cables are then connected to the rocker arms, the rotation of which acts as a pulley to provide the necessary seismic isolation, while eliminating the complexity of the curved metal surfaces of a real pulley and the bending of the thick cables (Figs. 2.23 and 2.24). Engineers do not usually like things that move, so it is unusual for a building to contain a moving mechanism. As such, the rocker is enthusiastically cele2.22

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2.20 Mode shapes, first three fundamental modes of vibration, cable net facade, Poly Corporation Headquarters, Beijing (CN) 2007 2.21 Poly Corporation Headquarters 2.22 Rocker mechanism, Poly Corporation Headquarters 2.23 Pulley testing, Poly Corporation Headquarters 2.24 Detail of rocker, scale 1:50, Poly Corporation Headquarters 2.23

brated and has become the centrepiece in the atrium. The other structural innovation is the glazed eight-storey glass “lantern”, suspended from the building structure within the atrium space, which houses a museum of bronze sculptures repatriated from the international auction market. The cross-braced steel frame is suspended from the cable-net wall’s diagonal cables, while a backup steel frame allows it to cantilever from an adjacent building service core in the event of damage to the cable system. This backup frame provides a stressing support for pretension in the diagonal cable, the forces of which exceed the entire weight of the “lantern”. This combination allows for its unique, seemingly lightweight structure to float above the foyer floor. The effect is that of a transparent rectilinear space that seamlessly interlocks with the building. Through their exposure, the structural elements inform the building design. The synthesis of structure and form at SOM, combined with a clarity of innovation, has evolved historically from a company culture of technology and design to the makings of a structural theory. This structural theory reveals an understanding of the complexity of creating clear and direct forms, which could further expand investigations of new structural form. The fact that SOM engineers realise the significance of designing even structures that are hidden, covered by layers of building materials, ensures an honesty of expression deep below the surface. Exposed rational structures express a direct structural meaning and purpose which play a double role as form.

References: [1] Rappaport, Nina: A Deeper Structural Theory. In: Architectural Design – Special Issue: The New Structuralism. Design, Engineering and Architectural Technologies. July/ August 2010, pp. 122 –129 [2] Ibid. [3] This is described in detail in: Rappaport, Nina: Deep Decoration. In: 306090 – Decoration, Vol. 10, 2006 [4] Structural Stainless Steel Case Study 10: Schubert Band Shell; available online at http://www.steel-stainless.org/ CaseStudies

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1 151 ≈ 15.2 mm strand cable, bundled, Ø 226 mm 2 121 ≈ 15.2 mm strand cable, bundled, Ø 206 mm 3 Metal plate to receive strand anchors 4 Plate metal clevis attachment 5 Secondary steel pin 6 Upper primary steel pin 7 Lower primary steel pin 8 Web stiffener plate 2.24

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34

Scale and proportion

Henry Petroski

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Clarity of design – giving things a name

Henry Petroski

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Sensory fields, self-reflection and the future

Mark Sarkisian

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Structural design of tall buildings

Bill Baker, Jim Pawlikowski

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Tall building case study – Burj Khalifa

Bill Baker, Jim Pawlikowski

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Scale and form are fundamental attributes of all natural and man-made creations. It is the form of a building or a bridge that determines both its architecture and its structure; the form that governs its interactions with its surroundings and with the forces of nature. It is the form that defines the available design space. Scale is a critical attribute in determining the systems that are most appropriate. In nature, questions of proportion can be paramount in developing the best solution – the proportions of the bones of an elephant, for example, are very different from those of a small cat. But proportions are just one variable; problems of largely divergent scales may require entirely different systems. The exoskeletons of small invertebrates represent a very different structural solution than that of either cats or elephants. So it is with man-made structures – problems of varying scale can sometimes be addressed by varying the proportions, but often they require entirely different solutions. Evolution has provided nature with a continuum of structural solutions from single-celled organisms to giant plants and animals. But man, freed from the restrictions imposed by gradual evolutionary change, can invent systems that are fundamentally different from previously existing solutions, giving rise to new building types and the ability to build at scales never before possible. It is in the realm of scale and form that SOM has made major contributions to structural engineering and architecture.

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Scale and proportion 1.1

[m]

Henry Petroski is the Aleksandar S. Vesic Professor of Civil Engineering and a professor of history at Duke University. He is the author of a number of books on engineering, invention and design.

Building tall structures appears to be an innate human drive. The oldest Egyptian pyramids date from about 4,600 years ago, with the Great Pyramid of Giza (completed in 2540 BCE) reaching a height of 146.5 m. This vertical achievement stood as the tallest man-made structure on Earth for almost four millennia; not until the 14th century was it surpassed by the central spire of Lincoln Cathedral. When that collapsed in 1549, the Great Pyramid regained the position of tallest structure, which it held it for another three and a half centuries, until wrought iron and then steel became preferred over stone for building tall. Most recently, of course, concrete has become competitive with steel for ultratall construction. Egyptian pyramids and Gothic cathedrals were made for purposes other than those driving most tall and supertall building construction today. The principal aims of ancient and mediaeval masonry structures were related to reaching and honouring a future and otherworldly plane of existence. As such, their scale seemed boundless. Many pyramids were built in the open desert, and cathedrals, by their very nature, were meant to be out of scale with the city from which they rose. The modern office or

residential tower is more firmly rooted in the urban present and serves the more immediate goals of shelter, pleasure and commerce. As such, the structural and architectural constraints of scale and proportion have entirely different meanings at different times in different places. With each side measuring about 230 m, the Great Pyramid has a footprint of around 53,000 m2, the original structure having had a volume of more than 2,500,000 m3. By comparison, the 8000 m2 footprint of Burj Khalifa (Fig. 1.3) is less than one-sixth of the pyramid’s, and the volume of concrete used in its construction is about 175,000 m3, or an order of magnitude less than the stone and rubble contents of the ancient monument. With the advances in structural engineering, especially in the last century, modern office and residential towers are built tall, but generally no broader at their base than is needed for stability (Fig. 1.2). Thanks to steel and concrete technology, modern tall structures no longer have to be built in pyramidal form. Having a burial chamber deep in the middle of a windowless pyramidal mass may have made sense, but having offices or flats far from a facade makes them dark and

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Burj Khalifa, Dubai, 2010, 828 m

Taipei 101, Taipei, 2004, 509 m

Petronas Towers, Kuala Lumpur, 1998, 452 m

Willis Tower, Chicago, 1974, 442 m

World Trade Center, New York, 1972, 417 m

Empire State Building, New York, 1931, 381 m

Trump Tower, New York, 1930, 299 m Chrysler Building, New York, 1930, 319 m

Woolworth Building, New York, 1913, 241 m

MetLife Building, New York, 1909, 213 m

Singer Building, New York, 1908, 187 m

Monadnock Building, Chicago, 1891, 60 m

Home Insurance Building, Chicago, 1885, 42 m

Great Pyramid of Giza, c. 25 600 – 2540 BC, 146.5 m

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undesirable. That is in part why the 60 m-tall Monadnock Building, completed in Chicago in 1891, remains to this day the tallest skyscraper with load-bearing masonry walls. The 2 m-thick ground floor walls could make such a building unsuitable for human habitation, but in the Monadnock the width of the windows and the narrowness of the building itself are compensating factors. Thick walls in a structure are generally undesirable. Those of the 169 m-tall Washington Monument are, of course, even thicker than those in the Monadnock. Yet, just as the pyramids, the obelisk-like masonry structure was never meant to be occupied by living people. What, then, are the desirable characteristics of a supertall building that is to be more than a monument to sheer height? And how does a structural engineer find the proper form when designing a tower taller than any that came before? What scaling rules, if any, do structures follow? The problem of scale Engineers have long wrestled with problems of scale. In his first-century BCE Ten Books on Architecture, the architect and engineer Vitruvius related a story concerning the city of Rhodes and its defence, which had long been the responsibility of an engineer named Diognetus. However, when a younger engineer named Callias came to town and gave a lecture in which he demonstrated a model of a crane that he claimed could sit atop the city’s walls and capture any enemy siege machine that approached, the responsibility for defence was transferred to him. When the city was later threatened by an approaching 150-ton machine, Callias was implored to scale up his model crane to capture it. It was only then that he confessed that it would not be possible. According to Vitruvius, “… not all things are practicable on identical principles, … there are some which appear feasible in models, but when they have begun to increase in size are impracticable, ...” It may have been known to Vitruvius that structures could not be scaled up indefinitely, but engineers into the Renaissance appear to have

forgotten that fact. In 1638, Galileo opened his Dialogues Concerning Two New Sciences by observing that large objects such as obelisks and ships were breaking inexplicably while being moved or launched. The situation was especially puzzling because the way such things were designed was by carefully scaling up geometrically an earlier successful design. Thus, every dimension of every piece of timber in a ship that was to be twice as long as one that had sailed the Mediterranean for decades was doubled. This rigorous adherence to a geometric design rule was expected to all but guarantee a successful ship of record size, but after a point it did not. Galileo took this situation as a clear indication that the mechanics of scale were not fully understood, and he supported his argument further by pointing out that objects occurring in nature do not scale linearly. His famous example consisted of the corresponding bones from two differentsized animals, one three times as long as the other (Fig. 1.4). By geometric proportioning, the width of the bones should also have differed by a factor of three, but they plainly did not. The bone of the larger animal was clearly thicker than that of the smaller. This observation drove Galileo to seek a rational explanation. He reasoned that nature must take into account something in addition to geometry and he believed it to be the strength of the material involved. (The limits to size were clearly understood when it was realised that the weight of any object is proportional to the cube of a characteristic dimension, whereas resistance to failure was proportional to cross-sectional area or the square of a characteristic dimension. With increasing size, the cube will eventually increase faster than the square, making failure inevitable.) What Galileo inferred from his observation of bones can be seen in the frames of modern buildings – as building height increases, the forces on the structure grow exponentially and the size of beams and columns, for example, may grow at a non-linear rate with height (Fig. 1.6, p. 38). Galileo saw each longitudinal half of a bone as an extension of the other half. With this insight

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A-frame struts, Broadgate Tower, London (GB) 2008 Comparison of base and height of different buildings from pyramids to Burj Khalifa Foundations, Burj Khalifa, Dubai (UAE) 2010 Comparison of corresponding bones from two different-sized animals Rebar cage of Burj Khalifa mat foundation reinforcing steel

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he posed the fundamental structural problem of a beam projecting horizontally from a vertical wall and loaded transversely by a large rock hanging from its end (Fig. 1.7). Relating the cantilever beam’s geometry and the strength of its material to the weight it can carry has come to be known as Galileo’s Problem. After a seminal analysis involving geometry, forces and equilibrium, he came to the conclusion that the load-carrying capacity of a beam supporting a weight at its end was inversely proportional to the length of the beam and directly proportional to the strength of its material, to its width and to the square of its depth. This last non-linear relationship explained nature’s departure from strict geometric scaling up. Scale and proportion were henceforth seen in a new light.

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Comparison of two steelframed structures by SOM a Fishers Island Residence, New York (USA) 2007, architects: Thomas Phifer and Partners b Willis (formerly Sears) Tower, Chicago, Illinois (USA) 1974 Cantilever beam, Galileo Galilei, from: Discorsi e dimostrazioni matematiche intorno a due nuove scienze. Leida 1638 Evolutionary topology optimisation to find optimal building shape for round building with fixed base diameter, height and total volume in response to wind load, Yongsan Tower, Seoul (ROK) Carefully scaled proportions of an expressed external concrete frame, Hartford Fire Insurance Building, Chicago, Illinois (USA) 1961

The building to house the Great Exhibition of 1851 held in London was an anomalous structure in that it was out of scale for its time. Although not a tall building (only less than 40 m high), at 560 m from end to end, it was certainly a very long and voluminous one, enclosing 92,000 m3 on a footprint of 70,000 m2. Among the chief characteristics of the Crystal Palace – the building’s more concise and descriptive, Punch-inspired nickname that stuck – was its modular construction. Cast-iron columns were set on 7.3-m centres and multiples of that, meaning that girders of only three lengths were required, making the construction project efficient and the structure eminently buildable. The modular nature of the structure gave it an appropriately repetitive and distinctly non-Victorian look. However, it worked exceptionally well both aesthetically and functionally for its purpose. Modularity has since been a hallmark of modern design and construction; indeed, SOM has been said to stand for “stay on module”. The Crystal Palace would have been out of scale in virtually any developed London location, including beside Buckingham Palace. Yet, its being erected in a meadow in Hyde Park put it some distance from other buildings and so made scale irrelevant. In fact, by virtue of its modular design, the Crystal Palace, especially

in its original flat-roofed form, could have been expanded or contracted to almost any reasonable length without suffering aesthetically. However, its height could not have been increased without reaching limits of practicality because Elisha Otis’s safety device for elevators had not yet been invented. (It would first be demonstrated publicly in 1853, in an event that took place in a subsequent world exhibition for which a derivative Crystal Palace was erected in New York City, where constraints of space did make scale matter.) It was an international exhibition held in 1889 in Paris that produced the opportunity for another unique structure. At 300 m, the Eiffel Tower – the world’s last great wrought-iron structure – was the first thing since the spire on Lincoln Cathedral to surpass the Great Pyramid in height. The Paris tower was certainly out of scale with the city and there was much opposition to the supposedly useless monstrosity, not least because people feared it would collapse on surrounding buildings. Opposition was especially vocal from the artistic community. They decried it vehemently, not only for its outlandish height, but also for its unadorned industrial material. When a statement signed by prominent cultural figures described the structure as the “baroque, mercantile imaginings of a machine builder”, Gustave Eiffel rebuffed the critics by declaring that the tower was “beautiful in its own right”. He defended engineers and their desire “to build beautiful, as well as solid and long-lasting structures” and maintained that “there is an attraction, a special charm in the colossal to which ordinary theories of art do not apply”. Eiffel’s belief that unadorned, functional structure can be beautiful in its own right is to this day an idea that is central to the aspirations of SOM structural engineers and designers. Size and structure In the 20th century, the mathematical biologist D’Arcy Thompson wrote his now classic treatise On Growth and Form, in which he revealed how mathematics and physical laws could explain “the forms of living things, and of the parts of

Constant wind pressure

Scale and proportion

ESO

ESO

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Constrained (constant) total volume, constant height, fixed diameter base, symmetric shape Shape optimised for minimum deflection at top of building 1.8

living things”, and maintained that, generally speaking, “no organic forms exist save such as are in conformity with physical and mathematical laws”. The same can, of course, be said of the forms of inorganic structures and, in particular, the products of engineering design known as tall, supertall and ultratall buildings. In his 1953 master’s thesis on the effects of scale on tall buildings, the future SOM engineer and architect Myron Goldsmith returned to the thinking of Galileo and Thompson and their conclusion that machines and structures – including natural ones such as bones and trees – could not grow boundlessly in size unless the structures became bulkier and heavier or new materials or designs became available. Goldsmith also made the perhaps counterintuitive observation that tall slender structures such as obelisks and chimneys can become more stable as they increase in size. To illustrate how structural form varies with size, Goldsmith referenced some work of bridge engineer David Steinman. Different bridge types are appropriate for different ranges of span length, the change from one type to another occurring when an increase in span becomes inefficient for a particular type. He gave the example of railway bridges, for which increasing the span length by a factor of four was accompanied by an 11-fold increase in structural weight. He cited a similar behaviour when the height of a prismatic building was increased: increasing the height by a factor of 10 means increasing the weight of the structure per unit building volume by a factor of 2.2. Goldsmith concluded that there was a practical maximum and minimum size for every structure, something D’Arcy Thompson also concluded for organic structures. Tall building height was limited by both structural and functional considerations and, when a certain height had been reached for a structural system, a new one had to be devised to build higher, which provided the opportunity for a new kind of architectural expression, if not necessarily unique to that scale. Many of the ideas in Goldsmith’s thesis can be seen reflected in the design of SOM buildings such

as the Hartford Fire Insureance Building in Chicago (Fig. 1.9). This is essentially what SOM’s Fazlur Khan did using the braced-tube concept in the John Hancock Tower and the bundled tube in the Sears (now Willis) Tower to reach new heights in Chicago and the world. Khan recognised that a “premium for height” (Fig. 2.2, p. 89) had to be paid for using a structural system beyond its practical maximum. The “tall building problem” In order to better comprehend these and subsequent structural systems, it is necessary to understand the issues of scale involved in what SOM structural partner Bill Baker calls the “tall building problem”. According to this, a tall building rising out of the ground can be considered the simplest of structures – a vertical cantilever beam. It is compressed and stabilised longitudinally by gravity and pushed sideways by seismic and wind loads, with these lateral forces being dominant. In other words, the tall building problem is essentially Galileo’s problem turned on its side. There are two critical concerns relating to tall buildings: gravity and lateral loads, primarily

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Structural elements Usable space Unusable space

If Willis Tower were directly scaled up to the size of Burj Khalifa, height, width and depth of the building would all increase by a factor of 1.57, resulting in an overall increase in floor area and overall volume of nearly 3.9 times. The Burj Khalifa floor plan, by contrast, has a much lower volume-tosurface ratio and allows increasing heights without adding excessive volume and unsuable floor area.

Willis Tower

Theoretical Willis Tower scaledup to height of Burj Khalofa

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wind. Baker’s notebooks are full of elementary hand calculations comparing how a tower’s cross-sectional shape affects its floor plates’ moment of inertia and hence its ability to resist overturning moments produced by the wind over the structure’s height. In an essay on building “beyond tall”, he characterises the interaction between a tower’s shape and the wind as the “single most important structural parameter”.

1.10 Comparison of floor plans of Willis Tower (442 m), Willis Tower scaled-up to the height of Burj Khalifa and Burj Khalifa (828 m) to illustrate the problem of scale 1.11 Atrium, Jin Mao Tower, Shanghai (CN) 1999

In a vertical structure, largeness is measured by a broad base and a tall height, so volume and weight can be used to the engineer’s advantage as they help to stabilise the structure against being tipped over. Thus, counterintuitively, building taller can be building stronger. This is contrary to D’Arcy Thompson’s observation for bridges, in which he notes that “if we build two bridges geometrically similar, the larger is the weaker of the two”. Thompson recognised that gravity was a foe of very large horizontal structures known as long-span bridges; Baker sees gravity as a helpful friend in the design of large vertical structures known as supertall buildings. Yet a supertall building is not just one that resists the wind without tipping over. It must also be practical by being affordable, buildable in a reasonable amount of time and have floor areas that are lettable. This last requirement means that no interior space must be too far from windows. The design of SOM’s tall Jin Mao Tower hollows out the centre of the upper floors of the tower to create a dramatic central atrium (Fig. 1.11), thus “adding air” to the structural system and moving structural elements to the perimeter where they are most efficient while simultaneously limiting the amount of floor area. The Y-shaped plan of Burj Khalifa, on the other hand, contains office, residential and hotel units located in relatively narrow structural wings, such that all units have window exposure. Next generation: ultratalls But what about a new generation of supertall buildings? Will they just be scaled-up versions of past record-holders? The example of the

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Sears (now Willis) Tower shows that just scaling up tall buildings is no more possible than scaling up bridges, cranes, bones or trees. Doubling the height of the Willis Tower would result in a super tower nearly 140 m on a side at its base (Fig. 1.10). Deep interior spaces would be so remote from windows as to present such serious psychological risks for office workers as to be virtually unlettable. As has been noted, building types cannot be scaled up indefinitely; greater heights must be achieved by introducing an entirely new paradigm. To go significantly taller than a kilometre or so with a Burj Khalifa-type building will require a structural concept as different from a buttressed core as it is from a bundled tube. What that successor concept might be is not easy to extrapolate from past structural concepts. It will be discovered and developed by following essentially the same design process that brought forth earlier discontinuous changes in the tall-building paradigm. This is an organic process uniting architects and engineers in the quest for a structure that will be affordable, buildable and lettable, and as much in scale with the spirit of the time and place in which it will be built as it will be out of scale with earlier record-holders. The only sure thing that can be said about its geometry is that it will be ultratall.

Bibliography: Baker, William F.: Burj Khalifa. A New Paradigm. Yale University, Yale School of Architecture Public Lecture Series, Gordon H. Smith Lecture, 26 January 2012 Baker, William F.: Beyond Tall. Issues of Scale and the Evolution of Tall Buildings at SOM. In: SOM Journal 7, 2011, pp. 140 –146 Galileo Galilei: Dialogues Concerning Two New Sciences. Translated by Henry Crew and Alfonso de Salvio. 1914 edition. New York 1954 Goldsmith, Myron: The Tall Building. The Effects of Scale. M. S. Thesis, Illinois Institute of Technology, June 1953. Khan, Fazlur R.: The Future of Highrise Structures. Progressive Architecture, October 1972, pp. 78 – 85 Steinman, D. B.: The World‘s Most Notable Bridges. Engineering News-Record, 9 December 1948, pp. 92 – 94 Thompson, D’Arcy Wentworth: On Growth and Form. New York 1992 Vitruvius: The Ten Books on Architecture. Translated by Morris Hicky Morgan. Cambridge, MA 1914; reprint, New York 1960

Clarity of design – giving things a name

Clarity of design – giving things a name 2.1

One commonly cited way of distinguishing between a scientist and an engineer is that the scientist studies what already exists, whereas the engineer creates things that never existed. Regardless of this distinction, both scientists and engineers are confronted with the problem of giving a name to a newly discovered species or a newly designed thing – and often a thing not yet fully comprehended. Depending on how thoughtfully this is done, the name can be a source of clarification or confusion. The need for a sentence or more to describe a structural system that has no simple descriptive name may indicate that the structure is not yet fully understood. For SOM, good design is more than reactive problem-solving. That is what can occur when an architectural concept is given to engineers who are charged with making the concept work structurally. Such a situation often results in ad hoc efforts to solve one sub-problem after another, which can lead to an overall design that is a hodgepodge of quick-fix solutions. The upshot is a structure that has no clear load path and is difficult to grasp in its entirety. Attempting to build such a structure can be fraught with conflicts, contradictions, cost overruns and delays.

Like the Sydney Opera House, SOM’s Hajj Terminal in Jeddah (Figs. 2.1 and 2.2) evokes the form of sails, but the name of the structural system – “tensioned membrane” – not only reflects the form of the roof, but even suggests how it would be built. The clarity of the tensioned membrane structural concept establishes and describes the architectural form and the building materials. But how are names chosen and what form do they best take? Scientific names When the 18th-century astronomer William Herschel discovered a new planet, he named it Georgium Sidus in tribute to King George III. Yet, that name was certainly not descriptive of the planet, nor did it place it within the context of the solar system. The French, not wishing to honour an English monarch, referred to it as Herschel, which was equally unhelpful. The German astronomer Johann Bode, who established the new planet’s orbit, proposed naming it after Uranus, the father of Saturn, thereby

Henry Petroski

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Structural axonometry, Hajj Terminal, King Abdulaziz International Airport, Jeddah (KSA) 1981 Tensioned membrane, Hajj Terminal, King Abdulaziz International Airport

Former SOM partner and director of the SOM structural engineering group Hal Iyengar cautioned engineers and architects not to draw a structural plan unless they had at least one idea about how to build it. Structural systems that are too complicated to name in less than a long, complex sentence or a rambling paragraph fall into this category. How such structures work will not be obvious or unambiguous, and how they are to be constructed cannot easily be fathomed from the drawings. The classic example of such a situation is the Sydney Opera House, whose competition-winning conceptual sketches of a cluster of what looked like sails was captivating, but gave no hint about how they would stand, let alone how they could be erected. Concise, intelligible names come only after full discovery and clear understanding. 2.2

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giving it a name derived from Western mythology in keeping with long-standing tradition. Even the English eventually accepted the internationally more neutral and fitting name. When modern biologists discover new species, they are constrained by the conventions of binomial nomenclature – even down to matters of Latin grammar and capitalisation – as to how they can choose and record a typically two-word scientific name. The first part of the name – the genus – is capitalised, but the second – the species – never is, not even if derived from a proper noun naming a person or place. The giant water lily, Victoria amazonica, for example, was originally improperly named “Victoria Regia”. Moreover, while this was honorific to Queen Victoria, it was not descriptive of the plant’s origins. There is, of course, much more freedom in creating a common name for a new species of flora or fauna, which explains in part why such names are not always unique. The same holds for non-organic minerals, invented devices, and engineered structures and structural systems. Non-living things tend to be named more descriptively than honorifically. For example, in the mid-16th century, an entirely new mineral that we now know as graphite was discovered in England’s Lake District. At first, it was referred to by names that described what it did and what it resembled. Whereas metallic lead had long been used to scribe faint guidelines on parchment and paper used for manuscripts, the new mineral made a heavy, dark black mark. Thus, imitatively, it began to be called by the Latin term “plumbago”, meaning “that which acts like lead”, in the sense of leaving behind a mark. In English, it was called, among other things, “black lead”, a simple descriptive name that helped understand the distinctive properties of the new mineral. This new substance was soon shaped to form the core of what we now know as a pencil. Prior to that, a pencil had been the name for a very fine-pointed brush. As a wooden shaft filled with graphite instead of animal hair could draw a fine line without having to be dipped into a wet medium, the implement was initially called

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a “dry pencil”. In time, as the newer form of pencil became more common than the older one, its name was shortened and simplified to the now-familiar “pencil”. The marking medium was eventually given the unique name graphite, the Greek root of which means “to write”. Naming an invented or designed thing is not always very easy or logical. The @ symbol, now ubiquitous in digital communication, had previously been widely used in commerce, which is why it typically appeared on typewriter keyboards. Its vestigial presence on the teletype machines used in early computer-to-computer communication made it an easy, if arbitrary, choice to designate early e-mail addresses. However, at least in English, the symbol still has no agreed name and is often referred to reflexively as the “@ symbol”, with @ simply pronounced “at”. In other languages, it is often named after what it resembles: “pig’s tail” in Norwegian, “rollmops herring” in Czech, “spider monkey” in German. Noun + adjective Individual finished bridges and buildings are typically named after the client or a politician, but the underlying structural genre is often associated with the engineer who invented and introduced it. Thus, the cable-stayed bridge is attributed to Fritz Leonhardt and the bundled tube building frame to SOM’s Fazlur Khan (Fig. 2.3). However, the common names by which structural genres are known tend to be more descriptive, like the straightforwardly designated bridge types of beam, arch and suspension. The meanings of truss and cantilever may be less self-evident to the layperson, but once understood they should also become a comfortable part of the enthusiast’s vocabulary. Of course, as bridges evolved, their structural systems often became more varied and complicated. The arch form, which once implied that stone was the constituent material, had to be paired with the qualifiers stone, iron, concrete and steel. Trusses that were not simple were called continuous. Among the latest variations on a traditional bridge form is the self-anchored suspension type, most

Clarity of design – giving things a name

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notably represented by the East Bay signature span of the San Francisco-Oakland Bay Bridge. Bill Baker observed that, for buildings, there are many structural systems – for example, “belt truss”, “bundled tube”, “buttressed core” – the names of which fall into the format of a noun modified by an adjective (noun + adjective), and this may be a preferred way to name any structural system. It could be seen as not unlike the binomial genus-species convention of biologists. However, rather than the species following the genus in the name, in structural engineering the species precedes it, at least in English, as in the examples given. This could be seen as appropriate, since it is in the qualifier of the noun that the innovation is captured. Yet not every structural system necessarily elicits the same noun + adjective pair from every structural engineer. As if there were a principle of relativity at work, the same tall building may be seen in different ways by different observers. Fazlur Khan saw the Sears Tower as a bundle of nine tubes acting together as a single “bundled tube”, but another engineer might see the building’s “noughts-and-crosses” ground floor plan and its curtailed versions in the building’s upper storeys as the basis for a framed tube or a modular tube (Fig. 2.3). Baker sees the layout as the cross section of a giant beam with four webs. However the structure is viewed and no matter what it is called, an engineer’s understanding of its behaviour as a tall cantilever emerging from the streets of Chicago will be shaped by how it is imagined. Given SOM’s emphasis on simplicity and structural clarity, the structural system of the Exchange House in London (see “Exchange House in detail”, p. 76 – 81) virtually names itself, as the hybrid building-bridge structure exposes the dominant tied arch (note the noun + adjective) that is central not only visually but also structurally. The appropriateness of this unique building-bridge structure over the wide expanse of railway tracks north of Liverpool Street Station might also be seen as a playful interpretation of the concept of a railway bridge. Similarly, the

long-span roof of McCormick Place Convention Center is a bold expression of a cable-stayed roof (taking its name from the bridge system of the same form; Fig. 2.6, p. 44). Both Exchange House and McCormick Place are long-span structures that share many of their traits with bridges, so it is not surprising that their structural systems evoke familiar bridge typologies. But what of buildings that do not so directly suggest a bridge or any other familiar structure? Naming the structure The frontispiece to Bill Baker’s Beyond Tall essay is an updated and augmented version of a graphic used by Khan in the 1960s to show how structural systems (and their names) evolved as the number of building storeys increased. As with bridges, a certain type of structural system is appropriate for a range of building heights. As the limit (whether structural, functional, economic or aesthetic) of an existing system was reached, a new system had to be created to build beyond the maximum height allowed by the old. As these systems evolved, they needed names to serve as a shorthand means of capturing the essential feature that distinguished them from one another. For concrete office buildings, the names of the structural system as the buildings grew from 20 to 75 storeys are shown in the graph to have been, successively: frame, shear wall, frame-shear wall, framed tube, tube-in-tube and modular tube (Fig. 2.5, p. 44). The systems for steel structures, which are shown to have grown from 30 to a possible 140 storeys, have been given the names, successively: rigid frame, frame-shear truss, belt truss, framed truss, truss-tube with interior columns, bundled tube and truss-tube without interior columns. (These names had evolved somewhat from those Khan had used, but that was to be expected as structural types were reinterpreted.) Certainly, all of these designations are clearly descriptive, but some are more mellifluous than others. How preferable it would be, from an aesthetic point of view, to stick to simple noun + adjective names.

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Bundled tube structure of Willis (formerly Sears) Tower, Chicago, Illinois (USA) 1974 Shaped truss, International Terminal, San Francisco International Airport, California (USA) 2000

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The observation that a tall building is, in essence, a vertical cantilever beam is a statement of clarity. Acknowledging this simple fact strips away the details of setbacks, facades and vortex-shedding strategies and enables us to see the building for what it is fundamentally. Yet, since every building is a cantilever, that word contributes no informational content when striving to distinguish one structural building system from another. It is here that the noun + adjective scheme, which focuses not on the length of the cantilever but on the nature of its cross section, expresses organisation of thought. In fact, whatever SOM building systems are called, they express simplicity and structural clarity. The 7 South Dearborn structure, for example, was conceived to have an exceptionally simple gravity load path: all of the building’s upper floor loads are carried directly by the core (or “mast”). The stayed-mast structural system represents a new conceptualisation of a tall building as a single vertical “mast” element that is laterally stayed. The John Hancock Tower in Chicago is another striking example. Its gently tapered shape makes the building recognisable even in silhouette, and the simple pattern of steel, expressing the fact that it is clearly a braced tube, distinguishes it both structurally and architecturally. Even if the layperson does not see every nuance to the bold

X-bracing, he or she cannot help but understand that it is what makes the building work. The Hancock building structure remains an archetypal precedent that has been and continues to be reinterpreted. The morphing diagrid for SOM’s Lotte Tower (Fig. 2.9), for example, starts with the same basic structural system as Hancock, but projects the mega-bracing on a surface that gradually changes from a square (like Hancock) to a circle. Likewise, the structure that comprises the diagrid-tube Tower of Hope sculpture at the University of Nebraska (Fig. 2.7) is immediately recognisable as the functional framework that provides vertical and lateral rigidity. The diagrid-tube is one and the same as the sculptural expression. New structural systems for higher buildings As buildings continue to grow in height, as they seem destined to do, there will be an ever-present need to devise new structural systems and hence new names for them. The convention of using a noun + adjective pair to characterise these new systems will certainly add a rational approach, not only to naming the systems themselves, but also to categorising what we can expect will be an ever-growing number of types. It was the conception and development of the buttressed core that made it possible to construct Burj Khalifa as tall as it is. The principle is

McCormick Place Convention Center, Chicago Completed in 1986, this significant expansion made McCormick Place the largest convention centre in the USA, a record it still holds. The project combined advanced engineering and the utilisation of air rights to enable development on land once thought unbuildable. Constructed over active railway tracks, the addition is divided into two distinct zones: a two-level exhibition hall and a storage area with loading docks. The roof is supported by steel cables hung from 12 distinctive pylons that rise up through the building. 2.6

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Tall building systems, their names and appropriate numbers of storeys Cable-stayed roof, McCormick Place Convention Center, Chicago, Illinois (USA) 1986 Diagrid tube, Tower of Hope, University of Nebraska Medical Center, Omaha, Nebraska (USA) 2011, artist: James Carpenter Finite element stress contours for connection nodes, Lotte Super Tower, Seoul (ROK), design 2008 Morphing diagrid, Lotte Super Tower

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made even more understandable when we actually feel the forces involved through Baker’s anthropomorphic model of a man holding an open umbrella and bracing himself against the force of the wind in a horizontally blowing rainstorm by planting one foot behind him, as if in a fencing stance. The backwards extended leg functions, of course, as a buttress. D’Arcy Thompson saw such a structure in trees that grow in a setting of strong winds. He observed that “anchoring roots form powerful wind struts, and are most developed opposite to the direction of the prevailing winds.” The buttressed core structural system will allow towers to be built about a kilometre high; to build taller than that is likely to require a new kind of structural concept. What that will be is not necessarily obvious. After all, in the evolutionary chain from skyscraper to supertall tower, the buttressed core did not spring to the mind of every structural engineer as the natural successor to the trussed tube. Taller structures will eventually be proposed, no doubt, but whether they are going to be built will depend upon a number of factors, not least of which will be the economic and political climate, which is intertwined with constructability and construction time. Of course, the basic structural concept will be the sine qua non. Baker does not believe that Frank Lloyd Wright’s mile-high Illinois Tower could be built as proposed as it would take some 10 years to build and, if completed as designed, would lack torsional stiffness. Yet, whatever present and future structural engineers see in their mind’s eye as a structural system that will make possible a tower approaching two or three kilometres, they will have to communicate it to other engineers, architects and clients. This task will be made easier if they can describe their structural concept not in a rambling sentence or disjointed paragraph, but in a few (preferably two) words. If they can do that, they will demonstrate that they themselves do, indeed, clearly understand the structure and can proceed with confidence

to its completion. However, a name evolves with or follows the discovery of the thing, and so it will emerge only after sufficient structural engineering and architectural thinking has been done to produce something on paper that is buildable in concrete and steel. Engineers will start by envisaging their concept non-verbally in their mind’s eye. They will then communicate it to colleagues and consultants in gestures, sketches and drawings. In time, these will be accompanied by words in presentations and prospectuses. Initially, the words may outnumber the drawings because the design may not be fully fleshed out and may not be fully understood. Eventually, the words will become fewer because the concept will have been given a name – a name that was effectively discovered in the process of thinking and working through the design.

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Bibliography: Baker, William F.: Beyond Tall. Issues of Scale and the Evolution of Tall Buildings at SOM. In: SOM Journal 7, 2011, pp. 140 –146 Goldsmith, Myron: The Tall Building. The Effects of Scale. M. S. Thesis, Illinois Institute of Technology, June 1953 Houston, Keith: Shady Characters. The Secret Life of Punctuation, Symbols, & Other Typographical Marks. New York 2013 Khan, Yasmin Sabina: Engineering Architecture. The Vision of Fazlur R. Khan. New York 2004 Petroski, Henry: The Pencil. A History of Design and Circumstance. New York 1990 2.9

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Sensory fields, self-reflection and the future Mark Sarkisian is the structural and seismic engineering partner for SOM.

Key to the development of new and the maintenance of existing infrastructure, neighbourhoods, campuses and cities is the balance of resiliency, self-sufficiency and regeneration. To avoid the depletion of natural resources, structures must be designed to be durable and adaptable, capable of coexisting with imposed environmental conditions while accommodating changes in use. Beyond sustainability, resiliency leads to environmentally sensitive buildings consisting of reused materials that are capable of adapting to future conditions such as climate change. Systems within these buildings require a design ethos based on performance whereby every component has multiple uses: structural systems capable of heating and cooling, exterior wall systems capable of absorbing and storing energy and building systems capable of operating with site-based water collection, power generation and distribution. Buildings must be completely self-sufficient, without relying on their

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neighbours. Advances in energy storage will be developed to bridge periods of limited or no power derived from solar exposure, while on-site water reclamation, purification and reuse will reduce demand on our most important resource. Morphogenetic planning of the future will consider weighted parameters for design beyond individual buildings (Fig. 3.2). Form, building material, embedded and operational carbon, daylight, use efficiency, site placement and other important parameters will be considered, even on the district or city scale, at early conceptual stages. The abundance of data will inform sensory fields, where the magnitude and direction of oncoming environmental changes can be anticipated, gathered, reported back and used to inform optimally performing structures. Structures will become self-reflective, capable of undergoing state changes of materials, allowing component properties to be temporarily altered to efficiently resist abnormalities in loading. Rheological systems will use the flow of materials

Sensory field data mapping of San Francisco Digital image of San Francisco Example of advanced analysis investigating the influence of the sensory field on specific structural response parameters 3.1

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Sensory fields The response of structures within the urban context is most significantly affected by imposed load and material characteristics, including stiffness, mass, shape and the connection to the earth. Mathematical models accurately depict the characteristics of these structures. However, loading magnitudes and directionalities are typically enveloped in the design, leading to conservatively estimated structural demands and, consequently, a waste of materials. Analytical models stand alone and are systematically subjected to different, imposed enveloped force conditions on a case-by-case basis. Defining the precise magnitude and direction of imposed loads results in optimal performance and the least amount of materials required for this response. Biologically based sensory systems used to monitor specific site conditions on a district or city scale can inform buildings systems of a required response to imposed demands from natural events. A deterministic assessment of seismic ground motions, for instance, results in a definition of force vectors and energy associated with an imminent earthquake. A similar technique could be used to evaluate wind conditions. A field of sensors, including accelerometers and anemometers, could pinpoint natural force flows on a district or city-wide grid (Fig. 3.1).

Interactively linking mathematical models to information sources fed from a sensory field would enable structures to respond intelligently, perhaps systematically changing internal properties or triggering active mechanisms (Fig. 3.3). Material or motion sensor systems would be placed within structures to evaluate in real time their behaviour under load, correlating actual and predicted performance and also providing a map of stress states of materials that could have experienced plastic stress states or permanent deformations.

Lotte tower diagrid angle Zone of maximum structural efficiency Zone of structural inefficiency Building elevation [m]

triggered by advanced analysis models interconnected with the building and the region’s sensory field. Structural systems and all building components will be designed to behave naturally in the environment, free of the potential for damage from extreme conditions such as seismicity. Ultimately, structures will exist in a true state of equilibrium in which umbilical reliance on services from other sources is eliminated and regeneration of resources is possible; structures will contribute to the environment rather than challenge it. This goal will only be achieved through innovative processes of collaboration, invention and integration.

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Huawei Technologies Corporate Campus, Shanghai (CN) 2007 Fused-pin linking trusses, Huawei Technologies Corporate Campus Frame with pin-fuse joint connections Link-fuse joint connection detail

Self-reflection Lengthening a structure’s characteristic (fundamental) period results in less demand from ground motions caused by earthquakes. Maintaining material elasticity is essential to achieving the highest performance with the minimum damage and the greatest chance of the building being put back into service following an extreme event. Seismic isolation is a technique of artificially lengthening a structure’s period by decoupling the building’s base from the ground. The rotation or “fusing” of joints accomplishes the same goal. Cast-concrete roof truss elements incorporating steel pinned connections at the research buildings of the five-storey Huawei Technology Headquarters (Fig. 3.4) unify the buildings into a complex with a consistent and lasting identity. The trusses are designed to separate or “fuse” at their apexes in major earthquakes, breaking the modules linked across the atria into smaller, simpler, stable structures with cantilevered trusses, and reducing potential damage (Fig. 3.5). Link elements that connect

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relatively stiff earthquake-resisting components, such as shear walls or steel-braced frames, usually experience significant ductility demand in severe earthquakes and undergo damage while dissipating energy. Link-fuse and pin-fuse joints Shear is typically the critical force component in link elements. The link-fuse joint was developed to withstand severe seismic events by dissipating energy through friction slip at a preset shear force level without experiencing significant beam or joint yielding damage (Fig. 3.7). By incorporating a joint that “slips” in friction at force levels just shy of potentially damaging shear forces, the link-fuse joint protects the integrity of the link beam components it connects, preventing yielding and damage in the components and, ultimately, costly post-earthquake replacement of the damaged elements. The goal of the link-fuse joint is to postpone inception of this flexural yielding by introducing mechanical energy-dissipating friction “slip” prior to plastic hinging of the beam at its ends. Yielding and damage thus occur at drift levels significantly greater than in structures with traditional link elements. The lateral system is allowed to behave elastically (no slip) in a frequent event (such as a 25-year event). In a severe event (such as a 2475-year event), the link-fuse joint can slip to accommodate larger displacements, with significant energy dissipation due to friction. This provides a high performance level with little or no structural damage and with minimal requirement for post-earthquake repairs. Additionally, there is the prospect of being able to loosen slipped link-fuse pins and utilise elastic energy stored elsewhere in the structure to recentre it if it has experienced permanent drift in a severe earthquake. Pin-fuse systems remain fixed during wind and moderate seismic events, and rotate or slide when subjected to high demand (Fig. 3.6). Damping systems can also be used to protect structures and mitigate violent responses in wind or seismic conditions by lessening the dynamic response and inherent imposed loads.

Sensory fields, self-reflection and the future

Wall Beam

Link-fuse joint The link-fuse joint involves a pretensioned pin that clamps two halves of a link (coupling) beam together until the shear force in the beam exceeds a certain threshold. As the lateral load continues to increase, the pin traverses the full length of the slots in a severe earthquake and eventually re-engages the two halves of the beams. Only then do the beam halves beyond the fuse attract additional force and eventually yield in flexure. The details of a typical link-fuse joint utilise conventional structural steel components.

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Informing the structure Similar to the way two plants of the same species react differently to different placements in the environment with variations in growth patterns, or the way luminary control devices determine required light levels for supplemental lighting, structures should be designed to be environmentally reactive, dynamic and selfreflective. Sensory information flows would inform structures of anticipated demand and allow an interactive response. Analytical mathematical models for the structure would be directly linked to the imposed magnitude and direction of load and inform structures of the next steps in response. These next steps could include activating strategically-placed behaviour-controlling devices or mechanisms that alter joint or base connections.

therefore the capability of force transmission between the ground and the structure. A perimeter membrane restraining system could be used to limit the possibility of uncontrollable property changes. The viscosity change in the soil would be similar to that of ketchup, which undergoes liquid property changes through shear thinning when shaken. The fixity of certain joints within the structure could also be modified on demand. For instance, if clamping forces could be temporarily relieved, the stiffness of the frames would be lowered, the natural periods of vibration lengthened and the attracted inertial loads reduced. This could be achieved by introducing energy in the form of heat into these joints, perhaps through fastenings, which would increase their lengths due to heat expansion and reduce the clamping forces. To control joint behaviour and allow for structural recentring, joint sinews could be introduced using counteractive high-strength strands or shape memory alloys such as nickeltitanium (NiTi), which would use the inherent elastic properties of the materials, or heat is applied, then cooled, changing the characteristics of the material through a martensitic transformation.

For instance, pneumatic dampers that incorporate compressed air or viscoelastic fluid would be activated and tuned by interactively correlating the actual dynamic response with the predicted mathematical dynamic response. Air is introduced into the handrail to change the dynamic behaviour of a long-span carbon fibre ribbon bridge to prevent vibration resonance. Damper activation would be introduced sequentially, focused on critical areas of the structure where their participation could be most beneficial. In the case of strong ground motions, a more effective and sophisticated response would be to create complete separation of the input source from the structure. Temporary levitation created by electromagnetic flow, or air cushions similar to the technology used by Poma Otis for the Skymetro train line in Zurich, would provide frictionless seismic isolation by separating the superstructure from its foundations. Soil liquefaction under foundation systems is typically mitigated (for instance by adopting deep foundations) to control superstructure responses during strong ground motions, but perhaps advantage should be taken of this soil behaviour and the subgrade rheology by using the ground motions to reduce soil shear strengths and

Reducing the mass A reduction in mass results in a reduction in inertial forces caused by seismicity. This reduction in mass results in less demand on vertical load-carrying elements, and usually also in a reduction of required lateral stiffness. Less mass results in a lower fundamental period; however, less stiffness results in a longer fundamental period (dynamic period is proportional to the square root of mass divided by stiffness). As the period of the structure lengthens, less ground acceleration is felt by the structure. It is known that approximately 25 % of the concrete placed in conventionally constructed buildings is not needed for strength but merely increases mass and demand on vertical load-carrying elements such as walls and columns. For example, most of the concrete placed in centre spans of structures is not required and is placed there for ease

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Recycled inclusion

3.8 Parametric city model This model interfaces with programs such as Grasshopper, used to define geometry, combined with tools such as Galapagos and Karumba, which allow generic algorithms and structures to be defined. The model accesses a database of hundreds of previously designed and built structures. The recorded data includes structural requirements relative to height, material type and site location (seismicity and wind conditions) along with space requirements for building systems, such as vertical transportation and mechanical systems.

of construction. In addition, the environment is becoming overburdened with waste materials that do not decompose and are not recyclable. Materials such as lightweight waste plastics and polystyrene could beneficially reduce mass if strategically introduced into structures where concrete is not needed. The Sustainable FormInclusion System (SFIS; Fig. 3.8) – originally conceived for creating air voids in structures by placing capped, empty plastic beverage containers into structural systems – achieves these goals. More practically, the system can utilise bricks composed of ground and formed plastics or waste foamed polystyrene (such as styrofoam) cast into a lightweight mortar. Environmental responsibility could be taken further by using zero-cement concrete with the use of products such as Greencem, whereby cement is essentially eliminated and waste blast furnace slag used as a replacement. Morphogenetic planning for the future Evaluating multivariable parametric building models on a district or city scale can be used to identify the best planning strategies for the

future. The parametric city model combines the weighted importance of form, structure, embedded carbon and efficiency of space use while considering orientation, including exposure to daylight and solar gain. The model is also capable of evaluating the embedded carbon impact of construction with regard to material type (steel, concrete, wood, masonry, etc.), fabrication and transportation of these materials, construction time and required equipment, and the number of construction workers and their transportation to and from the site. With the requirements for the structural and mechanical systems known, the commercial value of the net available space can be assessed based on its location within the building (such as floor level), access to daylight and views. The model is also capable of evaluating the environmental and financial benefit of incorporating advanced seismic systems into structures through the reduction of lifecycle carbon and anticipated damage over time, and the cost-benefit of addressing those risks at the time of construction. For slender structures or structures with complex geometries, parameters can be interactively evaluated with regard to the advantages of interlinkages or other geometric modifications. These models can be translated into more sophisticated structural analyses to determine where structural material should be placed in order to ensure that the least amount of energy is expended when work is done to resist load. In minimising the energy, forces and deformations should be distributed as evenly as possible throughout the structure by a synergetic placement of material. Forces will flow through the easiest, shortest and most natural load path of the structural form. Topographical optimisation techniques are used to map the structural response and define the most efficient placement of materials. On a district or city scale, the environmental impact of planning can be interactively evaluated based on proposed or anticipated building material type, geometry and site conditions. The type of use plays a significant role in the

3.9

50

Sensory fields, self-reflection and the future

3.8 3.9

Sustainable form-inclusion systems (SFIS) Parametric city model, illustrating darker areas where occupied efficiency falls below targets 3.10 Construction, Poly International Plaza, Beijing (CN) 2015 3.11 Diagram of facade, Poly International Plaza 3.10

overall plan as the requirements for building systems and structure vary when comparing, for example, office, residential and mixed use occupancies (Fig. 3.9). Rheological buildings for the future The envelope enclosure for structures represents the single greatest opportunity to consider flow and interaction between architectural, structural and building service systems (Figs. 3.10 and 3.11). Hundreds of millions of square metres of occupied area are enclosed each year with systems that essentially provide protection from the elements, safe occupancy and internal comfort. A closed-loop structural system integrated into an exterior wall and roof system that incorporates liquid-filled structural elements could provide a thermal store that heats up during the day and could be used for building service systems – such as a hot water supply or heat for occupied spaces – during the evening hours. A solar collection system could be integrated into the network and incorporated into double wall systems, where it could be used to heat the internal cavity in cold climates. Transparent photovoltaic cells could be introduced into the glass and spandrel areas to capture more of the energy of the sun.

The concept of flow can be further developed into structures that are interactively monitored for movement. Through the measurement of imposed accelerations due to ground motions or wind, structures could respond by changing the state of the liquid within the system. For instance, the structure could use endothermic reactions to change liquids to solids within the closed network. Sensor devices could inform structural elements of imminent demand and initiate a state change in liquids that could be subjected to high compressive loads during which buckling could occur. Magnetorheological or electrorheological (ER) fluids could be used to change the viscosity and therefore the stiffness of closed vessels and their damping characteristics. When subjected to a magnetic field, magnetorheological fluids greatly increase their apparent viscosity and can become viscoelastic solids. When subjected to an electrical field, ER fluids can reversibly change their apparent viscosity quickly, transitioning from a liquid to a gel and back again.

When storing liquids in very tall structural systems, pressures within the networked vessels become very large. With this level of pressure, for example, water could be supplied to the structure or to neighbouring structures of lesser height without requiring additional energy to move it. The energy required to store the water initially is minimised if water was collected at upper levels of the building, particularly roof and upper exterior wall areas. A continuous low velocity flow or a liquid with a low freezing point passing through these systems would keep it from freezing. Liquid in tuned-liquid dampers within the networked system would control motion, with fluid flow acting to dampen the structure when subjected to lateral loads from wind and earthquake events. 3.11

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Structural design of tall buildings

Wind overturning diagram

Gravity load diagram

Combined gravity wind load diagram: one wind direction

Potential tall buliding form 4.1

Bill Baker is the structural engineering partner for SOM. Jim Pawlikowski is an associate director for SOM.

free end

wind load

l fixed base

W 4.2

Using beam theory to understand tall building behaviour It is often useful to think of a tall building as a giant beam cantilevering from the ground. As with beams, the deflection of tall buildings is caused by flexure, shear and torsion. Flexural behaviour and shear behaviour are highly interrelated. In common beam theory, it is generally assumed that “plane sections remain plane”, implying that shear deformation effects (shear lag) do not significantly affect the flexural deformations. Furthermore, all vertical elements of a cantilevering beam are assumed to contribute based on their distance from the neutral axis. Because of the geometry and scale of tall buildings, there are shear lag effects that can be quite large and cause the behaviour to deviate from that of a purely flexural beam. In extreme cases, the vertical elements do not act together but behave as a collection of individual elements, and the single giant beam concept is not realised.

Tall buildings are one of the great achievements of structural engineering. The desire to build tall has long been a fundamental human aspiration, as evidenced by the Great Pyramids, the story of the Tower of Babel and medieval Italian tower construction (to name but a few examples). However, only in the last century has the aspiration been truly realised. The development of modern building materials, but more importantly, the birth of the discipline of structural engineering – through which structures can be conceived and designed using the applied principles of mathematics and physics – allowed humans, for the first time in history, to construct buildings that reach extraordinary heights. Many of the new tall building systems that have allowed and continue to allow ever-higher construction were and are created by SOM engineers. The defining characteristic of a tall, slender building is its lateral system. It is the tall building acting as one giant vertical beam and its resistance to lateral loads, such as wind or seismic forces and the lateral destabilising forces of gravity, that dominate the design (Fig. 4.1). It is the slenderness, even more than the height, that makes the design problem different from other structures. Slenderness (height divided by the width at the base) makes the structure more sensitive to movement caused by the axial deformation of vertical elements, such as columns and walls.

In the Burj Khalifa structure, the elongated threewing shape greatly enhances stability and stiffness by engaging the columns and walls at the tips of each of its long wings. A similar strategy was employed in the design of the Nanning Wuxiang Tower to maximise the distance of the vertical elements from the centre of the tower. Engaged nose columns for increased moment of inertia

Hammerhead walls, high fixural stiffness Walls resist shear

Hexagonal central core, high torsional stiffness

4.3

52

The support of the gravity loads is, of course, also very significant. For an economical tall building, it is important to use the vertical structure needed for gravity loads to resist the lateral forces as well. This also decreases the tension in the superstructure from wind loads and the potential uplift at the foundations. At SOM, this is often referred to as “managing gravity” and involves moving the gravity forces (and structure) to positions where they can also be used in the lateral system. The system used to resist shear can be readily used to move gravity loads to optimal locations. In general, the vertical elements should be arranged to maximise the flexural stiffness (a function of the moment of inertia) of the building, assuming that the shear system can move the forces to the columns and walls. In fact, the further the vertical elements are from the centre, the lower the uplift forces will be (decreasing inversely with distance from the centre) and the higher the stiffness (increasing with the square of the distance from the centre; Figs. 4.3 and 4.4).

4.4

Structural design of tall buildings

Mega-column Outrigger

4.1 4.2 4.3 4.4

4.5 4.6 4.7 4.8

Development of a potential tall building form Diagram of vertical cantilever beam Gravity load flow, Burj Khalifa, Dubai (UAE) 2010 Lateral system description which makes the analogy between a wide-flange beam and a closed shape – the buttressed core exhibits the best traits of each of these structural shapes, Burj Khalifa Structural components, Nanning Wuxiang ASEAN Tower, Nanning (CN), anticipated completion 2019 Diagram, Nanning Wuxiang ASEAN Tower Floor plan, ASPIRE Tower, Jeddah (KSA), design 2009 ASPIRE Tower

Reinforced concrete wall

Moment frame column

4.5

The Nanning Tower uses a three-wing core and plan configuration reminiscent of Burj but adds three pairs of mega-columns at the tips of each wing. The mega-columns are optimally positioned at the extremities of the floor plate to maximise overturning resistance and stiffness. The stiffness contribution of the mega-columns is proportional to the square of the distance of the columns from the centre of the floor plate – thus even a small increase in the structural “footprint” can have a large impact on the system’s stiffness (Figs. 4.5 and 4.6). It is clear that creating a stiff shear system that can deliver the overturning forces to the vertical elements is crucial. One way to measure the efficiency of a system is to look at how much it would deflect if it were a purely flexural beam with no shear deformation. For an efficient tower, this value should be at least 70 % of the actual deflection. In extremely efficient towers, it can achieve 90 %. Torsional effects are not often discussed, but a torsionally flexible building can have major problems affecting both strength and serviceability. Fortunately, adequate torsional behaviour can be provided in the form of a structural tube, which can usually be created by the core or other elements of the lateral system. Structures must be designed for both strength

and serviceability. The strength requirements not only consider issues of members and connections but also global behaviour, such as overturning due to wind or seismic loads. The ASPIRE Tower (Fig. 4.8) takes the concept of using gravity loads to assist in resisting lateral loads one step further. The tower was an entry in a design competition for a kilometre-tall building and utilises a strong central core as the only vertical structure – the core serves as both the gravity and lateral system. Floor framing is cantilevered off the central core by post-tensioned, cantilevered concrete girders without the use of any columns (Fig. 4.7). In this manner, all gravity load is directly placed within the lateral system; since there are no columns, there are no transfers, outriggers or beltwalls required to tie the structure together. Not only does this provide an efficient load path, it also helps facilitate construction because every floor can be treated as a typical floor – there are no atypical elements, such as outriggers, to interrupt and slow the construction process.

4.6 Commonly used lateral shear systems Common lateral shear systems include braced frames, wall systems, moment frames and coreoutrigger systems (Fig. 2.5, p. 44). Braced frames utilising diagonal or X-bracing can be very efficient, as can wall systems. Moment frames are less efficient but can have architectural advantages: the closer the columns and the deeper the connecting spandrel beams, the more efficient the moment frame becomes. A core-and-outrigger system is popular because of the way it permits an open perimeter. The walls of the core are very efficient in shear, but the core is too slender in flexure to resist the overturning moments over the full height. The outriggers then create zones of high shear in the core and outreaching arms in order to move the overturning forces from the core to the perimeter columns much further from the centroid. Cores can also be used with perimeter braced frames or moment frames to create a combined system.

Wind issues for very tall buildings Serviceability in the wind is a major concern. The aspects of serviceability for tall buildings are generally related to motions and the perception of motion. The cladding, fit-out and building services have to be designed and detailed to

1 2 3 1 Composite metal deck slab at center core 2 Structural steel columns and framing at central core 3 Reinforced concrete link beam 4 Reinforced concrete circular core wall 5 Reinforced concrete one-way slab 6 Post-tensioned cantilever beam 7 Open void

4 5 6 7

4.7

4.8

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Rapid vortex shedding

Strouhal number The rate at which wind vortices occur is described by a mathematical function called the Strouhal number S. The Strouhal number is a dimensionless parameter and relates the size of the building and the velocity of the wind to the frequency at which the vortices are created. S = B/V ≈ T S = Strouhal number; B = width of the building; V = mean hourly speed of air; T = period of vortex shedding (1/frequency) The Strouhal number has a fairly narrow range for common building shapes. For most shapes the range is from 0.11 (square) to 0.125 (octagon). A noteworthy exception is a circle with a Strouhal number of approximately 0.20.

54

Moderate vortex shedding

Slower vortex shedding

accommodate the motion of the building without damage. The perception of motion is much more complicated. Motion can be perceived in many ways: visually, audially and inertially. Visual detection of motion is uncommon, unless there are adjacent buildings of comparable height or the building twists in the wind. These conditions allow the occupant to detect motion through the movement of outside objects relative to objects inside the building. Audial detection of motion is common. The nonstructural elements can creak and groan as they try to move with the structure. This can be partially ameliorated through details that permit the relative motions to take place quietly or by using structural systems that minimise shear-racking. Inertial perception occurs through the detection of inertial forces on the occupant. This sensation can be experienced in the inner ear or feeling the need to balance. The inertial forces and frequencies can be estimated based on the dynamic properties of the building in conjunction with wind-tunnel testing. Even though the perception of inertial forces is highly variable and depends on the individual, there are industry guidelines (for example, by the Council on Tall Buildings and Urban Habitat (CTBUH), ISO, and the Architectural Institute of Japan – AIJ) for evaluating a design. They provide guidance on acceptable accelerations and velocities that building occupants will tolerate associated with various return periods. The less frequent the “event”, the higher the motion or acceleration tolerated. Although these guidelines and methods of estimating motion are very approximate, they often control the design of tall buildings. The engineer may need to address movement issues by adjusting the shape of the tower or the structural properties (mass, stiffness, periods, mode-shape and damping). One method commonly used to address these issues is to reduce the forces on the building (which also results in a reduction of motion), and to improve the aerodynamics of the tower. Key items to consider are the rate of vortex shedding, the directionality of the wind and

Improved vortex shedding

Wind

Crosswind movement

Wind

Crosswind movement

Wind

Crosswind movement

Wind

Crosswind movement

Wind

Crosswind movement

Vortex shedding force

Excellent vortex shedding 4.9

the harmonics, mass and damping of the tower structure. Vortex shedding Typical building shapes are not streamlined objects; the wind cannot flow around them like it does an aircraft wing or a racing car. Buildings are complex objects in a highly turbulent flow that comes from variable directions. The wind flow tends to detach or separate at edges and corners to form a region of separated flow and a turbulent “wake”. This vortex shedding is a common phenomenon in fluid mechanics and happens in air flow at scales as small as a wire (the sound of an Aeolian harp is caused by vortex shedding) to as large as a mountain. In tall buildings, vortex shedding is extremely important because the vortices cause alternating zones of low pressure which tend to rock the building from side to side in a rhythmic manner (Fig. 4.11). If the frequency of these pulses is close to the natural harmonics of the building, it can result in very large forces perpendicular to the direction of the wind (in the across-wind or lift direction), and are often much larger than the forces in the direction of the wind (drag direction). In order to access the sensitivity of a building to vortex shedding early in the design, SOM engineers tabulate plots of “critical building width” for various wind speeds. The frequency of vortex shedding can be estimated using hand calculations taking into account the wind speed and the building shape (see sidebar “Strouhal number”). The building width at which the frequency of vortex shedding would match the structure’s fundamental frequency of vibration is referred to as the “critical width” because it could result in resonance. Overlap between the critical width plots and the actual building width indicate a potential for undesirable resonance and point to means of tuning the structural design, either by changing the building shape or the dynamic properties. The SOM design team of engineers and architects often goes into the wind tunnel very early in the design process to assess different tower geometries (Fig. 4.9).

Elevation [m]

Structural design of tall buildings

600

1-year wind 10-year wind 100-year wind 1000-year wind 10,000-year wind Building width +/-5 % Rare wind with higher energy impacts small portions of the building at low heights for minimising overturning moments

500

400

300

4.9

Tall building aerodynamics and structural system shape efffciency 4.10 Critical building width with 8.35 second period for varying wind speeds 4.11 Dynamic frequency analysis showing the first three fundamental modes of vibration (exaggerated deformed shape), Burj Khalifa, Dubai (UAE) 2010

200

100 0 0

20

40

80

60

100 120 Building width [m] 4.10

The wind speed is a product of nature and is taken as a given. It varies in magnitude along the height of the tower, with slower speeds near the ground because of upstream ground roughness. Normally, wind speed is assumed to vary in the ratio of (z/z0)1/α where z is the distance from the ground and z0 is a reference height. In many cases, α is taken as 7. In general, frequently occurring wind speeds such as those with return periods of one year, five years and 10 years are considered for issues of occupant comfort. Return periods of 25 or 50 years are often considered for tower movement effects on items such as cladding and partitions. The return periods for strength considerations vary, depending on the design codes and load factors (factors of safety) used. These return periods might be 50, 100, 1000 or 1700 years. SOM often looks at 10,000-year wind events to Mode 1 Translation P = 11 sec.

Mode 2 Translation P = 10 sec.

see if there are unusual aerodynamic sensitivities (Fig. 4.10). Depending on the harmonics of a tower, resonance caused by vortex shedding can occur at relatively frequent return periods, so the designer should determine the structural forces at several wind speeds. The strength of the pulses from vortex shedding is related to the air flow separation. If a tower has a sharp corner, the flow separation of the air from the boundary of the tower will occur at the corner, and the forces can be quite large. If the corner is rounded, notched or canted, the point of flow separation may not always occur at the same location, and the forces will generally decrease, often substantially. Other treatments – such as blades, vents or other physical changes to the shape (including the plan proportions of Mode 2 Translation P = 4 sec.

Confusing the wind Key to successfully managing the wind is to “confuse” it by encouraging disorganised vortex shedding over the height of the tower. Commonly used techniques include varying the width of the tower and the shape of the floor plate over its height, considering dominant wind directions in orienting the building, varying the orientation of the floors and “tuning” the building dynamics / harmonics. The shape and proportions of the floor plate are key concerns. A square plan with sharp corners can have large across-wind forces. A rectangular shape can have benefits for certain wind directions. Easing the corners by rounding them or having chamfered or re-entrant corners is also beneficial. Other techniques include creating vents, holes or “bleed air” that will inhibit the formation of vortices, tapered shape, porous top, fins, strakes and out-of-alignment mode shape. 4.11

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a

b

c

d 4.12

the floor plate) – can also have remarkable effects on the across-wind forces. The circular tower shape as compared to a square or octagonal shape deserves some discussion. The circular shape has the disadvantage of shedding vortices almost twice as fast as a square, which results in resonance occurring during more frequent wind events. Also, the tower will generate the same strength of across-wind forces regardless of the wind direction. This is partially offset by lower drag forces and somewhat weaker vortex shedding forces.

Porous crown

wind

Wind directionality Depending on the climate, the directionality of the wind can have a degree of variability. This can be used to the advantage of the building. For example, the across-wind effects of the wind

Tapered form

Chamfered corners

seismic

More members at tower base to resist seismic forces

4.14

56

Structure harmonics and damping Because of the speeds of normally occurring winds and the shape and size of normal tower floor plates, it is not unusual to have wind storms generating vortices at rates that approach or match the harmonics of the tower (periods, mode shapes and generalised mass). The harmonics of the tower can be manipulated to help mitigate wind forces. The effects of having forces occurring at the harmonics of a tower can be understood by observing a child on a swing. The child is able to swing to great heights just by kicking its feet in time with the natural period of the swing. This height represents the large amount of dynamic energy being generated by the harmonic forces of the child’s feet. The analogous event is the large structural forces generated by the pulsing of the wind at the natural frequencies of the tower. To help address these issues, SOM has developed computer programs that give guidance on the proportioning of a tower to achieve target dynamic properties (Fig. 4.11, p. 55). An important parameter in wind dynamics is the damping of the building. Shock absorbers on an automobile are dampers that dissipate the dynamic energy caused by bumps in the road vibrating the vehicle. In buildings, there are low levels of natural damping in the structure and contents of the building. In a properly designed building, this is usually enough to limit the vibrations. If more damping is needed, devices such as tuned-mass dampers, piston dampers or water-sloshing dampers can be added. Often these devices are expensive and need special staff to operate and maintain the dampers. In

Ductile links/dissipation of seismic force

Optimised frame geometry/ minimised member sizes

4.13

blowing onto the corner of a square tower (diagonal direction) is much less than the wind blowing onto the face of the tower. In general, a square tower has four critical wind directions onto the four faces. If the faces do not align with the dominant wind directions of the local climate, the probable maximum wind forces will be less than if they are in alignment (see sidebar Strouhal number).

Structural design of tall buildings

a

b 4.15

Seismic issues for very tall buildings Seismic issues for very tall buildings are fundamentally different to wind issues, and the desired solution is often diametrically opposite. For most tall buildings, the higher the stiffness, the lower the dynamic wind forces, but the higher the seismic forces. In tall buildings, the higher dynamic modes are particularly important for seismic design, but are not dominant in wind design. While ductility is always desirable, the member forces due to wind do not approach the yield point of the materials, while in seismic design, ductility is a key consideration in limiting the forces and absorbing energy. In tall buildings located in a high seismic region, a good wind system, such as concentric bracing, is not desirable because of issues of stiffness and ductility, unless a seismic fuse system can be introduced to limit the forces on the tower. To address the sometimes opposing needs for ductility and stiffness on the Liansheng project, SOM engineers developed a mega-braced frame system that is inherently stiff, but augmented it with an embedded tied braced frame (TBF; Fig. 4.12) with ductile “fuses” that can be tuned to allow a specific level of seismic energy dissipation. The TBF system acts similarly to a steel eccentrically braced frame (EBF); but in the Liansheng project, the ductile link zones were tied with vertical elements to allow redistribution of ductility to multiple levels. The linkedtwin tower design expands the TBF frame concept to the macro level by linking the twin towers to effectively form one giant stiff but ductile system (Fig. 4.13). Subsequent to the Liansheng tower design, SOM engineers have further advanced the mega TBF ductile frame concept to incorporate both the ductility aspects of Liansheng – most notably the corner fuses that limit seismic forces by dissipating seismic energy through

4.12 Hybrid tied braced frame (TBF) megaframe system, Liansheng, Taiyuan (CN) 2012 (design) a Concentrically braced megaframe b Introduction of eccentric brace frame (EBF) c Introduction of tie columns d Inclusion of ductile links at all levels 4.13 Linked hybrid megaframe system, Liansheng 4.14 Schematic design of CITIC Financial Centre, Shenzhen (CN), anticipated completion 2018 4.15 Brace geometry of CITIC Financial Centre (b) as an evolution from John Hancock Center, Chicago (a) 4.16 Evolution of brace geometry: optimal truss geometry applied to tower, CITIC Financial Centre

shear yielding – and the latest results from SOM research into optimal bracing patterns (Fig. 4.12). In much the same way that SOM engineers decades ago created a new megabraced frame tube system with the John Hancock Center, the mega TBF frame proposed for the CITIC Financial Centre, Shenzhen represents a fundamentally new way of providing ductility and stiffness to supertall building frame systems (Fig. 4.15). The CITIC tower brace patterns are based, in part, on Michell truss forms. Brace angles change continuously and morph over the height of the building as a direct reflection of the optimal brace geometry. The result is at once a striking geometric display of the underlying structural system and a fundamentally new high-rise brace typology (Fig. 4.16).

P/2 P P

H

P

2B

P

z=3 –H 4

P Design domain

Optimal truss geometry

Optimal geometric proportions

n=2

n=1

n=3

n=4

n=5

Vertical stretch

almost all cases, SOM has been able to avoid the devices by designing the building based on an understanding of the wind.

n=1

n=3 4.16

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Tall building case study – Burj Khalifa 5.1 Bill Baker, Jim Pawlikowski

Creating a successful supertall building design is an exercise in developing an efficient lateral system, managing the gravity loads and the important role they play in the lateral system, understanding and manipulating the wind behaviour and environment, and fostering simplicity and constructability within the structure. All of this must be done in collaboration with the architects and the other disciplines, resulting in a design that embodies these principles while respecting and enhancing the design intent. Although the chapter “Structural design of tall buildings” (pp. 52 – 57) describes the typical process SOM undergoes for its tall building designs, one SOM project in particular stands out as an example – Burj Khalifa in Dubai, UAE. Burj Khalifa not only represents a successful implementation of structural design and wind management philosophies, but also illustrates the level of coordination required between the structural and architectural requirements of a supertall building. Burj Khalifa (Fig. 5.2) is the centrepiece of a new multi-billion dollar development located just outside of the previous financial centre of Dubai. Located in what had formerly been largely open desert, this development now contains the world’s tallest building, the world’s largest shopping mall and dozens of residential and office

towers. The tower has over 160 storeys of occupied space, consisting of a Giorgio Armani hotel, serviced apartments, luxury residential units and office floors. Six technical zones, three levels of elevator skylobbies and two separate observation decks are distributed throughout the height of the tower. A mostly open spire zone above the occupied levels completes the structure. Structural system description At 828 m in height, the Burj Khalifa stands more than 300 m taller than the previous world’s tallest building. Such height required that the structural engineers and architects work closely together from the beginning of the project to determine the shape of the tower in order to provide an efficient building in terms of its structural system and its response to wind, while still maintaining the integrity of the initial design concept. Keeping the structure simple and fostering constructability were also key goals. The resulting shape of the tower illustrates the forces influencing a tall building structure – a stout three-winged base is provided where gravity and wind loads are greatest, which tapers to a slender top as gravity and wind loads decrease. This tapering is established through the tower’s spiralling setbacks as it ascends in height. As it rises from a flat base, setbacks occur at each element in an upward spiralling pattern,

5.2

58

Tall building case study – Burj Khalifa

Wind

Tail Nose Upper plan Nose / cutwater

Wind Tail Middle plan

Nose / cutwater Wind Tail

Lower plan 5.3

reducing the tower’s mass as it reaches skyward (Fig. 5.2). At the pinnacle, the central core emerges and is sculpted to form a spire. The setbacks are organised in conjunction with the tower’s grid: the stepping is achieved by aligning columns above with walls below to provide a smooth load path. Early on, the team established the project rules of “no transfers” and “stay on module”. This enabled construction to proceed without the normal delays associated with column transfers. At each setback, the building’s width changes. The advantage of the tower’s stepping and shaping is, in essence, to “confuse the wind”. Wind vortexes can never sufficiently coalesce because the wind encounters a different building shape at each tier (Fig. 5.3). Utilising high-performance reinforced concrete, the Y-shaped structure can be described as a “buttressed core” system. The buttressed core is a response to the primary imperative of using the lateral system to manage gravity − getting gravity load out as far as possible to create a wide, stable stance, and at the same time maintaining a torsionally stiff, central, closed-shape core. Each of the building’s wings buttress the others via a six-sided central core, or hexagonal hub (Fig. 5.5; Fig. 4.4, p. 52). The central core provides the torsional resistance of the structure, acting as a strong axle and keeping the building from twisting. Corridor walls extend from the central core to near the end of each wing, terminating in thickened cross walls. These corridor walls and cross walls behave similarly to the webs and flanges of a beam to resist wind shears and moments. Perimeter columns and flat-plate floor construction complete the system. At mechanical floors, outrigger walls are provided to link the perimeter columns to the interior wall system, allowing the perimeter columns to participate in the lateral load resistance of the structure. The result is an extremely efficient structure, in that every vertical element is utilised to resist both gravity and lateral loads. As such, all vertical elements are tied together as one cohesive

lateral system, similar to one giant concrete beam cantilevering out of the ground. This allows gravity loads to be used to stabilise the structure – the weight of the structure itself is used to resist the wind. The tower foundations consist of a pilesupported raft, with 194 bored cast-in-place piles supporting a solid reinforced concrete raft 3.7 m thick. The piles are 1.5 m in diameter and approximately 43 m long, with a capacity of 3000 tonnes each (pile load tested to 6000 tonnes). The diameter and length of the piles represented some of the largest and longest piles conventionally available in the region at the time of construction. A unique situation for this scale of project arose from the site conditions: the ground water, which is quite high at approximately 2 m below the surface, is extremely corrosive, containing approximately three times the sulphates and chlorides as sea water. As such, a rigorous programme of anti-corrosion measures was followed to ensure the long-term integrity of the tower’s foundation system.

5.1 5.2 5.3 5.4 5.5

5.4 Construction, Burj Khalifa, Dubai (UAE) 2010 Burj Khalifa in its urban context Disorganised vortex shedding at different heights of the tower Construction of floor slabs Lateral system of buttressed core and construction systems

Wind engineering For a building of this height and slenderness, wind forces and the resulting motions in the upper levels become dominant factors in the structural design. An extensive programme of

Centre core wall

Automatic selfclimbing system

Wing core wall

Automatic selfclimbing system

Nose column

Circular steel form

Slab

Panel form system

5.5

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Wind tunnel testing Reduction of wind forces Wind tunnel test result: model frequency related to the recurrence interval for wind events. The vertical axis is proportional to the resonant dynamic forces divided by the square of the wind velocity. a Original building configuration b Configuration after several refinements of the architectural massing 5.9 Burj Khalifa 5.10 Tianjin CTF Financial Centre, Tianjin (CN), anticipated completion 2018 5.11 Wind tunnel workshop, Tianjin CTF Financial Centre, BMT Wind Tunnel 5.12 Wind tunnel tested schemes, Tianjin CTF Financial Centre

Normalised spectral energy of across-wind modal force

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Initial scheme Base moment Acceleration 5.6

wind tunnel testing was undertaken, during which the structural and architectural teams refined the tower’s shape to increase its performance. Wind tunnel testing was performed in Rowan Williams Davies and Irwin Inc.’s (RWDI) boundary layer wind tunnels in Guelph, Ontario. The wind tunnel programme included rigidmodel force balance tests, full multi-degree of freedom aeroelastic model studies, measurements of localised pressures, pedestrian wind environment studies and wind climatic studies (Fig. 5.6). Using the wind tunnel to understand and optimise wind performance was crucial to the tower’s design. Several rounds of force balance tests were undertaken as the tower’s geometry evolved and became refined.

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After each round of wind tunnel testing, the data were analysed, the building was reshaped to minimise wind effects and the building’s harmonics were refined (Fig. 5.7). In general, the number and spacing of the setbacks changed, as did the shape of the wings – originally, the setbacks were arranged in a spiralling counter-clockwise manner, which was reversed during testing to clockwise. Wind directionality was also studied, with respect to considering the direction of the frequent and strongest winds. As a result, the tower orientation was changed so as to better accommodate the most frequent strong wind directions for Dubai: northwest, south and east. Through wind-tunnel testing, the tower’s struc-

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ture was “tuned” to minimise the effects of the wind. This was accomplished by using the results of the tests to perform parametric studies on the effects of varying the tower’s stiffness and mass distribution (Fig. 5.8). Along with this effort, the process described above of establishing and refining the shaping of the tower resulted in a substantial reduction in wind forces by “confusing” the wind and encouraging disorganised vortex shedding over the height of the tower. These efforts also resulted in reduced wind forces and motions, such that the predicted building motions are within the ISO recommended values, without the need for auxiliary damping.

formwork system, allowing for quick floor cycle times with a minimal amount of crane usage. Only the rebar cages needed to be hoisted by cranes. Concrete is distributed to each wing using concrete booms attached to the formwork system. Two of the largest concrete pumps in the world are utilised to deliver concrete to heights over 600 m in a single stage. The core and wing wall areas utilised an “up up” construction process where the walls and wind wall column could proceed several floors above the slab pours. This was much faster than the typical construction process. Utilising concrete construction for Burj Khalifa was a natural choice. Concrete offers higher stiffness, mass and damping for controlling building motions and accelerations, which was critical in designing the world’s tallest building. In fact, due to the stiffness of the system, SOM was able to design the tower to satisfy motion and acceleration criteria without the use of supplemental damping devices. Additionally, the tower’s flat-plate floor construction offers increased flexibility in shaping the building, as well as providing the minimum possible floor thickness in order to maximise the ceiling height.

SOM engineers and architects often work in partnership together using a wind tunnel to develop the design of a tall building. The development of the Tianjin CTF Financial Centre (Fig. 5.10) included intensive experimentation in the wind tunnel to test the effects of various building configurations, including the shape and porosity of the top, the shape of the corners, possible slots or vents and several other geometric details (Fig. 5.11). The wind tunnel testing revealed that the total wind overturning forces on the tower could be reduced by more than 50 % by adjusting the geometry of the tower. The resulting geometry resulted in great material savings and a more striking architectural form that directly expresses wind-engineering principles (Fig. 5.12; see sidebar “Confusing the wind”, p. 55). Construction process Material technology and construction methods have a significant impact upon the design of supertall building systems. These elements must be incorporated early in the design process so as to provide a system that facilitates efficiency and constructability. The construction sequence for Burj Khalifa has the central core walls being cast first, in three sections; the wing walls next; then the slabs for the core and wing wall areas; and the wing nose columns and slabs after these. Walls are formed using an automatic self-climbing

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The importance of hierarchy

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Structure as poetry

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Hierarchy is in concept that can help to identify, distinguish and organise the components of a structural system or building. Not all parts of a structure or complex building system are of equal importance. The relative significance ascribed to the different elements – their hierarchy – will affect the story that the building tells. The concept of hierarchy can therefore serve as an organising principle that helps to focus the design process and identify the most critical aspects of the problem. Hierarchy is usually, but not always, related to the load path in a structure, but it is not necessarily the same. The hierarchy of systems and subsystems also often represents a conscious choice made by the designers and reflects their subjective judgements regarding the relative importance of the parts comprising a building. SOM believes that a hierarchical approach to finding a structural solution, especially as it pertains to constructability, is crucial in achieving a coherent result: a building that expresses its function clearly and candidly.

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The importance of hierarchy Bill Baker is the structural engineering partner for SOM.

Not all things are of equal importance. It is often helpful and sometimes necessary to distinguish things of higher importance from those of lesser importance. In conversation, people modulate their voices to emphasise a point. In a sketch, some lines are emphasised, others are not. The emphasis clarifies the conversation or the sketch. The clarity imparted can be intellectual, visual, physical or relate to whatever attributes connect the “things”. The hierarchy or relative importance of any group may vary, depending on the aspects being considered. The same group of students, for example, may have different hierarchies for different activities, depending on whether the activity is, say, an athletic or an academic event. In structural engineering, hierarchy is an organising tool that works at many levels. Hierarchy

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helps to organise the thought process during the design of a structure. It helps to determine how to express a structure or how to resolve the conflicts that always occur when various components come together. It helps to identify the most important components that define the structural concept and differentiate them from those that may be essential but are not defining. It helps to communicate the nuances of the structure to other design disciplines as well as clients and contractors, and identify the most important aspects of the structure, which must be respected and given priority when issues arise. Hierarchy helps to guide the design of the structural details and gives direction to how the structure may be formed and assembled. Establishing a structural hierarchy is an important conceptual process that applies to all structures. It is somewhat difficult to put a fine point on the definition of hierarchy as used here. It is not the same as load path, nor does it simply mean key elements (as the term is used in progressive collapse studies). It is often related to size, but not always. Expression may require a different hierarchy to detailing. At times, the hierarchy used for the structural expression or philosophic understanding of a structure may be somewhat different than the manner in which it is detailed to be assembled. On occasion, a tension member may be allowed to pass through a philosophically more important compression member in order to avoid issues concerned with the failure or economy of spliced tension members. Hierarchy and construction technology may sometimes be at odds, for example, at a large weldment where priority may have to be given to issues such as minimising the residual stresses from welding or avoiding through-thickness stresses that might lead to laminar tearing. All that being said, establishing a hierarchy is important to structural design, whether expressed or not. In any case, a structural engineer should always design a structure so that an architect feels bad if the structure is covered up.

The importance of hierarchy

A classic example of the expression of hierarchy can be found in the John Hancock Center, Chicago (Fig. 1.1). There is a myriad of essential structural elements, but they are not all of the same aesthetic or technical hierarchical rank. At the highest level are the diagonals, the corner columns and the mega-module horizontals. If a layperson were to sketch the building, these are the elements they would draw. At the next level are the other perimeter columns. These are conceptually important because the tower is a “braced tube” rather than simply a “trussed tower”. In the braced-tube concept, all the perimeter columns participate in resisting gravity and wind forces, not just the corner columns. The final major elements of the structural expression are the perimeter spandrels. It is here that the aesthetic hierarchy diverges from the

technical hierarchy. In the expression of the structure, all of the horizontals, apart from the horizontals at the mega-modules, are rendered equal and subservient to the structural elements mentioned above. This is not true from a technical point of view. One very important aspect of the tower is that the diagonal always meets a column at a horizontal and the horizontals between the intersections form important ties on each face of the tower, essentially creating a series of tied arches. This is a key factor in making the tower act like a tube of steel; thus, in technical terms, these ties are hierarchically more important than the other horizontals, but this is not expressed. A hierarchically important issue is the way a structure meets the ground. In the case of the Hancock, the corner columns, diagonals and mega-module horizontals all meet one storey above the ground (Fig. 1.3). The resolution of the structure is very clear, but this connection is technically not in the optimum place. A purely technical solution would have been to have these bottom nodes occur at the foundations one storey below ground level. The load path would have been straightforward, but the expression of the tower would not. Sometimes a structure must be distorted in order to be made clear.

1.2 Baxter International Inc., Deerfield, Illinois The headquarters of Baxter Travenol (now Baxter International) was designed to provide a flexible framework for innovation and expansion. SOM’s master plan called for a cluster of highly flexible, two and threestorey modular office pavilions that could expand both north and south from the central facilities building. The main structure – containing an auditorium, training center and cafeteria – is capped by a stayed cable-suspended roof supported by two steel pylons rising three storeys above the metal deck. The entire roof is supported by two giant piers which are hierarchically the most important elements, followed by the cables, then the roof framing. Visible from a nearby expressway, the twin masts have become a well-known regional landmark.

It is important to note that at SOM, even though structural elements have hierarchy, the source of design ideas does not. As a group practice with the name of three long-deceased men on the door, good ideas are welcome from all participants, ranging from summer interns to partners.

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Gravity load paths, John Hancock Center, Chicago, Illinois (USA) 1970 Baxter Travenol (now Baxter International) Headquarters, Deerfield, Illinois (USA) 1975 Intersection of corner column, diagonal and megamodule horizontal one storey above ground, John Hancock Center

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Structure as poetry Sigrid Adriaenssens is an assistant professor in the Department of Civil and Environmental Engineering at Princeton University, USA. Before that, she worked at the engineering design practices Jane Wernick Associates (London) and Ney and Partners (Brussels). Her research interests lie in novel structural systems, form finding and optimisation. She has written extensively and curated exhibitions on these topics.

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Demonstration of cantilever bridge system, Forth Bridge, Edinburgh (GB) 1890, John Fowler, Benjamin Baker Assembly of roof truss on site, International Terminal, San Francisco International Airport, California (USA) 2000 Design sketch for the roof, International Terminal, San Francisco International Airport Detail of truss connection, International Terminal, San Francisco International Airport Interior view, International Terminal, San Francisco International Airport

Good architecture, according to early writings by Vitruvius (1 CE), can be described by three main values: firmitas (durability), utilitas (utility) and venustas (beauty). Following these three values, successful designers translate a concept or abstraction into a physical reality. However, for durability and appropriateness to be achieved in a delightful design, the laws of physics must be addressed. Structure can enhance architecture by accounting for the laws of gravity in the realisation of a physical form. Generally, there is a distinct difference between the priority of values regarding architecture and structure. Architecture focuses on the aesthetic qualities and spatial programme that a form can offer, while structure addresses gravity and horizontal loads. However, achieving excellence in an architectural structure is more complex than creating a visually appealing form or resolving external loads. Instead, it requires scaling, positioning and connecting structural members in a way that embodies the architectural design intent, suits its purpose, facilitates constructability and is safe. SOM excels at integrating architecture and structure in its designs by balancing Vitruvius’s three key values. The practice has a profound understanding of the interconnectedness of a structure, from the interrelated members at the elemental level to balancing aesthetics and function at the macro scale. Its projects possess a natural structural hierarchy that works at many levels. In analogy, the English poet Samuel Coleridge (1772 –1834) once described poetry as “the best words in their best order” [1]. A poem is just a grouping of words. However, a poet chooses specific words within certain stanzas and combines them to create a unique poem. As individual parts, the words are not as strong as the feeling or idea they express as a whole. How different is poetry to a structure? Just as a poet begins with a thought or motivation, a designer begins with a need or intent. The challenge is translating this idea into a physical reality.

To achieve the best expression in the physical realisation of the design, there must be an ordering at every level, from the most global sense to the most specific connection. This implies that a hierarchy exists within a structure just as in a poem. First, there is the overall structure, which expresses a distinct style and serves the specific needs of a project, much like the verse of a poem. Next, the subsystems of the structure define an organised framework, like the grammar. Finally, elements such as members, connections and materials make up the basic components similar to the words of a poem. Hierarchy is first and foremost an expression of the relative importance of the elements that comprise the structural system. The importance may be a direct reflection of the loads, but it may also derive from more subjectively ascribed aspects of the design. SOM uses this hierarchy as an approach to organise the design process, to identify priorities, resolve conflicts between elements, express the significance of (sub)systems and components and as a guide for fabrication and construction. Structural poetry In the design process, the holistic concept is established first before smaller elemental structural subsystems and elements are envisaged. When SOM strategises to develop a design concept, both the architectural design intent and the natural configuration of the structure need to be considered. What is the relationship between the two aspects? In practice, there are a couple of ways of relating the structure to the architectural design: by concealing or by exposing and accentuating it. Depending on the specifics of the project and by comparing multiple solutions, SOM decides which strategy is best and either exposes or highlights the structural system. In exposing the structure, the system primarily provides the form for the desired architecture. However, by accentuating the structure, it is exploited as a design driver to emphasise both the form and the materiality of the architectural intent.

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The International Terminal at San Francisco International Airport provides an example of using the structure as a form-maker at the macro level. The airport terminal is both a practical facility for transportation and the source of the first impression of the city for travellers. Therefore, this design must be functional and iconic. The architectural intent of the project, defined by the project architect, was to create an airport terminal “founded upon the qualities of light and lightness”. This concept was translated in the roof structure “as a floating, sheltering plane and as a symbol”. The design team decided to use the style of grand 19th-century European railway stations as an inspiration for the roof. SOM is known for being innovative in its solutions. This innovation does not start from a

tabula rasa, but builds upon incremental knowledge, accumulated by the discipline. The extent to which the terminal roof design has advanced since the Forth Bridge in Edinburgh, Scotland (1890) is representative of the inventiveness and sophistication SOM brings to its projects. The roof uses the same cantilever principle as its forebear over the Forth (Fig. 2.1). In this bridge, two cantilevering trusses support a light central girder. The overall shape of the original bridge elegantly expresses the structural diagram of forces due to bending, as does the form of the terminal roof structure. As depicted by a sketch from the preliminary design stage (Fig. 2.3), it consists of three pin-connected truss systems: two double cantilever convex trusses and one lighter central

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convex truss. The entire structural assembly steers the design; it generates the form of the roof and expresses the architectural motivation by suggesting a wing-like abstraction. The quality of lightness desired by the design team is further enhanced by the use of natural light. At the top chords of the roof trusses, skylights are introduced to flood the concourses with pleasant levels of natural light and draw attention to the refined roof trusses (Fig. 2.5, p. 67). The San Francisco International Airport terminal is an example of SOM’s legacy of articulating the architectural design intent by celebrating structural clarity at the macro level.

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Lee Hall III, Clemson University, Clemson, South Carolina (USA) 2012, architects: Thomas Phifer and Partners, McMillan Pazdan Smith Architecture Vertical section, scale 1:50, Lee Hall III, Clemson University Interior view, Lee Hall III, Clemson University

A more recent venture the Lee Hall III expansion project in Clemson, South Carolina, further exemplifies the notion of structure as a design driver. In fact, during design development, the idea for the global concept shifted from structural minimalism to that of simplicity and constructability. This shift occurred in part as a result of the findings of the structural design process. The client, Clemson University, expressed the need for an additional building to the School of Architecture that would facilitate collaborative learning and interdisciplinary work between its students and the faculty. Through the expressive clarity of its structural and functional components, the architects Thomas Phifer and Partners wanted the building to instruct, inspire and enlighten its users. A lightweight roof with expressive supports would be an identifying feature of the design. This intent implied a hierarchically ordered system, not a structural surface system (such as a shell). The desire to have open spaces suggested that the principal structural support elements were to be linear in nature, rather than planar. Ideas about tensioned cable trusses supported on expressive columns were suggested. A large compression truss around the perimeter of the roof could resist the horizontal forces applied by the cable trusses. In this case, the columns would be left to carry vertical loads only. This initial solution was possible, although the bulky compression

truss seemed to be visually unattractive in comparison to the delicate cable trusses and columns. The initial solution conflicted with the design intent of structural minimalism. It also became clear that these systems would require special components (cables) and connections (turnbuckles), which assumed specialised knowledge of prestressing and tuning. The culmination of these factors contributed to a substantial economic cost. In addition, the proposed connection vocabulary appeared very “techy” and withdrew attention from the intended lightness of the roof support. The trade-offs in terms of aesthetics and budget did not seem to outweigh the associated complexity and cost. The initial objective for a light cable truss then appeared inappropriate. The aspiration of economic feasibility and aesthetic minimalist design intent seemed mutually exclusive with the cable truss solution. The design team visualised a creative way to minimally and aesthetically support a curved roof. As the design evolved, the importance of the skylights and the columns themselves as the organising principle of the design became clearer. First, an attempt was made to combine cable truss elements (e.g. tie rods and clevis connections) with traditional column forms. However, prestressing was also necessary in this option, resulting in higher construction costs. Prioritising the specific needs of the client and the project was imperative in leading to an optimal design: simplicity and constructability were more crucial than being structurally lightweight. Therefore a compromise was made, not as a means of settlement but to satisfy the design intentions most fully. A simple beam roof was chosen while the “column trees” could still be featured as a way to accentuate the structural steel and provide lightness in the design. The realised design of Lee Hall III integrates both function and form in its exposed steel structural system (Fig. 2.6). Lee Hall III, at 5110 m2, is almost entirely comprised of steel and glass. Its design includes an internal 10.7 m

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tall space with mezzanine levels and bridges, a curving planted green roof and 25 skylights supported by steel “column trees” (Figs. 2.7). While there were other functional means of supporting the structure, these steel columns have become the building’s main interior feature by enhancing the 2.13 m roof skylights and emphasising the architectural expression. The “trunks” of the column trees are steel tubes, each with four slender “limbs” made of thick, flat steel plates. The four arms extend to the ceiling where the circular skylight is positioned to cast light onto the columns (Fig. 2.8). While the geometry of each of the 25 column trees is unique, their connections all aim to be easily assembled and discreet. The planted roof requires a subtle curvature for optimal drainage and extends past the north and south glazed curtain walls. Therefore, exterior columns are necessary for support. To reflect the featured column trees inside Lee Hall III and continue the architectural expression of branching arms, the exterior columns were chosen to be “Y” shaped (Fig. 2.6). The design team carefully refined the design so that the fabrication and details could be completed in a conventional manner and Lee Hall III built without unnecessary costs. Despite the building having a curved roof, iconic columns and a contemporary form, the design team was meticulous in their structural organisation at the element level. In the drawings, connections were fully detailed to understand physical alignment and architectural appearance. In doing so, the connections could be communicated in simple two-dimensional drawings, and construction costs were kept in check.

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1 W 14 (Å-section 35.3 ≈ 25.4 cm) roof steel beam 2 Ø 1 in (2.54 cm) slip-critical bolts 3 Light-weight concrete composite metal deck slab: metal deck 3 in (7.62 cm), concrete topping 2 in (5.08 cm) 4 W 14 (Å-section 35.3 ≈ 25.4 cm) steel beam 5 W 10 (Å-section 25.9 ≈ 14.6 cm) steel skylight framing 6 ∑ 4 ≈ 4 in (10.16 ≈ 10.16 cm) skylight framing 7 1.25 in (3.17 cm) strut-arm plate 8 Ø 10.75 in /1.25 in thick (27.3 cm /3.17 cm) steel plate 9 Ø 10.75 in /1 in (27.3 cm /2.54 cm) seamless steel pipe column 10 Finished floor: Slab-on-grade foundation Continuous vapour barrier 6 – 8 in (15.24 – 20.32 cm) compacted granular fill Reinforced concrete footing 11 Ø 1.75 in (4.45 cm) anchor bolts, galvanised 2.7

Structural grammar In poetry, grammar defines the set of structural rules governing the composition of clauses, phrases and words in any natural language. Similarly, SOM employs structural grammar as a framework for different subsystems that contribute to a larger system to achieve a specific performance. This design strategy is exemplified by the design and construction of the Cathedral 2.8

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of Christ the Light in Oakland, California. In the design of the Cathedral of Christ the Light, the architecture needed to convey the same values and impressions that a place of worship offers: spirituality, solace and light. These objectives were achieved by geometric manipulations of the vesica piscis image (Fig. 2.9). This Christian symbol, two intersecting circles of equal diameter representing fusion and the reconciliation of opposites, was interpreted as a three-dimensional form and distilled into structural subsystems. At the same time, the new cathedral was to be designed for longevity in an area of high seismic activity. To cope with large earthquakes, a second hierarchical organising strategy was introduced. It resulted in a horizontal stratification with a robust (earthly) layer dug into the ground and lighter (heavenly) layers in the sanctuary above. These two main layers were separated by an isolation layer. The materials and form selected were also to complement the local architectural style of the San Francisco Bay Area in an ethos of sustainability.

2.9 Vesica piscis diagrams 2.10 Cathedral of Christ the Light, Oakland, California (USA) 2008 2.11 Construction of wood louvres, Cathedral of Christ the Light 2.12 Exploded axonometry, Cathedral of Christ the Light

The design process began by considering the congregation seating layout. The vesica piscis was translated in the layout by situating the church within the round arrangement and placing the altar to one side. The symbol is reflected by the outline of the reliquary walls in plan. These walls are exposed reinforced concrete

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and articulate the sensorial architectural intent of the cathedral at ground level. The walls are cast in place in a highly detailed form including openings to smaller side chapels and niches for audio and mechanical systems. Self-compacting concrete and high-density plywood formwork were used in the construction of the exposed concrete walls. The shape of the vesica piscis symbol returns as a hierarchical geometrical rule in defining the interior, a naturally lit sanctuary enclosure. The height and width of the three-dimensional geometry for the interior form is derived from its shape in plan, defined by the vesica piscis symbol. The proportions are related by the square root of hree, a value commonly found in nature and in the order of the universe itself. This inner form consists of curved connected ribs made of continuous glued laminated timber, which connect to a compression ring and horizontal roof diaphragm. This interior shape is further articulated by a series of closely spaced, glued, laminated timber louvres (Fig. 2.14, p. 72). The weight of these louvres is transferred to the interior ribs through bracket connections. The louvres are multi-purpose: they circumferentially brace the slender interior ribs and allow sunlight to flood the sanctuary space throughout the day. The outer glazed enclosure also relates to the vesica piscis symbol. The outer members, which slope inwards, are made of straight and continuous, glued, laminated timber elements. These are positioned on the conical ground plan, yet are aligned with the radial geometry of the inner rib members, as defined by the vesica piscis symbol. The conical shape of the outer form allows the cladding – fritted glass unitised panels – to be uniform planar facets. The glass is mechanically attached to horizontal tubular steel members, which carry mainly wind loads but also support the dead load of the cladding. The horizontal elements further serve to brace the exterior ribs circumferentially. The outer and inner forms are connected and behave together as a hybrid frame. This frame

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is made of linear elements (glued laminated ribs) connected three-dimensionally to each other with glued laminated compression struts and high-strength steel tension rods. As a result, the frame system can carry significant lateral loads, such as wind and seismic forces. The vertical section (Fig. 2.13, p. 72) shows that the hybrid frame consists of four types of elements: the outer and inner laminated timber ribs, to which the fritted glass and wooden louvres transfer their loads respectively; the connecting timber struts; and the diagonal steel tension rod bracing. The connections of the timber ribs and compression struts, as well as the steel tension rods, all contain true “pinned” connections to allow rotation and ensure the member loads are purely axial. The hybrid timber and steel frame sits on top of the reinforced concrete reliquary wall with a delicate pin connection, allowing both gravity and lateral loads to be resolved in the concrete walls and eventually through the base isolation system, substructure and foundations. Due to the location of the Cathedral of Christ the Light, seismic activity is an important concern to address in the structural system. In this case, a steel friction pendulum seismic base isolation system was used to support the sanctuary floor and reliquary. At the same time, the base isolator facilitated the architectural concept of horizontal stratification into earthly and heavenly layers. Although the base isolation system, which also holds the mechanical plenum, is not apparent to the visitor’s eye, it is crucial in defining the horizontal hierarchy. The system separates the robust mausoleum, which is below ground and capable of resisting earthquake loads, from the sculpted reliquary walls accessed from the ground floor and the intimate enclosed volume above the congregation area.

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The design and construction of the Cathedral of Christ the Light relied heavily on two hierarchal rules imposed in defining both the architecture and structural systems. The first rule was the geometry, the result of spheres and cones, also

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Curtain wall, unitised aluminium Fritted glass, laminated Wood rib, glue-laminated Steel tension rod, galvanised Tapered compression strut wood Tapered curved wood rib, glue-laminated Wood louvre Aluminium coping, powdercoated 20 in (50.8 cm) mullion extensions with stainless-steel inserts laminated on south-facing side of each vertical mullion beyond top of window wall Aluminium bar grating 4 ≈ 6 in (10.16 ≈ 15.24 cm) tube

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found in the Christian vesica piscis symbol; and the second was the base isolation, crucial for the cathedral’s long-term survival in a high seismic region. Geometry was used as a unifying rule to define forms in plan and section. The horizontal base isolation layer imposed horizontal stratification, which suited the architectural concept of a more earthly layer (the mausoleum) and the more heavenly layers (the congregation area and the light enclosure above it). As a result, the structural systems are design features and architectural form-makers, successful in evoking devoutness, consolation and light. Structural vocabulary To achieve the best expression in the physical realisation of a project, there must be quality at every level – from the most global sense to the most detailed individual elements and connections. The designer has to make choices at each hierarchal level. The manner in which forces are transferred from one subsystem to the next, and how an entire structural system performs as one unit, significantly depends on the elements and types of joints used. To achieve economically efficient structures, strategies for selecting and joining structural elements must also aim to avoid connections requiring extensive fabrication or time-consuming assembly. However, in SOM projects, the detailing can exceed this objective by also having a crucial role in establishing the overall design concept of a project. SOM strives to produce structural clarity in its details for even the most physically complex and technologically advanced structures. Its strategy is to design for pure load paths with minimal interruptions caused by eccentricities between members.

reflect a high-tech, machine-like aesthetic that expresses engineering excellence. This design intent was captured by giving hierarchical importance to the lightweight, efficient tensile elements and their connections. The Entrance Pavilion is a lenticular building with anticlastic, prestressed cable nets. The lenticular geometry was adopted to coordinate with the facade cable nets, consisting of horizontal, circular, segmented and vertical parabolic cables (Figs. 2.15 and 2.16). This anticlastic, prestressed cable net resists all positive and negative wind pressure applied to the glass facade of the pavilion. The parabolic vertical cables are prestressed against the roof truss, which is supported at the vertical apex trusses. The cables, with their lightweight, high-tech appearance, were given hierarchical priority over all other structural members. To further express the presence of tension forces in the design of the pavilion, the structural elements used were specific to the type of force they carry: tension in cables, and compression and bending in tubes. As a

2.13 Section, scale 1:200, Cathedral of Christ the Light, Oakland, California (USA) 2008 2.14 Interior view towards altar, Cathedral of Christ the Light 2.15 Isometric drawing, General Motors Entrance Pavilion, Detroit, Michigan (USA) 2004 2.16 General Motors Entrance Pavilion

In the design of the Entrance Pavilion for General Motors’ (GM) Global Headquarters in the Renaissance Center in Detroit, Michigan, for example, the detailing was key in realising the architectural concept and defining the hierarchy within the structural system. Given the worldclass corporation’s role as a vehicle manufacturer, the architectural design was meant to 2.16

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result, the cable net is form defining, and the entire pavilion appears lightweight and efficient. Compression only appears in the compression chords of the roof truss and the end post trusses, which are designed to be slender and not to distract attention from the tensile cable network. Their slenderness was made possible by partial bracing. Together, all elements form a self-stressed system with very clear articulation of tensile cable and slender compression elements, a minimalist structure emphasing function and efficiency. This self-stressing global structure is organised based on a sequence of subsystems that resist gravity and wind loads. The lenticular geometry of the pavilion has a curvature that allows each of the single laminated glass panels to be planar and equally sized. The detailing of the glass panels allows the glass to pass the compression chord of the end post truss and governs the spacing of the cable net. The glass facets follow the curve of the horizontal cables with radial geometry. The dead and wind loads acting on the glass panels are transferred through specially designed spider fittings to prestressed vertical cables by the extension of a cable net compression strut. These vertical cables are prestressed against the roof structure and foundations. The cables are designed not to go slack under any loading combination. As a result, they greatly inhibit deflection and rotation of the roof and can be thought of as thin columns supporting the roof.

2.17 Schematic drawing of roof truss, General Motors Entrance Pavilion, Detroit, Michigan (USA) 2004 2.18 Cable net truss to roof truss connection, scale 1:20, General Motors Entrance Pavilion 2.19 End-post truss, scale 1:10, General Motors Entrance Pavilion 2.20 Interior view, General Motors Entrance Pavilion

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While dead loads are simple to consider in design, wind forces are less predictable. They cause both positive pressure and suction, which may be uniform or vary in magnitude. Although cable and glass structures can handle these forces with significant deflections, the GM Entrance Pavilion was designed to be stiff and deflect minimally. Given its lenticular shape, the structure alone could handle uniform suction, but would not be able to withstand variable wind pressures. Therefore, the prestressed cable net provides stability, strength and stiffness for vari-

able wind forces through its anticlastic prestressed form. As one set of cables in the net (e.g. the horizontal ones) takes larger tensile loads, due to wind suction for example, the forces in the set of opposite orientation (e.g. the vertical cables) are reduced. If non-uniform loads act on the structure, the cable net resists the forces with small allowable displacements. These opposite sets of cables are separated by a compression strut made of stainless steel. Additionally, a stabilising cable was added to the third points of the vertical cables to avoid out-of-plane translation of the strut and vertical cables under certain load combinations. The vertical cable system needs to be prestressed against the roof truss (which spans onto the end post trusses and further resists horizontal wind loads) in its horizontal plane, and gravity forces in its vertical plane. In the horizontal plane, the truss consists of one central stainless steel compression member with stainless steel ribs radiating from it in a leaf-like pattern. These ribs transfer the prestressed vertical cable forces and the dead load of the roof glass panels back onto the truss. In the vertical plane, the truss forms a double convex shape and spans the end post trusses. The shape of the bowstring truss mimics the bending moment diagram due to gravity and prestress loads. The truss has a top and bottom chord and was triangulated for stability during the erection process. It was important in this project to ensure all eccentricities between members were eliminated to provide clear load paths and concurrent connection points. Resolving the component forces into a single concurrent connection point is not a simple endeavour, especially in three dimensions. For example, the connection between the roof truss and the cable net involves seven intersecting elements. Therefore, a stainless steel casting is used to join the multiple elements at a single point to avoid creating eccentricities and bending stresses. The desired high-tech efficiency was further expressed by articulation of the forces through

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the choice of structural members, e.g. tension in cables, compression and bending in tubes. At the detail level, SOM stayed true to its overarching goal of structural simplicity. The strategy behind the connection design was to reduce the size of clamps and plates so that all tensile elements visually bisect the compression members. For example, in the pre-stressed cable net, the vertical and horizontal cables intersect the compression strut and spider arm. When taking a closer look, it can be seen that the clamping bolts, which connect the cable net to the compression strut and spider arm, are hidden for greater visual appeal. The detailing design philosophy of the GM Entrance Pavilion clearly illustrates that the best realisation of a project relies on careful choices being made at the element and connection level. The form and nature of the components express the forces they carry. Connections accentuate and do not distract from this force flow through the different subsystems. This design philosophy can be traced back to one of SOM’s original sources of inspiration, Mies van der Rohe, who claimed: “God is in the detail”.

elements. The hierarchy in this project is defined by the grammar rules of analytical geometry and horizontal stratification. At the micro scale, the design of the GM Entrance Pavilion prioritises tensile elements, their connections and their relationships to other elements with the objective of obtaining a clear visual load path and evoking engineering excellence. Reference: [1] Coleridge, H. N.: Specimens of the table talk of the late Samuel Taylor Coleridge. London 1835

It is said that “an engineer is a (wo)man who can do for a dime what any fool can do for a dollar”. While it is customary for an engineer to be responsible for achieving a specific technological need at the lowest economic cost, this saying is crippling to both the engineer’s creativity and the design’s potential. Within the context of the large scale and programmatic complexity of the majority of SOM’s projects, the engineered structure is often expressed to accentuate the architectural design intent at the macro, meso and micro scales. The organising tool at these different levels is hierarchy. The San Francisco International Airport and Lee Hall III show how the expression of the convex trusses and the column trees are the global form-makers for the design concept, distinguishing themselves from other systems that are also crucial but not determinative. At the meso scale, the Cathedral of Christ the Light illustrates that hierarchy does not necessarily equate to load path or key 2.20

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Exchange House in detail 3.1 Dmitri Jajich is an associate director for SOM. Bill Baker is the structural engineering partner for SOM.

Broadgate Development The master plan, architectural and structural engineering design of 10 buildings in the Broadgate complex is part of the largest single development in the City of London. The multi-use complex expands the financial district of London by providing new office space and trading floors and enhances the surrounding urban district with retail and leisure facilities. Three public squares and the terraces and landscaping of Exchange Square, with spaces designed for performance and recreation, provide a focal point for the complex.

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Site location Structural system components Typical floor plan, scale 1:1000 Erection of steel structure

Challenges and constraints Completed in 1990, Exchange House is an office tower situated just north of London’s Liverpool Street Station in the Broadgate Development, directly above the convergence of multiple working rail lines. The site was developed as an “air rights” project whereby the existing property remained unaffected – the rail lines had to remain intact and usable throughout and after construction – but building could occur in the empty space – the “air” – above the property (Fig. 3.1). The site precluded the placement of large columns on a regular grid except in two zones spaced 78 m apart. Therefore, realising Exchange House required combining aspects of a bridge and an office building. Rather than concealing the bridge aspects of the design (or constructing a conventional building on top of a bridge), the designers decided to celebrate and express the bridging nature of the design. SOM identified the design’s principal objectives: the structure must efficiently span 78 m over the existing tracks, and a clear structural language should seek to reveal the workings of structure and simultaneously echo the existing iron and steel structures of Liverpool Street Station. Structural solution Numerous solutions were initially considered, including a diagonalised truss, a hanging catenary suspension system and several variations of the arch. Early engineering studies revealed that the diagonalised truss system required approximately 30 % more steel than the final tied-arch scheme because of inherent inefficiencies related to a longer load path – in a conventional truss, loads transmitted through the diagonals must travel a significant distance to the top ends of the truss before being transferred down to the supports. The catenary suspension system is naturally efficient at spanning; however, it requires a 10-storey tower to support the suspension elements, which increases the load path. The suspension system, furthermore, would have required a complex erection sequence. The final tied-arch system offered the most direct load path into the piers (and thus a min-

imal amount of steel), simpler connections and a relatively conventional construction sequence. The resulting structure also presented a simple and legible form, clearly expressed the hierarchical function of its key elements and allowed connections to be articulated with a consistent structural aesthetic (Fig. 3.2). Primary and secondary structure The Exchange House structure supports 11 (10 storeys) 78 m ≈ 52 m levels on four segmented, tied, parabolic arches spanning the 78 m distance over the tracks below. Two interior arches traverse the building internally, and two exterior arches are left exposed on the north and south faces of the building. All components of the exterior tied arches, as well as the exterior columns and hangers, are offset 1.8 m from the building enclosure to fully expose and express the primary structural elements. The offset from the facade places the primary structure outside the potential fire zone and means it does not have to be fireproofed. This emphasises the relative importance of the structure and begins to reveal the internal hierarchy of the building’s systems: it was a conscious design decision to designate the tied-arch system, which gives the structure its ability to span, as the most important aspect of the building. The window wall forms a smooth metal and glass screen behind the structure that enhances the clarity of the steel elements. In addition to this clear expression of the exterior arches on the north and south facades, a large atrium space inside the building was provided to reveal the structure of the internal arches. Fire-rated cladding and rational fire engineering analysis allowed the primary exterior structural steelwork to be expressed without the need to apply conventional fire proofing. The structuralarchitectural form, expression and articulation of the connections are realised primarily in exposed, painted structural steel. The proportions of members and connection details follow a strict structural logic to directly express the workings of the structure and the relative import-

Exchange House in detail

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ance of each element in the overall hierarchy. Even to the untrained eye, the hierarchy of the system components is intuitively obvious: the most important system (the tied arch and hangers) is presented in the outer layer and, within that system, the size and shape or members and the connections between members convey each element’s place in the hierarchy. Each floor plate is supported by composite floor trusses spanning four lines of columns (above the arches) or hangers (below the arches) in the planes of the arches. The columns and hangers are in turn supported by the tied arches at the arch node points. At the east and west ends of the building, an exposed vertical truss connects to the floor diaphragms to provide lateral stability and resist wind loads applied to the broad (north and south) faces of the building. In addition to the direct connections of the floor framing to the exterior hangers and columns, a secondary system of horizontal bar bracing in the plane of the floors was provided to serve as an extension of the floor diaphragms (across the 1.8 m gap between the facade and the arches) out to the plane of the arches – this system of horizontal bracing stabilises the arches and columns at the arch node points (Fig. 3.14, p. 80). The bracing between the slabs and the external frames reflects the need to brace the columns. Above the arch, the vertical load-carrying elements are columns in compression, thus rods and a spandrel beam are provided to brace the column at each floor level. Below the arch, the vertical elements that were columns become tension hangers. Elements in tension are not subject to buckling and require no bracing; thus the spandrel beam and the bracing rods are eliminated inside and below the arch. The result is a less obstructed facade under the arch, which accurately conveys the stability requirements at the different locations.

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allel steel plates. The primary ties connect to the base of the arch, near the point of bearing. A secondary tie formed by the first-floor framing connects to the arch about 1 m above the primary ties. All gravity loads ultimately lead to the arch bearing joints, which are supported by tapered reinforced concrete piers (Fig. 3.9). The piers continue down to track level, where they are supported on hand-excavated, underreamed pile groups. Groups of two and three piles with bell diameters of up to 6.2 m support loads of approximately 30 MN below the exterior bearing joints and 60 MN at the corresponding interior positions. The use of hand-excavated piles (i.e. no piling rig) allowed the trains to operate without interruption during construction.

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Modular frame The parabolic arch represents the most direct load path by axial compression when subjected to uniform loading. The actual building frame is laid out on a modular basis with a grid of columns, hangers and beams, which induces a series of discrete point loads on the arch. SOM engineers compared the behaviour of a smooth parabolic arch to a faceted arch, and determined that the latter more accurately reflected the discrete nature of the loading and eliminated bending moments in what would otherwise be a purely axial force member. The faceted arch was thus more efficient and presented a more accurate representation of the structure’s load flow. The faceted form of the arch may be thought of as the compression analogue of a funicular shape. The optimal shape for an arch carrying point loads is a faceted parabolic arch in which the slope of each segment is proportional to the vertical load carried (Fig. 3.8). Thus the centre or “key stone” segment of the arch is flat because the total shear at the centre of the span is zero and the segment carries only the horizontal thrust of the arch. Thinking of the tied arch as a giant simply-supported beam, the horizontal thrust is equal to the simple moment of the load (wl2) ⁄ 8 divided by the height of the

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arch. The segment adjacent to the centre carries the vertical load of one hanger/column line, and the inclination of the segment changes in order to resolve the vector sum of the horizontal thrust and the total vertical load. Thus the slope progressively increases as each successive arch segment picks up an additional column /hanger load. Support nodes Near the base of the arches at the bearing joints, the arches develop large horizontal thrusts. These thrusts are not imparted to the piers but are resisted by the tension ties, thus making the overall arched frame a selfcontained system that transmits only vertical forces to the top of the piers. The bearings at the interface between the steel arch and the top of the piers were designed to allow rotational movement and provide selected lateral restraint. A specific combination of lateral freedom and restraint allows the structure as a whole to expand and contract on top of the piers with changes in temperature, while providing a load path by which lateral wind loads are transmitted to the foundation. Bearing supports on one side of the arch allow horizontal movement, while the other end is restrained. Wind forces on the broad face are transferred to the middle two piers only, thus allowing the building to expand and contract in the short direction. The movement restraints at the bearings were provided by bulkhead stiffeners attached to the bottom of bearing plates keyed into U-shaped slots in the upper bearing plate (Figs. 3.5 and 3.6). By presetting the clearances around the slots, specific horizontal movements can be permitted, while others can be restrained. While the tied-arch system is highly efficient at carrying uniform gravity loads, a typical arch system proved to be sensitive to unequal live loads. A set of diagonals in the plane of the arch connecting the arch with the primary tie were provided to eliminate this sensitivity and improve the buckling strength of the overall 3.9

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system (Figs. 3.13 and 3.14). The diagonals are important to the overall system, but they are not the primary load path. Thus they are expressed hierarchically lower than the tied arch by the use of smaller round-shaped members.

components on the system to depict clear paths of load flow. For example, the columns and hangers were detailed to pass through the arch segments without interrupting the in-plane geometric continuity (Figs. 3.15 and 3.16). This choice serves to emphasise the hierarchical relationship between the two elements and clearly communicates that the building grid is suspended from the arch. The detailing of vertical connections further elucidates the nature of the forces carried in different parts of the structure: hanger splices carry tension (Fig. 3.11) and are detailed as visible lap splices, whereas column splices carrying compression are expressed as butt splices (Fig. 3.10).

Hierarchy of components The principal components of the system are the arch segments, the arch nodes, the base node, the primary ties, the hangers /columns, the arch diagonal, the horizontal bar bracing and the end trusses. The detailing and expression of the structural steel components was intended to convey a consistent character and emphasise structural logic and hierarchy by the use of crisp, open forms. After discussions with the design team, a structural “bridge” detailing aesthetic rather than a “machine” aesthetic was chosen. Equally important as the aesthetics was the need for simplicity, clarity and ease of fabrication and erection. The functional hierarchy of the arch system was expressed by layering the

The secondary nature of the arch diagonals is expressed by the use of smaller circular steel shapes which bypass the hangers with direct connections to the arch and primary tie. Floor framing truss extensions protrude through the facade and are connected directly to the column /hangers, thus making clear that the

3.10 Typical exterior column splice detail, not to scale 3.11 Typical exterior hanger splice detail, not to scale 3.12 Exposed structural system components 3.13 Unsymmetrical load deformations a Without diagonals b With diagonals 3.14 Exterior arch elevation 3.15 Detail of arch node connection, not to scale 3.16 Construction of exterior arch 3.17 View of Exchange House from plaza

Bibliography: Iyengar, Hal; Baker, Bill; Sinn R. C.: Broadgate Exchange House – Structural Systems. In: The Structural Engineer, Vol. 71, No. 9, May 4, 1993 3.14

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Exchange House in detail

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floor system loads flow from the floor to the column /hangers, then to the arch, and finally down to the bearings. Realisation A high degree of dimensional control and craftsmanship during fabrication and erection was required to realise the structural details in an aesthetically consistent manner. To that end, SOM structural engineers provided highly detailed, fully-engineered structural connection details that were painstakingly coordinated with the architectural design. In contrast, the connections in a conventional steel building are typically designed and detailed by the steel fabricator, and the exact configuration of the details is often not fully understood during the architectural design process. SOM engineers worked collaboratively with both SOM architects and the steel fabricators early in the design process, prior to bid, to determine a comprehensive system of tolerances for fabrication and erection and to ensure that structural details allowed the necessary access and adjustment to correct for distortions, movements and errors. Every detail was reviewed with the fabricator and erector, then modified as needed to optimise construction and maintenance.

To a layperson, the simplicity of the structural concept and clarity of the detailing may belie the high degree of design and craftsmanship required to realise Exchange House. The paradoxical – but now commonly accepted – conviction of Mies van der Rohe that “less is more” could be amended in the case of Exchange House to “less is more work”. Indeed, the success of Exchange House is a testament to the perseverance of the design team. The engineering goal of a minimum-material, well-proportioned and hierarchical structure was complemented by the sparsely elegant architectural expression that takes its form and beauty from the underlying hierarchical organisation of the building. Exchange House represents the successful transformation of a difficult engineering challenge into architectural expression, and exemplifies the SOM tradition of building designs in which architects and structural engineers can, by working together, produce an elegant solution.

3.16 AIA Twenty-five Year Award The Broadgate Exchange House has been selected for the 2015 AIA Twenty-five Year Award. Recognising architectural design of enduring significance, the Twenty-five Year Award of the American Institute of Architects is conferred on a building project that has stood the test of time by embodying architectural excellence for 25 to 35 years. Projects must demonstrate excellence in function, in the distinguished execution of their original programme, and in the creative aspects of their statement by today’s standards. Other SOM projects that won this award were Lever House, New York City (1980), Air Force Academy Cadet Chapel, Colorado Springs (1996), John Hancock Center, Chicago (1999), Weyerhaeuser Corporate Headquarters, Federal Way (2001) and Hajj Terminal at King Abdulaziz Airport, Jeddah (2010).

The tied arch was erected on temporary shores, with adjustable jacks at each hanger acting as columns during erection. The shores were supported on the plaza structure, which was capable of supporting the steel and metal deck structure (excluding the floor slab concrete) up to the eighth floor – the level which would complete the arch (Fig. 3.17). Upon completion of the arch and tie system, the shores were removed by jacking up the entire structure at the eight supports by 50 mm. This operation ensured that load was removed from all shores simultaneously and that the arch was uniformly loaded. The continuation of the hangers below the primary tie (to allow temporary support) was intentionally expressed as a remnant of the construction process. 3.17

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Structural art

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Constraints spur creativity

Maria E. Moreyra Garlock, Annette Bögle

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Optimising design goals

Maria E. Moreyra Garlock, Annette Bögle, Nils Ratschke

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The economy of construction

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Integrating discipline and play

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SOM’s structural engineers strive to create efficient and economical designs, where efficiency relates to minimisation of materials and economy to minimisation of cost. Since these two goals are not always aligned, engineers must strike a perfect balance to find solutions that are at once efficient, economical and practical. Key to the search for efficient structural systems is the geometry, where architecture and structure intersect. The complex interplay between efficiency and economy is addressed in the initial design phase using a variety of optimisation tools, where optimisation refers to maximising the most important design characteristics while minimising expensive or scarce resources. Often, optimising construction speed is crucial. Material expenditures can also be critical, but merely minimising the size of individual structural members is not sufficient, as their arrangement and the form of the overall system are often more important. The efficiency of a system may be gauged through the study of load paths, where the load is defined as the product of force and member length. Most of the time, a form in which load path length is minimised is the form that couples the maximum stiffness and strength with a minimum of material. The demands placed on the optimisation of forms often lead to new architectural modes of expression. But optimisation alone is not enough. SOM’s process of benchmarking and systematic bracketing and study of structural system options is as important as the optimisation results. Achieving efficiency and economy in a structure involves the parametric analysis of the effect of many different variables on the cost and efficiency of a design.

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Structural art Maria E. Moreyra Garlock is an associate professor at Princeton University in the Department of Civil and Environmental Engineering where she is the Director of the Architecture and Engineering Program. Her scholarship is in resilient building design and in studies of the best examples of structural designs of the present and past. Annette Bögle is a structural engineer, author, curator and full professor for Design & Analysis of Structures in the Department of Civil and Structural Engineering at HafenCity University in Hamburg.

References: [1] Billington, David: The Tower and the Bridge. The New Art of Structural Engineering. Princeton, NJ 1985 [2] Fazlur Khan: A Philosophic Comparison Between Maillart’s Bridges and Some Recent Concrete Structures. Presented at Second National Conference on Civil Engineering: History, Heritage and the Humanities, Princeton University, 4 – 6 October 1972 [3] Kielar, Richard M.: Construction’s Man of the Year. Avantgarde High-Rise Designer Fazlur R. Khan. In: Engineering NewsRecord, 10 February 1972 [4] Khan, Yasmin Sabina: Engineering Architecture. The Vision of Fazlur R. Khan. New York 2004

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The Industrial Revolution ushered in the age of new materials, and with the new materials – such as industrialised iron, structural steel, reinforced concrete, pre-stressed concrete, structural fabrics, glass and composites – came new forms for structures. It is in this context that “structural art”, defined by David Billington as the art of the structural engineer, was born [1]. Structural art encompasses three ideals: efficiency, the true ethos of engineering, characterised by the conservation of natural resources through the minimisation of materials; economy, the ethic of engineering, which strives for cost reduction by intimately connecting design to construction; and elegance, the aesthetic of engineering, represented in the creation of beautiful forms. Designers who seek and achieve these three ideals in their completed works are referred to as structural artists by David Billington. These designers seek to integrate elegance and efficiency rather than superimpose one on the other. They illustrate how the best technical design leaves room for ethical and aesthetic choices. The principles of design creativity that make great structures possible are timeless, and remain highly relevant today. SOM engineers aspire to continue this tradition of idealistic engineering by seeking efficiency and economy through design. In the realm of tall buildings, SOM was one of the first firms to express art through structure, through the innovative designs of structural engineer Fazlur Khan (1929 –1982). Famous, now iconic buildings and entirely new structural systems such as the Willis (formerly Sears) Tower (“bundled tube”), the John Hancock Center (“braced tube”), the DeWitt Chestnut Apartments (“frame tube”) and the Hajj terminal (“stressed membrane”) were all products of Khan’s innovations at SOM. He was a teacher, designer and scholar. As a designer, he worked closely with architect Bruce Graham, and as a teacher with architect Myron Goldsmith. Khan began the strong SOM tradition of engineering innovation and creativity in tall building design that still continues today. He believed in the “beauty and strength of the natural forms a structure tends to

take” [2] – in other words, that to achieve the right visual impact “a building’s natural strength should be expressed” [3]. His daughter Yasmin writes about his belief in “rational architecture – ranking structure as a determinant in design rather than an afterthought” [4]. Expressing the “natural form” and “natural strength” of a building means to convey how the structure functions – how the gravity or wind loads are resolved down to the foundations. It is within this context that an evaluation of structural art can be made. In a typical building, the structural system is hidden behind a facade; such a building expresses architectural art rather than structural art. Regardless of the art type, the building can be efficient and economical. SOM continues the tradition begun by Khan and his partners, architects Graham and Goldsmith, of expressing elegance through an expression of structure. This elegance is so seamlessly integrated with efficiency and economy that it is not possible to separate it from them. The strong relationship that SOM continues to foster among architects and engineers is significant particularly in tall building design. From an engineering perspective, form controls forces, whereas for architects, it controls spaces. In a building in which both forces and spaces must be synergistically controlled, the integration of the two disciplines is vital. Creativity requires bold and sometimes unconventional choices – it takes courage. When faced with serious constraints, engineers at SOM illustrate their creativity by innovating structural systems and structural forms and applying these unprecedented approaches to their designs. Creative thinking is also demonstrated in the use of both very modern and very old but forgotten tools to optimise forms for a minimum use of materials – the essence of efficiency. The economy of a structure is not only related to the reduction in materials use, but is also decidedly linked to the construction of a built work. Ultimately, both efficiency and economy are essential to the sustainability of a structure, which in the context of structural engineering is most strongly related to its embodied energy.

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Constraints on efficiency and economy in design do not stymie creativity; in fact, more often than not, they engender the creation of iconic structures, innovative forms and novel construction techniques. All buildings are in large part defined by the spatial constraints imposed by the site and the programme. The structural solution is further constrained by aspects such as budget, time, logistics, availability of equipment, expertise and experience, cost and availability of materials and customs and preferences. The importance of limiting material expenditures is paramount in tall and large structures, in which even a small increase in efficiency can translate into large material and economic savings. Many historical examples exist to illustrate the importance of constraints. Pier Luigi Nervi’s style, for example, featured ribbed surfaces for thin-shell concrete structures, which are difficult to form using traditional construction methods. Arguably, his design innovations were conceived and realised because he was building in Italy during the 1930s and 1940s, when construction materials were scarce. Likewise, Felix Candela designed iconic double-curvature, thin-shell concrete structures, mostly in Mexico City in the 1950s while working under tight economic constraints. SOM’s roots lie in the design of buildings of unprecedented height, which impose more extensive constraints and a greater imperative for efficiency than more conventional, smaller structures. This increased degree of attention to efficiency carries over into their approach to other types of work. Overcoming the tough challenges imposed by constraints requires “design engineering”, defined as problem-solving through creative thinking. “Technician engineering” – generating calculations without innovative thought – is not sufficient for achieving a solution. Fazlur Khan, working with Graham and Goldsmith at SOM, built breakthrough high-rises, the forms of which were developed under both spatial and economic constraints. It is arguably because of those constraints that innovation and creativity found their full expression. The three projects

examined in the following sections – the DeWitt Chestnut Apartment Building of 1964, the Broadgate Exchange House of 1990 and the Broadgate Tower of 2008 – exemplify the power of constraint-driven inventiveness. Fazlur Khan and the DeWitt Chestnut Apartments SOM received the commission to design the DeWitt Chestnut Apartments in 1961 (Fig 1.1 and 1.2), the same year that they received the commission to design the Brunswick Building. The 1960s marked a period of time in urban design during which efforts were being made to avoid the proliferation of “urban canyons” and to introduce more open spaces and plazas. The associated loss of ground-level square footage resulted in the need to build upward, since urban spaces were expensive and owners did not want to lose rent revenues in exchange for more pleasant street-level experiences. The constraints imposed by this building climate drove the innovative engines of Khan, in particular, to develop new forms and systems that would be efficient for tall concrete structures such as the Brunswick Building and the DeWitt Chestnut Apartments. The innovations that led to the development of the framed tube began with the design of the Brunswick Building (Figs. 1.3 –1.5, p. 19). In response to the office-space modules proposed by the architects, Khan suggested a column spacing of 2.84 m on the building perimeter. This decision was economically advantageous because the close spacing would eliminate the need for an expensive curtain wall, since the window panes could be secured directly between the concrete framing elements. The decision to leave the columns structurally exposed, without curtain walls, was difficult to make. Although it resulted in significant savings, there were concerns about the aesthetic aspect, as well as the impact that the harsh Chicago environment would have on the concrete. Khan perceived that the closely spaced columns would have a structural effect on the response of the shear walls located in the ser-

Maria E. Moreyra Garlock, Annette Bögle

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Construction work over railway, Broadgate Tower, London (GB) 2008 Construction of A-frame struts, Broadgate Tower Structural frame and steel plate girder raft over railway tracks, Broadgate Tower Broadgate Tower in its urban context

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vices core, which were designed to handle the lateral wind forces. He undertook a simple but careful study of these closely spaced perimeter columns and learned that they were capable of resisting nearly as much load as the shear walls. He then did more detailed research on the interaction between the interior shear walls and the exterior perimeter columns, the results of which were published and well received by other members of the profession. The Brunswick Building was arguably the first application of his tube-in-tube innovation [1]. In the time period between 1961 and 1963, while Khan was learning about the interaction between the shear walls and closely spaced columns of the Brunswick, he was also studying another structural system, which he referred to as the “framed tube”, for use in the DeWitt Chestnut apartment building project. In simplest terms, the framed tube can be described as a four-sided building in which the solid concrete walls forming the perimeter have had holes cut through them. The result is similar to the closely spaced columns of the Brunswick Building, except that the columns in this case are even closer together. To avoid congestion in the street, the designers of the DeWitt Chestnut decided to set back the building, giving it a smaller footprint. In order to preserve the total square footage of the original design, the height had to be increased from 26 to 43 storeys. Raising the height of a building increases the wind forces and consequently the moment and shear forces at ground level. Khan knew that for a building this tall, reinforced concrete with shear walls in the service core was the most economical solution. However, shear walls in apartment buildings are more difficult to situate than in office buildings. Apartment buildings are typically designed with a footprint that maximises perimeter area and therefore window exposure, resulting in long oblong rectangular cross sections. This aspect ratio results in insufficient wall lengths in the shorter dimension. The conditions that had to be met by this design were very restrictive. The solution was the

framed tube, which is essentially a vertical cantilever with a hollow tube profile cross-section in which the tube walls are perforated to create daylight openings. The columns were spaced at 1.68 m intervals and connected by 61 cm high spandrel beams. The small distance between the columns and the stiffness of the beams are the major reasons for the building’s stability. This exterior lateral system required only gravity columns inside the services core, which gave the structure functional flexibility. Because they concentrate the lateral-resisting frame on the perimeter of the building and thus allow the full footprint of the building to be utilised to resist lateral loads, framed tubes have the potential to be far more efficient than conventional building systems. This increase in efficiency allows buildings to grow taller and more slender without prohibitive increases in material use and cost. SOM engineers compare the quantities in their designs against other buildings, and to this day the DeWitt Chestnut building remains one of the most, if not the most, efficient concrete building for its height in terms of the volume of concrete used per area of built space. Exchange House In a 1993 publication [2], SOM authors wrote: “The art of structural engineering lies in the transformation of engineering challenges into aesthetically elegant architectural solutions”. This statement is partially based on their experience in designing the Exchange House (see “Exchange House in detail”, pp. 76 – 81), which is a building-bridge hybrid spanning a 78-metre congested railway yard at Liverpool Street Station in London. When faced with difficult constraints, designers do not immediately arrive at their final solution. Typically there are several options which are evaluated according to the three ideals of structural art: efficiency, economy and elegance. In the case of the Exchange House, several alternative designs were considered. Engineers and architects eventually selected a system of four tied arches, which “presented the most

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direct load flow to the piers, simpler connections, a conventional construction method, and was the most efficient system. The resulting arch structure also presented simplicity, clarity of form and function, and an ability to be architecturally articulated.” The engineering design for this project was led by Bill Baker and used labour-intensive parametric studies to develop the geometry [3]. Baker recently illustrated a less labour-intensive way to arrive at optimal geometric forms using Maxwell’s load path theorem, which is a 150-year-old method for understanding efficient geometries. This theorem, essentially lost to the modern engineer, is an elegant, simple and powerful tool for optimising efficiency (see “Maxwell’s theorem of optimal load paths”, p. 89f.). Broadgate Tower The Broadgate Tower in London, completed in 2008, represents another integrated architectural and structural solution driven by a highly constrained site. It is formed over a deep steelframed raft structure, which bridges the railway lines running directly into Liverpool Street Station (Figs. 1.3 and 1.5). The raft structure was designed by SOM during the mid to late 1990s, and was originally conceived to support a 10 to 12-storey groundscraper that would cover essentially the entire site. During the early 2000s, the development goals for the site changed as the demand for smaller, more flexible office floors emerged. At the same time, the desire to extend the urban realm and the north-south connection became more imperative, and a one-third public space requirement was imposed on the site. In order to maintain the same office area with a reduced ground plane contact, successive proposals saw the building grow vertically to 35 storeys. At 150 metres, the Broadgate Tower ranks among London’s tallest structures (Fig. 1.6). In addition to the rail lines below the building, the St. Paul’s “view corridor” was one of the primary drivers, dictating a taller massing on the western portion of the site. The limitations on structural support imposed by the raft below the ground plane, and the inability to connect the lift core to

the foundations due to the rail lines, together defined the structural challenge. However, rather than limiting the design, these constraints inspired the development of a thoroughly embedded structural solution within an architectural form and aesthetic. The lateral X-bracing and the large A-frame struts (Fig. 1.4) which support the eastern face of the building over the rail tracks below are clearly expressed and uniquely articulated. The A-frame struts, furthermore, help to activate and define a covered outdoor plaza between buildings. For towers of this height, a stiff shear wall core would normally be used as the main lateral stability element. However, as there is no way of connecting a large core with the foundations below, a cross-braced lateral system on each face of the tower envelope was employed, producing a clear structure and logic that are truly integrated with the architectural aesthetic.

References: [1] Khan, Yasmin Sabina: Engineering Architecture. The Vision of Fazlur R. Khan. New York 2004; p. 69 note 8 [2] Iyengar, Hal; Baker, Bill; Sinn R. C.: Broadgate Exchange House – Structural Systems. In: The Structural Engineer, Vol. 71, 09/1 [3] Baker, William: Structural Innovations. Combining Classic Theories with New Technologies. Higgins lecture at Northeastern University’s Department of Civil and Environmental Engineering, 13 February 2014

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Maria E. Moreyra Garlock, Annette Bögle, Nils Ratschke is a structural engineer and PhD researcher in the Department of Civil and Structural Engineering at HafenCity University in Hamburg.

To optimise is to make something as effective as possible within a prescribed set of constraints. In the context of structural and architectural designs, effectiveness requires a balance of efficiency and economy. The definitions of efficiency and economy given in the beginning of this chapter appear to harmonise; however, these two design goals can have an antithetical relationship, meaning that they can be in opposition to or in disagreement with one another. For example, the most optimised form for material efficiency may not be the most optimised form for economy, since it may result in increased construction costs due to construction time and complexity, in addition to which it may have a larger carbon footprint. Optimised design is hence a balancing act between efficiency and economy, and to some extent elegance as well. Dealing with compromises, weighing the various different optimums and generating solutions that can handle changing boundary conditions make a design project challenging, but it is during this process that the holistic quality of the result is forged. SOM’s history of structural optimisation SOM has always recognised that the decisions made regarding the form and layout of a structure have the most impact in optimising both efficiency (minimising materials) and economy (minimising construction cost). Already in 1953, Myron Goldsmith wrote his master’s thesis on structural efficiency at the Illinois Institute of Technology under the guidance of professors Mies van der Rohe and Ludwig Hilberseimer. His thesis included studies of the weight-to-span ratio of railroad bridges and the limits of scale (span) for different bridge structure types (truss, suspension, etc.; Fig. 2.1). He extended this discussion about the effects of scale – one major aspect of efficiency – to tall buildings and developed new exterior lateral systems for high-rise buildings. Goldsmith and Khan taught at the Illinois Institute of Technology, where they jointly supervised the research projects of graduate students for 22 years. This research was often about new

88

forms and optimised design solutions, such as the trussed tube concept of the John Hancock Center. Khan wrote about optimised steel structural systems for tall buildings of different scales, where “scale” in this case refers to the number of storeys . For example, he illustrated that a moment-resisting frame is not efficient or economical for a building of more than 30 storeys, and if one were to increase the height beyond this boundary, there would be a rapid rise in cost – a “premium for height” related to the lateral load effects such as wind. Khan’s “premium for height” concept is shown in Fig. 2.2. If a tall building is in a hypothetical glass dome, it is unaffected by wind. In this scenario, the cost of the building will increase modestly as the number of storeys increases. A structural system that is unaffected by the impact of wind is referred to as an “ideal system”. In reality, of course, tall buildings are not in domes. The taller the building, the larger the wind forces, and the more expensive the building becomes. Traditional structures that are designed for wind show an exponential increase in cost with number of storeys. Khan’s optimised innovations aimed to keep the price of tall buildings closer to the “ideal system” curve rather than the “traditional structures” curve. The framed tube system of the DeWitt Chestnut apartment building discussed earlier is an example of an optimised design. For example, placing the columns that resist the overturning effects of wind on the perimeter – in other words, as far away as possible from the centre of the building – instead of in the interior is the most efficient approach for both increasing the stiffness (reducing lateral deflections) and decreasing column forces (reducing the total required column area). However, forming many small columns on the perimeter is more expensive than building fewer, larger columns that are distributed on both the interior and exterior of the building, as is typical. Because of this cost discrepancy, the framed tube design may seem at first glance not to be optimised for economy, but a more holistic review reveals otherwise. The

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added expense of forming many columns is tiny compared to the significant savings realised by the obviation of a curtain wall, since the windowpanes can now be secured directly between the closely spaced columns. In the final analysis, the framed tube system is optimised for both efficiency and economy. As the need for designing taller buildings grew, SOM innovated with other structural systems such as the trussed tube (John Hancock Center), the bundled tube (Sears Tower), and the buttressed core (Burj Khalifa). While designers did not refer to their innovations as optimisations, the concept was nevertheless there: for a given scale, they were finding and developing forms – systems – for buildings that would result in designs that were efficient (least weight), stiff enough to resist lateral forces (least deflection) and economical (buildable). Over the past five decades, the tools available to the engineer have become quite advanced and sophisticated. In contrast to the early work of SOM, engineering today without computerTruss type

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based design and calculation techniques is hard to imagine. Though the goal to find optimised structures remains the same, the tools have changed. Recently, SOM engineers have rediscovered optimisation and design tools that are more than 100 years old, such as graphic statics and Maxwell’s load path theorem. These venerable tools are generally no longer taught at schools, nor are they known and used by most designers. Yet SOM engineers creatively utilise these old methods in combination with modern ones for optimising designs (see also “Structural optimisation – developing new design tools”, pp. 111–121)

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Bridge structural systems that represent the longest spans of each type at the time and the effects of scale, in terms of span length, requiring different structural systems for bridges. From: Myron Goldsmith: The Tall Building. The effects of scale. Chicago 1953 Fazlur Khan’s definition of “premium for height” Comparison of load paths for various cantilever truss configurations. The load path is inversely proportional to the hypothetical material efficiency of the truss.

Maxwell’s theorem of optimal load paths In general, the emphasis for structural efficiency lies in minimising the material quantity (structural weight) while maximising the stiffness (minimising deflections). While it may sound like a paradox, minimising the structural weight can actually maximise the stiffness. This is proven in Maxwell’s theorem of optimal load paths from 1870 [1].

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The theorem essentially states that, for a given set of applied loads and boundary conditions, the sum of the tension force load path minus the sum of the compression force load path equals a constant, where “load path” refers to the force in the member times the member length. The power behind this simple theorem is that if one increases the tensile load path, the compressive load path will also increase by an equal amount for a given set of applied loads and boundary conditions. The amount of material is directly related to the force and length of each member – that is, to the load path. An increase in the tension load path and the associated increase in the compression path therefore

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In 1904, Anthony George Maldon Michell presented his research based on Maxwell’s groundbreaking paper [Michell, 1904]. While Maxwell focused on the theoretical background of the load paths, Michell actually developed structures based on this theoretical approach. He discovered that the structural elements of an optimal truss (that is, with minimal structural volume and maximum utilisation of elements) must be arranged along the trajectories similar to principal stresses. Due to the fact that the external loads remain constant, the minimum structural volume implies the maximum stiffness for a given structure. This knowledge was lost, or rather unheeded, for a long time. Design optimisation considers more than just the minimisation of material and deflection. The designer must consider “complexity, cost, usability, aesthetics, multiple loading conditions, and permitted stresses” [2] that can be different for tension and compression. Michell’s truss solution is the geometry of the unbounded (not limited to depth B) load path structure; however, it is impractical to build. Nevertheless, it is useful as a benchmark for other cantilever designs and as a concept of structural creativity.

Early study for Elizabeth House, London (GB), design 2011, architects: David Chipperfield Architects Model, Elizabeth House Elizabeth House Structural elements, Elizabeth House redevelopment Axial force diagram for exterior trusses, Elizabeth House Bridging structure typology studies based on structural volume, Elizabeth House

Elizabeth House Maxwell’s theorem of load paths served as the solution for the structural system of the Elizabeth House redevelopment (Fig. 2.6). The project consists of two buildings: a nine-storey office 2.6

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imply a doubling of the additional amount of material. On the other hand, optimising the tension load path also optimises the compression load path. This theorem is illustrated by a simple cantilever structure that is three times as long as it is wide (Fig. 2.3, p. 89). A cantilever truss can be designed in the shape of a moment diagram, which is a good starting point for optimising form. Using Maxwell’s theorem demonstrates forms that are more favourable than those of the moment diagram, leading to the optimal form: a Michell truss.

1 Reinforced concrete pier 2 Primary tension tie 3 Cantilever diagonal 4 Built-up structural steel main truss member 5 Typical gravity column 6 Secondary tie/strut 7 Rolled-steel secondary truss member 8 Typical infill floor 9 Primary compression strut

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building on the southern site and a 26-storey mixed-used office and residential building on the northern site. The completion is planned for 2019. Because of its location near Waterloo Station in the inner city of London, the foundation constraints imposed mainly by the underground posed a challenge for the structural engineers. The superstructure of the taller building requires deep pile foundations, but their placement is severely limited by the underground infrastructure. Having previously gained experience with similar boundary conditions, the SOM team developed a bridgelike superstructure spanning 108 m of several underground subway lines’ tunnels. But in contrast to Broadgate Exchange House, the architecture here is more eccentric, making the search for an optimal loadbearing structure more complex. To find the best structural system with high stiffness and less weight, the engineers used Maxwell’s theorem of minimised load paths to define different topologies of the bridging

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structure. After this theoretical approach, a full finite-element analysis was completed; the topology study in preparation for the detailed analysis allowed a quick evaluation of different structural geometries (Fig. 2.8). These are judged by their structural weight as compared to the depth of the frame, equivalent to the number of storeys of the frame. Obviously the A-frame becomes increasingly efficient (with comparatively less weight) as the height of the frame rises. The structural solution also needed to be in sync with the various uses of the building. Using these optimisation techniques on the Elizabeth House provided a clean structural system with well-defined load paths and a clear member hierarchy (Fig. 2.7). The system is clearly organised according to three levels: the first is the main outer frame; the second are the stability diagonals; and finally there are the typical elements such as standard columns, floor framing beams and hangers. The clarity of this structure is visually apparent and reveals the structural thinking behind it.

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100 Mount Street Building The development of the structural system for the 100 Mount Street Building in Sydney, which will be completed in 2019 (Fig. 2.13), provided an even more complex optimisation challenge. Again the goals were maximum stiffness with minimal structural weight. But additional goals included maximum openings for prime views to the south, east and north. The core containing building services is located on the western end, where the building abuts existing construction. The eccentric non-central placement of the core improves the architectural layout, but exposes the building to undesirable torsional or twisting forces. To address this issue, SOM engineers placed a mega-braced frame on the eastern facade. The frame balances the stiffness of the core walls to the west and pulls the tower’s centre of stiffness back near the geometric centre of the building. In combination with the shear wall core, the mega-braced frame resists winds on the broad face and the torsion that they create. This frame needs to be as stiff as possible to balance the lateral stiffness of the shear wall core and to prevent torsional vibration from becoming a dominant mode. At the same time it should be as slender and elegant as possible to promote the least disrupted sight lines. Different bracing layouts were studied and found to be suitable, but too much material was called for to achieve the required stiffness. Thus the engineers referred back to their experience with the Maxwell method and started with some hand sketches. This was followed by a computer-aided topology optimisation study to determine the best bracing layout, in which the SOM engineers made use of their previous research in topology optimisation. What emerged was an X-bracing system with an elevated central node at about threequarters of the height of the X (Fig. 2.12). At this node height, the stiffest brace geometry is achieved. To realise the full potential of this frame geometry, the form-determining optimisation process was followed by an intensive study of structural

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behaviour resulting from a released node and the stability issues of the long diagonal compression members. Last but not least, there was an intensive discussion about structural detailing and the question of whether to choose concrete or steel for the frame. Because of the stiffness and strength requirements, concrete was recommended for both the core and the frame. The Polestar Tower Requirements for optimisation design goals are complex and potentially antithetical (see “Structural Art”, p. 84). In some situations, benchmarking structural solutions may be a helpful tool to obtain reasonable arguments for one or the other solution. Detailed system-

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2.10 Structural system components, 100 Mount Street Building, Sydney (AUS), design 2012 2.11 Typology optimisation process to find ideal truss geometry, 100 Mount Street Building 2.12 Topology optimisation: optimal bracing geometry a Problem statement b Free-body diagram c Topology optimisation results 2.13 100 Mount Street Building 2.14 Slenderness study for various heights, Polestar Tower, Gothenburg (S), anticipated completion 2019 2.15 Study of different outrigger configurations and normalised resulting material quantities, Polestar Tower 2.16 Comparison of deformed shapes due to lateral wind load for various outrigger configurations, Polestar Tower

atic parameter studies serve to determine the best system and evaluate its efficiency. These studies should be carried out even before a schematic design is developed so as to facilitate a new approach to the structural solution. When it is finished in 2019, the Polestar Tower in Gothenburg (Fig. 2.18, p. 94) will be 230 m tall, making it the tallest building in Scandinavia and scaling up the traditional development patterns in the country. The tower will be mainly for residential use, with a variety of flats positioned over several mixed-use podium blocks. Other facilities may include a public restaurant, a gym, a residents’ lounge, a podium-level garden and a rooftop observation deck overlooking the city and the waterfront. The design is highly efficient in terms of both floor space and energy use. Its fluid geometry is accentuated by the balconies that provide each flat with flexible living space. The aim of the benchmarking process is to determine whether the design of the structure is to be primarily strength /gravity-driven or serviceability /motion-driven. With increasing slenderness, the effect of the dynamic wind forces on and the resulting movements of the tower have more and more influence on the structural design process and thus on the evaluation of the structural material to be used. A first step involves an intensive study of local construction costs and the consent of the client, with the aim of arriving at unit costs of structural elements depending on the material and installation methods. This allows simple calculations and comparisons to be made. For the Polestar project, the process demonstrated that concrete was by far the most cost-effective material for the tower in this market. In the next step of the benchmarking process, a series of different standard floor framing methods are considered and sketched. In addition to the quantity and cost of materials, particular aspects of the construction have an influence on the evaluation. For example, the total depth of the system has a feedback effect on the overall tower height and efficiency, the speed of

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construction, the availability of local contractors and skills and the flexibility of the layout. The floor framing discussion is followed by an overview of different SOM projects to point out the influence of the construction programme on the overall costs. For this project, the designers presented several lateral structural systems and discussed the pros and cons of each. They selected the outrigger system for further examination because it is a suitable system and can be used as a benchmark for other systems. A series of parametric studies were undertaken to consider the material quantities required to frame the tower with a variety of outrigger locations and numbers. The

resulting table shows concrete quantities associated with each scheme (Fig. 2.17). This information allows the designers to make early price estimates and helps them identify the best approach for the project. Material quantities are not the only measure of efficiency. Speed and ease of construction can be equally – if not more – important in determining the most efficient system. A building that is fast and easy to build is often less expensive than a more complex building with less structural material. Another consideration is the suppression of shear racking to minimise potential noise when the building moves in the wind. A second parametric study focused on the relationship between total concrete quantity and increasing height. This allowed the design team to determine the ideal height of the building. Fig. 2.15 (p. 93) shows the quantity of material versus the height and illustrates when a building is “gravity-controlled”, and at what point of increasing height the wind forces and the movements of the structure begin to control the design. When the latter conditions take over, increases in height cause an associated rapid increase in structural materials. The process of benchmarking and then parametrically assessing the efficiency of different options provides valuable information that can inform the architectural and structural design rationally before the building plans are finalised. SOM believes this process is key to the success of tall buildings. Though the process often results in the choice of the least expensive structural system, it does not dictate this choice. The client and design team may choose other options that have other benefits. The point is that the optimisation process provides a framework in which to evaluate the costs and benefits of a variety of different solutions. References: [1] Maxwell, J. C.: On Reciprocal Figures, Frames, and Diagrams of Forces. Philosophical Magazine and Journal of Science, Vol. 26, 1864 [2] Baker, William: “Structural Innovations: Combining Classic Theories with New Technologies”. Higgins Lecture at Northeastern University’s Department of Civil and Environmental Engineering, 13 February 2014

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Efficiency through optimisation leads to the use of fewer materials, which in turn often leads to more economical designs. However, a building involves more than just the material related to the basic structure. A significant amount of material is for architectural purposes. Integrating the two types of material – that is, having the structural material act as architectural material as well provides opportunities for more economical designs. Construction costs play a significant role in the economy of a design, and construction is often more expensive than the material. “Time is money” is one of the oldest clichés, but in the construction of a large-scale structure, time is also risk. The longer it takes to finish a construction project, the higher the risk that political and /or economic events could postpone or halt the project. Consequently, in planning a design, construction must be considered not only with regard to feasibility and direct expense, but also to time. These two aspects of economy – integration and construction – are presented in this section. Case studies of SOM designs are used to illustrate designs that are successful by all standards – efficiency, economy and elegance – achievable through creative thinking and strong collaboration between architects and engineers. These case studies range in scale from a single-family residence to the tallest building in the world.

The project site was a challenge. There are no contractors located on the island and few who even work there. Further, all materials and equipment had to be brought to the island on small boats. Given these site constraints, conventional construction materials had to be used for the project to be economically viable. To achieve the modern, simple aesthetic, the design team built the residence using a variety of materials that are commonly available and often used in construction, though not necessarily in combination. The house has reinforced concrete foundations and basements walls, wood framing on the ground floor, steel stud and plywood shear walls, steel columns, steel and metal deck roof framing and aluminium and steel canopies. In the Fishers Island Residence, SOM was able to eliminate one architectural system by affixing glass directly to the structural columns. Traditionally, buildings have steel columns that hold up the roof and mullions that support and laterally brace the glass. Affixing the glass directly to the structural columns integrated the architectural and structural systems such that the steel column and the mullion are now one and the same. This solution led to an efficiency of materials, economy of construction and elegance achieved through a minimalist, clean aesthetic. The approach requires carefully managed construction, as different trades must work together to align the steel correctly. An

Maria E. Moreyra Garlock, Annette Bögle

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Fishers Island Residence, New York (USA) 2007, architects: Thomas Phifer and Partners Construction, Fishers Island Residence Like the Fishers Island Residence, the Manulife Pedestrian Bridge, Calgary, Alberta (CDN), eliminates a structural system of mullions by affixing the glazing directly to the structural steel vertical truss members.

Economy through integration The concept of economy through integration refers to combining structural and architectural systems so that the structure becomes the architecture. It requires an architect who is willing to accept the raw structural aesthetic and work with it rather than covering it up. One example in which economy through integration is achieved is the Fishers Island Residence (Figs. 3.1 and 3.2). SOM collaborated with architect Thomas Phifer and Partners to provide structural engineering services for the Armstrong residence located on Fishers Island, NY, on the eastern end of Long Island. The residence is modern in style with an open floor plan, a simple aesthetic, glass walls and exposed steel. 3.3

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1 Ø 18-in (45.7 cm) HSS steel pipe 2 Open spelter socket, galvanised 3 HSS 8 ≈ 4 structural horizontal steel mullion, painted 4 Ø 8-in (20.32 cm) steel pipe center post, painted 5 0.5-in (1.27 cm) steel plate built-up, tapered cruciform shape, painted 6 Glazing system 7 Ø 42-mm full-locked cable, pre-tensioned, galvanised 8 Cable clamp, galvanised 9 Pin with cap plate 10 1-in (2.54 cm) flat steel connection plate, continous through pipe 11 Ø 2-in (5.08 cm) high strength steel pin with cap

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additional feature that marries structure to architecture can be seen in the 50 steel canopy “trees” that are detailed to be reminiscent of actual trees. Another project design that integrates architectural form and structure is the award-winning Virginia Beach Convention Center. Since its completion, the Virginia Beach Convention Center (Figs. 3.4 – 3.6) has become the centrepiece of a 40-year master plan to green Virginia Beach and reinvigorate the classic resort town. A soaring 45-m glass and steel tower pays homage to the city’s historic lighthouse. Beside the tower, a central glass hall with 12 m-high ceilings and a 73 m-long clear roof span attracts notice; its dimensions make it one of the largest column-free spans in the US and represent a significant feat of engineering. SOM engineers developed lightweight, prestressed cable trusses to support the curving glass curtain wall. The combination of the primary structural system and the supporting system of the glazing expresses structural as well as architectural clarity. The structural elements are part of the architectural intention, underlined by careful detailing. Integrating structure with architecture can be uneconomical as it can involve expensive or unconventional fabrication techniques, special finishes, exotic connections or the higher tolerance architecturally exposed structural steel

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(AESS) requirements. SOM works to overcome these typical obstacles to economy by utilising mainly conventional materials and fabrications. The design team makes every effort to develop standard simple connections and fabrication techniques that will not incur undue expense. In addition, AESS requirements are applied only to critical aspects of the project. For example, while tighter tolerances may not be required, removal of mill marks from the steel may be. These projects illustrate how architecture and structure can be successfully integrated to achieve elegance of form and style without compromising the economy of a project. They represent one end of the spectrum of building design: small, complex spaces. At the other end of the spectrum lie the extremely large, mostly modular spaces, which also benefit from economy of construction. One of these is the tallest building in the world.

ciples is one of the fundamental rules of structural design: “Don’t draw anything unless you have at least one idea how to build it” [1]. The consequences of ignoring this rule are very high – sometimes prohibitively high – construction costs. Achieving simplicity and efficiency in construction is not easy and can be at odds with other architectural and structural goals, but it is essential to making supertall buildings buildable. The economy of construction of the Burj Khalifa revolved around two simple but important considerations: (1) no transfers; (2) to stay on module (the second meaning of the acronym SOM). With regard to the first consideration, it is important for the design team to be educated

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Curving glass curtain wall, scale 1:100, Virginia Beach Convention Center, Virginia Beach, Virginia (USA) 2007 Conceptual sketches, Virginia Beach Convention Center Central glass hall, Virginia Beach Convention Center

Tall buildings and Burj Khalifa Many tall buildings are proposed by design firms all over the world, but most are never built. Many of them are too complex, take too long to build or are too expensive. SOM engineers have learned that efficiency and economy in construction are as important in tall buildings as structural quantities, and a critical aspect to optimise. SOM’s thesis states that tall buildings that do get built must feature any or all of the following attributes: • Rational forms with structural systems that reflect the physics of tall buildings • An appropriate relationship between architecture and structure • Fast construction that is economical, reliable and feasible • Efficient forms that are optimised for stiffness and weight • Elegant design that reveals its underlying logic and is attractive in its own right. These were the underlying principles that informed the conceptual design of the world’s tallest building, the Burj Khalifa in Dubai (Fig. 3.7, p. 98; see “Tall building case study – Burj Khalifa”, pp. 58 – 61). Central to these prin3.6

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in recent advances in technology and construction methods. The structural team, led by Bill Baker, “didn’t want to have a system that required something that didn’t already exist … We wanted to use conventional systems arranged in a unique manner … If you get too bizarre, too complicated, it gets too expensive, a lot of times contractors are too afraid to bid it, …, you can’t get competitive bids” [2]. For large projects like this, the economy of the project will be sensitive to staying on module, meaning establishing a grid and sticking with it. Such an approach allows reuse of construction materials such as formwork. The Burj Khalifa project was able to reuse some of its formwork about 160 times. Although the floor plates all varied on the horizontal surface, they could be reutilised because, vertically, they were identical. A special challenge for the construction of this supertall structure was pumping the concrete to an altitude of well over 600 m. A horizontal pumping trial, simulating the appropriate drop in pressure, was done to ensure that it was possible. To facilitate the operation, reduced aggregate sizes were specified by the contractor working with material specialists for the upper floors. The final system used two of the largest pumps in the world, capable of pumping concrete at up to 35,000 kPa through a 150 mmthick pipeline. While the Burj Khalifa project did not require the invention of new construction techniques and materials, the existing ones it did employ were state of the art. For example, the walls were formed using the latest advances in automatic, self-climbing formwork. Also, given the limitations of conventional surveying techniques, a special GPS system was used to measure the verticality of the tower as it was being constructed. State-of-the-art, high-performance concrete was used for the columns and walls, with varying strength for different floor heights.

Level 128 1002 m2

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References: [1] Baker, William: Interview at Princeton University, 5 October 2010; available online at http://www.princeton.edu/ engineering/video/player/?id=6033 [2] Ibid.

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Isometric diagram, typical floor plates within each programmatic zone and stack of individual floor plates, Burj Khalifa, Dubai (UAE) 2010

Sustainability

Sustainability This chapter began with a discussion of “structural art”, which came into existence with the advent of the Industrial Revolution; but this same revolution also had negative consequences that today’s society must learn to manage. The increased burning of fossil fuels (carbon dioxide producers) and the depletion of forests (the second-largest natural carbon dioxide sink) have contributed to a significant increase in the atmospheric concentration of carbon dioxide, a main contributor to climate change. As a result, modern structural designers must consider the impact that their design choices have on the environment. Context and importance Sustainable – also referred to as “green” – design has always been one of the prime goals of design, as sustainable design is synonymous with an efficient use of resources, both with regard to the material used in a structure and the construction process. Far from viewing it as an impediment, SOM engineers exploit the need to limit carbon as a measure of structural performance. Optimising for minimal cost or material may not be important to clients who can afford to build inefficiently, or to an architect who values form over efficiency. But when building codes encourage or require limiting carbon production or energy usage, the engineer has another tool at his or her disposal to measure and advocate for efficient designs. Because large amounts of energy are required to produce both steel and concrete, minimising carbon emissions is often synonymous with minimising material. The increasing interest in sustainability has not only bolstered the need for efficient design with traditional structural materials, but has also encouraged SOM engineers to think more holistically about the expenditures associated with the entire construction and design life of a building, thus leading them to consider new materials and construction techniques and re-evaluating materials such as wood. Sustainability, therefore, is in line with the timeless engineering values of economy and efficiency and how the

design decisions based on these values affect our natural environment. Optimising for minimum materials to achieve efficiency should be combined with the practical considerations of constructability and cost, and the ethical considerations of minimising carbon dioxide emissions and reducing the consumption of our natural resources.

Maria E. Moreyra Garlock, Annette Bögle

Buildings are a major contributor to climate change. Of all US greenhouse gas emissions, the majority are related to energy production and consumption, and most of those are carbon dioxide (CO2). According to the Energy Information Administration (EIA), “from 1990 to 2013, energy-related carbon dioxide emissions in the United States increased on average by about 0.3 % per year. Of the total amount of US greenhouse gases emitted in 2013, about 84 % were energy related and 92 % of those energy-related gases were CO2 emissions from the combustion of fossil fuels.” The carbon emissions associated with a building can be classified as “operational” – lighting, heating and cooling – and “embodied” – the energy needed to produce the materials and construct the building. A life cycle assessment of buildings shows that the production of materials used in the structure of a building is the largest source of embodied energy. Therefore, material choice (steel, concrete, timber, mixed) and the quantity of material needed (efficiency) have an important impact on carbon emissions. Typically, the ratio of embodied to operational carbon emissions is 0.10 to 0.30 [1]. Current initiatives are trying to reduce the operational carbon footprint to zero. For example, Architecture 2030, a non-profit organisation, established a 2030 Challenge: to reduce carbon emissions so that, by 2030, all new buildings will be carbon neutral. The best known initiative for the consideration of environmental impact on building design in the US is LEED (Leadership in Energy and Environmental Design), developed by the US Green Building Council (USGBC). LEED is a rating system that qualitatively evaluates the

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environmental “friendliness” of a building design. It does not, however, attempt to measure carbon emissions. Policies such as more stringent building codes are one approach to reducing carbon emissions; however, the initiative for a sustainable design goal must come from the designers, code or no code. The designers must have a green design objective and make a conscious decision to minimise the negative impact of materials, construction and form on carbon emissions. The most important decisions affecting the success of this design goal happen in the early stages of the project, so it is critical for both engineers and architects to work together to identify the proper form, building orientation, material, etc., since these will have enormous impacts on carbon emissions.

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Learning and Development Center, Roche Diagnostics Corporation, Indianapolis, Indiana (USA) 2015 Comparison of embodied carbon for Timber Tower Research Project versus the original concrete design a Standard materials b Sustainable alternatives

Tall buildings and urbanisation “More than one half of the world population lives now in urban areas, and virtually all countries of the world are becoming increasingly urbanised”, according to the United Nations. As the population density increases in urban areas, the need to build tall grows. This density in urban areas can have a positive impact on carbon emissions by promoting sustainable transport, since transport is currently the second-largest source of carbon emissions. With proper urban planning, residents and commuters can rely on mass transit, bicycling or walking instead of cars. For example, when Sears was still in the Willis Tower, they had a large portion of the tower’s 405,000 m2 of floor space on one city block located close to the trains. Nearly everyone came to work by mass transit as there is very little parking. When Sears moved to the suburbs, they built low-rise buildings covering many more acres of land. Everyone needs to drive to work, which means more emissions. Furthermore, a large car park is required to accommodate all the cars, and each parking space is larger than the office space in a typical urban office. Sears’s move from downtown Chicago to the suburbs had a significantly negative impact on its carbon footprint.

In addition to reducing carbon emissions by encouraging the use of mass transportation, tall buildings can have a positive impact on operational energy. Modern technology leads to efficient central services, which consume less energy per square footage of floor space than single-family homes. In many cities, multiple buildings are linked so that they can share energy sources. Tall buildings also have a larger volume-to-surface ratio, so their indoor heating and cooling are less sensitive to the influence of the outdoor climate. The challenge in tall building design is to minimise the potentially negative impact of embodied energy. Relative to low-rise structures, more material is needed to support a tall building, for example larger columns and bigger lateral loadresisting elements. The first-floor columns of a 60-storey building will be much larger than the first-floor columns of a three-storey building with the same floor plan. The taller building’s columns need to support 59 storeys, whereas the shorter building only needs to support two. Lateral loads due to wind or earthquakes produce “bending moments” that are proportional to the height of a building squared. Therefore, taller buildings will have much larger bending moments and consequently larger structural members. The carbon cost of a building is very sensitive to its height. The earlier discussion of the “premium for height” (Fig. 2.2, p. 89) clearly applies not only to the cost in dollars, but also to the cost of the carbon footprint. Tall buildings must therefore be designed to minimise the premium for height. In 2003, when SOM won the competition to engineer the Burj Khalifa, the sustainability movement was not as strong as it is today, but sustainable design was considered good practice by the SOM designers and thus incorporated from the very start. For example, the Y-shaped plan form provides for self-shading, so that only one sixth of the facade is in direct sunlight at any time, thus reducing the energy needed to cool the interior. In addition, a stainless-steel vertical rib, 20 cm deep, is placed at every mullion to reflect light, which serves to keep light

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from heating the building. The Burj Khalifa also harvests water. The building is located in a humid environment, and the condensed water from the air conditioning is captured and used in the building. The innovative cooling condensate recovery system can provide 14 Olympicsized swimming pools of fresh water annually. In addition, the height was used to good advantage via “sky-source sustainability”. The temperature and humidity decrease with elevation. Engineers took advantage of this thermodynamic fact and pulled the cooler, fresher air from higher zones to lower zones. Solutions for reducing the carbon footprint Sustainable engineering is integrated into the design process from the very beginning, because it is at this stage that one has the opportunity to make the largest impact on reducing the carbon footprint. One of the best solutions to reducing the operational energy is proper building form and orientation to take advantage of natural lighting. For example, in the Indianapolis campus of Roche Diagnostics, completed in 2014, daytime lighting is provided a combination of north-facing skylights and extensive exterior glazing (Fig. 4.1). Exterior mechanical shading devices provide protection from thermal gain. Other features that reduce operational energy include chilled beams, radiant panels and raised floors for mechanical and electrical distribution. Three strategies are employed by SOM designers to reduce embodied energy: optimising the form to reduce the quantity of material required; using predictive tools for carbon emissions to inform design decisions; and selecting materials that use less carbon. The first strategy was discussed in “Optimising design goals” (pp. 88 – 94). The second strategy should be employed in the conceptual design phase to evaluate design options for various forms and materials, and again in subsequent design phases as needed. SOM has developed a new tool that estimates the equivalent carbon dioxide emissions embodied in structures of various building types: the environmental analysis (EA) tool. This tool

takes initial construction, service life, repair after hazardous events and deconstruction into consideration. Using inputs such as gross floor area, number of storeys, material type and quantity, location, seismicity and wind, the EA tool can be used to evaluate and assess the carbon implications of various design alternatives. The tool was developed with SOM’s advanced material quantity estimation algorithm generated from hundreds of previous SOM project designs. Project-specific inputs can override all parameters as projects progress through the final design and construction phases. The EA tool not only evaluates estimated carbon for building structures, it also performs a costbenefit analysis of enhanced structural systems (base isolation, for example) and estimates the damage expected over a building’s service life. As mentioned previously, the production of materials used in the structure of a building is the largest source of embodied energy. Currently there are four principal construction materials: steel, concrete, masonry and wood. Steel, concrete and mixed steel-concrete construction are the most common materials for commercial buildings, in particular tall ones. The reason for this exclusive use of steel and concrete is that these materials are stronger than masonry or wood; also, non-combustible materials are usually required for buildings more than four storeys high. Timber Tower Research Project SOM recently performed a research project on timber as an alternative building material for tall buildings. The results show that timber can reduce carbon emissions by 60 – 75 % as compared with that of the benchmark concrete structure. Carbon sinks remove carbon dioxide from the atmosphere, and the largest natural sinks are oceans and forests. In fact, one half of the weight of dried timber is carbon. In addition, the overall amount of energy needed to produce wood (mass timber and glued elements) is significantly smaller than that for other construction materials, so SOM designers are examining the potential of wood for tall building construction.

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Built-up timber columns Solid 8 in-thick timber floor panels Reinforced concrete spandrel beam

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The Timber Tower Research Project, completed in 2013, compared the design of a prototypical 42-storey building built with timber to an existing concrete benchmark building, the DeWitt Chestnut Apartments. By keeping the dimensions and location of the timber prototype the same as the benchmark, a direct comparison could be made between the carbon emissions of a tall building designed in timber and one in concrete. The research objectives included: • developing a conceptual structural system for tall buildings designed in timber • comparing the designs of the prototype timber building to the benchmark concrete building in terms of material quantities and embodied carbon footprint • interpolating results to shorter buildings • proposing construction sequences • providing recommendations for additional research and testing.

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The full report is available online [1]. The following paragraphs address only the results that are related to the embodied carbon footprint. The focus of the carbon footprint study was a “cradle-to-gate” analysis, meaning that it only included the embodied energy of the material used and the energy associated with the construction of the building. The analysis also took into consideration the carbon sink properties of wood. In the prototype timber design

(Figs. 4.3 – 4.5), the materials used were a laminated wood composite for walls, floors, columns and interior beams, and concrete for joints, link and spandrel beams, lower level floors, foundations and topping slabs. Two scenarios were considered. In the standard scenario, the material energy inventory does not include sustainable options such as cement replacement or air-drying of wood (the wood is kiln dried). In the sustainable scenario, these options are considered. The results of the comparative study show that using sustainable material options decreases the embodied carbon by a significant amount (Fig. 4.2, p. 101). But even more significant is the energy difference between the prototypical timber design and the benchmark concrete design. Most of this difference is due to the embodied energy of timber as compared to that of concrete, and also to the fact that, since timber is a carbon sink, a negative embodied carbon measure is observed for timber. Overall, this project illustrated that timber is a viable alternative for high-rise construction. It has the potential to reduce the embodied carbon footprint typical for a concrete building by more than one half. The project report recognises that there are still obstacles to overcome before tall timber towers become a reality. However, it states that such towers are “technically feasible and efficient on an architectural, structural, mechanical and interior design basis”.

Structural system components, Timber Tower Research Project, 2013 Proposal, Timber Tower Research Project Proposed columns, core and concrete spandrel beam, Timber Tower Research Project

References: [1] SOM: Timber Tower Research Project. Final report. 6 May 2013; available online at http:// www.som.com/ideas/research/ timber_tower_research_project 4.5

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Integrating discipline and play

Integrating discipline and play Through examples, this chapter has shown that architects and engineers must work collaboratively to achieve the goals of efficiency and economy in building design. In an interview, SOM Partner Bill Baker notes that “… this collaborative influence is where we don’t have firm barriers at SOM where you have architects and engineers working together…. It’s architects who like to work with engineers, and engineers who like to work with architects. [Engineers] never think twice about suggesting architectural ideas, and architects never think twice about suggesting structural ideas.” This type of collaboration is especially important at the very beginning of a project, in the conceptual design phase, since it is at this time that the form for a structure is chosen. The form will have the greatest influence on material quantity, cost, carbon emissions and elegance. Once the form is chosen, it limits the influence of any optimisation scheme applied later to improve efficiency or economy. The greatest designs are disciplined: they remain within the boundaries imposed by efficiency (related to statics and calculations) and economy (related to construction). Yet the best engineers recognise that, inside of this space, there is room to play in the search for elegant forms. Legendary engineer Felix Candela said, “But an efficient and economical structure has not necessarily to be ugly. Beauty has no price tag and there is never one single solution to an engineering problem. Therefore, it is always possible to modify the whole or the parts until the ugliness disappears.” [1]

eering as the work of teams of technologists and committees of experts. In short, the neglecting history has the direct effect of dehumanising modern engineering. There are structural engineering professors who recognise that these critical parts of an engineering education are missing. They teach the history of structural engineering by drawing on examples of the best engineers and designs, including designers and projects from SOM. It is to be hoped that practising engineers will use and draw inspiration from these teachings, and that they will influence the future of building design education and practice in order to inspire generations to come.

Maria E. Moreyra Garlock, Annette Bögle

5.1 Ideal brace configurations and alternate force diagram, sketches from Bill Baker’s notebook References: [1] Candela, Félix: New Architecture. In: The Maillart Papers. Edited by David P. Billington et al. Princeton, NJ 1973, pp. 119 – 126

Architects and engineers should all be educated so that they are adept at finding approximate dimensions rapidly using simple formulas; the complex analyses come later. In addition to rigorous technical training, an engineer should be introduced to important historical works of construction and know how to critique them. This study of history and critique is not common in engineering education. The profession often seems to have little interest in the recent history of engineering and therefore tends to see engin5.1

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Research into new technologies and typologies is central to SOM’s ethos and essential to the continued evolution of structural systems. Early SOM research into optimisation focussed on energy methods for maximising stiffness and minimising weight in structures controlled by drift. SOM was one of the first firms to use such techniques to proportion members in large structures. More recently, their research has been increasingly concerned with finding optimal forms and geometries that not only minimize weight and material, but also construction complexity, which includes making use of opportunities for repetition in the construction process. Whenever research yields new forms, these must be rigorously tested. To date, large-scale testing has been used to validate deep beams and, more recently, pin-fuse joints and frames. In the future, SOM will be using its own wind tunnel to make qualitative comparisons between new structures. Though a lot of research is specific to a particular project, “blue sky” studies are directed forward to identify and address the issues of tomorrow, and ideas that emerge from these studies inform the approach to new structural challenges. Research encourages engineers to look beyond their own discipline and their own project constraints to encompass the work of others, and thus facilitates creativity and the generation of new ideas.

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Quo vadis – megatalls as the focus of the SOM Research Gang Annette Bögle is a structural engineer, author, curator and full professor for Design & Analysis of Structures in the Department of Civil and Structural Engineering at HafenCity University in Hamburg. Nils Ratschke is a structural engineer and PhD researcher in the Department of Civil and Structural Engineering at HafenCity University in Hamburg.

Form and structure are often viewed as conflicting aspects of objects such as buildings. Form, a result of creative endeavour driven by aesthetic goals, is generally regarded as belonging to the domain of architecture. Structure, on the other hand, is assigned a rational, technical motivation, based on the clear, abstract logic of the natural sciences, and thus belongs to the traditional remit of engineers. Viewed in this light, architecture and engineering appear to pursue competing interests. Yet though their perspectives may differ, the two disciplines are ultimately concerned with one and the same object. The relationship between form and structure in the context of buildings necessarily mirrors the relationship between architecture and engineering. In an ideal world, neither architecture nor engineering should be forced to make false compromises; instead, they should act synergistically to complement each other. Whenever this happens, the result is more than the sum of the individual contributions of both disciplines. Such collaboration forges a path to innovation and pushes the boundaries of what is known and possible. Innovations can encompass design tools and construction types or technical solutions. The closer that building gets to the limits of technical feasibility, the more important these innovations become, and the more critical it is that architects and engineers work together intensively. New forms, such as bigger dimensions and heights in high-rise construction, are impossible without the development of new engineering technologies for computation, form-finding, design, construction and material application. Especially in the case of the highly integrated supertall structures, the resolution of this dichotomy is crucial to delivering successful results. The SOM Research Gang – a small group of young engineers who share a fascination for structural design issues – is the inevitable product of the company’s historical evolution into a global player in high-rise architecture

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and engineering. It is the legacy of exceptional individuals at SOM who, for more than 80 years, have masterminded innovative developments and pioneering technical solutions with passion and empathy, and who, with their teams, have blazed a trail in high-rise construction by erecting the world’s tallest buildings. Yet their stellar achievements have never been attributable to a single discipline. Architects and engineers have worked in tandem ever since the firm was formed, which was unusual then and is still unusual today. At SOM, this close collaboration from the very outset of a project has remained an enduring practice up to the present. The work fostered by this interdisciplinary environment culminated in ground breaking engineering solutions, such as the first “superstructure” for the John Hancock Center (1970) or the tube-in-tube system for the Sears (now Willis) Tower (1974) in Chicago. Here, exceptional architects and engineers worked side by side in the systematic pursuit of ever more efficient structures, with the goal of steadily increasing the heights of buildings while at the same time expanding SOM’s structural potentialities. Both the interdisciplinary working environment and the search for new technologies inform SOM’s research activities. Research, specifically the search for new forms in close coordination with technological development, can be done in many ways in many different places. Traditionally, it is based in an academic institution and focuses either on the acquisition of fundamental knowledge or on the solutions of practical problems. Complex projects, however, require innovative solutions developed through applied research at the interface between academia and practice – where the main focus can be either academic or practical. Given SOM’s commitment to innovation and their reliance on up-to-the-minute expertise, research features prominently in the company’s everyday routine, and the Research Gang is a key focus

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of the corresponding investment. Projectrelated research is complemented by “blue-sky studies” – that is to say, research done for the sake of research and aimed at promoting staff development, stimulating discussion, generating synergies and identifying ways to solve future challenges. History The current research initiatives of the Chicago Research Gang can be traced back to Myron Goldsmith. Though he was trained as an architect, Goldsmith’s career and working practices make a precise professional classification difficult, as he effectively blended the roles of architect and engineer. In 1961, while still working for SOM, he was appointed professor of the School of Architecture at the Illinois Institute of Technology (IIT). Goldsmith’s ideas and visions, reflected in his research interests, were decisively influenced by Fazlur Khan, who joined SOM at the same time as Goldsmith. At the IIT, Goldsmith initiated the “Saturday Sessions” [1], a forum for hands-on analysis and criticism of students’ work by professionals, including both architects and engineers. For Goldsmith and Khan, this provided a direct link between cutting-edge research and everyday practice at SOM. By the same token, it allowed pressing practical issues to feed into and add meaning to research activities. When Goldsmith’s partnership with Khan was cut short by Khan’s death in 1982, various other SOM engineers stepped in to ensure the continuation of project-oriented research at the IIT School of Architecture. The collaboration between SOM and the IIT persisted even after Goldsmith’s death, though it became increasingly sporadic and completely ceased at the start of this century, in part because of changes in the academic curriculum. Khan, like Goldsmith, was a cross-disciplinary thinker, with an education and skills that perfectly matched those of Goldsmith. After obtaining an engineering degree, Khan studied theoretical mechanics, which later enabled him to explore the interface between structural engineering

Pin-ups on Bill Baker’s office wall of ongoing research projects Samples of components used in various SOM projects

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and the then evolving theoretical computational models. His interest motivated him to introduce numerical engineering methods to the firm and spur the development of in-house software. The development work was carried out in the office and was directly related to projects. Interdisciplinarity and practical relevance are the hallmarks of research at SOM. Neither of them is intrinsic to a typical university education, where disciplines are all too often taught in isolation. This was, in fact, manifest in SOM’s own university research activities; though they were based on an interdisciplinary collaboration between architects and engineers, they took place entirely at the School of Architecture. They had no impact on the engineering curriculum, and the potential benefits there remained unexploited. The widespread tendency in engineering is to focus exclusively on the calculability of structures, the upshot of which is often expensive, over-dimensioned solutions. These not only fall short of aesthetic expectations, they also fail to capitalise on existing technical potential. Bill Baker, in his role as chief engineer at SOM in Chicago, identifies this as the crunch issue when he warns against complacency and the simple reliance on existing knowledge: “Engineering is an evolving profession. People think we’re done, but we’re a long way from done. What’s the optimal shape for a bridge? We’re still trying to discover that.” [2] He emphasises the fact that today’s technical possibilities are neither fully exploited nor fully integrated into the daily routine of engineers. With enthusiasm and empathy, he challenges himself and his team with many open-ended questions in the search for insights and interdependencies. As Baker sees it, engineers have a responsibility to devise fully optimised, efficient structures – in keeping with a professional ethic that enshrines efficiency as the basis of technical development. It was this conviction that prompted Baker to establish the Research Gang. At the weekly forum, he joins committed SOM employees to

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Discretised Michell truss with inner angle notations, hand sketch from Bill Baker’s notebook Projected 20 tallest buildings in 2020. All of them are more than 500 m high, eight can be classified as megatall (more than 600 m). Testing of pin-fuse joint, designed to protect buildings in areas of high seismicity a Test set-up b In-plane buckling of brace at -6.9 % storey drift Movements of pin-fuse joint

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discuss fundamental issues relating to the unity of form and structure, such as form-finding methods, topology optimisation or even general attitudes to design. They focus not only on developing new design principles, but also on changing the working practices of engineers: the participants analyse and discuss potential methods and concepts – both traditional and novel – to identify simple, clear-cut and sustainable solutions to structural problems. Motivation: Tall, taller, tallest By 2020, the average height of the world’s 20 tallest buildings will be around 600 m (Fig. 1.4). All of them will rank as “megastructures” [3]. In

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8 Makkah Royal Clock Tower Hotel (601 m) 9 Goldin Finance 117, Tianjin (597 m) 10 Lotte World Tower, Seoul (555 m) 11 Doha Convention Center and Tower (551m) 12 One World Trade Center, New York City (541 m) 13 Chow Tai Fook Guangzhou (530 m) 

1 Kingdom Tower, Jeddah (1000+ m) 2 Burj Khalifa, Dubai (828 m) 3 Ping An Finance Centre, Shenzhen (660 m) 4 Seoul Light DMC Tower (640 m) 5 Signature Tower Jakarta (638 m) 6 Shanghai Tower (632 m) 7 Wuhan Greenland Center (606 m)

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the evolution from the “normal” high-rise via the (approximately 300 m tall) superstructure to the megastructure, the main focus was on optimising structural behaviour, specifically as it relates to stiffness and deformation. Although problems and requirements grow exponentially with increasing height and slenderness, these very challenges are what drives progress in technical knowledge and practical construction techniques. The resulting developments and optimisations have been made possible by numeric methods, complex geometric models and ever more powerful computers and software. The initial focus on structural behaviour engendered a monocausal optimisation and an increase in

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the importance of the structure itself, due to its architectural prominence and visibility. Yet the growing aesthetic and functional requirements that accompany the evolution towards the “megatall” must be seen not only in terms of the building proper, but also in the wider context of the urban environment. The past 25 years have seen a decisive shift in high-rise occupancy types: while pure office facilities accounted for some 90 % in the 1990s, this share had fallen to 43 % by 2012 [4]. The proportion of residential and mixed-use development, on the other hand, has risen correspondingly, imposing complex requirements on utility services (supply and disposal), internal layout, flexibility for future use, daytime lighting provision, views and special features such as sky gardens. Most of these tall and supertall buildings are built in Asia and the Middle East, in densely developed urban environments where expansion is driven by high population growth. As a consequence, these high-rises no longer represent isolated corporate icons, for instance, but rather elements in a three-dimensional urban space. Cost-effectiveness, efficient design and rapid construction are the main priorities here. Complicating site factors, such as earthquake loads and extreme wind and climatic conditions, also tend to play a role, given the geographical location of many such high-rises. Moreover, closely linked to the climatic issues are the ever more stringent sustainability demands placed on buildings. The monocausal optimisation of structural behaviour – the key design criterion for the first milestones in supertall construction – no longer constitutes an adequate response to today’s complex requirement profile. Sustainable highrise development must now factor in a multitude of different, sometimes conflicting, criteria. The overriding goal is to develop a framework that is not simply dictated by the given structural parameters, but which is capable of inspiring powerful architectural concepts. While the ever tougher requirements necessitate increasingly specialised knowledge in

individual disciplines, the frequently neglected interface management tasks are also growing in importance. SOM guarantees close coordination between the various areas of expertise by uniting most of the relevant disciplines under a single roof: architects, engineers, urban planners, interior and landscape architects routinely work together on projects from the very outset. The prioritisation of intensive interdisciplinary collaboration during the earliest possible project phase, with the aim of delivering the best overall result, also involves modifications to established working procedures. Particularly in the case of complex design processes, where numerous, sometimes conflicting design parameters have to be reconciled and ever tighter schedules must be met, the mere development of new engineering solutions is not enough. Other factors, notably the establishment of new forms of communication, are equally vital [5]. Creativity and visual thinking in engineering The current challenges posed by supertalls call for a paradigm shift in engineering, a readiness to relinquish established comfort zones such as the traditional focus on structural dimensioning. The precondition for successfully synthesising the dual exigencies of form and structure is a new style of responsible and creative thinking in engineering. In adopting this new paradigm, engineers will no longer content themselves with ad-hoc problem-solving using the latest technology, but will go one step further by identifying the problems of tomorrow. A fundamental understanding of structural behaviour and, particularly, flow of forces is a crucial prerequisite for contemporary developments. At the interdisciplinary boundary, the geometry of the structure assumes particular significance. This is where architecture meets structure and where the two, ideally, are reconciled and harmonised. Visualisations are essential here, both for picturing the loads and engineering requirements placed on the structure and as a vehicle for interdisciplinary communication.

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Steel plate back span Water tank back span Water tank Diagonal high-strength rod, cable size varies from 50 to 36 mm at end Stainless steel guardrail supported by steel plate at end Chock to prevent over-rotation Operating gear Top chord high-strength rod or cable, varies from 150 to 50 mm at end Cross bracing

Balance Bridge, Bergen Experimentation with forces and equilibria is an enjoyable and instructive element in the work of the Research Gang. The dynamic equilibrium of the Balance Bridge is a case in point: large water tanks as counterweights above the shore allow the use of a surprisingly slender truss assembly for the two cantilevered spans – a transparent presence above the water, the tensile loads of which are all but visible. Pumping water between internal chambers in the tank back span structure will alter the location of the water mass and allow gravity to open the bridge without hydraulic actuators or mechanical drive mechanisms.

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Structural elements of the pedestrian drawbridge, Balance Bridge, Bergen (N), design 2005 Balance Bridge

Modus operandi of Research Gang Given the increasing complexity of the requirements and background factors outlined above, the hunt is on for new design solutions for supertalls and megatalls. These should seek to achieve an optimum synthesis between form and structure. Innovative solutions demand creative, virtuoso architects and engineers capable of pushing the envelope in their disciplines while at the same time operating effortlessly across professional boundaries and taking on interdisciplinary challenges. This is the tradition espoused by the informal SOM Research Gang, which comprises mainly engineers, though it also includes a number of architects. Baker has always advised younger generations to “get as much education as they could and take up theoretical classes”. He adds: “If things go wrong on a project, the important thing is to relax, think and go back to the basics.” The Research Gang can draw on a profound body of knowledge, augmented by the firm’s long track record with structurally challenging projects. Over time, SOM has developed a vast array of tools, which it constantly refines. The weekly meetings provide an opportunity to analyse problems from a theoretical standpoint and discuss them in abstract terms at a high

scientific level. The Research Gang members are also able to contribute wide-ranging expertise in numerical techniques. This makes it possible to examine structural issues using diverse methods and numerical tools, thereby viewing them from a variety of perspectives. These methodological investigations are conducted independently of any specific project. Nonetheless, the findings continuously feed into routine project work. Indeed, the continual process of theoretical reasoning and contemplation can sometimes pave the way for the prompt application of innovations to a specific project, even within a narrow time frame. The principal aim is to deliver comprehensive, forward-looking solutions for present and future challenges facing the built environment. The solutions must give broad consideration to sustainability issues as well as to the situational factors governing the project brief, with particular attention given to the engineering dictate of efficiency. In supertall design in particular, this can typically lead to a focus on structural optimisation tasks which may, among other things, combine structural behaviour, form and production processes. In some cases, however, extreme conditions such as heavy winds or earthquakes may make a monocausal perspective necessary as well. References: 1 Neveu, M. J.; Saliklis, E. P.: Myron Goldsmith – the Development of the Diagonally Braced Tube. In: Structures & Architecture. Edited by Paulo J. S. Cruz. London 2010, pp. 223f. 2 Volner, Ian: Well-Oiled Machine. In: Metropolis, March 2014. 3 Council on Tall Buildings and Urban Habitat (CTBUH): The Tallest 20 in 2020. Entering the Era of the Megatall. Chicago 2011 4 Busenkell, Michaela; Schmal, Peter Cachola: Best HighRises 2010/11. Berlin 2010 5 Beghini, Lauren L. et al.: Connecting Architecture and Engineering Through Structural Topology Optimization. In: Engineering Structures 59, 2014

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Structural optimisation – developing new design tools The problems and objectives of optimisation are closely linked to those of responsible and innovative engineering. Were it not for the constant pursuit of ever-improving technical performance, inventions such as the automobile and the aeroplane would have been impossible. The motivation that underlies this pursuit is the far from straightforward quest for maximum performance combined with an optimal use of resources. The complex relationship between these two factors drives a continual search for the best compromises among a number of different and sometimes conflicting factors. In the context of structures, the quest for optimum performance requires focus on a number of different goals. The building must exhibit (1) efficient structural behaviour combined with (2) a simple, coherent construction method. These are invariably linked to (3) cost, a factor that is in itself always contingent on societal values and/or global economic conditions and is rarely Altair SOM tool Commercial software

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clear-cut and absolute. Finally, (4) the appropriateness of the result and its overall impact are also crucial assessment criteria.

Annette Bögle, Christian Hartz is a structural engineer at SOM. He received his PhD from the Technical University Berlin.

A look back through the history of the art of engineering reveals many examples of how, at the interface between form and structure, new technologies, materials and methods of calculation have led to new forms of architecture. The notions of lightweight structures, greatly influenced by the Stuttgart School of Engineering Design [1] and in particular by Frei Otto (1925 – 2015) and Jörg Schlaich (*1934), have been particularly significant, shaping the work of generations of engineers – not least those at SOM – and encouraging them to address issues of form and structure. Various form-finding and optimisation methods have emerged in the field of structural engineering (Fig. 2.1). Though their development is based on experience gained using experimental methods, today these methods are generally Tall building design SOM knowledge

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used in combination with computer-assisted processes. Form-finding and optimisation methods focus on different levels of a structure and different phases in its design process and so have a varying influence on the overall appearance of a building (Fig. 2.2). Where they concentrate on details of a design – that is, on the structure and its elements – in order to identify the optimum cross-section of a beam or thickness of a slab, for example, the process is referred to as size optimisation. Here the requisite dimensions (values such as thickness, height, cross-sectional area, moment of inertia, etc.) are determined according to the load on the structural elements for a given design; that is, the chosen statics system or the building’s shape remains unchanged. Size optimisation constitutes part of the traditional role of the engineer, and it therefore unfortunately tends to draw a disproportionate amount of attention in everyday engineering situations. The art of architectural engineering, on the other hand, demands the virtuoso handling of shapes and/or topology. Innovative solutions can be found only if the relationship between form and design is addressed during the design process, and there are various topology and/or shape optimisation methods available to do this. In the design process, it is most likely the topology of the basic structural system – consisting of the nodal positions of the

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structural elements and their connectivity – which is established first. An optional next step is to modify, adjust and optimise the shape, followed by the fine-tuning of the result of the topology optimisation process using shape optimisation methods. The shape optimisation process may necessitate further topology optimisation depending on the task at hand. Thus, structural optimisation methods need not adhere to a strict protocol or procedure. Rather, they are tools to be used individually and as needed, depending on the problem to be solved. Their efficacy and/or appropriateness is evaluated based on the end result. Topology optimisation Benchmarking the footprint layout The topology of a skyscraper – its basic structural composition – is essentially determined by the cross-sectional layout described by its external form (circular, square, triangular, etc.) and by the arrangement of structural elements such as the core and lateral columns. These topological elements have as significant an impact on the footprint of a building as on the structural behaviour under wind or seismic loads. While wind load demands the stiffest structural solution possible for low deformations, an optimum structure for seismic load requires maximum ductility. Both affect footprint development and thus the efficient use of space in the building. Obviously it is now possible to perform this function using computer-assisted topology optimisation, developing the basic structure outside the black box . In addition to computer-assisted topology optimisation, however, the conscientious, committed engineer has another tool at his or her disposal: experience. Building on decades of such experience, SOM has benchmarked the optimal crosssectional shape for supertalls by performing a systematic analysis of a variety of building footprints and determining their associated efficiencies and their structural characteristics. Fig. 2.4 illustrates the efficiency of material required under wind load in relation to the layout

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Idealised workflow for given design domain using topology, shape and size optimisation Topology optimisation of a beam based on material intensity using an optimisation filter Various footprint shapes ranked by a Moment of inertia b Moment of intertia divided by wind sail

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of the footprint. If only the moment of inertia is taken into account, the triangular footprint with explicit columns at the vertices is the most efficient choice. But as soon as the wind sail is considered, it comes as no surprise that the squared footprint with mega-columns at the corners and a central core offers a more efficient solution. Continuous topology optimisation The aim of computer-assisted topology optimisation [2] is to establish the optimal basic design of a structure under load within a predetermined, continuous design space. A look at nature provides numerous examples of topology optimisation. The structure of a bone, for example, varies according to its load, rather than being materially homogenous. In areas of high load, the bone is made up of tiny, lattice-shaped units, whereas areas subject to smaller loads are hollow or contain bone marrow. Most of the computer-based calculation methods used in topology optimisation today were developed in the automotive industry and aerospace engineering. At SOM, these methods are adapted for the whole of the building design process and applied specifically at the interface between architecture and engineering. This is done using commercial software programs and applications developed in academia as well as programs developed in-house exclusively for SOM (Fig. 2.1). The basic procedure can be described as follows. First, a design space is defined based on the volume in which it is possible to create structural elements: the footprint and height (in the case of a tower) or span and width (in the case of a bridge) of the planned structure. Displacement constraints and loads are established based on the available locations for supports and on loading conditions. The topology optimisation stage iteratively determines the optimal distribution of the structural material (the material density) throughout the design space, where a negligibly small material density can be interpreted as a void within the structure. In addition, optimisation filters and post-processing techniques can be applied to provide topological

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variance, which can be used to influence the delicacy of the structure, for example (Fig. 2.3, pp. 112f.). Since topology optimisation starts with a continuous body, its results can be verified using what is known as continuum mechanics, in which a stress state is represented by principal stress trajectories that indicate the directions of tensile and compression stresses. Principal stress trajectories are always perpendicular to one another and therefore intersect at an angle of 90 degrees. If as a result of topology optimisation, the structure follows these principal stress trajectories, a primary structure subject to either tensile or compression stress is created in line with the principles of efficiency. During the process of topology optimisation, the nature of the design space changes, and the structural solution moves from an initially isotropic space to one that is concentrated along paths. The tension and compression load paths tend toward orthogonality. These paths are similar to, but not the same as, principle stress trajectories, as they are discrete and not a solution on a continuum. The comparison of a fixed, vertical member under lateral wind load and the stages in the P/2

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topology optimisation process show an optimum match between the analytical and the optimisation results. These patterns were used, for instance, in the development of the superstructure of the office tower at 100 Mount Street in Sydney, Australia (Fig. 2.6). The primary objective of topology optimisation in the design process is to develop the most material-efficient structural solution possible. However, it is currently difficult for optimisation to be carried out for more than one stress state, though in reality a structure is exposed to many, sometimes conflicting, stresses. This results in conflicting optimisation objectives and evaluation criteria, such as minimum deformation, maximum rigidity, ductility, natural frequencies and stability parameters. The topology optimisation process generates a number of design alternatives, which provide vital data for the discussion about the relationship between form and structure that should ideally take place between architects and engineers. Discrete topology optimisation In contrast to continuous topology optimisation, discrete topology optimisation is based on discrete members. The Michell truss provides a clear example of this type of analytical optimisation. The calculus of variations, a prerequisite for the mathematical solution of optimisation problems, was developed in the mid-18th century. It was later applied to structural questions, in particular in the work of Scottish physicist James Clerk Maxwell (1831–1879; see “Maxwell’s theorem of optimal load path”, p. 89f.) and Australian mechanical engineer Anthony George Maldon Michell (1870 –1958). In his 1904 publication “The limits of economy of material in framestructures”, Michell developed a particularly interesting frame structure of minimal weight for given support and load conditions (single load) and a defined design space. The individual frame members follow the principal stress lines, resulting in a “balloon-shaped” support [3]. Michell structures can be derived from continuous topology optimisation by applying a single load to a continuum with a defined external geom-

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etry and a semi-circular support configuration (Fig. 2.7), for example. In the continuum, the principal stress trajectories represent the principal stress paths. The continuum is then reduced to these principal stress lines, which are transferred to a structure. If one transfers the continuum solution to a discrete one, the angles at the intersections deviate from orthogonality and vary based on the density of the pattern. When designing in accordance with this principle, it is possible to create a structure of extremely high rigidity. Theoretical knowledge about simple model structures and their comparatively simple load scenarios can be applied on a project-by-project basis using a number of optimisation software

programs. Much of this software has grown out of collaborative work with scientists. The topology optimisation software PolyTop, developed by Professor Glaucio H. Paulino’s research group at the University of Illinois at Urbana-Champaign, was put to the test and used in the conceptual design of a SOM project while still in its developmental phase. Later on, SOM engineers and interns from the same group working at SOM expanded this software to allow for pattern repetitions and other features, tailoring it to better address architectural problems [4]. It is available throughout the company to both engineers and architects. In addition, SOM developed an in-house implementation of the ground structure method, extending the application of topology

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3D density-based topology optimisation of a bridge PolyTop proof of concept for optimised brace configuration, 100 Mount Street Building, Sydney (AUS), design 2012 Development of a classic Michell structure using the Ground Structure software Michell structure, CITIC Financial Centre, Shenzhen (CN), anticipated completion 2018

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Examples of the form-finding process using the force density method a Input with fixed corners and two high points b Resulting anticlastic surface Examples of the form-finding process using the force density method a Input with fixed outer edges b Synclastically curved surface as a result of constant pressure Family of different surfaces, optimised in accordance with the heavy-soap film principle and the force density method Examples of surface generation to find new structural forms using the inversion circle technique a Torus with obvious lines b Torus with Villarceau circles c Deformed torus Various mesh layouts for given surfaces with different design goals

optimisation to three-dimensional space, with structural problems as a main objective (Fig. 2.5, p. 114). At SOM, Maxwell’s theorem, the Michell truss and the methods of graphic statics are combined and applied to project work. Studies and findings of Michell structures have been used in actual projects, such as the CITIC Tower in Shenzhen, China (Fig. 2.8, p. 115), where the goal was to obtain a structure with maximum stiffness – using the design principles of Michell structures – without the need for intermediate vertical columns. Shape optimisation Once the basic configuration of the design elements, the topology, has been established, the shape is optimised within the associated boundary conditions, which include the geometrical parameters of the structure, such as component loads, radii of holes and curvature, the positions of nodes within the space, etc. In addition to

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achieving optimal structural behaviour, other important optimisation objectives include node configuration and the manufacturing process. Classic experimental form-finding methods, such as the hanging models devised by Frei Otto as part of the design process in structural engineering, are the simplest and clearest of these methods. They are based on obtaining a spatially balanced structure under a given load and result in double-curved surfaces in either tension or compression. The load affine shape defines the most efficient load path. Form-finding of cable and membrane structures using the force density method Computer-aided form-finding of double-curved surfaces subject to tensile stresses can be achieved successfully using the force density method. Developed in the 1970s by Hans-Jörg Schek [5] and Klaus Linkwitz from geodetic equalisation calculus, it is used today in a number of software programs. The force density method is primarily applied in mechanically or

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pneumatically pre-stressed constructions and is responsible for the large variety of membrane and cable-net structures. To use this method, one must first determine the support points, boundaries and load state for a given nodal connectivity. The shape-giving load is either a mechanically applied pre-stressing force, which gives anticlastically curved surfaces (curving in opposite directions; Fig. 2.9), or internal pressure, which results primarily in synclastically curved surfaces (curving in the same direction; Fig. 2.10). These enveloping surfaces, which are subject to tensile stress only, can be realised – using the inversion principle – as shell structures subject to compression stress only. Optimising grid shells: surface optimisation Shape optimisation can be used to particular advantage in the design of grid shells. These lightweight, transparent shells are employed primarily in entrance areas and to cover interior courtyards, thereby forming elements within

building complexes. As they are generally glazed, the underlying structure is visible and therefore of architectural significance. The aim is thus to achieve both optimum load transfer, primarily under compression stress, enabling the structural elements to be as delicate as possible, and a mesh layout that underscores the design concept. When it comes to shape optimisation, however, manufacturing issues such as node equality and the planarity of covering elements are also relevant as they have a crucial impact on the cost efficiency of the structural solution. At SOM, the first stage of shape optimisation can be carried out using the shell optimiser program developed in-house, which is based on the heavy soap film model. Soap film models are characterised by even stress distribution at every point on their surface. Shell optimiser can be used to create a whole family of shells based on supports, soap film density, boundary length and the desired height of the shape. Though they look quite different, the shells are all based on the same principle (Fig. 2.11).

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The problem of the efficient and artistically appealing segmentation of a spherical surface has occupied entire generations of engineers. Examples include geodesic domes, associated primarily with the name of Richard Buckminster Fuller, and Jörg Schlaich and Hans Schober’s grid shells. In both cases, the goal was to combine optimum load transfer with a manageable manufacturing process, thereby minimising the number of different member lengths and nodes, and, for reasons of economy, to cover the area in question with flat glass panels. Last but not least, the structural arrangement was integral to the aesthetics of the grid shell. Although manufacturing techniques have advanced significantly, they continue to be assessed using the same criteria. Fig. 2.13 (p. 117) shows a study of the surface structure of an oval grid shell with the following optimisation objectives: • a uniform node configuration • planar panels • identical edge lengths • a uniform aspect ratio. Graphic statics The principles underlying the graphic determination of forces can be traced back to antiquity, and graphic statics has formed an integral part of the study of statics since the first clear definition of the term was provided by German structural engineer Karl Culmann (1821–1881).

Unfortunately, despite offering simple solutions to complex structural problems, it has been increasingly marginalised as a discipline over the years due to continual developments in computer-based analytics. It is currently undergoing something of a revival, however, particularly as it can now be integrated into computer programs to represent a structural layout and its corresponding forces quickly and clearly. Using graphic statics, it is possible to determine the force on truss members in an explicit and unequivocal manner by producing a force diagram on which the inclination of the members and the scaled size of the forces are plotted [6]. Fig. 2.14 shows a simple, triangular roof girder subject to a load at its nodes, with the shape of the bottom chord not yet defined, and the corresponding force diagram. This can then be used to create an optimised form diagram, which in turn makes it possible to develop a shape for the bottom girder that will cause all elements of the upper chord to experience the same stresses. Thus, two-dimensional structures can be modified to ensure a design based on force optimisation. This sheds light, for example, on Robert Maillart’s roof girders at the Chiasso railway station in Chiasso, Switzerland. Efficient trusses with optimal forces and an unmistakable design language can clearly be designed for both long-span roof structures and the outriggers inevitably required in the structures of supertalls subject to high loads. Graphic statics therefore serves not only as a calculation but also as a design tool. The Airy stress function The principles of graphic statics apply to discrete elements and can be extended to faceted two and three-dimensional structural shapes (Fig. 2.16). However, to achieve force equilibrium using planar elements, it is necessary to use a plane stress theory. Plane stress theory uses the Airy stress function F(x,y), introduced by English mathematician and astronomer George Airy (1801–1892), to solve the relevant differential equations at equilibrium. Like graphic

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2.14 Examining the shape of a roof truss using graphic statics, Railway Station, Chiasso (CH) 1925, engineer: Robert Maillart 2.15 Shape optimised large roof structure, Tanggu Convention Centre, Tianjin (CN), design 2009 2.16 Airy stress function 2.17 Form-finding using genetic algorithms, ASPIRE Tower, Jeddah (KSA), design 2009 2.18 1 km-tall ASPIRE Tower Following double page: Timeline showing relationships between research and projects at SOM 2.16

statics, Airy’s stress function is not limited to the calculation of stresses, but can also be used to obtain reasonable architectural geometries – that is, structures comprising plane-faced polygons that are in equilibrium for a unique load. Planar n-sided polygonal elements are used to create a three-dimensional surface by defining their angles and lines of intersection. They can be arranged so as to produce a planar overall structure. In this way, it is possible to generate extremely variable shapes – a fact which will undoubtedly have a major impact on the design process for new kinds of loadbearing structures. Genetic algorithms Evolutionary algorithms are optimisation processes that mirror nature. They use the approach adopted in bionics, in which nature is considered to be the best builder, and apply generation and selection principles – rather than shapes taken from nature – to an artificial creation process. These algorithms can be used to take into account the fact that it is impossible to know all the boundary conditions for a given process, and that even the known design criteria

are often in conflict. Nature serves as a model, providing many examples of how living creatures have adapted to very diverse living conditions. The process of adaptation happens through evolution, without any theoretical underpinnings, by means of mutation, heredity and selection (survival of the fittest). The principles of natural evolution are now being codified into computers, which use the evolutionary algorithms to generate a wide range of different solutions. Instead of using an analytical approach to calculate just one “correct” solution, they generate a whole population of equally good “optimum” results. This method was used, for example, in the design of the office tower ASPIRE proposed for Jeddah, Saudi Arabia (Figs. 2.17 and 2.18). Its elongated conical form is derived from a structural form optimised for strength and wind performance. The overall profile was determined from genetic algorithm techniques that optimise the behaviour of the structure. Its circular shape provides the building with inherent strength and stability.

References: [1] Bögle, Annette; Kurrer, Karl Eugen: Jörg Schlaich and the Stuttgart School of Engineering Design. In: Federal Chamber of Engineers (Ed.): Art of Engineering 2015. Berlin 2014 [2] Bendsøe, Martin Philip; Sigmund, Ole: Topology Optimization – Theory, Methods and Applications. Berlin 2003 [3] Baker, William F. et al.: Maxwell’s Reciprocal Diagrams and Discrete Michell Frames. In: Journal of Structural and Multidisciplinary Optimization, Vol. 48, 02/2013, pp. 267– 277 [4] Stromberg, Lauren L. et al.: Application of Layout and Topology Optimization Using Pattern Gradation for the Conceptual Design of Buildings. In: Journal of Structural and Multidisciplinary Optimization, Vol. 43, 02/2011, pp. 165 –180 [5] Schek, Hans-Jörg: The Force Density Method for Form Finding and Computation of General Networks. In: Computer Methods in Applied Mechanics and Engineering, 03/1974, pp. 115 –134 [6] Beghini, Lauren L. et al.: Structural Optimization Using Graphic Statics, Vol. 49, 03/2014, p. 351– 366 Bibliography: Liu, Kai; Tovar, Andrés: An Efficient 3D Topology Optimization Code Written in Matlab. In: Journal of Structural and Multidisciplinary Optimization, Vol. 50, 06/2014, pp. 1175 –1196 Sigmund, Ole: A 99 Line Topology Optimization Code Written in MATLAB. In: Journal of Structural and Multidisciplinary Optimization, Vol. 21, 02/2001, pp. 120 –127 Sokól, Tomasz: A 99 Line Code for Discretized Michell Truss Optimization Written in Mathematica. In: Journal of Structural and Multidisciplinary Optimization, Vol. 43, 02/2010, pp. 181–190 Talischi, Cameron et al.: PolyMesher  – A General-Purpose Mesh Generator for Polygonal Elements Written in Matlab. In: Journal of Structural and Multidisciplinary Optimization, Vol. 45, 03/2012, pp. 09 – 328 Zegard, Tomas; Paulino, Glaucio H.: GRAND – Ground Structure Based Topology Optimization for Arbitrary 2D Domains Using MATLAB. In: Journal of Structural and Multidisciplinary Optimization, Vol. 50, 05/2014, pp. 861– 882

2.17

2.18

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RESEARCH + FUTURE

Before

2005

2006

2007

2008

2009

IIT collaborations Design methodologies for tall buildings

CFD Studies of tall buildings

Wind engineering studies

Force density

Studies of built buildings

Evolutionary structural optimisation

Instrumentation of tall buildings

Study of important engineers, architects and artists

Optimal shell form finding Studies on ribbed shells

Topology Optimisation (UIUC collaboration)

Quantities for tall buildings

Post-buckled tubular geometries Genetic algorithm Study of Maxwell’s paper: “On Reciprocal Figures, Frames, and Diagrams of Forces”

Energy sizing methods & lagrange multiplier for deflections and harmonics

Studies of Cremona Ultratall tower studies

Maxwell’s load path theorem

Precast wall atudies

Michell’s trusses / paper

Principal stress trajectories

Study of Michell’s layouts Optimisation with Altair Hyperworks

Study of discrete Michell trusses

Tall building optimal shape and surface study

Optimal shell form finding

Hyperbolic paraboloid shells (studies on Candela’s paper)

Shanghai Center, Shanghai (CN)

120

Studies of Prager

In-house wind tunnel feasability

Yongsan Tower, Seoul (ROK) Tanggu Hotel and Convention Center, Tianjin (CN)

Ninth Avenue Plaza, New York (USA)

Discovery of geometrical rules in discrete optimum trusses

Studies on ribbed shells First ice shells experiments

Tianjin High-Speed Rail Station, Tianjin (CN)

Research timeline

2010

2011

2012

2013

2014

2015

Minimum load path shells

Topology optimisation using polygonal finite element (POLYTOP)

Graphic statics – dual optimum structures Airy’s stress function

Bridge problem Multiple objectives optimisation

Ground structures optimisation and minimal load path structures

Graphic statics

Airy’s stress function for trusses

Thrust network analysis and grid shells L

Mechanisms vs. self-stressed structures

d/2

Graphic Statics and minimal load path structures

q

Studies of Rankine Application of topology optimisation in high-rise design

Optimum bracing geometry

Ground Structure optimisation in 3D

Grid / panel optimisation

Timber Tower Report

3D Rankine and Cremona

In-house wind tunnel construction

100 Mount Street, North Sydney (AUS)

The CTF Tianjin, Tianjin (CN)

Nanchang Greenland Expo Center, Nanchang (CN) Timber Tower

Ningbo Eastern New City, Ningbo (CN)

Wind tunnel at SOM’s Chicago office

121

PROJECTS + PEOPLE

Catalogue of projects

124

People

140

Picture credits

144

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PROJECTS + PEOPLE

Client Nanning Skyfame Yujun Investment Co., Ltd. Building type Mixed-use tower Structural concept Central, outer cores with perimeter frames, mega-columns and outriggers Anticipated completion 2019

Nanning Wuxiang ASEAN Tower, Nanning (CN) The superstructure of the 86-storey, mixed-use Nanning Tower, which gently tapers from its base, is a core and frame system with the core consisting of reinforced concrete shear walls in a hexagonal arrangement linked to three corner reinforced concrete cores. The central core extends to the full height of the tower. The outer cores extend to level 62. The perimeter frame consists of columns spaced at 9 m in combination with six composite mega-columns, two at each corner. Linking the perimeter frame to the core are outriggers and belt trusses in structural steel located at four mechanical levels distributed over the height of the building. This linkage greatly increases the building’s rotational stiffness and limits lateral deformation. The total height of the tower to the top of the crown is 501 m.

Client Serneke Group AB Building type Mixed-use residential tower Structural concept Core and outrigger with incremental slab edge translation Anticipated completion 2019

Polestar Tower, Karlavagnsgatan, Gothenburg (S) At 230 m, the proposed 65-storey residential Polestar Tower will be the tallest building in Sweden. The result of a competition, it features a design that “takes inspiration from ribbons blowing in the wind”. The slender tower, constructed in reinforced concrete and approximately 30 m square, rises in a uniform extrusion emphasising verticality and slenderness for approximately 40 floors, then gradually twists 45° and back again from levels 41 to 65. The twist is realised by a simple, incremental translation of the slab-edge geometry. The lateral system is comprised of a central reinforced concrete core with multi-storey outriggers to perimeter belt walls engaging the perimeter columns. The twisting shape helps to confuse the wind and reduces the frequency and severity of wind-induced movements.

Client Bank of Beijing Co. Ltd. Building type Research campus Structural concept Distributed cores with perimeter moment frames Anticipated completion 2018

Bank Of Beijing Science And Technology Research And Development Center, Beijing (CN) Located in northeast Beijing’s Shunyi New Town City, the new research and development campus for the Bank of Beijing encompasses diverse functions across multiple buildings, including a 15-storey office tower, data centre, employee residences and training facility in a park-like setting without sacrificing the bank’s unified identity and culture. A common concourse level links the structures while creating a strongly defined central courtyard that serves as a natural space where employees and campus visitors can gather. In response to high seismic loading, the structural system for the tower efficiently takes advantage of the stiffness, mass and damping characteristics of the four reinforced concrete shear wall cores located near the four corners of the building, in combination with the perimeter structural steel moment frame to resist all lateral loads.

Client CITIC Securities Company Ltd., Goldstone Zexin Investment Management Co. Ltd. Building type High-rise tower complex with retail podium Structural concept Center core with optimised external diagrid bracing Anticipated completion 2018

CITIC Financial Centre, Shenzhen (CN) Located on Shenzhen Bay, approximately 9 km west of the city centre, the CITIC Financial Centre consists of two mixeduse towers with an adjoining retail podium. The taller, 65-storey office tower – at a height of 312 m – consists of condominiums above office floors. The hotel tower is shorter, at a height of 212 m. Each is framed in structural steel, with the architecturally expressed exterior of composite steel/concrete diagrid braced frames linked to reinforced concrete shear walls at the cores. Based on the Michell truss, the similar but distinct exterior diagrid geometry of each tower gracefully morphs from bottom to top, creating continuous variation in the brace angles over the height and maximising the stiffness of the overall structure in each tower. Furthermore, the exterior diagrid structure creates completely column-free interior spaces beyond the cores in both towers.

Client Zhongtian Urban Development Group Co. Ltd. Building type Mixed-use complex including towers Structural concept Tube in tube Anticipated completion 2018

Guiyang World Trade Center, Guiyang (CN) The Guiyang World Trade Center is a mixed-use development on a 16-ha site. The focal point is a 380-m, mixed-use landmark tower with a series of five adjacent residential towers, as well as a number of low-rise commercial, retail, and entertainment components across the site. The site is characterised by a significant elevation variation of proposed grading platforms against an existing hillside adjacent to the Nanming River. The structural system of the landmark tower consists of a conventional ductile reinforced concrete central core with a highly efficient perimeter reinforced concrete ductile tube moment frame with SRC (steel-reinforced concrete) columns at the base. Lateral loads are shared between the core and the perimeter tube by means of composite metal deck slabs without requiring belt trusses or outriggers and thus facilitating a faster construction cycle.

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Manhattan West, Ninth Avenue, New York, New York (USA) The site for the Manhattan West project is on land currently open to railway tracks west of Penn Station in midtown Manhattan. The project is a large mixed-use development with two tall office towers that rise from a plaza over active railway tracks. The northeast tower, with 71 storeys above grade, is 304 m in height. The structural system of the northeast tower consists of a central reinforced concrete core with a structural steel perimeter moment frame and gravity framing. Due to the tracks below, the entire perimeter structure is transferred back to the building core, resulting in an exceptionally slender lateral system with an approximate 1:16 aspect ratio. The southeast tower currently under design is tentatively 65 storeys in height (280 m) with a lateral system also comprised of a reinforced concrete core with perimeter structural steel frame cantilevered from it.

Client Brookfield Properties Building type Office development Structural concept Concrete core with exterior steel framing Anticipated completion 2018

Tianjin CTF Financial Centre, Tianjin (CN) The 96-storey, 530-m Tianjin Chow Tai Fook Binhai Centre uses undulating curves to subtly express its three programmatic elements of apartments, hotel and offices in an overall bold monolithic expression on the skyline. Its gently curving glass skin conceals eight sloping perimeter columns that lie behind the primary bend of the elevations and increase the building’s stiffness in response to lateral loading. The tower’s aerodynamic shape reduces vortex shedding, which in turn dramatically minimises wind forces. The structural system consists of a central reinforced concrete shear wall core and perimeter moment frame in structural steel, with composite mega-columns following the curves of the building and belt trusses increasing the stiffness of the perimeter frame, which is linked to the central core at mechanical levels.

Client NW Project Management Ltd. Building type Supertall office tower Structural concept Core and mega-columns Anticipated completion 2018

Gemdale Shenzhen Gangxia Development, Shenzhen (CN) The Shenzhen Gangxia project consists of a 33-storey office tower set in the northwest corner of a larger parcel incorporating a landscaped ground level over three basement levels with retail, parking, and other services. The tower is square in plan, with sides 47.20 m long and re-entrant corners creating distinctive architecturally expressed frames on the four facades. The lateral system is a frame-core system consisting of reinforced concrete shear walls at the core in conjunction with perimeter moment frames composed of composite columns and reinforced concrete perimeter beams. The perimeter frames are enhanced by structural steel tube braces creating a distributed braced frame on each facade, thus increasing the stiffness of the perimeter frames and providing a unique architectural identity for the building.

Client Shenzhen Gemdale Dabaihui Real Estate Developmment Company Ltd. Building type Office tower Structural concept Core and distributed braced frame Anticipated completion 2017

Greenland Group Suzhou Center, Suzhou (CN) Like sculpting a high-performance automobile, aerodynamic modelling was critical to shaping this 77-storey, 358-m, supertall mixed-use tower. The plan configuration is elliptical in the lower office portion with a split-core arrangement, consisting of two curved bars on either side of an atrium, in the upper hotel portion. Acting as the “lung” of the building, the tower’s atrium is defined by a 30-storey operable window that invites cool airflow during the summer months and floods the interior with natural light. The structural system is that of a “frame shear wall” consisting of reinforced concrete core walls, with structural steel outrigger and belt trusses at the mechanical levels connecting to the perimeter frame columns. The two halves of the core on either side of the atrium are connected together with structural steel braces, providing the stiffness of the combined “coupled core”.

Client Greenland Group Building type Mixed-use supertall tower Structural concept Coupled core Anticipated completion 2017

Manhattan Loft Gardens, London (GB) Manhattan Loft Gardens is located in Stratford in the gateway area to the site of the 2012 Olympics, and encompasses a seven-storey hotel base with 35 floors of residential construction above. A feature of the tower is a series of sky gardens that carve into the building, creating a striking profile on the London skyline. The hotel superstructure consists of reinforced concrete walls and columns from the ground floor to the underside of the roof terrace at level eight. The primary structure for the residential tower is a central reinforced concrete core extending from the hotel below. At levels 10 and 28, a storey-high grid of post-tensioned concrete transfer walls cantilevers from the core. These walls in turn support exterior structural steel belt trusses, which support concrete columns and post-tensioned concrete residential floors above, creating the ceiling plane for multi-storey sky gardens below.

Client Manhattan Loft Corporation Building type Hotel, residential Structural concept Central core with transfer walls Anticipated completion 2017

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Client Shum Yip Real Estate Co. Ltd., Shum Yip Southern Land (Holdings) LTD Building type Mixed-use towers Structural concept Integrated ladder-core Anticipated completion 2017

Shen Ye Upperhills Mixed-Use Development, Shenzhen (CN) The Shen Ye Upperhills Mixed-Use Development consists of two mixed-use towers 380 m and 280 m tall, along with a freestanding ballroom pavilion. Tower One, the taller tower, has a structural system incorporating a ductile reinforced concrete core linked with eight concrete-encased, structural-steel mega-columns at the perimeter, utilising composite coupling beams along the entire height of the building and creating a ladder-like extension of the core footprint. With only eight mega-columns, the perimeter girders span 28 m and cantilever 9 m to the corner in an efficient balanced cantilever configuration. The structural system of Tower Two incorporates a ductile reinforced concrete core and a structural steel perimeter moment frame linked at two locations along the building height by two-storey steel outriggers and belt trusses. Each system is efficient in resisting wind and seismic loads and supporting gravity loads.

Client 111 Main, LLC Building type Office tower with air rights easement over new performing arts facility Structural concept Central ductile reinforced concrete core with steel hat truss and suspended perimeter columns Anticipated completion 2016

111 Main Tower, Salt Lake City, Utah (USA) The 111 Main Tower project consists of a 24-storey office tower in the financial district of Salt Lake City built adjacent to and extending over a new facility for the Utah Performing Arts Center, with the requirement that exterior columns of the tower over the performing arts facility cannot extend to the ground. Thus the challenge of unbalanced loads for a highrise in a high seismic zone. The solution is a performancebased design of a central ductile reinforced concrete core for the tower designed for all lateral loads and supporting a 3D structural steel roof hat truss system to carry suspended perimeter columns. Six friction pendulum structural bearings are used under the roof truss system to manage lateral and gravity load transfers at the top of the core walls.

Client US General Services Administration Building type Office and courthouse tower Structural concept Central concrete core ans steel frame with optimised cantilever roof trusses and suspended perimeter columns Anticipated completion 2016

Federal Courthouse, Los Angeles, California (USA) The Los Angeles Federal Courthouse consists of a major new 10-storey facility located in the city centre, occupying an entire block. The courthouse has been designed with no perimeter columns extending to the foundations, thereby eliminating elements potentially vulnerable to threat at ground level and providing a building that seems to float above a large civic plaza. The building has been designed with primary structural steel framing which is selectively encased in reinforced concrete throughout the floor plan, creating cores and shear walls which resist all lateral loads. These central cores and walls support a storey-height hat truss system of structural steel cantilever trusses that carry the suspended perimeter columns. The design of the trusses has been optimised in accordance with principles set forth by mathematician John Michell in the early 1900s.

Client Shenzhen Rural Commercial Bank Building type Office tower Structural concept Core and diagrid Anticipated completion 2016

Rural Commercial Bank Headquarters, Shenzhen (CN) Adjacent to a public park and only three blocks from the sea, this striking 41-storey, 150 m tall mixed-use tower in Shenzhen’s Bao’an District will offer spectacular views at every floor and be a world-class benchmark for sustainable design and high-end, flexible office spaces. The structural system for the tower is tube-in-tube, with the internal tube of the central reinforced concrete core working with the perimeter structural steel diagrid that forms the external tube. The architecturally expressed diagrid is emblematic of SOM’s rich tradition of integrating architectural design and structural engineering and enables flexible, column-free interior spaces while doubling as a solar shading device for the recessed glass curtain wall.

Client Japan Tobacco International Collaborating engineers Ingeni – Ingenierie Structurale Building type Corporate headquarters Structural concept Full depth facade cantilever trusses Completion 2015

The JTI Building, Geneva (CH) The landmark headquarters for Japan Tobacco International located near Lake Geneva is a 10-storey building framed in structural steel, triangular in shape around a central open courtyard. Each of the three perimeter and three courtyard building elevations are sloped, full-depth, nested Pratt trusses tied together by moment connections in the spandrel beams. As such, they act together to resist all gravity and lateral loads. Limited supports result in cantilevers of 48 and 60 m and clear spans of up to 75 m to create the distinctive elevations around the building. Floor framing consists of composite steel beams and lightweight composite metal deck slabs.

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Learning and Development Center, Roche Diagnostics Corporation, Indianapolis, Indiana (USA) The use of crisp white metal and glass in the Learning and Development Center, including painted exposed steel framing, establishes a modern aesthetic and identity for the Indianapolis campus of Roche Diagnostics, a Swiss-based pharmaceutical company. Comprised of three distinct zones, the two-storey building, constructed primarily in structural steel, is organised around double-height, sky-lit spaces. The primary lateral load system consists of reinforced concrete shear walls distributed throughout the building, thus allowing the structural steel columns to be minimised in profile and to be constructed of slender bar stock.

Client Roche Diagnostics Building type Office and research facility Structural concept Post and beam Completion 2015

OKO Tower (Plot 16), Moscow (RUS) OKO Tower complex on Plot 16 is part of the thriving Moscow city zone located between the new Expocentre and the Third Ring Road. The OKO Tower is an 85-storey residential tower, 354 m in height and adjacent to a 47-storey office tower, 224 m in height. Both towers are constructed in reinforced concrete. For the taller residential tower, the structural system consists of core walls and perimeter belt walls. The shorter office tower’s structural system consists of core walls linked to the perimeter moment frame for resistance to lateral loads. As the two towers rise, their in-line facades progressively chamfer inward, thus opening a distance and creating a changing dialogue between the towers with height.

Client Capital Group Building type Office and residential towers Structural concept Core and belt walls (residential tower), core and moment frame (office tower) Completion 2015

Pertamina Energy Tower, Jakarta (RI) As the dynamic centrepiece of a new consolidated headquarters created for the state-owned energy company, the 99-storey Pertamina Energy Tower will rise more than 523 m above Jakarta as a stunning landmark on the capital’s skyline. For the relatively high seismic forces at this site, the tower efficiently utilises a dual structural system for resisting lateral loads that consists of a ductile reinforced concrete core system coupled with a perimeter moment frame via outrigger and belt truss systems at the mechanical levels. The perimeter frame consists of steel framing with composite columns of concrete-encased steel members. The tower’s shape with its continuously tapering geometry and large notches is engineered to reduce vortex shedding and minimise wind-induced acceleration and forces.

Client PT Pertamina Building type Supertall office tower Structural concept Reinforced concrete core with perimeter moment frame with outrigger and belt truss Design 2015

Poly International Plaza, Beijing (CN) The Poly International Plaza is located on the east side of Beijing, approximately halfway between the Forbidden City and Beijing Capital Airport, in a prominent position along the Capital Airport Expressway. The project consists of three office towers, two of modest height, and the Icon Tower, 32 storeys in height (161 m) and oval in plan. The primary lateral resistance system for the Icon Tower is a faceted, fourstorey module, cross-braced, diagrid exoskeleton in structural steel enveloping an elongated reinforced concrete core and a pair of arc-shaped office spaces in the clear span between the outer diagrid and the core. The diagrid brace members consist of concrete-filled circular pipe sections in steel, with the horizontal steel beams integral with the floor framing and linked to the core for maximum efficiency in distributing lateral loads.

Client Beijing Poly Ying Real Estate Development Co., Ltd. Building type High-rise tower complex with retail podium Structural concept Core and diagrid Completion 2015

United States Air Force Academy, Center for Character & Leadership Development, Colorado Springs, Colorado (USA) This new addition to the US Air Force Academy is a singlestorey academic and office building with a sunken forum constructed in the courtyard of one of the existing academic buildings. The building is topped by a dramatic 32 m steelframed, inclined skylight tower directed toward Polaris, the North Star, a guiding symbol at the Academy and essentially on axis with and complementary to the iconic Cadet Chapel. The skylight consists of architecturally exposed structural steel plates organised on each of the four faces in a diagrid pattern. The depths of the plates increase toward the centre of each face to create a uniform stiffness profile across each face of the structure. The skylight structure itself is very stable due to the triangulation of all its faces and is supported by braced frames on the ground floor.

Client United States Air Force Academy Building type Classroom and office building Structural concept Diagrid Completion 2015

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Client Mumbai International Airport Limited Building type Airport terminal Structural concept Long span two-way roof truss system Completion 2014

Chhatrapati Shivaji International Airport Terminal 2, Mumbai (IND) The new integrated terminal building at Mumbai’s Chhatrapati Shivaji International Airport combines international and domestic operations at one of the busiest airports in India. The 410,000 m2 building has a capacity of 40 million passengers per annum. The primary design feature of the building is a long-span roof covering a total of 6.9 ha, making it one of the largest roofs in the world without an expansion joint. The terminal roof’s 30 mega-columns are spaced at 64 m in the north-south direction and 34 m in the east-west direction, producing a large column-free space ideal for an airport. In response to site constraints, the mega-columns were designed to serve also as hoist mechanisms, so that the entire roof could be constructed without tower cranes. The terminal building also includes the longest cable wall system in the world.

Client Continuum Partners, Denver Union Station Project Authority, East West Partners Building type Railroad station Structural concept Steel arched trusses with PTFE stressed fabric roof Completion 2014

Denver Union Station, Denver, Colorado (USA) One of the most comprehensive intermodal transportation and transit-oriented developments in the United States, Denver Union Station brings together commuter, inter city and light rail lines, including airport access, along with a bus station for regional and local circulating buses. The centrepiece of the development is a new open-air “train hall” canopy, expressive and dramatic in form, and a contemporary interpretation of historic train sheds. Constructed in structural steel spanning six rail lines adjacent to the historic station, the primary structural system consists of 11 arch trusses spanning 54 m from single pin connections atop reinforced concrete columns designed to take the thrust of the arches and all gravity loads. A PTFE fabric roof is stressed between steel framing members to form the membrane roof surface. The large central opening over the tracks adds to the sinuous quality of the canopy roof.

Client Cayan Investment and Development Building type Residential tower Structural concept Tubular frame with circular core Completion 2013

Cayan Tower, Dubai (UAE) A helical skyscraper that makes a distinct mark on the Dubai skyline, the 73-storey residential tower’s building form directly follows its structural framework. While all the floor plates are identical, each is slightly rotated from the storey below, resulting in a full 90° twist on the course of the tower’s 307 m height. Besides the aesthetics, this unique form also results in reduced wind forces compared to a rectilinear building of the same height and affords a greater number of tenants the desirable views of the nearby marina, coastline and gulf.

Client Jiangau Goldenland Real Estate Development (Group) LTD Building type Office tower Structural concept Reinforced concrete frame core with steel diagonal braces Completion 2013

Kingtown International Center, Nanjing (CN) This next generation, 54-storey tower maximises performance, efficiency and occupant experience. Its faceted form is derived from the juxtaposition of the innovative double-skin facade and the external bracing system that wraps the tower from crown to base and defines the dimensions and folds of the building envelope. The lateral system consists of a reinforced concrete framed core and a perimeter steel diagonal brace system. The core in the upper level is eroded as permitted by strength demands to bring light into the hotel atrium. Steel diagonal braces on the exterior facade limit the drift from lateral forces and are connected to the corner of the base building at every 16 m.

Client China National Tobacco Corporation, Guangdong Tobacco Corporation, The Guangzhou Pearl River Tower Properties Co. Ltd. Building type Office tower Structural concept Core and outrigger with X-braced end frames Completion 2013

Pearl River Tower, Guangzhou (CN) This 71-storey office tower along the Pearl River in Guangzhou provides a highly sustainable design by incorporating the latest green technology and engineering advancements. The tower’s sculpted body directs wind to a pair of openings at its mechanical floors, where travelling winds drive turbines that generate energy for the building. Other integrated sustainable elements include solar panels, a double-skin curtain wall, a chilled ceiling system, under-floor ventilation and daylight harvesting, all of which contribute to the building’s efficiency. Structurally, the tower is a composite of reinforced concrete and structural steel with a reinforced concrete core and corner composite mega-columns linked to exterior structural steel columns by outrigger and belt trusses to resist seismic and wind loading as well as gravity.

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Catalogue of projects

Weather Field No. 1, Santa Monica, California (USA) The Weather Field No. 1 sculpture enlivens Santa Monica’s Tongva Park through its contemporary design that engages prevailing wind patterns to create beautiful “flocking” patterns. SOM collaborated with artist Iñigo Manglano-Ovalle by providing structural engineering services for the piece, which consists of 49 tightly grouped, stepped, grade 316L stainlesssteel vertical poles. The poles vary in height from 5.44 m to 6.25 m and in diameter from 25 mm at the top to 75 mm at the base. During the design process, the team paid special attention to fatigue at the connections and to aerodynamic instability due to vortex shedding. Each pole height and each mode shape underwent Scruton number evaluation, dynamic analyses and critical wind speed determination. A grid of stainless steel plates with socketed clamp connections forms the sculpture base that is supported on four piers.

Artist Iñigo Manglano-Ovalle Client City of Santa Monica Building type Sculpture Structural concept Slender cantilever poles Completion 2013

100 Mount Street, North Sydney (AUS) This 33-storey office tower rising 144 m in the commercial heart of North Sydney features highly transparent double-skin glazing articulated by exposed, architecturally clad structural elements. The superstructure is of cast-in-place concrete, with floor construction consisting of post-tensioned, one-way slabs and beams. The lateral load system consists, of an offset reinforced concrete core shear wall system at the west end, where views are marginal, and an architecturally expressed, mega X-braced frame at the east end. Acting together, they resist broad face wind forces and torsion, while the core resists narrow face lateral loading. The frame at the east end utilises a system of raised node X-bracing in structural steel, with the number of panels and the position of the nodes in near optimal position for maximum lateral stiffness for the least material.

Client Laing O’Rourke Building type Office tower Structural concept Optimised X-bracing Design 2012

Lee Hall III, Clemson University, Clemson, South Carolina (USA) Lee Hall III is a 5112 m2 addition to Clemson University’s College of Architecture, Arts and Humanities. Conceived as “a building that teaches”, it encourages informal learning through observation of its energy-efficient design and exposed functional and structural systems. Nearly all of the superstructure components in Lee Hall are structural steel, and the structural systems and architectural expression are inseparable. Beyond the window walls on the north and south faces of the building, a row of super-slender Y-columns support a steel trellis of exterior exposed steel beams and perforated metal panels. The exterior Y-columns are reflective of the 25 interior “column trees” supporting the light steel framing of the roof. The building showcases the expressive use of structural steel on a limited budget.

Architects Thomas Phifer and Partners, McMillan Pazdan Smith Architecture Client Clemson University Building type Education building Structural concept Exterior Y columns, interior column trees Completion 2012

Liansheng Financial Tower, Taiyuan (CN) The flow of Taiyuan’s Fen River provided inspiration for the arrangement of this complex of multiple towers with office, hotel and residential functions plus low-rise convention and retail facilities. The centrepiece is a supertall tower constructed in structural steel, 420 m in height and consisting of two functionally distinct but complementary towers linked intermittently by bridging levels that connect the towers so that they behave as one. The structural system is perimeter mega-bracing with composite mega-columns taking advantage of the full tower footprint to resist seismic and other lateral loads. Tied braced frames act as “fuse” beams sized to develop plastic hinges at the specified seismic intensity level. Structural steel bracing at the core of each wing provides additional local resistance to lateral loads at floors intermediate to the perimeter megaframe modules.

Client Liansheng Group Building type Mixed-use tower Structural concept Dual towers with fused braced frames Design 2012

Skyspace at Rice University, Houston, Texas (USA) The Skyspace (or “Twilight Epiphany”) is a permanent, outdoor, experiential art installation consisting of a 22 m square roof atop a berm-like, two-storey viewing gallery below ground. Conceived by artist James Turrell, Skyspace integrates light, sound and space in order to complement the natural light present at sunrise and sunset. The structural system of the roof is comprised of an octa-symmetrical, tapered, cantilevering structural steel frame supported by a lateral system of eight slender, cantilevered columns in structural steel. Skyspace’s innovative engineering enabled the design team to push the limits of cantilever span, slenderness and structural concealment. With minimal visual interference, the smooth planes of the roof beautifully capture the soft light of a day’s beginning and end, ultimately transforming the hues and the sky into an artful experience.

Artist James Turrell Architects Thomas Phifer and Partners Client Rice University Building type Art installation Structural concept Tapered cantilever roof Completion 2012

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Client Ajial Real Estate and Entertainment, Al Hamra Real Estate Building type Office tower Structural concept Hyperbolic shearwall stiffened core Completion 2011

Al Hamra Tower, Kuwait City (KWT) Rising to a height of 412 m, this iconic 80-storey skyscraper is the tallest building in Kuwait. A desire to maximise views of the Arabian Gulf while minimising solar heat gain inspired the building’s asymmetrical form. The tower’s sculpted geometry posed significant challenges to its design and construction. Long-term shortening, or creep, affects all concrete towers; but for the Al Hamra Tower, this creep has a twisting as well as a vertical component. To offset this, the entire tower was constructed with an opposite direction twist built in. The structural system for the tower is a reinforced concrete core plus moment frame. Hyperbolic paraboloid walls and a punched wall on the south facade are used to supplement the stiffness of the central core.

Architects David Chipperfield Architects Client Chelsfield Partners LLP + London and Regional Properties (JV) Building type Mixed-use Structural concept “Bridge” trusses Design 2011

Elizabeth House, London (GB) SOM provided preliminary structural engineering services for Elizabeth House, a 140,000 m2 commercial, mixed-used development at Waterloo Station designed by David Chipperfield Architects. The project consists of two mid-rise towers, separate above grade, but linked at the lower levels. Because the site is characterised by a complex network of underground transportation structures and tunnels, SOM proposed a building, framed in structural steel, which acts as a 108 m bridge above four active London Underground lines, permitting a more significant development than otherwise would have been possible. Limited access for locating new foundations informs the skewed geometry of the columns at ground level, which in turn creates a series of simple spans and cantilevers of varying lengths to support the orthogonal building form above.

Artist James Carpenter Design Client University of Nebraska Medical Center Building type Campus art tower/sculpture Structural concept Cylindrical diagrid Completion 2011

“Hope”, University of Nebraska Medical Center, Omaha, Nebraska (USA) SOM collaborated with James Carpenter Design on a 37-m open tower, 3.50 m in diameter, entitled “Hope” and designed to act as a visual identifier for the campus from a distance. The design features hundreds of triangular, perforated, titanium-coated, stainless-steel panels attached to an openframe structure constructed of solid stainless-steel members. Vertical members are round stock varying in diameter, welded to horizontal hoop plates to form a cylindrical diagrid. Stainless-steel panels and frame reflect the stunning natural light effects of the open prairie sky in Nebraska while contributing to the scientific character of the Medical Center. The panels decrease in size towards the top of the tower to eliminate possible design concerns related to the effects of vortex shedding.

Client Huawei Technologies Co., Ltd. Building type Corporate campus Structural concept Reinforced concrete roof trusses with seismic fuse links Completion 2011

Huawei Technologies Corporate Campus, Shanghai (CN) The new corporate campus of Huawei Technologies occupies a large site in the Pudong District of Shanghai. The low-rise buildings, generally five storeys in height above two levels of parking, are arranged in an elongated Z-configuration with two primary rectilinear wings. Each wing comprises three modules housing office and software production areas linked by a central hub. The modules are 110 m long by 22 m wide and framed in reinforced concrete, with a central circulation spine and reinforced concrete trussed roof with skylights and a special fuse link at the apex that allows the spaces on either side of the circulation spine to separate and act as simpler building modules under severe earthquake loading. The lateral system for each building module is a dual system of shear walls in conjunction with moment-resisting frames. Major beams spanning 19 m are post-tensioned.

Client Finance Street Jinta (Tianjin) Real Estate Co. Ltd. Building type Office tower Structural concept Steel plate shear walls with concrete filled steel tube moment frames Completion 2011

Tianjin Global Financial Center, Tianjin (CN) Located in the historic heart of Tianjin, across the Hai River from the new high-speed rail and transit hub also by SOM, this 337 m, 75-storey tower framed in structural steel marks the city centre and provides an iconic visual point of reference. The tower is oval in plan with the floors tapering towards the top. At the perimeter there are triangular cantilevered slabs between the columns creating a pattern of graceful, vertical, tapering ribs over the height of the building. The lateral system for the tower consists of a perimeter moment frame with composite concrete-filled steel columns linked to the central core of stiffened steel plate shear walls. For drift control, there are four levels of outrigger trusses over the height of the building linking the core to the perimeter moment frame.

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Burj Khalifa, Dubai (UAE) Soaring 828 m above Dubai, this 162-storey office, retail, hotel and residential tower, constructed in high-performance concrete, is currently the world’s tallest man-made structure and the centrepiece of a large mixed-use development. The tower is Y-shaped in plan, an efficient shape in resisting wind forces on the tower, while maximising the views of the Arabian Gulf. The structural system is that of a buttressed core, with each wing having longitudinal and cross walls that act like the web and flanges of an I-beam buttressing the other wings via a six-sided central core. The result is a tower that is extremely stiff both laterally and torsionally. The wings of the tower step back in a pattern that spirals up the building, thus changing the shape of the tower at each setback and disrupting the wind, which greatly contributes to the efficiency of the overall structure.

Client EMAAR Properties Building type Mixed-use tower Structural concept Buttressed core Completion 2010

“World Voices”, art installation, entrance hall of Burj Khalifa, Dubai (UAE) “World Voices” is a dynamic metal art installation in the residential entrance hall of Burj Khalifa in Dubai, for which SOM provided structural design. It consists of 114 slender stainless-steel tubes of varying heights with 196 cymbals, each representing one of the countries of the world, attached in various locations along their height. The tubes lean above two shallow pools of water, and special equipment releases drops of water from the 18-m ceiling height onto the cymbals, producing sound. The longest tube is 12 m long and 60 mm in diameter, and the tubes bend significantly due to the large slenderness ratios, adding to the dynamic quality of the installation. Accurate prediction of the flexible rods’ deformation as well as providing the smallest diameter possible for strength were the primary challenges of the structural design, which required non-linear, large deflection stability analysis.

Artist Jaume Plensa Building type Art installation Structural concept Ultra-slender cantilevered rods Completion 2010

ASPIRE, Jeddah (KSA) Developed for a competition for a mixed-use, kilometre-tall tower, the elongated conical form of Aspire is derived from a structural form optimised for strength and wind performance. The efficiency of the structure is facilitated by the purity of its circular shape, which provides inherent strength and stability to the building. The overall profile was determined from genetic algorithm techniques that optimise the behaviour of the building. The design provides a building that is efficient in materials usage and easy and fast to construct. The primary structural system is a reinforced concrete tapering circular core wall. No columns or outriggers are required for the perimeter structure, which simplifies and quickens construction in addition to providing unobstructed views. The floor plates are cantilevered off the core wall via post-tensioned, tapered cantilever girders.

Client Jeddah Economic Company Building type Supertall mixed-use tower Structural concept Super core with cantilevered floor plates Design 2009

Takshing House, Hong Kong (CN) Proposed for an area in the heart of Hong Kong, the Takshing House Redevelopment project replaces an existing mid-block building with an office tower and lower-level retail space. Inspired by the natural architecture of trees, the proposed office tower is lifted 90 m out of the dense mass of existing buildings, allowing southern sunlight to reach the ground level and exposing the once-covered facades of existing historic buildings. Above 90 m, office floors are cantilevered like branches from the tower’s reinforced concrete core, which also supports all gravity loads and resists all lateral forces. The office floors are cantilevered off the central core by tapered post-tensioned girders in reinforced concrete, which in turn support other post-tensioned beams and conventionally reinforced one-way slabs.

Client Tak Shing Investment Co. Ltd. Building type Office Structural concept Central core with cantilever floors Design 2009

Tanggu High-Speed Rail Station, Tianjin (CN) China’s premier high-speed rail line is capable of travelling upwards of 400 kilometres per hour across the northeastern part of the county. The train’s terminal station will be located in the new mixed-use Tanggu District of Tianjin City, east of Beijing, where SOM has designed an intermodal hub to serve over 6,000 passengers during peak hours. The design incorporates a lightweight, structural-steel, domed, latticed roof structure in the form of a grid shell that seamlessly flows into the landscape of the surrounding park, its parabolic ribs radiating outward in accordance with Maxwell’s load path theorem. The ribs are structural-steel box sections, with the thrust contained by a perimeter tension ring in structural steel. Maximum length of the dome is 167.60 m with a dome height of 28.3 m.

Client Tianjin Innovative Finance Investment Co. Ltd. Building type Transit hub Structural concept Grid shell Design 2009

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Client The Trump Organization Building type Hotel and residential tower Structural concept Core and outrigger Completion 2009

Trump International Hotel and Tower, Chicago, Illinois (USA) Rising an impressive 415 m at a prominent site on the Chicago River, the Trump International Hotel and Tower was, at the time of completion, the tallest residential building in North America. The reinforced concrete tower is constructed with flat plate floor construction. Lateral loads are resisted by a central core wall system and outrigger walls to the exterior columns along the long faces at selected levels. In the narrow direction of the building, the core remains constant in dimension and steps back in the long direction as the building steps back.

Client Greenland Group, Nanjing Guozi Greenland Financial Center Co. Ltd./ Nanjing Greenland International Commercial Center Co. Ltd. Building type Mixed-use tower Structural concept Corewall and perimeter moment frame with outrigger and belt trusses Completion 2009

Zifeng Tower, Nanjing (CN) Rising from a retail podium and including a separate 24-storey office tower at a bustling intersection in the city of Nanjing, this 88-storey, 450 m-tall, triangular tower was the seventh tallest building in the world and the second tallest building in China at the time of its completion. Articulated and stepped in response to its various functions of office, hotel, restaurant and observatory, the tower’s primary lateral load-resisting system is comprised of a triangular centralised reinforced concrete core and perimeter moment frame system consisting of composite concrete encased steel columns and structural steel beams. The core and the perimeter is linked at three locations along the height of the building with structural steel outriggers and belt trusses. All gravity framing consists of structural steel to reduce the seismic demand on the building structure.

Client Catholic Cathedral Corporation of the East Bay, Roman Catholic Diocese of Oakland Building type Cathedral Structural concept Base isolated, composite reinforced concrete, glulam timber and steel rod diagonal bracing Completion 2008

Cathedral of Christ the Light, Oakland, California (USA) The Cathedral of Christ the Light resonates as a place of worship and instils a sense of solace and respite from the secular world. It was conceived as an inner wooden vessel contained within a veil of glass and anchored to an architectural concrete base. The superstructure of the cathedral’s main body above the base consists of a composite system of curved, tapered, glue-laminated (glulam) timber rib members and high-strength structural steel rods paired with glulam timber compression struts, and a steel friction pendulum seismic base isolation system. A total of 724 closely spaced glulam louvre members interconnect and provide lateral bracing for the inner rib members and allow natural daylight to be cast in the sanctuary. This system also supports the primary vertical mullions of the exterior glazed “veil” window wall, thus producing a building of extreme lightness and transparency.

Client Lotte Group Building type Supertall tower Structural concept Diagrid Design 2008

Lotte Super Tower, Seoul (ROK) Planned for southeastern Seoul, this 555-m tower is a mixeduse building, including retail, office and hotel spaces and an observation deck. The tower’s design transforms smoothly from a square base to a circle at the top. This geometry greatly decreases the wind forces by varying the rate of vortex formation along the height. The exterior is an architecturally expressed, robust, perimeter, structura-steel diagrid in a tapered, conic form. Coupled with a ductile interior concrete core tube system, the overall lateral system limits wind and seismic effects in an efficient manner. The floor framing systems are in structural steel.

Client British Land, M3 Consulting Building type Office tower on preconstructed raft structure over active railroad tracks Structural concept Braced tube with A-frame base Completion 2008

The Broadgate Tower, London (GB) Built as part of London’s Broadgate Development, which has been constructed as an air rights project above the active train lines of Liverpool Street Station, this project consists of two primary office towers with a glazed arcade in between. Broadgate Tower is a 35-storey, structural-steel, braced tube supported at its base by six transverse, multi-storey A-frames spanning the railroad tracks. One leg of the A-frames on each column line is incorporated in the tower, and the other extends into the retail galleria between the buildings. X-bracing extending above the outer legs in the narrow face of Broadgate Tower provides resistance to lateral loads and is expressed in the stainless-steel architectural cladding. The adjoining 201 Bishopsgate is a 12-storey office block supported on the previously constructed raft – also designed and engineered by SOM.

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Poly Corporation Headquarters, Beijing (CN) Prominently located northeast of the Forbidden City, this 110-m headquarters building is a unique structure, triangular in shape, that includes 24 storeys of office space with an L-shaped floor plate, a 90 m tall atrium enclosed by one of the world’s largest cable-net glass walls, and an eight-storey museum that hangs from the main structure in the plane of this wall. The structural system for the tower is a conventional composite frame and shear wall system consisting of reinforced concrete shear walls at the cores and structural steel moment frames at the perimeter, including a 12-storey deep Vierendeel truss spanning the south atrium window. This window, 60 m wide and 90 m tall, is divided into three cable net segments fitting around the hanging museum and expressed with V-shaped folds and large-diameter tensioned steel cable boundary elements connected using cast split rocker mechanisms.

Client Beijing New Poly Plaza Real Estate Company, China Poly Group Corporation Building type Office building Structural concept Rocker mechanism isolated cable net wall Completion 2007

Poly Real Estate Headquarters, Guangzhou (CN) This striking mixed-use development near the Pearl River consists of two slender 35-storey office towers, each coupled with a podium that houses exhibition and retail space and a belowgrade exposition hall and trade centre. These energy-efficient towers are defined by their expressed structure and offset cores. The long south facades of the two towers incorporate double-depth diagonal mega-braces in structural steel that resist lateral loading from seismic forces and typhoon winds in both primary directions. This exposed structural framework on the south facade also serves as a shading device, and the buildings further achieve sustainability through natural ventilation, floor-by-floor mechanical systems, under-floor air distribution, shaded outdoor space and green roofs. Both towers are constructed in a combination of structural steel and reinforced concrete.

Client China Poly Real Estate Company Limited Building type Office building Structural concept Exterior diagonal mega-braces Completion 2007

Private Residence, Fishers Island, New York (USA) SOM collaborated with architect Thomas Phifer and Partners to provide structural engineering services for the Fishers Island residence. The design is a response to the unique island setting at the eastern end of Long Island and the client’s very specific personal interests. The house is modern in its open floor plan, minimalist in aesthetics and makes use of exposed steel and glass while being equally organic in relation to its setting. Constructability on the remote site while maintaining minimalist aesthetics led to maximising the use of standard residential-style materials, such as wood framing on the ground floor and use of plywood shear walls in conjunction with structural steel framing that has been designed to minimise visual interruption of the glazing. Fifty freestanding steel canopy “trees” surround the house and shield the full height of the glass from direct overhead sunlight.

Architects Thomas Phifer and Partners Building type Residence Structural concept House: solid bar column /mullions Canopies: eccentric cast ductile iron trees Completion 2007

Virginia Beach Convention Center, Virginia Beach, Virginia (USA) Since its completion, the award-winning Virginia Beach Convention Center has become the centrepiece of a 40-year master plan to green Virginia Beach and reinvigorate the classic resort town. The Convention Center consists of four reconfigurable halls under a roof 256 m in length by 122 m in width, a linear entry and prefunction spine and a glass and steel observation tower 45 m in height that pays homage to the city’s historic lighthouse. The roof structure of the main halls consists of a series of structural steel trusses, 9 m on centre with a principal span of 75 m, providing one of the largest column-free spaces in the USA. The prefunction spine has a dramatic glass wall 23 m in height, with inclined bowstring cable trusses as the principal support.

Client City of Virginia Beach Department of Public Works, Virginia Beach Convention and Visitors Bureau Building type Convention center Structural concept Bowstring cable trusses Completion 2007

Bergen Balance Bridge, Bergen (N) The Bergen Balance Bridge, a proposed 132 m long pedestrian bridge spanning the Puddefjorden in Bergen, Norway, is a technologically advanced, zero-energy drawbridge with two cantilevered spans constructed in structural steel with highstrength steel cables. Simple and elegant in form, the bridge relies on photovoltaic cells to power a unique hydraulic pumping system that operates the two cantilevered spans extending across the navigation channel. Pumping water between internal chambers in the tank back-span structure will alter the location of the bridge’s centre-of-mass and allow gravity to raise the two spans without hydraulic actuators or mechanical drive mechanisms.

Client City of Bergen, Norway Building type Pedestrian bridge Structural concept Hydraulic shifting center of gravity over rocker and cable-stayed cantilever Design 2005

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Client General Motors Corporation Building type Glazed entrance pavilion Structural concept Stainless steel and high strength cable trusses Completion 2005

General Motors Entrance Pavilion, Detroit, Michigan (USA) The principle public access to the front of the General Motors Building is the Entrance Pavilion, a lens-shaped, all-glass, freestanding element for the display of new automotive products from General Motors as well as a dramatic covered entrance. The pavilion is designed with a unique cable-andpost suspension system to support the all-glass walls. The structural elements are all of stainless steel. Two columns, one at each end, support a bow truss that sustains the glass roof and, in turn, all the cable elements of the wall and roof glazing. One of the special aspects of this pavilion is the kinetic way in which the structure responds to the natural light.

Client University of California, Merced Building type Central plant Structural concept Structural simplicity and utilitarianism Completion 2005

University of California, Merced Central Plant Complex, Merced, California (USA) The new central plant facility is at the core of an ambitious sustainability strategy for the university in providing power and water while serving as a “living laboratory” for environmental science students. The facility consists of three buildings: a telecommunication hub, a two-storey thermal energy storage tank, and a three-storey plant that houses most of the university’s power and infrastructure operations. The various buildings are framed in structural steel at the upper levels and in reinforced concrete at the lower levels. Perforated metal panels clad the exterior for the most part, while interior functions are on display, enabling the buildings and their functions to be used as a teaching tool.

Architects Frank O. Gehry & Associates Client Chicago Department of Transportation, Chicago Public Building Commission, City of Chicago Office of the Mayor, Lakefront Millennium Managers Inc. Building type Concert pavilion and open trellis Structural concept Cantilever steel space truss (bandshell), arched grid shell (trellis) Completion 2004

Jay Pritzker Pavilion, Millennium Park, Chicago, Illinois (USA) Along Michigan Avenue in the centre of Chicago, in a corner of Grant Park that had been open to active railway lines, SOM provided master planning, architectural and structural services for the design of the 7 ha site that creates one of the largest green roofs in the world, providing parkland above car parks and the railway lines. A major component of the park is the Jay Pritzker Pavilion, an open-air concert pavilion, that backs up to another theatre for backstage facilities. The roof of the pavilion consists of structural steel trusses cantilevering up to 27 m from the proscenium arch, supporting bands of stainless-steel on structural-steel sub-framing. Arched steel tubular members create an open trellis over the lawn to support a speaker system.

Client Samsung Construction Co. Ltd. Building type Residential tower Structural concept Core and outrigger in Y plan Completion 2004

Tower Palace III, Seoul (ROK) Tower Palace III is a multi-use residential complex consisting of a 69-storey residential tower and an adjacent sports club with parking facilities that allows the Korean electronics giant Samsung to showcase its new digital home concept in one of Seoul’s most luxurious and visible residential areas. The tower is constructed in a combination of high-performance, reinforced concrete and structural steel. In plan, there are three wings extending from the reinforced concrete central core that rises the full height of the building, and this “Y” arrangement efficiently increases the stability of the tower with its relatively small residential floor plates. Exterior composite columns are linked to the core by a series of reinforced concrete belt walls and steel outrigger trusses at the mechanical levels located at mid-height and at the top of the building. Floor framing of the tower is in structural steel.

Artist James Carpenter Design Associates, Inc. Collaborating engineers schlaich bergermann und partner Client Saint Paul Parks and Recreation Building type Band shell Structural concept Grid shell Completion 2002

Schubert Club Band Shell, Raspberry Island, St. Paul, Minnesota (USA) Designed by James Carpenter Design Associates and engineered by SOM, the band shell on an island in the Mississippi River between Minneapolis and St. Paul utilises a novel system of structural steel pipes and rods configured as a tensioned lattice, which serves as an economical means of support for an elegantly minimalist 7.60 ≈ 15.20-m, saddle-shaped, etched-glass roof. Attached to two precast reinforced concrete buttresses, the tensioned latticed rod roof system proved to be both stronger and more economical than a traditional welded-steel grid. Utilising a geometry consisting of a curved line that translated along another curved line allowed all of the shell’s glass panels to be flat, thus avoiding the more difficult and expensive requirement for warped glass.

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7 South Dearborn, Chicago, Illinois (USA) Although never built, the design for a 610-m tall, 118-storey, large-scale mixed-use tower in the centre of Chicago incorporated architectural and structural engineering advances that marked an important milestone in high-rise development. The stayed-mast structural system developed and introduced for this project enabled smaller, less expensive floor plates than would have been required by more traditional structural systems for a building of this height. The central supercore constructed in high-strength, reinforced concrete provides both support and resistance for all gravity and lateral loads, with floor construction cantilevering from the core walls at the top three zones of the tower. An outrigger truss system below the lowest gap provides the stays for the mast above. The tower’s rounded corners and slots greatly mitigate the wind loading, thus contributing to the efficiency of the central supercore system.

Client European American Realty Building type Supertall mixed-use tower Structural concept Stayed-mast Design 2000

Korea World Trade Center Expansion, Seoul (ROK) Developed in collaboration with several local firms, SOM’s design doubled the size of the existing facility and established KWTC as a world-class competitor in the field. Oriented around a series of convention halls, the expansion incorporates a 34-storey office tower, 28-storey luxury hotel, parking complex and retail space, in addition to greatly expanding the exhibition and convention spaces. The primary exhibition space is framed by structural-steel trusses spaced at 9 m and spanning 81 m between reinforced concrete core walls bounding the hall below. The trusses have a cantilever at each end of 18 m for a total roof area of 117 ≈ 171 m, producing one of the largest convention spaces in the Far East.

Client Korea International Trade Association Building type Convention center Structural concept Long span steel trusses Completion 2000

San Francisco International Airport, International Terminal, San Francisco, California (USA) The centrepiece of San Francisco International Airport’s expansion, the International Terminal is highly visible from the air and surrounding land and gives the airport – the US gateway to the Pacific Rim – a strong visual cohesiveness and iconic sense of identity. The main roof, dramatically wing-like in form, consists of a series of linked lenticular trusses. Each principal truss line consists of two outer, balanced, cantilever trusses supporting a central truss for a total roof span of 254 m. The central clear span is 116 m, thus minimising the need for supporting columns and allowing the building to bridge existing access roads. Subject to demanding seismic requirements, the building incorporates base-isolation technology, permitting it to move as a single entity in a seismic event, the largest building to do so at the time of construction.

Client Bureau of Design and Construction, San Francisco Airports Commission Building type Airport terminal Structural concept Base isolated lenticular long span steel roof trusses Completion 2000

Jin Mao Tower, Shanghai (CN) At the time of its completion, this 88-storey, 421-m high, octagonal mixed-use tower with a hotel above the office space was China’s tallest building, and it remains its most iconic. The Jin Mao Tower has set the standard for supertall tower design throughout China, both architecturally and structurally. Responding efficiently to high seismic and typhoon loading, the superstructure is a combined system of a reinforced concrete, octagonal supercore linked by outrigger steel trusses to eight exterior, composite, mega-columns. All other framing is relatively light structural steel in combination with composite metal deck slabs.

Client China Jin Mao Group Co. Ltd. Building type Mixed-use tower Structural concept Core and outrigger Completion 1999

Atlantico Pavilion (now MEO Arena), Lisbon (P) Evoking the magnificent sailing ships of Portugal’s Golden Age of Discovery, SOM’s competition-winning Atlantico Pavilion, built to house special events at Expo ’98, is a glue-laminated (glulam), wood-framed arena with a maximum span of 115 m, ovoid in plan, with an oxidised zinc roof that appears to float above the glass-walled vestibules. At the time of its construction, this was the longest one-way span of glulam timber in the world, and one of a few built in an active seismic zone. The main exhibition hall is framed by 16 two-hinged, arched, portal truss frames spaced at 9 m, supporting glulam purlins and a plywood diaphragm. For seismic loading, the roof structure was completely analysed in three dimensions and based upon the structure remaining essentially elastic in a seismic event.

Client Parque Expo ’98 SA Building type Arena Structural concept Long span glulam wood arch roof Completion 1998

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Architects Frank O. Gehry & Associates Client Consorcio Del Proyecto Guggenheim Bilbao Building type Museum Structural concept Modular lattice grid Completion 1997

Guggenheim Museum, Bilbao (E) Designed by Frank O. Gehry & Associates and one of the most visually unique buildings of the 20th century, the museum is a series of organically interconnected buildings featuring undulating walls and roofs. A large atrium provides a central focal point. Challenged to design a structural framework to accommodate the interplay of the compound curvilinear forms of the walls and roofs, SOM proposed an innovative, economical system in structural steel, allowing for prefabrication in a shop environment using computer-controlled techniques to achieve a high degree of accuracy for assembly in the field under tight tolerances. SOM structural engineers then created and developed a modular lattice steel grid system that could be adapted to the various curved surfaces of the undulating walls and roofs.

Client HKCEC, Hong Kong Trade Development Council, Wong & Ouyang (HK) Ltd. Building type Convention center Structural concept Long span steel roof trusses Completion 1997

Hong Kong Convention and Exhibition Centre, Hong Kong (CN) Surrounded by Victoria Harbour on three sides, the Hong Kong Convention and Exhibition Centre is in a highly visible location in the city. Built for ceremonies for the handover from British to Chinese rule on 30 June 1997, the centre was designed and constructed on a very tight schedule. The aluminium-clad roof structure is comprised of curved planes that overlap like wings and have been achieved by prefabricated, structural-steel trusses delivered by sea and lifted in place, creating a space 23 m high with an 80-m clear span. The primary trusses support a system of secondary trusses, stringers and galvanised steel roof decking. The interior reinforced concrete core walls that provide the lateral load-resisting system for the convention centre.

Client Korean Air Lines Building type Corporate headquarters and aircraft maintenance hanger Structural concept Trussed arches with balanced cantilever trusses Completion 1995

Korean Air Lines Operations Center, Seoul (ROK) The Operations Center for Korean Air Lines at Gimpo International Airport combines a dramatic clear-span hangar facility with perimeter office and operations facilities. The roof structure of the hangar bay spans 90 ≈ 180 m and can accommodate two wide-body aircraft or other combinations of smaller aircraft. In order to minimise overall building height while maintaining a clear span at the hangar doors, an innovative roof structural system was developed that essentially carries the weight of the entire roof on three columns. Tied arch trusses span from the corner columns at the hangar doors to a major column in the centre of the rear wall of the hangar bay. Intersecting rib trusses serve as balancing cantilevers on each side of the arch trusses. The graceful shape of the arch and rib trusses reflects an approximation of the moment diagrams under loading.

Client British Rail Property Board, Rosehaugh Stanhope Development PLC Building type Office tower on preconstructed raft structure over active railroad tracks Structural concept Fire-engineered exposed structural steel framing Completion 1993

1 Fleet Place, Ludgate Development, London (GB) The Ludgate Development is built over the Thameslink Railway in a densely populated area where the City of London adjoins the West End on a site dating to Roman times. 1 Fleet Place is framed in structural steel on a concrete raft above the active tracks, with the framing isolated for vibration control. Architecturally, the exterior exposes the structural-steel framing, recalling the Roman grid of this part of the City of London and SOM’s historical background as a partnership founded on Miesian principles of structural rationality. The structural steelwork is fire-engineered to enable the majority to remain unprotected and exposed. The structural skeleton of the building is further emphasised by the changing planes of the exterior walls.

Client The Travelstead Group Building type Mixed-use tower Structural concept Megaframe exoskeleton Completion 1992

Hotel Arts, Barcelona (E) The Vila Olimpica master plan, designed for the regatta and sailing programs of the 1992 Summer Olympics, uses a network of civic spaces and commercial facilities to connect the City of Barcelona with the Mediterranean Sea. The 45-storey tower – the centrepiece of the development that includes hotel and residential floors – is constructed in structural steel with a distinctive X-braced exoskeleton in white that is structural and establishes the strong architectural character of the building. The window wall system is set back 1.50 m, thus satisfying fire engineering criteria while allowing the exoskeleton to be fully expressed. The exoskeleton is an efficient exterior braced tube designed to resist all lateral loads.

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Broadgate Exchange House, London (GB) Exchange House, a 10-storey office block, sits at the heart of the multi-building Broadgate Development that has been constructed as an air rights project above the active railway lines of Liverpool Street Station. The building spans the 78-m railway tracks like a giant bridge. Four parallel, seven-storey tied arches – two expressed externally and two internally – extend to supporting foundation piers. The building is braced in the transverse direction by X-bracing. Fire-rated vision glass, set back from the structure, allows the exterior steel arch to remain exposed and untreated. The bridge structure provides for large, column-free office and trading floors as well as a large open plaza at ground level on the plaza structure above the tracks, also designed and engineered by SOM.

Client Rosehaugh Stanhope Development PLC Building type Office tower spanning over active railroad tracks Structural concept Steel tied faceted arch Completion 1990

McCormick Place North Building, Chicago, Illinois (USA) For the North Building expansion of Chicago’s McCormick Place convention centre, SOM used advanced engineering techniques to develop air rights over an active railway to enable the development of land previously thought unbuildable at an affordable cost. The roof of the main hall consists of a grid of 4.60 m-deep structural-steel trusses suspended by cables hung from 12 reinforced concrete pylons, spaced to provide the greatest flexibility for exhibition configurations and miss the active tracks. Each of the pylons supports 12 cables, which in turn support the 146 ≈ 238-m roof. The pylons also serve as air supply ducts keeping the upper hall free from ductwork. The main hall is clad in light grey aluminium and polished stainless steel with a band of vision glass just below the roof emphasising the structural nature of the cable support system above.

Client Metropolitan Pier & Exhibition Authority Building type Convention center Structural concept Long span, cable-stayed roof Completion 1986

National Commercial Bank, Jeddah (KSA) The desire to take advantage of the spectacular views of Jeddah and the Red Sea led to the design of this distinctive, triangular, travertine-clad, 27-storey office tower flanked by a helical parking garage. The verticality of the tower is interrupted by three large, seven-storey, triangular courtyards alternating at two of the facades with a linear service core at the third facade. The resulting V-shaped floor plates have glazing facing the shaded courtyards, producing an inward orientation typical of Islamic traditional design in response to the intense Saudi Arabian sunlight. The tower is framed in structural steel braced at the exterior for lateral resistance and trussed at the large atrium openings to accommodate columns for the floors above.

Client National Real Estate Company of Jeddah Building type Office tower Structural concept Steel braced frame Completion 1983

Hajj Terminal, King Abdul Aziz International Airport, Jeddah (KSA) The Hajj Terminal serves as a welcoming, culturally symbolic and structurally innovative portal for more than one million pilgrims during the annual pilgrimage to nearby Mecca. Appropriately, SOM utilised the highly identifiable form of the Bedouin tent to create a marvel that was the world’s largest cable-stayed, fabric-roofed structure at the time of its construction. The roof consists of 10 modules of 21 semi-conical, Teflon-coated, fibreglass units, each measuring 45 m per side, stretched and formed by 32 radial cables. The modules are supported by 45-m high structural-steel pylons and paired at the perimeter of each module for lateral support. The inherent long-span characteristics of steel cable structures allow for the spacing of the columns to be far enough apart to create a very open feeling and great flexibility to the large support area covered by the roof.

Client Kingdom of Saudi Arabia – Ministry of Defence and Aviation Building type Airport terminal Structural concept Cable stayed stressed fabric roof and steel pylons Completion 1981

Baxter Travenol Laboratories Corporate Headquarters (now Baxter International), Deerfield, Illinois (USA) This large multi-building campus and corporate headquarters, consisting of four office pavilions, an executive pavilion, two garage structures and a central facilities building, was designed to provide a flexible framework in which a pharmaceutical industry leader could continue to innovate and expand. The main central facilities building, containing an auditorium, training centre and cafeteria, features a stayedcable suspended roof supported by two steel pylons rising three storeys above the ground floor plane. The cable-hung suspended roof of the central facilities building provides a dramatic focus for the complex and permits a bold solution to the need for a large column-free interior space for the cafeteria.

Client Baxter Travenol Laboratories, Inc. Building type Campus of multiple buildings Structural concept Long span cable supported roof Completion 1975

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Client Sears Roebuck & Co. Building type Office tower Structural concept Bundled tube Completion 1974

Willis (formerly Sears) Tower, Chicago, Illinois (USA) The world’s tallest building until 1996, this 109-storey, structural-steel tower rises 442 m above Chicago’s Loop. The structural concept is that of a bundled tube, consisting of an arrangement of nine (3 ≈ 3) tubes, each 23 m square, of column-free space. The bundled-tube concept provides high structural efficiency while being very robust in resisting lateral loads for a tower of this size. The effective walls of each tube are constructed of prefabricated structural-steel tree elements, consisting of two-storey columns with attached half spandrels, resulting in columns spaced at 4.60 m. At the exterior, these elements are clad in aluminium for a clear expression of the structure. Above level 49, the tubes begin to step back, thus diminishing the wind sail area of the tower and providing smaller floor plates with increased window wall perimeter in relationship to the floor area.

Client Rayanna Corporation, The Republic Building type Office and printing plant Structural concept Beam and post Completion 1971

Republic Newspaper Office and Printing Plant, Columbus, Indiana (USA) Located in an architectural Mecca of the Midwest, SOM was commissioned to design a new office and printing plant for the historic local newspaper, The Republic, in a downtown location. The one-story, glass-enclosed building makes the printing process visible and celebrates the industrial design and sculptural quality of the offset presses on full display. The transparency and lightness of the building has been achieved by an integration of the exterior glazing mullions with the perimeter building structure, both to provide the greatest expanse of glass in conjunction with the optimised, minimal structural support for gravity and lateral loading.

Client John Hancock Mutual Life Insurance Company Building type Mixed-use tower Structural concept Braced tube Completion 1970

John Hancock Center, Chicago, Illinois (USA) This iconic 100-storey tower, the world’s first mixed-use supertall high-rise, effectively consists of a residential tower above an office tower in one single envelope tapering vertically as the floors diminish in area. The lateral load-resisting system consists of an exterior diagonally braced tube in structural steel, providing enhanced structural efficiency and resulting in a steel weight per unit area equal to that of a conventionally framed building of 50 storeys. The strong architectural character of the building has been achieved by directly expressing the framing of the tube. The exterior structural elements have been clad in anodised aluminium to clearly and elegantly express the structural system.

Client Alcoa Aluminum Co., Golden Gateway Building Corporation Building type Office tower Structural concept Exterior braced tubed frame Completion 1967

Alcoa Building, San Francisco, California (USA) This 25-storey office tower and corporate headquarters is set astride a three-level public car park at the southern end of the Golden Gateway Center, a residential and commercial development near the waterfront. A search for the most economical and efficient means for resisting gravity as well as seismic and wind lateral loads led to a proposal for an exterior braced tubed frame in structural steel in conjunction with conventional steel framing within the building. The diagonals allow eliminating every other column at the base. The architectural expression of the building was dramatically established when it was decided to expose the bracing and set the curtain wall behind, thus allowing the mega-diagrid X-bracing to be seen on all facades and creating a powerful image on San Francisco’s skyline of contemporary office design.

Artist Pablo Picasso Client Chicago Public Building Commission Building type Sculpture Structural concept Exposed steel plate structure Completion 1967

Chicago Picasso at Daley Center, Chicago, Illinois (USA) When SOM began the design for the Chicago Civic Center, now Daley Center, along with other collaborating firms, senior partner William Hartmann envisioned a piece of large-scale public sculpture to anchor the building’s front plaza, which today is Chicago’s prime civic square. An admirer of Pablo Picasso, Hartmann travelled to the artist’s home in France to present the project. Working from a maquette provided by Picasso as the basis of design, SOM translated the work into a piece of monumental sculpture standing 15 m tall. At full scale, some artist-approved modifications were needed to resist wind loads. Fabricated in preweathered steel like that of the exterior of the Daley Center, which weathers permanently to a deep brown, it was publically unveiled in 1967, dubbed the “Chicago Picasso” and has since become one of the most important symbols of the city.

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Catalogue of projects

Brunswick Building (now Dunne Cook County Office Building), Chicago, Illinois (USA) The Brunswick Building represents an early collaboration of SOM high-rise pioneers Bruce Graham, Myron Goldsmith, and Fazlur Khan. For the Brunswick, Khan conceived a firstof-its-kind tube-in-tube structural system, with an exterior reinforced concrete framed tube of closely spaced columns and spandrels. An interior tube of reinforced concrete shear walls around the service core allows for column-free interior spaces and an efficient system for resistance of lateral loads while establishing the exterior architectural expression of the building.

Client Washington-Dearborn Properties, Inc. Building type Office tower Structural concept Tube-in-tube Completion 1965

DeWitt Chestnut Apartments, Chicago, Illinois (USA) This 42-storey residential tower introduced the framed-tube structural concept for resistance to lateral forces, a revolutionary structural system developed by Fazlur Khan that still informs the architectural and structural design of many tall buildings. Here the framed tube is in reinforced concrete, consisting of an exterior grid of closely spaced columns and spandrels clad in travertine that efficiently carries gravity and wind forces while setting the architectural character of the exterior of the building. With the lateral resistance system at the exterior, there is no need for concrete walls at the stair and lift openings. Maximum flexibility is provided with the placement of interior columns, thus facilitating apartment layouts. Variations of this system would later give rise to structural systems of the John Hancock Center, the Willis (Sears) Tower and other notable tall buildings.

Client Federal Housing Administration, Metropolitan Structures, Inc. Building type Residential tower Structural concept Framed-tube Completion 1965

United States Air Force Academy, Cadet Chapel, Colorado Springs, Colorado (USA) The Cadet Chapel is the culminating architectural element of SOM’s original master plan and design of the entire US Air Force Academy campus along the Front Range of the Rocky Mountains. This striking building, which has become the defining iconic element of the academy, features a succession of 17 glass and aluminium-clad A-frames reading as a hightech interpretation of Gothic spires. Each spire is composed of steel-framed tetrahedrons arranged to create sharply pointed A-frames and the connecting infill from spire to spire. Bands of stained glass between the tetrahedrons dramatically allow diffused and coloured natural light to enter the building

Client United States Air Force Academy Building type Chapel as part of a multi-building campus Structural concept A-frames formed from tetrahedron shaped trusses Completion 1963

McMath-Pierce Solar Telescope, Kitt Peak, Arizona (USA) Sculptural in form and simplicity, the housing and structure for this landmark solar telescope rises from Kitt Peak above the Arizona desert with a strong, direct profile. The simplicity of the form is the result of careful architectural and structural coordination in conjunction with the technical requirements of this major solar telescope and its housing. The heliostat of the telescope is mounted on a 30-m concrete tower with a circular concrete shaft for the mirror extending 152 m at an angle of 32° with the horizontal, and is buried for the most part in the ground where the temperature fluctuates less than in the air. The outer steel jacket of the above-grade housing is supported on separate structural steel framing.

Client Association of Universities for Research in Astronomy, Inc. Building type Telescope Structural concept Reinforced concrete cylindrical support tower and angled tunnel with outersteel enclosure Completion 1962

Inland Steel Building, Chicago, Illinois (USA) The use of stainless steel as the cladding material for this structural steel-framed office tower remains timeless in its architectural quality. In plan, there is a linear single clearspan open bay with the columns projecting beyond the glazed curtain wall. Services and entrance hall spaces are in an adjoining tower providing a strong and clear expression of the two functions. With its open planning, the introduction of cellular metal structural flooring with provisions for electrical, data and air distribution, modular flexible ceiling with integrated lighting fixtures and high-performance floor-toceiling double-glazed facade, it is considered the seminal modern office building for integrated architecture and engineering solutions.

Client Inland Steel Company Building type Office tower Structural concept Longspan floor framing with offset core Completion 1958

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PROJECTS + PEOPLE

Partners

Bill Baker

Mark Sarkisian

Charles Besjak

David Horos

John Zils

Bill Baker PE, SE, FASCE, FIStructE, NAE, FREng With firm since 1981 • Chicago University of Missouri, University of Illinois Major Projects: Burj Khalifa (UAE) • Broadgate Exchange House (GB) • General Motors Renaissance Centre (USA) • Trump International Hotel and Tower (USA) • Tianjin CTF Financial Centre (CN) • Korean Airlines Operaitions Centre (ROK)

Mark Sarkisian PE, SE, LEED AP With firm since 1985 • San Francisco University of Connecticut, Lehigh University Major Projects: Poly Corporation Headquarters (CN) • Federal Courthouse Los Angeles (USA) • St. Regis Hotel and Residences (USA) • Harvard University Northwest Science Building (USA) • Cathedral of Christ the Light (USA) • Poly Real Estate Headquarters (CN)

Directors Charles Besjak SE, PE, AIA With firm since 1987 • New York University of Illinois

Stan Korista

Preetam Biswas

John Gordon

Dmitri Jajich

Major Projects: Chhatrapati Shivaji International Airport (IND) • Pearl River Tower (CN) • Zifeng Tower (CN) • Busan Lotte Town Tower (ROK) • KAFD Conference Center (SA) • US Air Force Academy – Center for Character & Leadership Development (USA)

David Horos PE, SE, LEED AP With firm since 1989 • Chicago Lousiana State University, The University of Texas at Austin, Northwestern University Major Projects: The JTI Building (CH) • Lee Hall III, Clemson University (USA) • National Oil and Gas Headquarters (Q) • Fishers Island Residence (USA) • Jesse Brown Medical Center (USA) • USG Building (USA)

Senior Engineers, Emeritus Ronald Johnson

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John Zils SE, FAIA, FASCE With firm since 1966 • Chicago University of Illinois

Major Projects: Willis Tower (USA) • Hajj Terminal (KSA) • Guggenheim Museum (E) • Hotel Arts (E) • Broadgate Exchange House (GB) • Onterie Center (USA)

Stan Korista

SE, PE, CEng, SECB, FASCE, FIStructE,

MICE, MHKIE, CCES, MSIE

With firm since 1965 • Chicago Bradley University, University of Illinois Major Projects: Burj Khalifa (UAE) • Jin Mao Tower (CN) • First Wisconsin Center (USA) • Canary Wharf (GB) • The Republic – Newspaper Plant & Offices (USA) • Three First National Plaza (USA)

Associate Directors Preetam Biswas PE With firm since 2004 • New York University of Illinois, University of Bombay Major Projects: Chhatrapati Shivaji International Airport (IND) • Zifeng Tower (CN) • Manhattan West, Ninth Avenue (USA) • Shum Yip Upperhills (CN) • KAFD Conference Center (KSA)

John Gordon PE, SE, CEng, MIStructE, Eur Ing With firm since 1987 • San Francisco Heriot-Watt University Major Projects: Harvard University Northwest Science Building (USA) • Poly International Plaza, Guangzhou (CN) • Shanghai Huawei Technologies Corporate Campus (CN) • Moscone Center Expansion (USA) • Ningbo Guohua Financial Tower (CN)

Dmitri Jajich CEng, MIStructE, SE, PE With firm since 2000 • London Macalester College, University of Minnesota Major Projects: Manhattan Loft Gardens (GB) • Denver Union Station (USA) • Lee Hall III – Clemson University (USA) • Glenstone II (USA), North Carolina Museum of Art (USA) Ronald Johnson SE, SECB With firm since 1978 • Chicago University of Illinois

People

Major Projects: Minneapolis City Center (USA) • Broadgate Development (GB) • Ludgate Development (GB) • Hong Kong Convention Centre (HK) • Korea World Trade Center and Convention Center (ROK)

Bonghwan Kim PE, AIA, LEED AP With firm since 2005 • New York University of Illinois Major Projects: Zifeng Tower (CN) • Allied Riyadh Bank Tower (SA) • 201 Bishopsgate (GB) • Lotte Super Tower (ROK) • Chhatrapati Shivaji International Airport (IND)

Peter Lee PE, SE, SECB, LEED AP With firm since 1980 • San Francisco University of California, Berkeley Major Projects: 111 Main Tower (USA)• San Diego Central Courthouse (USA) • San Bernardino Justice Center (USA) • Cathedral of Christ the Light (USA) • San Francisco International Airport, International Terminal, San Francisco (USA)

Eric Long PE, SE, LEED AP With firm since 2000 • San Francisco University of Illinois at Urbana-Champaign Major Projects: Cathedral of Christ the Light (USA) • 350 Mission (USA) • Federal Courthouse Los Angeles (USA) • Jinao Tower (CN) • Tianjin Global Financial Center (CN) • St. Regis Museum Tower, San Francisco (USA)

Stuart Marsh Eur Ing, CEng, MICE, MIEAust CPEng With firm since 2008 • London Queensland University of Technology Major Projects: The JTI Building (CH) • Manhattan Loft Gardens (GB) • Four Seasons Hotel (KSA) • 51 College Road (GB) • United Nations Office (CH)

Neville Mathias PE, SE, LEED AP With firm since 1985 • San Francisco Madras University/ Regional Engineering College; University of California, Berkeley

Major Projects: Al Hamra Tower (KWT) • Poly Corporation Headquarters (CN) • Poly International Plaza (CN) • Industrial and Commercial Bank of China (CN) • Asian Development Bank Headquaters (RP)

Bonghwan Kim

Aaron Mazeika PE, SE, AIA With firm since 2000 • Chicago University of Cambridge Major Projects: Poly Corporation Headquarters (CN) • Al Hamra Tower (KWT) • Greenland Group Suzhou Center (CN) • Jiangxi Nanchang Greenland Central Plaza (CN) • Beijing Finance Street (CN)

Peter Lee

Eric Long

James Pawlikowski SE, LEED AP With firm since 1996 • Chicago University of Illinois Major Projects: Burj Khalifa (UAE) • NATO Headquarters (B) • Plot 16 (RUS) • KAFD Muqarnas and Medina (SA) • Tower Palace III (ROK)

Dane Rankin PE, SE, P.Eng With firm since 1993 • Chicago Iowa State University, Purdue University Major Projects: Trump International Hotel and Tower (USA) • GM Renaissance Center (USA) • 601 Congress Street (USA) • GSA Denver Office Building (USA) • Pazhou Poly Village Redevelopment (CN)

Brad Young PE, SE With firm since 2000 • Chicago Southern Illinois University, Texas A & M University Major Projects: Burj Khalifa (UAE) • Cayan Tower (UAE) • Virginia Beach Convention Center (USA) • Muqarnas Tower (SA) • Nanchang Greenland Expo Center (CN)

Stuart Marsh

Neville Mathias

Aaron Mazeika

James Pawlikowski

Dane Rankin

Associates Gabriel Albano PE, CPIC With firm since 2011 • New York University of Buenos Aires Major Projects: Diagonal Tower, Yongsan Business District (ROK) • Changsha North Star Tower Competition (CN) • Godrej BKC (IND) • Manhattan West, Ninth Avenue (USA) • All Aboard Florida (USA)

Brad Young

Gabriel Albano

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PROJECTS + PEOPLE

Alessandro Beghini

Alessandro Beghini PE, SE, LEED With firm since 2005 • San Francisco Politecnico di Milano, University of Tokyo, Northwestern University

Dieter Feurich Dipl.-Ing, PE With firm since 2010 • New York Gottfried Wilhelm Leibniz Universität Hannover, City University of New York, Baruch College

Major Projects: CITIC Financial Centre (CN) • Federal Courthouse Los Angeles (USA) • 111 Main Tower (USA) • Salt Lake City (USA) • Tianjin CTF Financial Centre (CN)

Major Projects: WKL Hotel and Tropicana The Residences (MYS) • LaGuardia Airport (USA) • University of Connecticut Innovation Partnership Building (USA) • Manhattan West, Ninth Avenue (USA)

Aurelie Ble PE, LEED AP With firm since 2006 • New York Massachusetts Institute of Technology

Benton Johnson PE, SE With firm since 2007 • Chicago University of Minnesota

Major Projects: Manhattan West, Ninth Avenue (USA) • KAFD Conference Center (KSA) • Denver Union Station (USA) • NATO Headquarters (B) • National Oil and Gas Headquarter (Q)

Major Projects: The Christ Hospital (USA) • 100 Mount Street (AUS) • Nanchang Greenland Expo Center (CN)

Aurelie Ble

Christopher Brown

Jin Chen

Ashpica Chhabra

Dieter Feurich

Benton Johnson

Rupa Garai

Lindsay Hu

Jeffrey Keileh

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Christopher Brown SE With firm since 1986 • Chicago Ohio Northern University, Purdue University Major Projects: AT&T Corporate Center (USA) • Korean Air Lines Operations Center (ROK) • Cayan Tower (UAE) • The JTI Building (CH) • USG Building (USA)

Rupa Garai PE, SE, LEED AP With firm since 2005 • San Francisco Sardar Patel College of Engineering, Stanford University Major Projects: Poly International Plaza (CN) • San Diego Central Courthouse (USA) • 222 South Main Street (USA) • Pioneer Park (IND)

Jin Chen PE, SE With firm since 2007 • Chicago Hunan University; University of California, San Diego

Lindsay Hu PE, LEED AP With firm since 2004 (2002 Intern) • San Francisco University of California, Santa Barbara; University of California, Berkeley

Major Projects: Jiangxi Nanchang Greenland Central Plaza (CN) • Beijing Aonan Shidai Olympic Park (CN) • Beijing CBD East Expansion (CN) • Tianjin Global Financial Center (CN) • Pazhou Poly Village Redevelopment (CN)

Major Projects: Moscone Center Expansion and Improvement (USA) • San Bernardino Justice Center (USA) • Al Hamra Tower (KWT) • Cathedral of Christ the Light (USA) • US Consulate General, Guangzhou (CN)

Ashpica Chhabra PE, SE, LEED AP With firm since 2007 • Chicago Delhi College of Engineering, Stanford University

Jeffrey Keileh PE, SE, LEED AP BD+C With firm since 2006 • San Francisco University of California, Davis; University of California, Berkeley

Major Projects: Wujiang Greenland Tower (CN) • Jiangxi Nanchang Greenland Tower (CN) • China World Trade Center Phase 3B (CN) • Chhatrapati Shivaji International Airport (ND) • San Bernardino Justice Center (USA)

Major Projects: The Strand-American Conservatory Theater (USA) • UCSF Neurosciences Laboratory and Clinical Research Building 19A (USA) • US Consulate General, Guangzhou (CN) • San Francisco Veteran Affairs Medical Center (USA)

People

Andrew Krebs PE, LEED With firm since 2009 (2008 Intern) • San Francisco North Carolina State University, Stanford University Major Projects: Federal Courthouse Los Angeles (USA) • Sunset La Cienega (USA) • San Diego Central Courthouse (USA) • Poly International Plaza (CN) • San Bernardino Justice Center (USA)

Xuemei Li SE With firm since 2003 • Chicago Southeast University, University of Illinois Major Projects: Pearl River Tower (CN) • China World Trade Center Phase 3B (CN) • Greentown Qingdao Tower (CN) • Manhattan West, Ninth Avenue (USA) • Broadgate Tower (GB)

Arkadiusz Mazurek PE, SE With firm since 2005 • Chicago University of Wisconsin-Milwaukee, Cracow University of Technology Major Projects: NATO Headquarters (B) • Mashreq Bank Headquarters (UAE) • Emaar Sky View (UAE) • KAFD Muqarnas (SA) • Nozul Lusail Marina (Q)

Scott Murin PE, SE With firm since 2008 • Chicago University of Minnesota, Southern Methodist University Major Projects: Lee Hall III, Clemson University (USA) • James Turrell Skyspace at Rice University (USA) • The Christ Hospital (USA) • The JTI Building (CH)

Georgi Petrov PE, AIA, LEED With firm since 2008 • New York University of Illinois, Massachusetts Institute of Technology Major Projects: Jubilee Park Pavilion (GB) • City Lights Design Competition (USA) • Pertamina Energy Tower (RI) • KAFD Conference Center (KSA) • Lotte Busan (ROK)

David Shook PE, LEED AP With firm since 2007 • San Francisco Texas A & M University Major Projects: 350 Mission Street (USA) • Transbay Redevelopment Project Block 9 (USA) • Harmons (USA) • Sichuan Aviation Plaza (CN)

Alexandra Thewis PE With firm since 2007 • New York University of Illinois, Lehigh University Major Projects: Chhatrapati Shivaji International Airport (IND) • FBI Biometric Technology Center (USA) • Longgang Tian’an Cyber Park (CN) • 35 Hudson Yards (USA) • Lotte Super Tower (ROK)

Rebecca Ulaszek PE, SE , P. Eng With firm since 2004 • Chicago University of Illinois Major Projects: Manulife Place (CDN) • Cleveland Clinic Weston NICI (USA) • OKO Tower (Plot 16) (RUS) • Central Trust Bank (USA)

Syed Uzair Ullah PE With firm since 2012 • New York Illinois Institute of Technology

Andrew Krebs

Xuemei Li

Arkadiusz Mazurek

Scott Murin

Georgi Petrov

David Shook

Major Projects: Shenzhen Shum-Yip Tower One (CN) • All Aboard Florida (USA) Alexandra Thewis

Joanna Zhang PE, SE, LEED AP With firm since 2008 • San Francisco University of California, Berkeley Major Projects: UCLA Teaching and Learning Center for Health Sciences (USA) • The Strand – American Conservatory Theater (USA) • Monterey Conference Center (USA) • Fuzhou Worldwide Tower (CN) • Shanghai Century Link (CN)

Rebecca Ulaszek

Syed Uzair Ullah

Joanna Zhang

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Picture credits

Picture credits The authors and publishers would like to express their sincere gratitude to all those who have assisted in the production of this book, be it through providing photos or artwork or granting permission to reproduce their documents or providing other information. All the drawings were specially produced for this publication. Photographs not specifically credited are taken from the archives of the magazine “DETAIL Review of Architecture” or SOM. Despite intensive endeavours, we were unable to establish copyright ownership in just a few cases; however, copyright is assured. Please notify us accordingly in such instances.

Cover photo: Richard Leeney for British Land, London (GB)

INTRODUCTION p. 6 Tim Griffith Past and future – reaching new heights p. 9, fig. 1.3 Christian Schittich, Munich (D) p. 10, fig. 1.5 Leslie Schwartz, Chicago, IL (USA) p. 10, fig. 1.6 Peter Barreras Photography p. 11, fig. 1.7 Nick Merrick for Hedrich Blessing, Chicago, IL (USA) p. 13, fig. 1.12 Timothy Hursley p. 13, fig. 1.13 Ezra Stoller / ESTO p. 14, fig. 1.15 iStockphoto.com / Gilles Delmotte p. 15, fig. 1.16 D & W

Tall building case study – Burj Khalifa p. 58, fig. 5.2 iStockphoto.com / Gilles Delmotte p. 59, fig. 5.3 Spectra Maxima p. 59, fig. 5.5 Samsung C & T Corporation, Seoul (ROK) p. 60, fig. 5.10 Nick Merrick for Hedrich Blessing, Chicago, IL (USA) HIERARCHY p. 62 SOM /Alan Williams, London (GB) The importance of hierarchy p. 64, fig. 1.1 Hedrich Blessing, Chicago, IL (USA) p. 65, fig. 1.2 Bill Engdahl for Hedrich Blessing, Chicago, IL (USA) p. 65, fig. 1.3 Ezra Stoller / ESTO Structure as poetry p. 67, fig. 2.5 Timothy Hursley p. 68, fig. 2.6; p. 69, fig. 2.8 Scott Frances, New York, NY (USA) p. 70, fig. 2.10; p. 72, fig. 2.14 César Rubio, San Francisco, CA (USA) p. 71, fig. 2.11 Jane E. Lee p. 73, fig. 2.16; p. 75, fig. 2.20 Justin Maconochie for Hedrich Blessing, Chicago, IL (USA) Exchange House in detail p. 77, fig. 3.4; p. 81, fig. 3.16 John Davies p. 79, fig. 3.9; p. 80, fig. 3.12 Alan Williams, London (GB) p. 81, fig. 3.17 Richard Waite

SIMPLICITY + CLARITY p. 16 Scott Frances, New York, NY (USA)

EFFICIENCY + ECONOMY p. 83 Richard Leeney for British Land, London (GB)

Architecture and engineering at SOM – in the genetic code p. 18, figs. 1.1, 1.2 SOM; Ezra Stoller / ESTO p. 19, fig. 1.5 Nicholas Adams, Skidmore, Owings & Merrill. The Experiment Since 1936. Milan 2006 p. 20, fig. 1.7 McShane Fleming Studios, Chicago, IL (USA) p. 21, fig. 1.10; p. 22, fig. 1.12; p. 23, fig 1.13 Ezra Stoller / ESTO p. 23, fig. 1.14 Jay Langlois /Owens Corning

Constraints spur creativity p. 85, fig. 1.2 Hedrich Blessing, Chicago, IL (USA)

Informing design p. 25, fig. 2.5 Scott Frances, New York, NY (USA) p. 26, fig. 2.7 Paul Hester, Fayetteville, TX (USA) p. 27, fig. 2.10 Tom Harris for Hedrich Blessing, Chicago, IL (USA) p. 28, fig. 2.11 Richard Barnes, New York, NY (USA) p. 29, fig. 2.14 Photo Image /Shutterstock p. 30, fig. 2.17 Richard Kress /James Carpenter Design Associates, New York, NY (USA) p. 31, fig. 2.19 Atchain, Shanghai (CN) p. 32, figs. 2.21, 2.22 Tim Griffith, San Francisco, CA (USA)

SCALE + FORM p. 34 Nick Merrick for Hedrich Blessing, Chicago, IL (USA) Scale and proportion p. 36, fig. 1.1 Richard Leeney for Hedrich Blessing, Chicago, IL (USA) p. 37, fig. 1.3 Kamran Jebreili /Associated Press p. 39, fig. 1.9 Ezra Stoller / ESTO p. 40, fig. 1.11 Hedrich Blessing, Chicago, IL (USA) Clarity of design – giving things a name p. 41, fig. 2.2 SOM /Jay Langlois /Owens Corning p. 44, fig. 2.6 Hedrich Blessing, Chicago, IL (USA) Sensory fields, self-reflection and the future p. 48, fig. 3.5 Tim Griffith, San Francisco, CA (USA) Structural design of tall buildings p. 57, fig. 4.13 a Ezra Stoller / ESTO

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Optimising design goals p. 94, fig. 2.18 Pixelflakes, London (GB) The economy of construction p. 95, fig. 3.1 Scott Frances, New York, NY (USA) p. 95, fig. 3.2 SOM / Thomas Phifer Architects p. 97, fig. 3.6 James Steinkamp Photography, Hinsdale, IL (USA)

RESEARCH + FUTURE Quo vadis – megatalls as the focus of the SOM Research Gang p. 107, figs. 1.1, 1.2 Christian Schittich, Munich (D) p. 108, fig. 1.4 Council on Tall Building and Urban Habitat, Chicago, IL (USA) p. 110, fig. 1.8 James Steinkamp Photography, Hinsdale, IL (USA) Structural optimisation – developing new design tools p. 111, fig. 2.1 left column: Altair Altair Engineering, Inc. Polytop Glaucio H. Paulino, Raymond Alle Jones Chair, Georgia Instutute of Technology, Atlanta, GA (USA) TOP 3D Kai Liu, Andrés Tovar, Indiana University, Purdue University Indianapolis, Indiana (USA) Ground Structure Tomasz Sokól, Warsaw University of Technology (PL) right column: Force density Schek, Hans-Jörg: The Force Density Method for Form Finding and Computation of General Networks. In: Computer Methods in Applied Mechanics and Engineering, 03/1974, pp. 115 –134 Rhino Vault BLOCK Research Group, ETH Zurich (CH)

PROJECTS + PEOPLE p. 122 César Rubio, San Francisco, CA (USA)

Catalogue of projects p. 125, first from top: Brookfield Properties, New York, NY (USA) p. 125, fifth from top: Hayes Davidson, London (GB) p. 127, first from top: Tom Harris for Hedrich Blessing, Chicago, IL (USA) p. 128 first from top: Robert Polidori for Mumbai International Airport Pvt. Ltd. p. 128 second from top: Ryan Dravitz Photography p. 128, fourth and fifth from top; p. 130, first from top; p. 130, fourth and fifth from top; p. 133, first and second from top; p. 134, second from top: Tim Griffith, San Francisco, CA (USA) p. 129, first from top: Leslie Schwartz, Chicago, IL (USA) p. 129, third from top: Scott Frances for OTTO p. 129, fifth from top: Rice University, Houston, TX (USA) p. 130, third from top: Richard Kress /James Carpenter Design Associates, New York, NY (USA) p. 130, fourth from top; p. 131, second from top: Nick Merrick for Hedrich Blessing, Chicago, IL (USA) p. 132, first from top: Tom Rossiter p. 132, third from top; p. 135, third from top; p. 138, first from top: Timothy Hursley p. 132, fourth from top: Seventh Art Group, New York, NY (USA) p. 132, fifth from top: Richard Leeney for British Land, London (GB) p. 133, third from top: Scott Frances, New York, NY (USA) p. 133, fourth from top; p. 136, second from top: James Steinkamp Photography, Hinsdale, IL (USA) p. 134, first from top: Justin Maconochie for Hedrich Blessing, Chicago, IL (USA) p. 134, third from top: H. G. Esch, Hennef (D) p. 135, first from top: www.flickr.com /j.fo p. 136, fifth from top: James Morris for Axiom p. 137, first from top: Alan Williams, London (GB) p. 137, second from top: Ben Altman for Sadin Photo Group, Ltd. p. 137, third from top: Wolfgang Hoyt / ESTO p. 137, fifth from top: Howard N. Kaplan for HNK Architectural Photography, Highland Park, IL (USA) p. 138, second from top: Ezra Stoller / ESTO p. 138, fourth from top: Mak Takahashi p. 139, first from top: Hube Henry for Hedrich Blessing, Chicago, IL (USA) p. 139, second from top: Hedrich Blessing, Chicago, IL (USA) p. 139, fourth from top: Ezra Stoller / ESTO

Cover: The Broadgate Tower, London (GB) 2008 Photos introducing main sections: p. 6 Poly Corportation Headquarters, Beijing (CN) p. 16 Fishers Island Residence, New York (USA) 2007, architects: Thomas Phifer and Partners p. 34 Burj Khalifa, Dubai (UAE) 2010 p. 62 Exchange House, London (GB) 1990 p. 83 The Broadgate Tower, London (GB) 2008 p. 105 CITIC Financial Centre, Shenzhen (CN) 2018 p. 122 Cathedral of Christ the Light, Oakland, California (USA) 2008

With its 80 years of experience in bringing together architecture and structural engineering Skidmore, Owings & Merrill (SOM) remains groundbreaking to this day, particularly in the construction of gigantic skyscrapers, the so-called “supertalls”. The fourth volume of the DETAIL engineering series presents the approaches, the roots and the theoretical background of the SOM Structural Group. One central question is how far architecture can go – in terms of height. Typical structural concepts and individual details from numerous projects across the world serve to illustrate processes of solution finding. The results illustrate the firm’s core values: simplicity, clarity, hierarchy, efficiency, economy and advancement. A number of iconic SOM buildings such as the John Hancock Center and the Sears Tower help to position the example projects – ranging from James Turrell’s Skyspace to Burj Khalifa – within the context of SOM’s complete work. The innovative structural solutions presented here indicate how SOM enables the creation of “next generation” buildings.

ISBN 978-3-95553-223-9

9 783955 532239

DETAIL Institut für internationale Architektur-Dokumentation GmbH & Co. KG, Munich www.detail.de