Bridges 9783955535643, 9783955535636

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Contents

Bridges – Potentialities and Perspectives – Preface 3 Bridge Construction Yesterday – Today – Tomorrow Designing Bridges – A prestigious discipline 6   Bridges for Slow-Moving Traffic – Pedestrians and cyclists 14   Road Bridges – Links for motorised traffic 22   Bridges for Rail Traffic – Track-bound rolling and hovering 32 Bridges and Traffic – Mobility in progress 44 Preservation and Evaluation of Bridges – Refurbish or replace? 50 Requirements Impact – Internal and external loads Function – Routing and bridge equipment Economic Efficiency – Using financial resources responsibly Sustainability – Thinking about tomorrow today

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Materials Materials – Properties, construction and gestalt 84 Designs Designs – Catalogue of options 94 Bridges in Detail Ten outstanding project examples 102

Appendix 156 Picture credits, sources, authors

Imprint

Authors  Thorsten Helbig, Michael Kleiser, Ludolf Krontal Co-authors  Markus Friedrich, Martin Knight

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Editing:  Steffi Lenzen (Project management), Cosima Frohnmaier (Project examples), Jana Rackwitz (Copy-editing German edition and layout), Charlotte Petereit (Editorial ­assistance), Carola Jacob-Ritz (Proofreading German ­edition)

© 2021, first edition DETAIL Business Information GmbH, Munich (DE) detail-online.com

Translation into English  Julian Jain, Berlin (DE)

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Bridges – Potentialities and Perspectives Martin Knight

It is natural to consider bridges as physical objects and from that to wonder about their materiality and how they are built, but it is important to first think of bridges in terms of why they are necessary and to whom they bring benefit, because our world would be unimaginable without the critical connec­ tions made by bridges. For millennia, bridge design has changed the world for the bet­ ter, offering nourishment to our hearts and our minds, while bringing access to the markets, the services and the relationships upon which society depends. The physical potential created by a bridge satisfies a basic human urge: to move, to explore, to go beyond ... The bridge draws lines of communication towards it to become a focal point that is both ­physical and emotional. And, in forming this new connection, the place of the bridge is itself changed and a new sense of place is created. In doing so, a struc­ ture which is by definition a functional piece of engineering simultaneously acquires an architectural meaning that is just as profound. Wherever we are in the world we associate bridges with a sense of place and this helps to explain the popular appeal of bridge design. This is as true for the ‘beau­

tiful ordinary’ bridges which form the back­ drop to everyone’s everyday lives as it is for the extraordinary ‘iconic’ structures that feature in postcards, films and Instagram. And of course, at its best, bridge design is the ultimate platform for engineering creativ­ ity where efficiency and elegance combine in public view; pure and unadulterated by decorative cladding. Great bridge design can seem to be a ­magical mixture of art and science, with the most memorable specimens fitting perfectly into their physical and cultural context. This is of particular significance because of the extended life of a bridge, which often serves many generations, and for decades or even hundreds of years, defining a strong sense of identity as well as the move­ ment patterns around it. The popular interest in bridge design per­ haps also lies in the perfect simplicity of its function: to conquer gravity and carry traffic safely across a void. In looking at the development of bridge design through history, and examining the best examples of contemporary design, it may be possible to imagine where this exciting field will head next. Indeed, it is helpful to look back at the ­evolution of bridge design precisely

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because the true value of infrastructure only becomes clear over time, which means bridge designers must plan, design and build with great care and the very best of intentions. The most valued bridges often reflect the technical peak of their age and this, in turn, represents the cultural and political ambitions of society. History is a good indicator of the possible ways that design will change – often not through gradual evolution but by radical step changes brought by new materials and production techniques, in the hands of relatively few, inspiring individuals. History also shows us that lessons are learned hardest and deepest following moments of failure and that we must always treat bridge design with respect, for human safety is paramount. However, where in the past we have under­ stood this to mean robustness and durabil­ ity, often leading to overdesign and lazy consumption of materials, we must now urgently consider the carbon footprint and energy consumption of bridges, whose designs must increasingly satisfy social and environmental scrutiny. As we look to the future, ours is the first generation which genuinely recognises we must change our ways if we are to leave a bene­

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ficial legacy that is also genuinely sustain­ able. This means seeking to reverse our contribution to climate change as well as to design for the effects of climate change upon the planet. In 2015, the United Nations defined seven­ teen goals for sustainable development, and it might appear to bridge designers that we can only directly influence one, or perhaps three if we consider energy infra­ structure, water and sanitation alongside transport infrastructure. But how much big­ ger must our thinking be? For millennia, great infrastructure has been the basis for sustainable communities and cities, from which flow the critical com­ ponents of social sustainability, ultimately bringing economic sustainability. These in turn have given us the means and momen­ tum to now demand environmental sus­ tainability. Of course, any one of these goals can and should be thought of as the catalyst for such positive change, but the message must be clear: as designers in the world of infrastructure, we have the ability – and responsibility – to shape the future posi­ tively. To achieve this, we must work harder, more creatively and more collaboratively. We must question the need for new infra­

structure, so that, according to the “reduce, reuse, recycle” principle, we build only what is necessary and ensure that it serves us better and for longer and with a much lower carbon footprint. The conservation and renovation of bridges has previously been driven by economic factors, or the desire to protect something of historical value, but a new awareness must drive innovative technologies to safely extend the lives of existing structures and to reduce the environmental cost of con­ nectivity. The design of new bridges must be more materially efficient, functionally effective and in harmony with their environment than ever before. The social value provided by bridge design has always been there, but it was often obscured by short-term focus on costs and time, only becoming appreciated in hindsight. It is now the high­ est priority. As bridge designers, we need to shift our mode of thinking from outputs to outcomes and from objects to people. We need to honestly and robustly answer the questions “why is this project needed?” and “who will it affect?” before we move on to the ques­ tions “what is the design?” and “how do we build it?”, and this requires a radical shift in emphasis.

Infrastructure is for people, who benefit from careful and sensitive design, and the future of bridge design will see collab­ orative working in projects of all types and all scales, involving communities and clients, engineers and architects, to ensure the social, economic and environmental benefits of excellent bridge design are fully realised without leaving an impossible debt for the future.

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Designing Bridges A prestigious discipline

The bridge — cultural monument and ­contemporary witness Bridges overcome obstacles to allow people and goods to pass quickly and safely. Bridges are functional buildings of civilisation but also part of the history and cultural monuments of a society. In addition to the technological capabilities of a society, they reflect its values and the spirit of the times. From opulent stone arch bridges lined with religious sculptures, such as Charles Bridge in Prague from the 14th century (fig. 1), to Brooklyn Bridge in New York, completed in 1883, with its two large granite towers and first use of steel for the main support cable (fig. 3), to athletic-looking, shallow-span suspension bridges made of steel sheets and cables, such as the Millennium Bridge in London from 2000 (fig. 2), bridges are eminent historical markers of their time.

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Sometimes they even lend a sense of identity to their respective place. Bridges clearly show the value that a society places on the civilised environment it has created and thus on itself. Bridges in public spaces are usually built for a long service life. With a current technically intended lifespan of no less than 100 years, bridges built today can still be used at least by the great-grandchildren of the gener­ ation that built them. However, bridges can also be used for a much longer period of time if they are structures that significantly shape their local landscape and have been planned with foresight. If the bridges are designed in an aesthetically pleasing manner, with a clear-cut and robust construction, they will also be maintained and preserved by society, and thus be usable for the general public for a long time. Bridges are difficult to convert. Once built, they can usually only be converted for higher loads, wider lanes or changed boundary conditions at dispro­portionately high expense. Their presence on site, shaped by form and materiality, is hardly modifiable, since it is, after all, the support structure that determines their shape. The old stone bridges of Rome or the Brooklyn Bridge in New York still function with their originally constructed supporting

1  Charles Bridge in Prague (CZ), 14th ­century

2  Millennium Bridge, ­London (GB) 2000 /2002, Norman Foster, Arup 3  Brooklyn Bridge, New York (US) 1883, John August Roebling

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structure. These bridges are still in use today, even though the loads to be carried have increased many times over and the environment has changed fundamentally in the last 2,000 (or even 137) years since their construction. This works because these structures were designed for robustness and durability when they were built. The historical bridge structures are contemporary witnesses to the way technology was handled and manifest the state of development of a civilisation. This begins with crossings made from locally available stone slabs and wooden trunks and is also evident in the (technological) euphoria still perceptible today, which since the beginning of industrialisation has been associated with ever larger spans and continuously further developed high-performance materials. In the early 21st century, the incipient fourth industrial revolution with its attribute of intelligent networking and associated social changes is also leading to a paradigm shift in bridge construction: the responsible use of natural resources and consideration of the emissions caused during manufacture, operation and disposal are increasingly entering into society's consciousness. In the future, this will also call into question

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some previously “self-evident” design approaches in bridge construction. ‘Less is more’ only remains valid if a lower volume of an energy-intensive, high-performance material and for a defined span does not in the end mean higher climate-damaging greenhouse gas emissions. Moreover, contrary to discussions of only a few years ago, lightweight construction in bridges is not by default sustainable but only if the visible, filigree support structure does not require anchoring in an invisible, massive concrete foundation hidden in the ground or in costly deep foundations. Finally, the achieved service life and necessary maintenance significantly determine whether a bridge structure can be considered sustainable. It would be desirable for every bridge designer to realise that, in addition to ­making possible a safe and efficient crossing of an obstacle, it is also a matter of ­creating something that reflects social ­values and makes them comprehensible for later generations – even in the case of a seemingly insignificant pedestrian and cycleway bridge over a stream, a road bridge supposedly going unnoticed (fig. 4) or a railway bridge that is not perceivable by the travellers themselves. What is at

4  Nanin Bridge, near Mesocco (CH) 1968, Christian Menn

stake today is nothing less than a sustain­ able bridge-building culture. Bridges connect places As part of a road or modern transport infrastructure, bridges have been and continue to be an essential catalyst of economic developments and civilising processes. The bridges of early times made it possible to establish efficient links over obstacles that were previously difficult for people and beasts of burden to overcome. A bridge was often built where there was a ford (e.g. in Frankfurt, lit. “Frank ford”) or where only short distances needed to be covered. The path or road to the bridge followed the topography. The motorisation of traffic around 135 years ago necessitated the integration of bridge structures into widely laid-out routes, which

often narrowly defined the spatial location as well as the width and length of the road to be supported. The bridge is oriented along the traffic route (fig. 5). Pedestrian or cycle bridges often allow greater freedom in the routing. Thus, for specific bridge designs, favourable routings can result in interesting alternatives for a crossing. However, careful integration into the existing route network is an essential design component for all bridges. Bridges create places Urban bridges such as the Pont Neuf in Paris from 1607 (fig. 6), the 14th century Charles Bridge in Prague (fig. 1, p. 6), or the Ponte Vecchio in Florence from 1345 (fig. 7) shape urban spaces and are often catalysts for urban development. Inner-city bridges were

5  Yavuz Sultan Selim Bridge, the third bridge across the ­Bosphorus, near ­Istanbul (TR) 2016, Michel Virlogeux, JeanFrançois Klein

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often used for trade and commerce. Thus, butchers, tanners and simple tradesmen first settled on the Ponte Vecchio; later, after the construction of the arcade as part of the Vasari Corridor (Corridoio Vasariano) by the Medici family, only goldsmiths were allowed to pursue their craft on the Ponte Vecchio. The old London Bridge, with its narrow, up to seven-storey building that at times formed its own London borough, or the Krämerbrücke in Erfurt were also inhabited and populated by craftspeople. It was not until the beginning of the Industrial Revolution and the empowerment of the ­middle classes at the start of the 19th century that bridges were built purely as pedestrian bridges for strolling. One such example is the Passerelle des Arts in Paris, designed exclusively for pedestrians in 1804, one of the first cast-iron bridges in France (fig. 8). In Isfahan, Iran, the Khaju Bridge (Pol-e Chādschu) and the Allahverdi Khan Bridge (Si-o-se-pol) (fig. 9), built in the 17th century, are not only important links across the Zayandeh River dividing the city, but also seasonal dams used to irrigate upstream gardens. Even today, tea houses invite visitors to linger under the arches of the brick viaducts.

6  Pont Neuf, Paris (FR) 1607 7  Ponte Vecchio, ­Florence (IT) 1345 8  Pont des Arts, also called Passerelle des Arts, Paris (FR) 1804 /1984, Louis-­ Alexandre de Cessart / Louis Arretche 9  Allahverdi Khan Bridge (Si-o-se-pol), Isfahan (IR) 1602 (start of construction)

10  De Lichtenlijn, ­pedestrian bridge, Knokke (BE) 2008, Ney & Partners

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Unity of structure and design With bridges, the load-bearing structure determines the design. Bridges are therefore designed in a fundamentally different way from buildings, as the load-bearing principle and the materials of the primary structure in buildings are usually hidden. The unity of the load-bearing structure and the design in bridge construction often already fulfil the demand for economic efficiency. And we know from gestalt psych­ ology that forms which logically and comprehensibly depict the mechanism of bearing and loading are generally perceived as pleasant. According to the philosopher and psychologist Theodor Lipps, every human being has an “unconscious mechanical knowledge” that functions more or less intuitively as a standard of judgement. Accordingly, the observer unconsciously recognises whether a structure is correctly or incorrectly designed. The aesthetic impression is thus based on the perception of the inner lawful “mechanical workings” [1]. A legible unity of structure and design is perceived as pleasant and thus encourages a positive overall impression. Beyond the logic of form as a basic pre­

requisite for the perception of aesthetic quality, bridge design offers a multitude of opportunities for expression and interpretation in the choice of form [2]. After dutifully designing a logically comprehensible and robust support structure comes the pleasure of playing with the form and refining the aesthetic expression. The gestalt psychologist Rudolf Arnheim speaks of forces and tensions that influence the immediate perception [3]. Oblique positions, curves, heightened complexities and special emphasis on the inner load-bearing state (see “Stuttgart Wooden Bridge in Weinstadt”, p. 104ff. and “A5.Ü20 near Wilfersdorf”, p. 132ff.) enhance the dynamics of form and are often perceived as exciting and thrilling (fig. 10). Such dynamic elements are frequently used either consciously or unconsciously to react to the immediate site, to respond to it, or to change it in a purposeful way. The Japanese engineer Kunio Hoshino speaks of the atmosphere of a place, to which the bridge structure responds in the form of deliberate accentuation or restraint, or even subordination – alternatively, the bridge itself shapes the place and creates a new atmosphere in the space [4].

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Technical and design challenges “Bridge building is one of the difficult construction tasks that attracts and challenges the vigorous, self-confident engineer.” [5] Fritz Leonhardt’s quote testifies to the high self-esteem of the engineer, who sees themselves as both technician and designer of the structures they construct. Whether the engineer tackles this “supreme discipline” alone or whether better solutions can indeed be developed in an interdisci­ plinary team of engineers, architects and other experts in a constructive dialogue, bridges represent a special and exciting design task. After all, in bridge design, the function, though quite simple compared to other construction tasks (i.e. the overcoming of obstacles), is confronted with multi-layered boundary conditions and highly complex requirements. In order to master the technical challenges, a thorough ana­lysis of site integration, precise staticstructural development, working out of all bridge components and finally a careful technical elaboration up to the consid­ eration of the manufacture and assembly

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of the bridge structure are required. The Roman architect and engineer Vitruvius already set three main requirements for buildings: firmitas (strength), utilitas (usefulness) and venustas (beauty). Nowadays, the first two of these three main requirements, supported by extensive standardisation and the availability of highly developed design tools and technologies, can safely be met for bridge structures. However, the third requirement, venustas, cannot be easily fulfilled by a purely tech­ nical approach. Until the Renaissance, ­master builders with extensive technical and artistic training strictly followed the design principles of Vitruvius and later those of Leon Battista Alberti who drew on the former’s treatises, both of which relied on the Greek scholars, including Plato and Aristotle, for observing the rules of order, scale and harmony. Art basically followed “a rule-guided activity” [6]. With the advent of the Enlightenment, an individualisation of the concept of beauty also took hold. The concept of beauty was now no longer bound to objective, i.e. generally valid rules

11  Millau Viaduct (FR) 2004, Michel Virlogeux, Norman Foster

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b 12  Vessy Bridge /Arve Bridge, Geneva (CH) 1936, Robert Maillart a Force diagram b View

Notes: [1] Lipps 1897, p. 39 and preface [2] Kleiser 2017, p. 402— 411 [3] Arnheim 2000, p. 13ff. [4] Hoshino 1972, p. 4ff. [5] Leonhardt 1982, p. 9 [6] Hauskeller 2013, p. 33 [7] Tasche 2015, p. 121

and criteria; instead, it was individual sensibility that decided on the attractiveness of an object. The consequences of this are still visible today in the great variety of forms in bridge construction, right up to the spectacular and expressively sculptural (fig. 11). Planning tools As with the available materials and manufacturing technologies, the available design and analysis tools have likewise always influenced the bridge constructions of their era (fig. 12). In pre-industrial times, bridge structures were designed on the basis of handeddown empirical values. It was not until the middle of the 19th century that the scientific foundation for a structural analysis of the support structures was created. The most innovative era in iron bridge construction, which roughly covers the period from 1850 to 1870 and in which almost all of the loadbearing systems still in use today were developed, ended at the moment when engineers attempted to calculate the loadbearing structures more precisely using newly acquired methods. Until 1870, however, bridge builders still intuitively preferred load-bearing structures with a high degree of static indeterminacy, which could

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only be inaccurately statically calculated, but which proved to be very durable due to their redundancies in load transfer. There­ after, however, the design process was reversed. Whereas previously, structures were based on empirical values, they were now adapted to the statics, preferably applying statically determined systems [7]. Today, almost anything seems doable; computer-aided static modelling and sophisticated construction techniques offer almost limitless possibilities. The use of static analysis tools for extreme value considerations of highly complex systems with non-linear calculations is no longer a problem. However, the designer must weigh up whether constructability can be ensured with reasonable effort and whether the technical risk remains safely controllable in any case. It is only through the interaction of the components of structure, design and construction that it is possible to build innovative, sustainable, long-lasting and distinctive buildings.

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Bridges for Slow-Moving Traffic Pedestrians and cyclists

People have been building bridges since time immemorial. From the beginnings of bridge building in the form of boardwalks over moorland, to the advent of the railway about 200 years ago and the developing automobile traffic a little more than 100 years ago, bridge building was geared solely to slow-moving traffic. Yet these bridge constructions laid the ­technological foundations for all later forms of transport. It was only with the constant development of the “new” means of transport such as railways and automobiles that further differentiating requirements for appropriately designed bridge types arose and continue to arise. Nowadays, from a constructional and design point of view, pedestrian and cycle bridges offer the greatest scope for experimentation and development. ­Innovative designs with partly new mate­ rials can be realised more easily here than under the conditions of the narrowly defined specifications that exist for bridge structures for modern rail or road traffic facilities. Compared to other forms of transport, bridges for non-motorised traffic have the longest history, and today, in times of the global ­climate crisis, bridge construction for use by pedestrians and cyclists is highly topical

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again. Movement by bicycle or pedelec is the focus of modern urban development in particular, and bridges for pedestrians and cyclists are now literally paving the way for a new (and not only) urban mobility. Bridge building – The art of what is ­possible From the tree trunk laid across a ditch (fig. 1) to the modern bridge structure made of high-performance materials, the state of the art in construction technology can be observed especially well in bridges. Bridges are therefore public witnesses of the techno­ logical possibilities of their time. Wooden bridges Wood is the oldest building material in bridge construction. Since early human

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1  Reconstruction of the bog path created around 65–45 BC in the Diepholz Moor

2  Two-storey root bridge in Meghalaya, northeastern India (IN)

3  Rope bridge made of bamboo in Malawi

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­ istory, streams and ditches have been h spanned by tree trunks laid on both banks. The locally available building material, which could be worked with simple iron tools, was already used in the Bronze and Iron Ages to build bridges by hand. For example, researchers found Bronze Age bridge remains in the Thames near Vauxhall as well as those of an Iron Age bridge in La Tène, Switzerland. Ancient Assyrian cuneiform texts attest to the existence of bridges in Anatolia. Probably the oldest documented wooden bridge in Europe was built on oak pillars in 1525 BC on what is now Lake Zurich between ­Rapperswil and Hurden. A crossing at this location existed until 1878, with various wooden bridge con­ structions. The Greek historian Herodotus reports on a bridge in Babylon across the Euphrates around 700 BC (Book 1, page 186). According to ­tradition, the first wooden bridge across the Tiber, the Pons Sublicius, was built in Rome at the same time. The first crossings probably consisted mostly of unhewn tree trunks laid side by side. Later, these were laid on piles driven into the ground (mostly made of oak for ­reasons of durability) or heaped stone walls. The development of processing tools and the increasingly specialised (carpentry) craft allowed the construction of ever more complex structures. The joinery, known from house construction, was accomplished by skilled manual work and made it possible to ­create dissolved framework constructions with larger spans. Rope bridges Simple rope bridges made of natural fibres for crossing gorges and rivers probably existed in India and China as well as in southern Africa as early as 2000 BC (fig. 3).

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The availability of long-fibre plants in trop­ ical areas, such as lianas, meant that slack ropes could be used for roping along or for laying individual pieces of wood directly on top as a walking surface. This is how the first suspension bridges were built in Asia (India, China) and South America, long before the first chain bridges were erected in Europe in the early 19th century. Simple bridges made of lianas and willow rods are still built and used today. In Japan and northern India, the tradition of living bridges developed: roots, tendrils, or lianas are influenced in their growth in such a way that they form bridge support structures that are mostly subjected to tensile stress. Since it is a living material, there is no danger of moisture decomposing the components. For example, the indigenous Khasi people in the region around Cherrapunji in the north-eastern Indian state of Meghalaya build or grow “living root bridges” (fig. 2). In this process, the aerial roots of the locally occurring Ficus elastica grow along hollowed-out tree trunks, bamboo poles, or palm trunks in a predefined direction and then root at the destination. Further root growth and interlacing eventually leads to a stable support structure. Some of the bridges found there are more than 100 years old.

Bridges for Slow-Moving Traffic

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Stone bridges Stepping stones are a rather simple form of crossing streams or rivers on foot, which has probably been practised since time immemorial (fig. 4). Flat stones laid in the water with gaps for the water to pass through make it possible to cross with dry feet in shallow places. These early crossings have been documented for Asia, Europe and North and South America. However, they required appropriately large and flat stones in the near vicinity. For stone slab bridges, slabs of about 2–4 m in size were laid on stone pillars

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­ sually built in dry construction (fig. 5). u Often these were built at places where already existing step crossings could be used as a substructure. Only rarely are traces of stonemasonry to be found. Since they were difficult to work on, most of the stone slab bridges had no railings. For the transport of goods with carts and cattle, the unsecured passage was a ­challenge. As the necessary large stone slabs were only available in a few areas, stone slab bridges are mainly found in the French, Spanish and Irish mountain regions. As early as the 2nd century BC, the stone arch bridges developed and built by the Romans became so stable and durable (fig. 2, p. 23) that they would serve as ­models for bridge technology well into the 19th century (fig. 6). For example, four stone bridges built across the Tiber in Rome between 174 BC and AD 260 (fig. 1, p. 78) have ­survived to the present day.

4  Stepping stones (lit. “Ox piano”) across the Pfrimm (DE) 5  Clapper bridge, ­Dartmoor (GB), around 13th century 6  Ponte dei Salti (lit. “Bridge of Jumps”), Lavertezzo (CH), 17th century

Bridges of iron and steel With the Industrial Revolution, iron and later steel became producible in large quantities as well as high quality, and usable for bridge building. In Coalbrookdale, the ­cradle of the Industrial Revolution, the first cast-iron arch bridge was built across the ­Severn in 1779 (fig. 7). The struggle to find a unique construction language for the new

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7  Built in 1779, the Iron Bridge near ­Coalbrookdale (GB) is the world’s first ­cast-iron arch bridge.

8  Monier Bridge following renovation. Haardt Castle Park, Neustadt an der Weinstraße (DE), around 1885, ­renovation 2003

material of iron is clearly visible here. The connections are still more similar to timber construction; for example, the joints were constructed as mortises and dovetails and connected with screws and dowels [1]. Bridges of steel-reinforced and prestressed concrete Iron-reinforced concrete was experimented with from the mid-19th ­century in France, the United Kingdom and North America. After the French gardener and landscape builder Joseph Monier obtained a first patent for ferro-concrete in 1867 for “mobile containers made of iron and cement for ­horticulture”, he also applied for a patent in 1873 for a “device for ­building bridges and

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footbridges of all kinds”. The castle moat bridge built by Monier at Chateau-de-­ Chazelet in 1875 was the first ferro-concrete bridge. As was fashionable in landscape gardening at the time, it was executed as “faux bois” (“pseudo-wood”). Similar garden bridges were later built elsewhere (fig. 8). Based on Monier’s patent, building with ferro-concrete spread rapidly throughout Europe and America. The advantages of the building material, which has been called (steel-)reinforced concrete since the 1940s, were soon also ­recognised for heavy traffic bridges (fig. 9). Today, the vast majority of bridges are made of reinforced concrete or prestressed concrete.

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9  Pont Camille-deHogues, Châtellerault (FR), 1900, François Hennebique

Bridges for Slow-Moving Traffic

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On foot and bicycles After bridges were initially built for communication and trade routes as well as for military purposes (pontes longi), pedestrian bridges were also used as deliberately designed elements in landscape gardening from the beginning of the 19th century. At that time, the middle classes, which was growing stronger with the Industrial Revolution, had bridges built “only” for strolling. For example, the Passerelle des Arts of 1804, one of the first cast-iron bridges in France, was designed exclusively for pedestrians (fig. 8, p. 10). From the end of the 19th century, bicycle traffic gained in importance. With the safety bicycle designed by John Kemp Starley in 1884, a further development of the pedalcrank penny farthing, and the invention of the air-filled tubed tyre by John Boyd ­Dunlop in 1888, cycling became safe and comfortable. The first cycle routes were laid out. One of these was the Great California Cycleway, opened in 1900, which was designed to connect Los Angeles and ­Pasadena (fig. 11). The cycleway ran at a height of up to 15 m as a bridge construction above the ground but was soon dismantled for various reasons. Today, a century later, approaches like the Cycleway are topical again.

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From the second half of the 20th century onwards, the greater leeway in the design of footpath and cycleway bridges compared to the strict requirements of railway and road bridges was increasingly perceived as an opportunity for experimentation and (sometimes expressive) design (fig. 10). With their support structure and construction details up to the handrail material, pedestrian bridges are part of the everyday experience of the users’ environment in terms of the perceptibility of the scale and (also in the literal sense) “tangibility” of the components. Thus, pedestrian bridges are understood as “furniture of the city”, passed, inhabited and touched on a daily basis. Since the 2000s, there has been an increase in pedestrian bridges

10  Stress ribbon bridge across the Main-­ Danube Canal near Essing (DE), 1986, Richard Johann Dietrich, Heinz ­Brüninghoff

11  California Cycleway in Los Angeles (US), 1900, partly completed, dismantled at the start of the 20th century

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12 12  Charles Kuonen Suspension Bridge, Randa (CH), 2017, Swissrope 13  Pedestrian bridge made of glass over the Grand Canyon of Zhangjiajie, Zhangjiajie National Forest Park, Hunan Province (CN), 2016, Haim Dotan Ltd. Architects and Urban Designers

designed for special experiential effects in order to make certain places and regions more attractive for tourists. Suspension bridges with spans of up to 500 m and a walking surface made of transparent glass attract thousands of visitors every day. In China alone, nearly 2,300 glass-bottomed bridges and walkways were built by 2020 (fig. 13). The Charles Kuonen Bridge in Randa, Switzerland, is part of the Europe Trail and, at 494 m, the longest-span foot-

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bridge in the world (fig. 12). With an emphatically sculptural design of expressively accentuated support structures, pedestrian bridges are used as attractions in urban spaces and often as catalysts for urban development. At the price of high costs for construction and maintenance, spectacular stand-alone objects are created that can – perhaps surprisingly for an infrastructural component – trigger a Bilbao effect (fig. 14).

14 Gateshead Millennium Bridge, Gateshead (GB), 2001, Wilkinson­ Eyre, Gifford and Partners

14

Bridges for Slow-Moving Traffic

19

15

Outlook Since the end of the 20th century, bicycle traffic has been decoupled from road and partly also from pedestrian traffic. In several urban regions in Europe and Asia, inner-city but also interregional cycle path networks have been created and are being further expanded. Cycle highways, which allow for shorter journey times compared to other forms of transport, are being established to promote cycling. In Copenhagen, a city where there are now (as of 2020) five to six times more bicycles than cars, about 12,000 km of routes in the urban area of about 88 km2 are reserved exclusively for cycling (fig. 15). In 2019, 50 % of commuters already used bicycles to get to work or school. In Xiamen, China, a 7.6-km inner-city elevated bicycle lane was opened in 2017, a 4.80 m wide lane running below the express busway exclusively for bicycles. In London, elevated cycle highways above the extensive inner-city rail network and a floating cycle path on the Thames (Thames Deckway Project) have been ­proposed to alleviate the city’s traffic problems (fig. 17). Similar approaches exist for other trafficcongested metropolises such as New York City.

20

The bridges for slow-moving traffic, long combined as pedestrian and cycle bridges, will in future probably be increasingly separated into separately usable lanes up to special “cycle superhighways” reserved exclusively for cycle traffic (fig. 18). The requirements for bridge equipment and the speeds of the users are too different. This trend is further reinforced by pedelecs and e-scooters, which reach speeds of up to 25 – 30 km/h. The requirements for the bridges for the new cycle highways must then be consistently adapted to the specific use. For ex­­ ample, the minimum widths for the separate cycle lane areas are specified in the recommendations for cycle superhighways that have now been drawn up as at least 3 m (for one-way traffic) to 4 m (for two-way ­traffic). This means that most of the pedestrian and cycle bridges that have been built to date, which often only have clear path widths of 2 to 3 m, are probably unsuitable for integration into cycle superhighways, as accident-prone bottlenecks will otherwise form. Pavement areas for pedestrians require an additional 2.50 m width, which would lead to clear bridge widths of up to 6.50 m for combined use. When constructing and integrating the bridge deck into the cycle superhighway

16 15  Pedestrian and cycle bridge Lille Langebro, Copenhagen (DK), 2019, WilkinsonEyre 16  Hovenring, Delft (NL), 2011, ipv Delft Design Agency

Note: [1] Dupré 1998

17

18  Pedestrian and cycle bridge, Geumgang, South Korea (KR), ­competition 2017, Dissing Weitling

network, care must be taken to ensure that the design is suitable for cyclists, and that it is designed for speeds of up to 30 km/h. Therefore, generous minimum curve radii in the layout design and appropriately designed cross-slopes for the bridge deck must be observed. In contrast to pedestrian bridges and their approach ramps, which must be acces­ sible and require a maximum longitudinal slope of 3 %, pure cycle bridges and their approach ramps can be designed with a slope of up to 6 % as a rule and up to approx. 12 % in exceptional cases. The road surface should have a good grip, be even and seamless, and have good ­rolling characteristics. Longitudinal grooves in the pavement should generally be avoided, while continuous pavement levels are preferable to slabs or planks. Cycle superhighway bridges often require lighting in urban areas and at least highcontrast road markings outside urban areas. Cycle-only lanes can be assumed to have lower load assumptions than pedestrian traffic. Dense crowds, as assumed in the load scenarios for pedestrian bridges, are not very likely for cycle path bridges and lead to oversized support structures with the ultimately larger bridge widths required.

17  SkyCycle, London (GB), design 2013, Foster + Partners

However, corresponding standard values have yet to be developed for this (see “Loads from traffic”, p. 59). Comfort criteria such as the avoidance of natural frequencies in the area of pedestrianinduced vibrations are also not necessary for pure cycle path use. The lower static and dynamic requirements will ultimately lead to more economical structures compared with the combined footpath and cycle path bridges.

18

Bridges for Slow-Moving Traffic

21

Road Bridges Links for motorised traffic

Character Road bridges, in contrast to pedestrian bridges, are characterised by the higher loads from motorised traffic and the increased safety considerations against vehicle impact and crashes, and are ­therefore marked by more solid support structures. A crucial factor in Nordic and Alpine countries is the required winter ­maintenance, which generates chloride loads on roads and their bridges due to salt spreading. Unlike railway bridges, road bridges can follow complex routing specifications and can also reach great widths in the form of multi-lane motorways. Due to the fact that vehicles are not bound to a lane, a multitude of load positions and ­configurations have to be taken into account. A wide variety of ­traffic routing during construction within existing structures also poses major ­challenges. Despite these general parameters, the design of road bridges still allows a great deal of freedom compared to the tight deformation criteria and extreme load requirements of railway bridges. Considering the local conditions, the planned ­routing, the appropriate and economical use of materials and systems, as well as

22

the official requirements and stipulations, a concept to successfully embed a bridge in its surroundings also requires a certain sensitivity (fig. 1). Archetypes The development of bridges from simple stone slab, wooden or filigree constructions suitable for pedestrians and light, ­non-­motorised traffic (see “Bridges for ­Slow-Moving Traffic”, p. 14ff.) to more solid support structures is due to the ­revolutionising of road traffic by the invention of the motor vehicle. The rapid expansion of functioning trade routes made it ­possible to transport ever greater loads over longer distances. Since the loads of carts and carriages increased ­constantly and arbitrarily, the Romans established a load regulation with a maximum load of initially 250 kg as early as 50 BC [1] in order to ensure the stability of road surfaces and thus also of bridge structures. The first bridges for higher loads were ­initially made with the proven building ­material of wood (see “Wooden bridges”, p. 14f.). However, not least for reasons of durability, other building materials such as stone and brick soon prevailed as the primary material of the solid bridge. After

1 1  Bridges as links in the landscape and in the road network. ­Storseisundet Bridge, Atlantic Road (NO), 1989 2  Two Roman bridges in Mérida (ES) a across the ­Albarregas River b across the Guadiana River

a

the first solid constructions across canals and smaller streams made of corbelled arches with a span of 3 to 4 m starting from the 4th ­century BC in Greece, the Romans refined the engineering technology to a standard unique for the time, with an enormous ­number and variety of bridge structures. The vaulting technique already developed under the Etruscans and perfected by the Romans made it

b

2

­ ossible to achieve spans of 35 – 38 m [2]. p The Roman stone arch bridge, as the first true archetype of a solid bridge, is ­convincing because of its comprehensi­ bility of the individual functional elements. Clearly visible from the outside is the ­separation of the radial stone arrangement’s load-bearing function in the stone arch from the ballast function of the horizontal stone layering in the spandrel areas, which was necessary due to the statically non-optimal shape of the circular arch. The offset cornice or a parapet on the bridge also visually ­illustrate its function as a lateral path ­delimitation independent of the structure (fig. 2). In the spandrel areas of low load concentration, openings were ­usually ­provided for flood drainage or there was room for ornamental stone masonry. Whereas, in structural ­elements with large internal forces – similar to ancient temple construction – it is evident that these are self-sufficient and do not tolerate any additional ornamentation [3].

Road Bridges

23

3

Calculation A further technical development of the road bridge and other milestones in bridge building only came about as a result of new mathematical findings from the ­engineering academies in France at the end of the 18th century. Jean-Rodolphe Perronet, the first director of the École ­royale des ponts et chaussées founded to the east of Paris in 1747, ventured into the formation of basket arches with specific ­calculations in order to realise larger spans with smaller arch rises. This made it possible to optimise flow openings and reduce ramp inclinations. With the help of the inwardly drawn reveals, the so-called “cow horns”, which also brought fluidic advantages, the bridge was given a ­slender and beam-like appearance that was unheard of at the time, using the ­example of the Pont de Neuilly, which no longer exists (fig. 3). For the first time, ­Perronet raised the falsework of the Pont de Neuilly in order to prevent dead weight deformations [4]. This new construction method can be seen as a precursor of the frame construction method, which was only developed with the help of iron additions at the beginning of the 20th century [5]. The introduction of iron and ferroconcrete as building materials and ultimately of ­reinforced and prestressed concrete (see

4

24

“Bridges of steel-reinforced and prestressed concrete”, p. 17) broke the ­centuries-long monotony of stone vault bridges, and new types of bridges were developed by the rising, independently ­acting profession of engineers. Filigree iron bridges as truss, chain and cable ­constructions aroused great resistance in the ­established architectural scene, to whom this technical-industrial design ­language did not appear to be “culturally conducive”. Architects like Paul Zucker also doubted the formal strength of the ferro­concrete beam due to the lack of ­material authenticity, as the decisive tensile reinforcement was hidden [6]. Decades would pass before the general public accepted the elementary expressiveness of the new engineering structures. The rather accidental appearance [7] of the George Washington Bridge (fig. 4) caused Le Corbusier to rejoice in 1937 that it was “the most beautiful bridge in the world” and that it “smiled like a young athlete” [8].

3  Pont de Neuilly across the Seine River by Jean-Rodolphe ­Perronet, as an engin­ eering feat from 1772 (replaced by new steel bridge in 1942)

Dissolution It was not only with iron structures that the objective was to keep material use to a ­minimum. At the beginning of the 20th ­century, the use of ferroconcrete also led to the breaking down of bridge struc-

4  George Washington Bridge, New York (US), 1931, Othmar Ammann 5  Merjubrigga, Stalden (CH), 1930, Alexandre Sarrasin a view b construction in ­progress

a

b

5

6 6  Mike O’Callaghan — Pat Tillman Memorial Bridge across the ­Colorado River (US) 2010, T. Y. Lin Inter­ national 7  New bridge developments made of ferroconcrete a Ponte del ­Risorgimento, Rome (IT), 1911, François ­Hennebique b Vorderrhein Bridge near Tavanasa (CH), 1905, replaced 1928, Robert ­Maillart c Inn Bridge near Zuoz (CH), 1901, Robert Maillart

tures into slabs and discs, even to the point of ­‘skeletisation’, in order to reduce mass, especially in road bridge construction. ­François Hennebique’s Ponte del Risorgimento in Rome from 1911 is con­ sidered one of the first designs of a hollow box girder, which – still outwardly percep­ tible as an arch – statically acts as a beam clamped on both sides (fig. 7 a). The Swiss civil engineer Robert Maillart, with his unique ingenious flair, developed the three-hinged arch at the Tavanasa Bridge (fig. 7 b) from the idea of simply omitting the spandrel areas in the ends of the discs 30.00 due to earlier cracking problems at the Inn Bridge near Zuoz (fig. 7 c). Constructional articulation was also carried out on the classical arched structure and resulted in a high degree of filigree and

­ legance for road bridges. If the arch e bridge as a solution over deep ravines was unsurpassable as a landscape-­defining ­element, its construction, however, required some effort. The structural mass had to be reduced in order to be able to build it, using ingenious timber falsework structures that were splayed into the rocky flanks. The bridge in Stalden, built between 1929 and 1930, is convincing due to the delicacy and fusion of the ­individual functional units of the arch, the deck and the supports into one overall unit (fig. 5). Today, long-span concrete arches are usually constructed using the cantilever method, as in the 51.00 example of the Mike O’Callaghan – Pat ­Tillman Memorial Bridge over the Colorado River (fig. 6) or the Tamina Bridge (see ­project example, p. 126ff.).

Clamping by counterweight W

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Clamping by counterweight

Road Bridges

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100.00

Prestressing The prestressed concrete construction method, developed in the mid-20th century under the leadership of Eugène Freyssinet, revolutionised road bridge construction in particular. Tension members made of high-strength steel braids are inserted into the cross-section and prestressed. By over-pressing the cross-section, ­important serviceability requirements for deformation limitation and durability can be fulfilled and, moreover, large spans of beam structures with extreme slendernesses of up to l/50 in the centre of a bridge become possible. The construction method for the bridge across Austria’s Lavant Valley has produced a modern translation of P ­ erronet’s bridge over the Seine in Neuilly (fig. 3, p. 24) that is reduced in form and material as a result of the building material (fig. 8). The curved bottom view with seamlessly connected twin columns emphasises the static frame system on the outside. In general, prestressed concrete and ­reinforced concrete bridges are now an integral part of most superordinate road ­networks worldwide (fig. 9). The inversion of the haunched prestressed concrete beam and the resolution into tension and compression components in the support area lead to the tension chord or extradosed bridge type, which are characterised by their flat cable or cable guides as ­support for the stretch girder. Designed by Christian Menn, the Sunniberg Bridge

10

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8

Reinforced concrete Prestressed concrete

Steel Masonry

Timber Other Department of Transport, Queensland (AU) Development Board, Tasmania (AU) mainroads, Western Australia (AU) Asfinag (AT) Civil service, Wallonia (BE) Ministry of Transportation, Québec (CA) Department of Roads (CL) Cerema (FR) Department of Transport (FR) Franchised motorways (FR) Közút (HU) RMTO (IR) Anas S.p.A. (IT) Department of Transport (JP) Motorway Association (KR) Transport Authority (NZ) Roads Administration (NO) Roads Directorate (PL) Roads Administration (SK) ZAG (SI) Ministry of Development (ES) Department of Transportation, Wisconsin (US)

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in Switzerland, with its slender super­ structure, represents the transition to the cable-stayed bridge and impresses with its outwardly bent support forms, which react functionally to the curved course of the road (fig. 10). This bridge is considered an icon in road bridge construction not least because of its integral construction method without the use of roadway transition structures (see “Function”, p. 66ff.). Long spans Long-span, cable-supported bridges are fascinating due to their self-explanatory mode of action of supports and loads. In the span range from approx. 250 m onwards, the cable-stayed bridge has established itself as an economical solution in road bridge construction. The nowadays very

9

8  Valley crossing over the Lavant Valley (AT), 1985, Alfred Pauser; extension 2007: Pauser ZT, Ertl Horn und ­Partner, FCP ZT 9  The number of bridges with respect to the ­construction material in superordinate road networks worldwide 10  Sunniberg Bridge, Klosters-Serneus (CH), 1998, Christian Menn, Andrea Deplazes

11  Skarnsund Bridge, Inderøy (NO), 1991, Johs Holt 12  Erasmus Bridge, ­Rotterdam (NL), 1996, Ben van Berkel 13  Ting Kau Bridge in Hongkong (HK), 1998, schlaich bergermann partner

14  Various means of expression for a cablestayed bridge a with straight pylon b with outwardly inclined pylon c with inwardly inclined pylon

a

b

c

14

narrow cable routing in the form of multicable systems in fan or harp shape and the resulting slender stiffening gir­ ders give cable-stayed bridges great expressiveness. Tilting the position of the pylon can lead to a decisive change in ­perception. The example of a ­singlepylon cable-stayed bridge is a good ­illustration of the change from a sublime, neutral appearance (fig. 14 a) to an expressive, suspenseful one (fig. 14 b) and a h ­ umble, subordinate one (fig. 14 c). The sober form of the Skarnsund Bridge nestles in the pleasant Norwegian fjord landscape and bears witness to the extreme minimalism of the materials used (fig. 11). With a span of 530 m, it still holds the record for the longest cable-stayed bridge constructed with a reinforced concrete deck. The deck measures only about 2 m in height. In contrast to this, the striking pylon bend below the cable fan inlet of the Erasmus Bridge creates a suspenseful dynamic that reflects the pulsating port city of Rotterdam (fig. 12). Since the turn of the millennium, multi-span inclined cable solutions have become increasingly popular to bridge larger distances. In doing so, the deformations from uneven field loading must be limited. In contrast to the rigid pylon design on the Rio-Antirrio Bridge over the Gulf of Corinth, the cable compartments on the Queensferry Crossing near Edinburgh overlap so that the pylons could be designed to be slender (see project example, p. 120ff.). The Ting Kau Bridge in Hong Kong, completed in 1998 as one of the first multi-span cablestayed bridges of a modern kind, solves the problem by using long stabilising cables to brace the central mast, which also provide safety in the event of a typhoon (fig. 13). As the crowning glory and special archetype of bridge construction, the suspension bridge conveys absolute efficiency as well as elegance through the dissolution of the

11

12

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support structure into a tension element and the suspension links. However, a suspension bridge is only economical for very large spans and is used especially for road bridge construction due to the softness of the system. With suspension bridges, the prestigious span chase also began, the record for which has been held since 1998 by the Akashi Kaikyō Bridge in Kobe, Japan, with a free span of 1,991 metres. Originally, this bridge was designed for a shorter span. However, an earthquake increased the pylon spacing by almost 1 m before the stiffening girder was installed.

Road Bridges

27

A slender stretch girder, a construction that became possible only with knowledge and mastery of the aerodynamic effects from wind excitation, possesses an extraordinary design quality. As the bridge over the Great Belt in Denmark shows (fig. 15), the sweep of the supporting cable and the slightly upwardly curved, extremely slender stretch girder also creates a special dynamic in terms of perception psychology, through the play of the oppositely directed curvatures [9]. Motorway bridges The development of the automobile at the beginning of the 20th century and the need for ever faster traffic connections led to the separation of roadways and the development of grade-separated traffic junctions. The first motorway-like road was constructed near New York in 1908 in the form of the Long Island Motor Parkway. In Europe, the Berlin Automobil-Verkehrsund Übungsstraße, or AVUS, and the Italian Autostrada Milano-Laghi project led the way [10]. However with the rapid expansion of motorway networks, especially in the second half of the 20th century, bridges became interchangeable and bland. The aim was to churn out roads and bridges with little ­funding in order to meet the economic boom of the post-war period, the new mass suitability of the motor vehicle and the expanded demands for mobility. In ­contrast to bridges with large spans or high local impact, which attract attention due to their size or publicity alone, today the ­priority for many bridges in motorway construction is functionality, practicality and rapid implementation. The flyover bridge is representative of many “forgotten” bridges with small and medium spans, which will be discussed below in order to explore expressive potential and avoid “soulless” bridges.

28

Over-bridging A characteristic feature of the motorway is the overbridge, which is even depicted in the corresponding traffic sign, not least for this reason. At regular intervals, overbridges enter the motorist’s field of vision from a ­distance and remain there for a long time. As recurring signatures, they are the calling card of a motorway and should therefore be given great care in the design – even overriding the route-bound support structures. The design of overbridges is primarily determined by the route characteristics, landscape integration and the perceptual experience from the driver’s perspective and less by the immediate location of the structure and its residents. Experts such as the early motorway architects Paul Bonatz and Wilhelm Tiedje prefer open-view type bridges instead of barrier-generating ones [11]. Column-free overbridges also highlight forward movement from a perceptualpsychological point of view [12]. From this, a wealth of variants can be derived, from the clearance-enclosing classic doublespan bridge to the support-free lightweight construction (fig. 17), which, depending on the design intention, contains different opportunities for expression from stringent to expressive forms [13].

15  Storebæltsbroen, Bridge across the Great Belt, Nyborg / Korsør (DK), 1998, COWI, Dissing+Weitling

15

Interlinking – Interweaving Bridges as nameless mass-built structures appear above all in the increasingly inter­ woven motorway junctions. In order to speed up the flow of traffic, interweaving areas of lanes are avoided and thus sep­ arate ramp structures are erected for each relation. The bridges pile up into multi-­ storey structures and are absorbed in ­complex support and structural systems (fig. 16). Renovations of these nodes as well as reconstructions for capacity increases will certainly become one of the challenges of the future.

Openness and Dynamics

16  Traffic junction in Los Angeles (US) 17  Overbridge variants

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16

Outlook In parallel with societal developments towards accelerated, flexible and largely independent – and therefore unpredictable – mobility behaviour, corresponding trends are emerging today for road bridges. Resisting The current high expenditures for main­ tenance promote a trend towards the use of high-performance materials that have a low material degradation and thus lower life cycle costs. This particularly affects road bridges that are either exposed to ­frequent freeze-thaw cycles in alpine regions and high chloride exposure due to the use of de-icing salt, or are subject to highly aggressive environmental conditions in coastal or industrial areas. There are currently a number of research projects and pilot applications of tension elements made of non-corrosive and chemically resistant materials that are waiting for regular implementation. For example, prestressing ­members and construction methods with carbon fibres are being developed under the catchword of “carbon concrete” in combination with higher-strength fine-grained concretes. The Wild Bridge in Carinthia, made of ultra high performance concrete (UHPC) with steel fibre reinforcement, shows that fine-grained, resource-saving construction is possible if the surface’s pore density promises high robustness and resistance (fig. 18).

Road Bridges

29

18

19

In Switzerland, ultra high performance ­concrete (UHPC) is today often successfully used as a structural reinforcement by applying several centimetre-thick layers. After the SIA 2052 code of practice came into force in 2016, construction companies in particular are pushing UHPC technology with their own developments by special manufacturers, so that this construction method without bituminous sealing can already compete with conventional retrofitting in terms of price (fig. 19). Introducing flexibility Another trend in road bridge construction is to react flexibly to changing uses. Road traffic is changing ever more rapidly in

terms of traffic volume, traffic flows and type of mobility. Although road bridges are usually designed for a theoretical service life of 100 years, many have to be renewed prematurely due to changes in use such as additional lanes and higher real loads. The latter result from the steadily increasing traffic volume, new concepts of mutually communicating, distance-reduced road trains (platooning), longer heavier vehicles (LVHs), but also from the increasing number of heavy goods transports. The increasing traffic demand with ever more frequently shifting and changing, ­time-of-day-dependent traffic volumes requires a high degree of flexibility on the available roadway surfaces (see “Requirements for future bridge structures”, p. 48f.). In terms of design, this means avoiding central supports on flyovers in favour of large spans that allow maximum manipulation options for the traffic flowing ­underneath. Alternatively, bridge systems are required that can be easily extended laterally with little disruption to flowing ­traffic or that also allow additional lanes on the underside of the bridge by means of longer end spans (fig. 17, 3rd ­variant from above, p. 29). Accelerating Fast and flexible construction with prefabricated elements is still prevalent, even if the experiences from the prefabricated construction boom of the 1970s and 1980s are rather ambivalent with regard to longevity.

20

30

20  The beams of the future bridge crosssection are folded out. Lahnbach Bridge, Fürstenfeld (AT), 2020, Kollegger, Schimetta ZT

18  Bridge made of ultra high performance ­concrete (UHPC) with steel fibre reinforcement, Wild Bridge, Völkermarkt (AT), 2010, TU Graz, Wörle Sparowitz Ingenieure 19  Bridge reinforcement with ultra high per­ formance concrete (UHPC)

Notes:  [1] Lee 1947  [2] Merckel 1899, p. 295   [3]  Pauser 2005, p. 3  [4] Bühler 2019, p. 143  [5] Pauser 1987, p. 76ff.  [6] Hartmann 1928, p. 31  [7] Billington 2014, p. 122  [8] Le Corbusier 1964, p. 75ff.  [9] Grütter 2015, p. 201f. [10] Kreuzer 2005, p. 12ff. [11]  Tiedje 1966, p. 10 [12] Arnheim 1980, p. 166 [13] Kleiser 2017, p. 402— 411 [14] Seidl et al. 2016, p. 126 —136 [15] Kollegger et al. 2020, p. 484 —494

21 Road transport system of the future with two storeys and bridge systems made of prefabricated elements

Transit lanes

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22 Optimised crosssection for elevated roadways made of ultra high performance fibre reinforced concrete

The consequence of these experiences is a necessary intersecting of durability requirements with the demand for flexible, fast and low-resource construction. In Germany, pilot applications are being carried out with prestressed, precisely laid and directly accessible segmental bridges [14]. In Austria, research is underway that combines a low-maintenance and easy-to-assemble construction method of semi-precast elements with in-situ concrete. Bridges of this type have already been constructed in Austria, for example (fig. 20). They are built using a folding process developed by the TU Wien that works like the mechanism of an umbrella [15]. The current trend towards megacities and converging conurbations of 100 million and more inhabitants calls for suitable transport routes. By raising lanes and creating gradeseparated intersections, the bridge will play a decisive role in road traffic systems of the future. This is also shown by a current research example: autonomously driving,

centrally controlled e-vehicles move in a closed roadway system with prefabricated open-box girders made of ultra high performance fibre reinforced concrete (fig. 22) and form individual flow traffic on elevated lanes in Level 1 that can match the efficiency of a metro (fig. 21).

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Road Bridges

31

Bridges for Rail Traffic Track-bound rolling and hovering

Responsibility and challenges Railway bridges are an important part of the rail infrastructure. As high-performance structures, they have to be designed for ­significantly higher loads than road bridges, and at the same time to be low in deform­ ation and vibration. Due to the age distribution of the structures, some of which are already 150 years old, conservation, maintenance and new replacement construction will become increasingly important in the future. Replacement investment in railway bridge construction for new structures or route expansion is significantly more expensive and technically more demanding than in road bridge construction due to building under rolling stock, the very high loads, and the comfort requirements for modern railway bridges. Railway bridges must be robust but don’t have to be ungainly and ugly. Intelligent structural systems can be used to build rigid structures with low structural heights that are aesthetically pleasing and, incidentally, sustainable with low maintenance and repair costs and through effective use of materials. In addition to the conventional railway ­networks, the high-speed lines with inter­ national interconnections offer an increase

32

in travel speed and can also be a substitute for short-haul air traffic, provided that appropriate, well-timed train lines exist. In add­ ition to the very high demands placed on high-speed traffic, international standardisation of the infrastructure (interoperability) is thus also required for the railway bridges. For rail-bound traffic, rigid point-to-point connections in the transport network by means of maglev trains, monorails or hyperloops, for example, are also conceivable models. Irrespective of this, however, railway bridges will remain an important part of the rail-bound infrastructure in the future.

1  Increasing the capacity of rail transport as a low-emission mode of transport is one of the great challenges of our time. Climate protection by reducing CO2 in the transport sector is only possible through an active transport turnaround, switching significantly more passenger and freight transport from road to rail.

1

a

b

c

2  Portageville Viaduct (US), 1852/1875/2017, Silas Seymour, George S. Morison, Modjeski and Masters a Wooden bridge, 1852 b Steel structure ­(trestle bridge), 1875 c Steel arch bridge, 2017

Development of railway bridge ­construction After the first public railway pulled by a steam locomotive began operation in 1825 on the Stockton and Darlington Railway in the UK with George Stephenson’s “Loco­ motion No. 1”, railway traffic developed ­rapidly in Europe, the USA and Asia. The pioneers in the early years were mainly the USA, ­Germany, France, the UK and Belgium. The first long-distance connection in ­Germany was the Leipzig-Dresden railway, which went into operation in 1839 with a length of 120 km. A number of engineering structures, mostly arched bridges, from this first period of railway construction have survived to this day and are still in use. In addition to the technical and financial challenges of building the railway lines, engin­ eering structures were required that had to meet completely new requirements compared to the previous road and pathway bridges (see “Road Bridges”, p. 22ff.). One of the first large railway bridges in Germany was built across the Elbe in Riesa for the Leipzig-Dresden line. This was still constructed as a multi-span wooden arch bridge with spans of 28 m on solid piers and carried traffic for almost 40 years from 1839 to 1878 until it was replaced by a steel arch bridge.

2

The increased demands on the bridges in terms of railway loads and speeds and the negative experiences with the combustible material of wood led to a radical break with wood as a building material in railway bridge construction from around 1850 – wood has since been banned for railway bridges in Germany [1]. Similar developments took place in Europe. From about 1850 on, the new options and the use of industrial materials such as brick and modern steels greatly changed railway bridge construction and very quickly displaced wooden bridges. However, wooden bridges continued to be built in the USA, and some are still in use today. The Portageville Viaduct is a good example of the developments in bridge construction in the USA (fig. 2). Since 1852, the ­Portageville Viaduct has carried a railway line across the Genesee River Gorge near the town of Portageville in New York State. At first, it was made of wood and was then replaced in 1875, after a fire, by the filigree steel construction of a trestle bridge. After more than 140 years of use, the structure no longer met the requirements and had to be renewed. The commissioned engineers, the municipality and the Norfolk Southern Railway Co. worked together to develop a new steel arch bridge, which went into service in

Bridges for Rail Traffic

33

2017. The history of railway bridges is tied to place and time, as here in Portageville – many similar examples demonstrate how different structures have changed on site over the decades, taking advantage of ever-evolving technologies. In the second half of the 19th century, the railway was the emerging transport system that enabled metropolitan and regional ­connectivity, as well as economic development. During this period, railway networks grew rapidly. The designs of the bridges were subject to continuous further developments resulting from new requirements, materials and technologies as well as more complex calculation methods. With industrialisation and the development of iron and later steel technology in the UK, the first steel bridges were built from cast iron. It was only with the establishment of efficient iron and rolling mills that wrought iron sheets and rolled sections of different

3

4

34

dimensions could be produced. Assembling a bridge from individual bars and sheets by riveting was much easier and faster than casting various elaborate cast parts in sand moulds. Basically, with wrought iron and the truss theory developed around 1850, completely different bridge shapes could be constructed than would have been possible with cast iron, which had little tensile strength [2]. New ­calculation methods for long-span bridge structures became necessary for the ­planning of the railway bridge. With the help of the “Graphic Statics” published by Karl ­Culmann in 1864 –1866, it was now possible to statically analyse bridge ­constructions that had previously been based on experience and thus to erect long-span, efficient support structures (figs. 3 and 4). In Germany, uniform regulations for dimensioning, technical processing and the ­preparation of construction drawings were established, and the first type designs for railway bridges with a span of up to 20 m were introduced. This laid the first foundations for the standardisation of ­railway bridges. The spread of steel railway bridges was very closely linked to the technological development of steel ­production and processing. In the 1930s, high-strength structural steels and the ­introduction of welding revolutionised the technological opportunities in steel bridge construction. The economic and qualitative advantages were obvious: the production of steel bridges could be largely transferred to the factories and final assembly at the construction site could be implemented at high speed [3]. The new building material concrete, which was used in bridge construction from 1880, initially only replaced natural stone in arch bridges and was rammed into the formwork without iron inserts (tamped concrete). Engineers quickly recognised the potential of the freely mouldable material in combin­

3  Firth of Forth Bridge, Queensferry (GB), 1890, John Fowler, Benjamin Baker 4  Pont de Mousserolles, Bayonne (FR), 1864, F. Daney, demolished and replaced by ­Bayonne railway bridge in 2013

5 5  With its 100 m spanning arch, the bridge designed by Karl ­Arnstein and completed by the Züblin company in 1914, the Langwieser Viaduct in Graubünden (CH) was at its time the ferro­ concrete bridge with the largest span worldwide. In order to absorb the changes in length of the superstructure due to temperature, the deck slab above the abutment piers is interrupted and clamped in a flexible reinforced concrete wall.

ation with reinforcing steel. The technical requirements of ferroconcrete shaped the bridge designs and gave engineers completely new opportunities. In the period of searching and experimenting from 1900 onwards, many integral, i.e. jointless, concrete bridges were built in railway bridge construction (see “Transition zone to the free track”, p. 67ff.). These bridges are characterised by high stiffness, integral load-bearing behaviour and economical manufacture combined with efficient use of materials (fig. 5). Reinforced concrete bridge construction developed rapidly in the following years and replaced steel bridges in the lower span range from about 1920 onwards. Compared to steel, the high stiffnesses for railway bridges could be achieved more economically in reinforced concrete. The bridge cross-section made of steel girders cast in concrete, which has been known in Europe since the beginning of the 20th century, occupies a special ­position. The so-called rolled girder in ­concrete is still regularly used in new ­construction today.

Prestressed concrete bridge construction is of outstanding importance for the development of railway bridge construction. It was only with the prestressing of the cross-­ sections that larger spans with high stiffness and robustness became possible from the 1930s onwards. The period from 1934 to about 1965, during which anchorages and construction methods still used today were developed, marks the most innovative time of prestressed concrete bridge construction [4]. After World War II, depending on the economic development of the countries and regions, railway networks worldwide grew. Especially in Japan, high-speed lines were developed from the 1950s onwards and put into operation in 1964 in time for the Summer Olympic Games in Tokyo. Japan was the starting point for numerous developments and innovations, such as the slab track, which became established worldwide. Subsequently, high-speed lines were built in numerous countries, especially in Europe, in order to quickly connect urban centres over long distances. China is currently the country with the greatest pace of

Bridges for Rail Traffic

35

expansion in high-speed traffic (HST). ­Currently, 80 % of the major cities are connected by the high-speed rail network, which amounts to more than 30,000 km of track (as of 2020). [5] Stiff prestressed concrete cross-sections with spans from 35 m to 70 m have become established worldwide for bridges for highspeed lines in order to meet the increased requirements from dynamic loads (figs. 6 and 7). Maglev bridges Maglev trains have been in development since around 1970 and were planned as a grand vision for high-speed transport and as an alternative to express trains. The ­technical and economic developments in the transport market over the last 30 years have meant that rail-bound transport at 300 km/h has also become suitable for long distances and, due to falling prices from low-cost airlines, travel over longer distances has become cheaper. In Germany, investment was made in the Transrapid system for many years and test tracks were built as elevated tracks made of prefabricated segment girders. Currently, the technology of the magnetic levitation train is being further developed and tested on a line in Sengenthal in the Upper Palatinate. The system was designed as a stand-alone maglev track for short ­distances (fig. 10). These elevated tracks represent bridges with high demands on

7

36

6

the construction. With a clear design and durable integral structures, effective guideways can be built that ensure free passage of crossing traffic and could be used, for example, in densely built-up urban centres. Hyperloop Today, concepts are emerging worldwide for innovative forms of propulsion that require completely new support structures for lane-bound passenger transport. ­Hyperloop systems consist of large tubes supported on the ground with pylons as an elevated guideway similar to the ­Transrapid (fig. 9). The tubes and pylons form integral support structures that can be routed close to the ground or spanned over valleys or obstacles, depending on the topography. Wide embankments and the associated land consumption as with railway lines are not necessary here. Passengers and goods travel in pressurised capsules that float through the tube on a frictionless magnetic cushion at speeds of over 1,000 km/h, extremely quietly and with zero emissions. With the Hyperloop, numerous issues will have to be addressed, ranging from propulsion and safety concepts for passengers to the construction, mainte­ nance, and upkeep of the infrastructure. For bridge construction, many new tasks will

6  High-speed bridge in Germany Unstruttal Bridge, near Karsdorf (DE), 2012, Marx Krontal Partner, schlaich bergermann partner, SMP Ingenieure 7  Railway bridge, Changsha (CN), 2013

Number of arch bridges

1,600

problem. The preservation of arch bridges is of great economic importance for the ­railway networks of Europe. Therefore, the focus for these structures must be on preserving the building stock and ensuring durability. A very effective protection and, at the same time, load-distributing method is the installation of a reinforced concrete track trough. There are numerous examples of the use of track troughs – also called ballast troughs in Switzerland – in Germany, Switzerland [7], and Austria [8].

1,400 1,200 1,000 800 600 400

2017

2010

2000

1990

1980

1970

1960

1950

1940

1930

1920

1910

1900

1890

1880

1870

0

until 1850 1860

200

Construction year 8 8  Year of construction of the arch bridges in Germany

arise here and innovative approaches to solutions will be required, including the development of new materials and technologies. Bridge systems for railway bridges Most railway bridges date from the time of the expansion of the railway network around 1880 or from the years 1900 to 1930.

9  Hyperloop 10  Magnetic levitation train Transport System Bögl (TSB)

Steel arch bridges – Langer’s beam In 1859, the Viennese civil engineer Josef Langer sketched out ideas for stiffened, anchored chain bridges, which later would form the basis for numerous arch and ­tied-arch bridges made of steel. He was aiming for the most economical construction with a high degree of robustness. Langer further developed the lattice girder system,

Arch bridges Arch bridges made of natural stone or brick, mainly built between 1850 and 1930, are the standard bridges in Germany with a share of 28 % [6], and form an important part of Europe’s railway infrastructure despite their age. The majority of arch bridges have thus been in service for more than 100 years and longer, because they are characterised by great stiffness and robustness, are dynam­ ically uncritical and have high load-bearing reserves for speed and load increases. Often, they are also characteristic of the landscape and historically valuable (fig. 8). Due to inadequate sealing and defective drainage systems, surface water often ­penetrates the structures, causing damage to joints, structures and surfaces. These damage patterns are very typical for arch bridges and do not represent a deficit in the functionality of the support structure from the outset but are primarily a durability

9

10

Bridges for Rail Traffic

37

11

which was well-known in Europe, by combining the lattice girder as a bottom and top-mounted truss or as an arch with cable tensioning or bowstrings. Today, the system is known as a tied-arch bridge. These are used in railway bridge construction for spans of approx. 50 m to approx. 150 m and are erected in various designs as pure steel or as steel composite bridges (fig. 12). Network arch bridges Network arch bridges have an optimised structural concept compared to tied-arch bridges. In comparison with the tied-arch bridge, the number of hangers in the network arch bridge is increased and arranged in two planes per arch, ascending and descending in opposite directions. This intersecting hanger arrangement creates a shear-resistant connection between the

arch and the stiffening girder, which now act like the chords of trusses. The bending moments in the arch are thus reduced to approx. 80 % and in the stiffening girder to approx. 15 % [9]. Network arch bridges are economically used for spans of 90 m and more. The welded flat steel hangers pose a static and structural challenge. Due to their own weight, deflections can occur and, if the tensile stresses from the carriageway are too low, compressive forces can also occur in the hangers. Such failing hangers must be excluded in the ­network layout. New structural concepts with tension rods made of carbon fibre reinforced plastic (CFRP) as hangers in network arch bridges are currently being planned (fig. 11). Due to the high fatigue resistance of carbon components, the hangers can be arranged purely under the aspect of utilisation in

11  Use of carbon hangers. New Oder Bridge Kostrzyn-Kietz (PL / DE), planned completion 2022, Knight Architects, Schüßler-Plan 12  Rhine bridge LustenauSankt Margrethen (AT / CH), 2013, ­Bernard Ingenieure

12

38

13  Loyola railway viaduct and station, DonostiaSan Sebastián (ES), 2017, Anta Ingeniería Civil, Estudio Lamela a Multi-span steel trough bridge (length 277 m) b Inclined column connection

14  DB standard bridge with very low deck height, thick plate bridge, Davenstedter Strasse railway overpass, Hanover (DE), 2017, Marx Krontal Partner a Construction ­drawing b View

a

a

b

t­ension without consideration of limited vibration widths. The cross-section of carbon hangers required for this is less than a quarter of that of steel hangers. In com­ bination with a low modulus of elasticity in CFRP, this leads to significantly larger expansion paths in the hangers, which are thus permanently under tension. By using carbon tensile members as hangers, the economically viable range of applications for network arch bridges can be extended well beyond a 300 m span [10].

A further development are the so-called thick plate bridges, where the deck is made of a continuous heavy plate with a thickness of up to 100 mm. The use of this material is limited to spans of up to 20 m due to the high lifting weights during assembly (fig. 14). Trough cross-sections can also be erected as composite structures with external reinforcement made of composite dowel bars. As a single-span structure, this type of bridge can also be used with monolithic integration into the abutments if the construction height is very limited. The external reinforcement is used as a load-bearing ­element in the longitudinal and transverse directions. The result is a trough cross-­ section with a grid-like arrangement of the steel cross girders in the deck and in the upper and lower chord of the trough ­stringers. The T-shaped steel sections are placed at the lower fibres of the cross-­ sections, where they very effectively influence the load-bearing effect. Usually two girder halves are produced from a rolled girder in

Bridges with low deck heights Bridges with shorter spans of up to approx. 30 m are often used in inner cities and must be designed with a low deck height. This ensures that the clear heights can be maintained without gradient adjustments in the case of new replacements. For such tasks, trough bridges with solid wall girders and an orthotropic steel deck are used. This type of construction is used today where space is limited and the available headroom is low (figs. 13 and 14).

13

b

14

Bridges for Rail Traffic

39

an industrial process and the shape of the dowel bars is cut in the middle in the flamecutting machine (fig. 15). Beams in continuous and in frame systems have to be provided with face plates in the steel mill [11]. These structures are characterised by high quality, a high degree of prefabrication, small corrosion protection areas and very economical acquisition and maintenance. Composite and rolled-girder-in-concrete bridges Composite bridges made of steel sections cast in concrete were first built around 1910 with the advent of the wide-flange ­Differdinger rolled section. Due to the high bending stiffness, the wide-flange profiles in the rolled-girder-in-concrete bridges ensure very low deflections with effective slendernesses achievable at the same time. The main advantage of this construction method is the almost complete encasement of the steel girders in concrete. This allows maintenance to be reduced to a ­minimum. Due to the high deformation restrictions for ­railway bridges, rolled-girder-in-concrete bridges are still mainly used in railway bridge construction today. From about 1950 onwards, composite steel bridges which, especially with regard to their robust construction and due to the connection between concrete and steel, offer considerable advantages for railway bridges, were further developed for spans over 25 m. Composite carriageways as a concrete component are significantly more

40

15  Trough cross-section with external reinforcement with composite dowel bar a View b Construction ­drawing 16  Composite precast girder cross-section for railway bridges. Railway bridge Flutgraben at Erfurt railway station (DE), 2010, Marx Krontal Partner a View b Construction ­drawing

a

b

15

a

b

16

17 17  Composite bridge with tubular truss for larger spans. New railway ­viaduct Pulvermühle, Luxembourg (LU), 2019, Leonhardt, Andrä und Partner, TR-Engineering, Aurelio Galfetti

robust and durable than comparable orthotropic steel carriageways. There is no need to renew the corrosion protection on the carriageway, so composite carriageways are more economical to maintain. The main girders of the steel superstructures are ­prefabricated in the factory and thus meet high quality standards. Parallel to prefabrication, the foundation work can be carried out on the construction site, e.g. under ­auxiliary bridges, with full railway operation. This ensures fast construction during ongoing operations (fig. 16). Railway bridges as prestressed concrete superstructures are executed as monolithic constructions of slabs, slab beams or hollow boxes, or as mixed constructions with prefabricated prestressed concrete girders for spans of more than 20 m. Due to their higher dead weight, reinforced concrete and prestressed concrete bridges are very capable of absorbing vibrations and dynamic loads from railway operations

and have become the standard bridges in railway bridge construction. With clean structural design of the concrete components and avoidance of joints and bearings, railway bridges can be designed to be very robust and durable. Different types of cross-sections have become established for reinforced concrete and prestressed concrete bridges, depending on the span ratio (fig. 18, p. 42). In the general perception, modern railway bridges made of reinforced concrete are often dull and heavy-looking structures that tend to “block” our landscapes rather than contribute to the building culture in their natural or urban environment. As part of the realisation of the Ebensfeld-ErfurtLeipzig / Halle high-speed line, several semi-integral structures were designed and constructed (Grubental Bridge, Scherkonde­ tal Bridge, Gänsebach­tal Bridge, Unstruttal Bridge, Stöbnitztal Bridge) that have a ­distinctly different appearance (fig. 20 and

Bridges for Rail Traffic

41

18  Different cross-section types for reinforced concrete and prestressed concrete bridges.

Slab cross-section slackly reinforced Span width: 2 to 25 m Slenderness: L/10 to L/15

Wide beam Prestressed concrete cross-section Support span: 30 to 45 m Slenderness: L/20 to L/25

Double-webbed slab beam, prestressed concrete cross-section Span: 20 to 35 m Slenderness: L/12 to L/20

Prestressed concrete box cross-section Support span: 40 to 60 m Slenderness: L/12 to L/20

42

18

Notes:  [1] Eisenbahnzeitung 1850, p. 63  [2] Tasche 2016, p. 118   [3] Kurrer / Weißbach 2009, p. 113  [4] Tasche 2016, p. 315  [5] Krüger 2020   [6]  Krontal 2014, p. 1  [7] SBB 2008  [8] Arbeitshilfe 2020   [9]  Gautier / Krontal [10] Haspel 2019, p. 159 [11] Seidl / Lorenc, p. 550 [12]  Marx 2015 [13] Schlaich et al. 2008

α

β

f1

19  Comparison of end tangent twists a with supported ­single-span beam b with frame

a

project example “Scherkondetal Bridge”, p. 142ff.). Especially in flat, longer valleys, these slender, almost fragile-looking ­structures blend ­harmoniously into the ­landscape [12]. Through a sensible choice of the support system, the design and location of the fixed points required for the transfer of large braking forces, as well as the length of the structure or ­section, the track can even be guided over the structure without rail extensions in some structures (fig. 20). [13] Particularly in railway bridge construction,

f2

b

19

the use of integral or semi-integral structures is very advantageous, as the rigid connections between the pier and the superstructure allow high stiffnesses and greater slendernesses to be achieved with simultaneously high stiffness and dynamic robustness (fig. 19). Furthermore, there are no maintenance-intensive bearings and joints, while additional structural redundancies are available that give reason to expect long-lasting structures (see “Transition zone to the open track”, p. 67ff.).

20  Gänsebachtal Bridge, new ICE line ErfurtLeipzig / Halle (DE), 2012, SSF Ingenieure, schlaich bergermann partner

20

Bridges for Rail Traffic

43

Bridges and Traffic Mobility in progress (Markus Friedrich)

Bridges and mobility “There are seven bridges to be crossed” goes the song by German rock group Karat, quoted here in the English version by Chris de Burgh. “There are seven bridges to be crossed to reach your destination.” – This is the result for an average person’s change of location in Germany when data on mobility behaviour is intersected with data on the traffic route network and its bridge structures. Mobility behaviour in passenger transport is regularly surveyed in Germany as part of the “Mobility in Germany” study [1]. For 2017, the study provides the following key data: • Each person covers an average of 3.1 journeys per day to get to desired places (e.g. to their home, or for work, shopping, leisure and education).

1

44

• To reach these places, a transport effort of about 40 passenger kilometres arises per person, without flights. Fig. 2 shows the distribution of this transport effort among the modes and the traffic route category. The largest share of passenger kilometres, 75 %, is accounted for by the private car (self-driver/passenger). Public transport accounts for 19 %, walking and cycling for 6 % of the passenger kilo­ metres. The breakdown by traffic route category illustrates the importance of ­federal arterial roads. • The time required to cover the 40 km is around 80 minutes. • A medium route is thus about 13 km long and requires around 25 minutes. There is no uniform data basis for bridge structures in Germany. Reliable data sources [3] are available for the federal arterial roads [2] and the DB Netz AG rail network. For the other categories of transport infrastructure, there is an estimate [4] based on OpenStreetMap data. Fig. 3 ­collates the data for bridges from these sources. The estimate provides an order of magnitude of about 230,000 bridge structures in Germany. The majority of these bridges are less than 30 m long. For federal arterial roads, about 6 % are large bridges

1  Road zipper technology on the Golden Gate Bridge, San ­Francisco (US)

2  Daily traffic volume per person in Germany by mode and traffic route category

Passenger kilometres by mode

Pedestrians and cyclists (6 %)

Public transport (19 %)

Passenger kilometres by traffic route category

Pedestrians and cyclists

LongCitydistance based public public transport transport

Bridge density [Bridges per km]

0.1

0.8

0

Car passengers (20 %)

Cars on federal motorways & federal roads

Cars on state roads

Cars on district roads

Cars on other roads

1.0

0.3

0.1

0.1

0.1

5

10

15

with a length of more than 100 m [5]. A bridge density can be ­calculated from the combination of network length and ­number of bridges. It describes the average number of bridges per km. The bridge density depends on the category of t­ransport route. Railways and arterial roads have a greater bridge density than other transport routes. When weighted according to transport demand, the average bridge density is about 0.5 bridges per km. On average, road users therefore use a bridge every 2 km. For a daily distance of 40 km, this corresponds Traffic route category

3  Number of bridges, bridge density, passenger kilometres and number of bridge crossings in Germany

Car drivers (55 %)

20

25

30 35 40 Passenger kilometres per day 2

to about 22 bridge crossings per day and seven bridge crossings per route. Regardless of how correct this value may be, it illustrates the importance of bridge structures to the mobility of each individual person and to traffic as a whole (figs. 2 and 3). Bridges as an element of transport network design Transport networks connect places and make them accessible. They enable people to move and goods to be transported. The task of transport network planning is to

Traffic route network in Germany

One person in Germany

Network length

Number of bridges

Bridge density

Passenger kilometres

Number of bridge crossings

[km]

[quantity]

[bridges/km]

[km/d]

[number/d]

Pedestrians and cyclists

570,000

83,000

0.1

2.5

0.4

Public Long-distance transport City-based

33,500

25,700

0.8

3.8

2.9

0.11)

3.3

0.3

Cars

Federal arterial roads

41,000

39,700

1.0

16.2

15.7

State roads

87,000

22,000

0.3

5.6

1.4

District roads

91,000

10,000

0.1

2.9

0.3

Other roads

570,000

53,200

0.1

5.6

0.5

Total per day

40.0

21.6

Total per route

12.9

7.0

1)

 

Assuming that the bridge density corresponds to the density of other roads

Bridges and Traffic

3

45

Beeline speed [km/h]Beeline speed [km/h]

4  Evaluation of the service quality of a route with the help of the parameters beeline speed (a) and diversion factor (b) in relation to the beeline distance. A The service quality is B described by six quality levels A (very goodC quality) to F (insuffiD cient quality). E

100 90 80 70 60 50 100 40 90

F A

30 80

B C

20 70 10 60 0 50 40

D E 0

50

100

150

200

250

300

350 400 450 F500 Beeline distance [km]

0

50

100

150

200

250

300

350 400 450 500 Beeline distance [km]

100

150

200

250

300

350 400 450 500 Beeline distance [km] 4

100

150

200

250

300

350 400 450 500 Beeline distance [km]

30 20 10 0 a Diversion factor

These requirements lead to a conflict of objectives. For example, the benefit of an additional diagonal traffic route might be at stake (fig. 5). In order to minimise construction and maintenance costs, but also land consumption, it is desirable to concentrate traffic flows on a few efficient roads. However, this leads to the fact that journeys between two locations cannot be undertaken directly. The mileage and thus the energy consumption increase. In many cases, higher mileage will also lead to more accidents. At the same time, diversions increase the time spent by motor vehicle drivers. This trade-off between concentrating traffic demand on a few traffic routes and appropriate diversion is a fundamental task of transport network planning. The conflict is solved in the network design guidelines [6] by setting specifications for two central parameters that can be used to quantify the quality of a transport service between two locations, the beeline speed and the degree of diversion: the beeline speed is defined as the ratio of travel time and beeline distance. The beeline distance thus describes the

Diversion factor

design transport routes in such a way that the requirements of network operators, ­network users and the environment are met as well as possible: • Requirements of the network operator - minimum construction costs - minimum maintenance costs • Requirements of the network user (car driver, public transport passenger) - minimum time expenditure - minimum operating costs or fares - high accident safety - high reliability • Environmental requirements - minimum land consumption - minimum fuel consumption - minimum fragmentation of space into   sub-areas - bypassing of sensitive areas (e.g. landscape conservation areas)

2.50 F

2.25 2.00

E

1.75 2.50

D

1.50 2.25

F C B

1.25 2.00 1.00 1.75

E A 0

b 1.50

D50 C B

1.25

A

1.00 0

46

50

time required for a change of location. The beeline speed is to increase with increasing beeline distance. The degree of diversion is given as the ratio of the travel distance and the beeline distance (diversion factor). The degree of diversion describes the spatial effort of a change of location. It should decrease with increasing beeline distance. Fig. 4 shows a diagram each for the beeline speed and the degree of diversion, which can be used to evaluate the quality of service of a relation. Depending on the beeline distance, different requirements arise in each case, which are evaluated as quality levels A (very good quality) to F (insufficient quality). The network depicted in fig. 5 shows that the decision to build an additional diagonal traffic route should depend on the length l

5  Transport network design: to build or not to build? Z Z

Central place Existing traffic route (with bridge)

l

Z

Possible new traffic route (with bridge) l

Objective

Build a traffic route?

Low time expenditure

Yes

Low degree of diversion

Yes

Low fuel consumption

Yes

High network robustness

Yes

Low land consumption

No

Low fragmentation

No

Low construction and maintenance costs

No 5

of the network mesh. In the case of a small length, as is common in urban networks, a diversion is reasonable because the additional time required is hardly significant in relation to the total travel time. In ­networks for long-distance traffic, the traffic routes are designed in such a way that higher speeds are possible. Therefore, detours are also justifiable here. Provided there are no special requirements from ­traffic demand, network densification in long-distance traffic – e.g. through a ­diagonal line – is considered from a length of about 100 km. Bridges have a special significance as an element of transport network design for ­several reasons: • Traffic routes have to overcome natural obstacles such as rivers and valleys, but also other transport routes. Bridges are necessary for this. • Traffic routes should be routed as directly as possible, minimise gradients and be passable at a given design speed. A route that meets these requirements needs bridges and tunnels. • Bridges are particularly expensive traffic routes. Therefore, it is particularly import­ ant to bundle the traffic demand of ­several routes. The structure is more ­economical if it is used by as many road users as possible. • The bundling of many routes into one bridge structure means that bridges are particularly vulnerable structures. The failure of an object can considerably impair the connecting function of the transport network. The importance of a bridge structure in a transport network can be determined with traffic demand models. For this purpose, a demand matrix that contains the traffic demand between the sources and destin­ ations of a study area is assigned to the transport network. This traffic assignment maps the route choice behaviour of users

Bridges and Traffic

47

and thus determines traffic volumes for each network element. The results of an assignment form the basis for benefitcost studies of a new transport route, such as those carried out in Germany in the ­planning of federal transport routes. However, assignments can also be used to ­estimate the traffic effects of a full closure that becomes necessary when a bridge structure fails. Fig. 6 shows the effects of a full closure in the event of the failure of a specific bridge – in this case the Kocher Viaduct on the A6 motorway. The full closure results in about 50,000 vehicles having to accept a diversion every day, which means a time loss of one hour on average. However, a corresponding examination of the network significance of a traffic structure has not yet been provided for in the regulations. Requirements for future bridge structures A large proportion of bridges, at least in western Europe, were built before 1990. In Germany, about 10 % of the bridges in the federal arterial road network are in an inadequate condition, and for 2 % the ­condition is assessed as insufficient [7]. The reasons for this, apart from delayed maintenance measures, are strong growth in heavy goods traffic and an increase in the permissible total weight of heavy goods vehicles. Since 1960, the average traffic volume on motorways has increased fivefold across all motor vehicles, and heavy goods traffic has increased almost fourfold [8]. The permissible total weight has increased from 32 t to 44 t ­(articulated lorries) during this period [9]. In Austria, the trend of the actual increase in traffic and the legal load increase on road bridges has also been upwards since 1945 (fig. 7). This development was hardly predictable 50 years ago. However, the question arises whether a better forecast of traffic volumes and vehicle weights would have led to

48

­ ifferent structures. Would it have made d sense to dimension all motorway bridges for a six-lane cross-section and for today’s vehicle weights as early as 1960? Higher investment and maintenance costs would have been incurred for these bridges although the structures would only have been fully utilised for part of their useful life. This question is also relevant for the dimensioning of future bridge structures. Any ­forecast of future developments in traffic is associated with uncertainties, and they are not made any easier by the intensively ­conducted climate debate. This concerns not only the volume of traffic but also the type of traffic and the traffic flow. Will

Time expenditure in 1,000 h Condition 1: 5,760 Condition 1: with blocking Condition 0: 5,710 Condition 0: without Difference:       50 ­blocking

6  Effect of a full route closure on the travel time expenditure of a day, here using the example of the Kocher Viaduct Assumptions: • Road users only have alternatives in the motorway network. The subordinate road network cannot be used for a diversion. • Only the traffic demand in car traffic with a source and destination in Germany is taken into account.

Full closure Additional traffic Condition 1 Full closure Additional traffic Condition 1 6

Notes: [1] BMVI 2019 [2] BASt 2019 [3] DB Netze 2018 [4] Arndt 2013 and Difu 2015 [5] ADAC 2013 [6] FGSV 2008 [7]  BASt 2019 [8] BMVI 2019 StVZO 2020 [9]  and 1960

45

7,000,000

40

6,000,000

35 30

5,000,000

25

4,000,000

20

3,000,000 Total permissible weight 2,000,000 of motor vehicle Total permissible weight of motor vehicle with trailer 1,000,000 Number of motor vehicles 0 1995 2005 2015 2025 Year

15 10 5 0 1945 1955

1965

1975

1985

targeted government initiatives lead to a significant shift of road traffic to rail or even to cycling for short distances? How will digit­ alisation and the autonomisation of traffic influence future bridge dimensions? Can higher line capacities be created through shortened train intervals? On the road, automated vehicles will probably be able to run at closer intervals in the future. Rather unlikely, but still conceivable, is a development in which automated vehicles with ­perfect lane keeping require smaller lane widths. Then, on a cross-section with three lanes today, four lanes could be arranged in the future. More efficient lane use such as lane sharing, direction-changing lane assignments, for example by means of road zipper technology (fig. 1, p. 44), or temporary breakdown lane clearance are already in use or being tested worldwide in order to better manage daytime-dependent traffic peaks. However, it probably does not make economic sense to dimension bridge structures in the future in such a way that all possible developments that could occur during their service life are taken into account. What is needed in future bridge planning is a high degree of flexibility in use (see “Introducing flexibility”, p. 30). From a traffic perspective, the following requirements can be formulated for future bridge structures:

Number of motor vehicles

Total weight [t]

7  Real (= number of motor vehicles) and legal (= permissible total weight) load increase on bridges in Austria The increase in 2019 is due to the increase in the permissible total weight for motor vehicles with electric drive.

8,000,000

50

7

• Load models should assume denser traffic with low vehicle spacing. • When planning a bridge structure, it is advisable to already consider requirements that are necessary for the construction of a replacement structure while fully maintaining traffic. • When designing traffic route networks, the availability of alternative routes should be included as an additional evaluation criterion in economic feasibility studies. In this way, the importance of critical network elements can be recognised and taken into account. This is especially true for bridges. • The scheduled maintenance of bridge structures and the associated con­ struction sites have a negative impact on traffic flow. Capacity reductions due to roadworks require long-term, ­network-wide roadworks management. The aim of this roadworks management must be to coordinate construction ­activities in such a way that no roadworks are set up on alternative routes at the same time. • The space under a bridge requires more attention in bridge design. In cities, this space often has a poor quality of stay, is dirty and inhospitable. A utilisation concept for this space should be developed for each bridge.

Bridges and Traffic

49

-

Preservation and Evaluation of Bridges Refurbish or replace?

Bridges are structures with a long service life. Road and railway bridges are designed for a service life of about 100 years. How­ ever, if they are robustly constructed and properly maintained, bridges can be reliably used for more than 100 years. The mainte­ nance and continued use of older bridges depends on numerous factors, such as the condition of the structure, the loads ­acting on it and current and future traffic developments. In recent decades, the demands on bridges have increased significantly due to higher traffic volumes and increasing heavy goods traffic, while at the same time structural ­conditions have declined. However, most of the bridge structures in Germany dating from the 1960s to 1980s were not designed according to today’s principles and are not designed for the current traffic loads; this is referred to as conceptual ageing. Since the 1980s, heavy goods vehicle traffic in particular has increased considerably (fig. 1). In addition to age-related deteriora­ tion, the increased loads are having a major impact on the stability and durability of bridge structures [1]. This is not only a European problem. In many industrialised nations, the bridge infrastructure is in great need of repair. In the USA, for example, more than a third of

50

the bridges need to be rehabilitated or replaced [2]. In addition to investment in expansion and new construction, however, the preservation of existing bridges is also crucial for a safe, robust, functional and resilient transport infrastructure. In addition to the investment volume, the construction of a bridge also consumes material, energy, resources and CO2 that are stored in the structure. For economic, resource-saving, but also cultural reasons, bridges should only be replaced when they can no longer be maintained for an additional period of use, i.e. when they can no longer be refurbished or repaired, strengthened or upgraded to the current state of the art at a justifiable financial cost. The preservation of a bridge depends above all on whether it is still geometrically suitable and sufficiently load-bearing and whether the existing condition of the struc­ ture permits repair. The preservation of the current bridge stock is a highly responsible task for ­infrastructure operators worldwide and ­presents bridge engineers with great ­technical challenges. In addition to the assessment of the load-bearing capacity and evaluation of the structure’s sub­ stance, which require a high degree of

Vehicle type

8

1984

2005

9

33 35 41 97 98 Others 0

10

1  Change in the vehicle collective (reduction in two-axle and increase in multi-axle truck ­traffic with articulated lorries) in heavy goods traffic on motorways in Germany, in 1984 and 2005 in comparison. Vehicle types (selection) according to TLS2012 (BASt 2012) for heavy goods traffic

2  Railway bridge Hermann-Lönspark, Hanover (DE), 1906 a Condition in 2014 b After superstructure renewal and abutment repair in 2019

20

60 30 40 50 Frequencies of the vehicle types [%] 1

experience and sensitivity, reliable future ­scenarios for traffic development, loads and status forecasts have to be worked out. This makes it possible to determine remaining useful service lives, maintenance intervals and risk assessments. In addition to retrofitting measures, monitoring systems and, in the future, digital, predictive status monitoring will be used to extend useful ­service life. Heritage conservation and building culture Many bridges are important in terms of urban development and building culture, as they are part of the environment in which they are used. People use these structures to cross obstacles and have become accustomed to them. For this ­reason, many historic bridge structures are important and identity-defining testimonies and are listed as historical monuments. The listed status of a bridge generally results from the two factors of conservational capacity and conservational worthiness. The reasons for protection, which can be used to prove the monument status and which partly overlap in content, are gener­ ally historical, artistic, scientific and urban planning-related criteria [3]. In the event of changes to the infrastruc­ ture or if the structures are in such poor

­ ondition that repair or upgrading would c be very costly and technically difficult, the demand for demolition and replacement often arises. A conflict between two societal needs is inevitable: While the building authorities are responsible for the economic, unrestricted and safe functioning of the bridge structures, and for these reasons often prefer new structures, the state repre­ sentatives for heritage conservation, on the other hand, advocate the preservation of each individual object (fig. 2) [4]. Not all listed buildings can be preserved, as they often no longer meet today’s ­technical requirements. Nevertheless, the aim should be to leave the original charac­ ter of the historic building fabric intact as far as possible during conversions and to replace components that cannot be reused with new ones, integrating them into the existing structure. This is the only way to preserve important evidence of bridgebuilding culture, such as special technical constructions, particular types of construc­ tion or rare building materials, for future generations.

a

b

Preservation and Evaluation of Bridges

2

51

Technical maintenance circle

Structural inspection, monitoring Consumptive repairs

Strategic regulation

Strategy for building preservation (Replacement) new construction

Recalculation, damage analysis

• Strategic objectives of the traffic load carriers • Schedule and cost management • Life cycle-oriented design and maintenance

Technical and economic ­assessment

Repair, reinforcement

3

As a result of the bridge inspection, for example in Germany, the stability, durabil­ ity and traffic safety are classified as grades 1– 4. From this, damage analyses can be made and possible future deterio­ ration can be predicted. Based on the results, the infrastructure managers must develop maintenance strategies with the support of technical experts and, if necessary, plan and implement repair or reinforcement measures. Maintenance strategies vary widely across European countries. In general, the necessary maintenance measures in the life cycle of a bridge are defined as follows:

Average condition score

Conservation The extension of the service life through ­conservation measures is becoming increas­ ingly important compared to the expansion and renewal of the bridge infrastructure. The service life of a bridge is shaped by the tech­ nical maintenance cycle and the strategic regulations in bridge management (fig. 3). On behalf of the bridge operators, bridge inspectors regularly inspect the existing bridges as part of structural inspections and record their actual condition. Damage and conspicuous features are documented, cat­ egorised and divided into damage classes (fig. 4). Reinforced concrete

3  Cycle of maintenance and strategic regulations according to DIN EN 1992:2-2010

Prestressed concrete

Steel

Arched

Rolled girder in concrete

1

2

Condition scores: 1 No measure required 2 Maintenance measures are to be planned 3 Renewal measures are to be checked 4 Renewal measures are to be planned

3

4 0

52

10

20

30

40

50

60

70

80

90

100 110

120 130

140 150 160 170 Age of bridge [years] 4

4  Progressive deterio­ ration in the condition of railway bridges by construction type at Deutsche Bahn

5  Examples of different strengthening measures a Retrofitted inlying external prestressing, Siegtal Bridge, Siegen (DE), 1969, refurbishment 2018, König und Heunisch Planungsgesell­ schaft b Subsequently supplemented external prestressing on the Rachenbach Bridge L 39 (AT), 2007, ­Zimmermann, Kuss & Partner c Reinforcement with prestressed CFRP lamellas

• Maintenance • Repair • Retrofitting • Renewal

a

b

c

5

The bridge operator has to define the ­utilisation condition with fulfilment of load-­ bearing and utilisation safety in a structurespecific manner, and is responsible for the maintenance and upkeep of the bridge. All measures to restore the target condi­ tion are referred to as maintenance. They are necessary when the actual condition ­deviates from the target condition. Typical maintenance measures are, for example, the renewal of corrosion protection, road surfaces and sealing, as well as the repair or renewal of transition structures, bridge railings, drainage facilities, bearings or caps. For retrofitting measures, the regulations valid at the time of planning the retrofitting apply. The infrastructure manager may specify load rates for specific projects. Examples of retrofitting measures are the subsequent installation of external pre­ stressing, reinforcement by means of ­carbon fibre lamellas (CFRP lamellas) or steel lamellas (e.g. transverse force rein­ forcement) as well as statically effective cross-section and reinforcement additions (fig. 5) [5]. Evaluation The assessment of the maintenance condi­ tion of existing bridge structures differs ­fundamentally from the usual planning ­process in new construction. Here, engi­ neers are required to deal with the existing structures and assess them according to the current state of the art. The method­ ology, the procurement of information, and the determination of essentials are com­ pletely different from those in new construc­ tion and are much more crucial, since ­structures can only be analysed with secure basics. The existing geometry, construction,

Preservation and Evaluation of Bridges

53

and static cross-sections cannot be changed easily. Often, there are only ­insufficient as-built documents or struc­ tural analyses that were planned with com­ pletely different dimensioning regulations than today. Therefore, only engineers with in-depth knowledge and experience in ­dealing with existing bridges should carry out the assessment. If there is no structural damage or design deficiency that affects the load-bearing capacity of the structure, it can be assumed that the corresponding load-bearing sys­ tems have proven their worth. Conversely, structural damage and constructional defects can also provide conclusions about the load-bearing behaviour of a structural system [6]. In the case of damage, changes in condition, load increases or conversions and widenings, existing structures must be assessed with regard to their continued usability. In Germany, a step-by-step approach has proven to be practicable [7]. The essential steps in the assessment of a bridge structure are the inventory, the static recalculation, short-term measurements and, in the case of railway bridges, often static or dynamic load tests. Inventory (diagnostics) The basis for the assessment of bridges is the stocktaking survey. For this purpose, after a review of the as-completed plans and structural analyses – if available – an examination concept is developed to record the current condition of the struc­ ture. Depending on the structure, condition and task, different examination strategies are to be carried out. The most important steps and types of examination include: • Ascertaining the condition of the structure through visual inspection • Ascertaining the geometry by deformation measurements, laser scan etc. • Recording findings on damage, cracks, internal dimensions, reinforcements, degrees of corrosion

54

6  Different test methods on prestressed concrete bridges a Specimen removal from tendons (lowdestructive) b Result plot (left) of a radar measurement for reinforcement and crack location (non-destructive), with associated drill core (destructive) c Backstrain measurement (destructive) on tendons

a

b

c

6

• Ascertaining the building material proper­ ties by taking samples for material char­ acteristics and chemical investigations Non-destructive, low-destructive or destruc­ tive testing methods are used (fig. 6). The aim of the examinations is to determine the geometric and building ­material properties, and to localise deficits with regard to loadbearing capacity, s­ erviceability or durability. Based on this, recalculations and service life forecasts as well as conclusions on the possibility and worthiness of repair can be drawn.

a 7  Building diagnostics as a basis for digital inventory models a Test specimen for recording internal load-bearing structures on a prestressed concrete bridge, Werntal Bridge, Arnstein (DE), 1965 b Digitised 3D crosssection of the tendons and reinforcement layers of the specimen c Ultrasonic multichannel measuring equipment for nondestructive testing (NDT) of concrete

b

Building diagnostics is closely linked to inventory documentation and will thus become part of a digital BIM (Building Infor­ mation Modelling) inventory model in the future, in addition to the 3D recording of the building component cubature (fig. 7). Numerous research institutions are currently working on the development of correspond­ ing techniques and interfaces, from the ­digital stocktaking of structures and dam­ age to the digital twin. Recalculation The requirement and procedure for a recal­ culation of bridges is regulated differently in different countries. In Austria, the principle of trust applies, which assumes that existing bridges without structural defects or dam­ age affecting the load-bearing capacity are considered to be tested, and that a recalcu­ lation based on the current set of standards is generally not necessary [8]. If there are indications of statically relevant damage, structural interventions in the load-bearing structure, a significant increase in traffic loads due to heavy load transports, for example, or a change of use, a recalcula­ tion is necessary [9]. In Germany, the ­recalculation of bridges is regulated by the recalculation guideline (NRR) [10] for road bridges and the guideline 805 [11]

c

7

for railway bridges. The recalculation guide­ line opens up an extended scope of action for experienced planners and engineers through special regulations or specifica­ tions, and offers the possibility to make greater use of the reserves of the structure and the building materials without restricting the reliability level required by DIN EN 1990. It does not release them from an independ­ ent evaluation of the results, which remains a responsible engineering task. While new buildings are designed on the basis of assumed material properties, recal­ culation for existing bridges uses specified material properties or those determined by the structure. The recalculation takes place step by step in up to four stages. In the recalculation guideline, material properties are provided as calculation values. The recalculation is carried out on the basis of an inventory and according to predefined targets and scenarios. The results are to be evaluated by engineers with regard to loadbearing capacity, serviceability, fatigue and durability. If necessary, special inspection instructions, including the specification of detection areas for expected damage (e.g. cracks), must be prepared for the structure. Where applicable, immediate measures to ensure stability must also be ordered (e.g. measures restricting traffic).

Preservation and Evaluation of Bridges

55

structure reactions. With these experimen­ tally determined values, the t­heoretical models or calculation assumptions can be improved and the structural safety can be assessed more reliably. The improved findings can often open up load-bearing reserves that can be used to avoid costly reinforcements or even a replacement con­ struction [12]. For existing bridges, there are different tasks for load tests (fig. 8): • Level 3 of the recalculation guideline as system measurement and for model ­calibration • Determination of static or dynamic proper­ ties of the structure •  Brake load tests on long railway bridges • Dynamic load tests with different exciters (ambient action, falling weight, vibration exciter, train crossing) • Verification of sufficient structural safety against defined actions

a

b

c

8

Load tests When assessing existing structures, how­ ever, ignorance of the inner workings of the structures can lead to conservative models and assumptions; a recalculation is then often not very meaningful. For this reason, experimentally supported verifi­ cation has become established in some European countries for the evaluation of existing s­ tructures (level 3 of the recalcu­ lation guideline), which is carried out by means of selectively applied actions directly on the structure as well as by the metrologi­ cal ascertainment and evaluation of the

56

Monitoring – predictive maintenance – ­digital twin Monitoring is the systematic and continuous supervision of influencing variables or of building reactions by means of electronic measuring systems. Like in medical moni­ toring, buildings can also be monitored 24 hours a day, 7 days a week; in the event of malfunctions, appropriate notifications are sent according to defined process chains (fig. 10). For this purpose, critical conditions (warning and threshold values) are defined in advance. Depending on the task, there are very different requirements for the technology, the time period and the evaluation [13]. In recent years, structural monitoring has developed into a reliable and recognised method for measuring structural behaviour and its changes. Due to new sensor and evaluation technology as well as digitalisa­ tion, the importance of bridge monitoring and the potential for bridge evaluation has increased considerably in recent years. Typ­ ical applications for bridge monitoring are:

8  Different load tests a Load test with hydraulic presses to determine the substructure stiffnesses at the Itztal Bridge on the new ICE line Ebensfeld-Erfurt (DE) from 2004 b Load test with heavy truck-mounted ­concrete pumps to measure the ­suspension forces and stresses in the support structure, Köhlbrand Bridge, Hamburg (DE), from 1974 c Brake test on the Stöbnitztal railway bridge, new ICE line Erfurt-Leipzig / Halle (DE) from 2013.

Notes:   [1]  BASt Booklet B 68  [2] 2020 ARTBA Bridge Report (USA)  [3] 2020 Working guide, p. 14  [4] Marx 2020   [5] ÖNORM B 40082:2019-11-15  [6] ibid.   [7] Fingerloos / Marx / Schnell 2015   [8] as note 5  [9] ibid. [10] Federal Ministry of Transport, Building and Urban Development, Department of Road Construction 05/2011 [11] DB Netz AG, Guideline Ril 805, 2012 [12] DAfStb Guideline for load tests 2020 [13] DBV leaflet 08/2018

Actual building

Digital twin

Geometry + data

BIM

Condition

Monitoring + structural inspection

9 9  From the existing ­structure to the digital twin for the visualisation of actual structural condition data of the Köhlbrand Bridge in Hamburg (DE) from 1974

10  Applying sound emission monitoring at the Stennert Bridge in Hagen (DE) from 1959, which is at risk of stress corrosion cracking, it is proven that no prestressing steel ­fractures occur under continuous load and that further use is possible even without announcement behaviour.

• Increasing safety during construction work next to bridges in use • Confirmation of structure behaviour when new types of structures or components are introduced • Extension of the planned service life of a bridge • Increase of utilisation requirements (traffic load increases, speed increases on railway lines) • Measurement-based verification when calculated limit values are exceeded

s­ tructure and evaluated using artificial ­intelligence (AI) and condition indicators (fig. 9). The aim is to use pattern recognition to detect changes in the structure before damage is visible. The development, test phase and introduction of predictive main­ tenance in conjunction with machine learn­ ing will become a new future field of bridge maintenance.

Predictive, i. e. anticipatory, long-term ­monitoring, as already established in many other industrial sectors, enables a reduction in maintenance and repair costs and leads to an increase in operational safety and reliability. Research is currently being conducted on the development of a digital, predictive maintenance concept for bridges that provides a direct overview of the condition of the structure in different depths of data detail on the basis of Struc­ tural Health Monitoring (SHM) and the entire data integration in a digital building model according to the BIM method. The central data collection and evaluation via the sensors in the structure is expanded to include the processing of data from the vehicle ­sensors, the traffic recording, the climate data, etc. The consolidation of all data takes place in a digital building model according to the BIM method. All data is merged in a digital twin of the bridge

10

Preservation and Evaluation of Bridges

57

Impact Internal and external loads

Load ratio [gK /(gA+p)]

Structure’s impact on itself A structure impacts itself due to the dead weight of the construction, essentially due 100 80

Railway bridges Road bridges Pedestrian bridges

60

to the raw densities of the building materials used (e.g. steel, concrete, ­timber) and the material-specific properties occurring in the case of deformation restraints as well as due to the prestressing in prestressed concrete cross-sections. Dead weight and support loads Bridges are significantly loaded by their own weight. The longer the span of the ­support structure, the greater the ratio of dead weight to all other acting loads and the smaller the slenderness (fig. 1). The dead weight is determined by the gross weights of the materials and their ­volumes in the structure. Also Slenderness [λ = L/d]

In order to determine the structural dimen­ sions of a bridge, all foreseeable actions on the structure must be reliably estimated. These factors, which depend on the spe­ cific use, geographical location and envir­ onmental conditions, are determined ­individually on the basis of the regulations applicable in the respective countries. The loads to be applied are to be subjected to appropriate safety factors for the static-­ constructive dimensioning of the bridge structure.

35 30

Railway bridges Road bridges Pedestrian bridges

25 20 15

40

10 20

5

0

0 0 10 20 30 40

a

58

50 60 70 80 90 100 Span [m] b

0 10 20 30 40 50 60 70 80 90 100 Span [m] 1

1  Comparison of a single-­ span rectangular concrete beam carrying pedestrian, road and railway traffic. a Ratio of construction loads (gK) and the sum of support loads (gA) and live loads (p) as a function of the span L of bridges. Here, the required girder height d is approximately calculated assuming a reinforcement content of 0.8 % in the ultimate limit state (state II). b Slendernesses λ = L /d as a function of the span. For short spans (approx. up to 15 m), the influence of the individual loads to be applied to road bridges on the girder design is clearly evident.

the loads of the extensions necessary for the specific function of the bridge have to be taken into account. Constraint Depending on the material, forces occur from internal constraint due to creep, shrink­ age, swelling or hydration processes of the building material over the service life of the structure. However, these constraining stresses can be relieved by cracking – especially when reinforced concrete is used – and therefore usually play a subordinate role for the ultimate limit state of the loadbearing capacity. Prestressing Prestressing can positively influence the stress state in a structural system by intro­ ducing tensile or compressive forces. A possible crack formation as in reinforced concrete is thus prevented and the durabil­ ity is increased. A stiffening of the system also occurs, which in turn can be used, for example, in cable structures to avoid slack tension members. The prestressing remains inherent to the system and there­ fore also represents a self-impact inherent to the object. Loads from traffic The traffic loads, i.e. the loads to be trans­ ferred by the bridge user, are also of great importance for the calculation and design of the bridge. Due to the load move­ ment inherent in the traffic over the bridge, dynamic effects, braking and starting forces as well as centrifugal forces also occur.

2  Load densities for ­different traffic loads caused by cyclists and pedestrians on a bridge (representation without scale and not referring to 1 m2)

Cyclist 1 kN/m2

Pedestrian 1 kN/m2

Vertical and horizontal live loads In addition to the dead load, the vertical load components from traffic are decisive for the dimensioning of the structure. The applied loads should take into account future developments with sufficient cer­ tainty, but also allow for an economic design. While in the case of footbridges and cycle bridges the increasing tendency to decouple pedestrian and cycle traffic could enable a specifically graded load assumption and thus also a partial reduc­ tion of the traffic loads in the future, a ­further increase in the load to be taken into account is to be expected for the other forms of traffic. In addition to the expected increase in real road traffic (see “Requirements for future bridge ­structures”, p. 48f.), train lengths that exceed the train length limit of 740 m ­currently valid in Europe are being con­ sidered for rail-bound traffic. For e ­ xample, Europe-wide organisations are already ­discussing train lengths of up to 1,500 m with loads of up to 5,000 t and 25 t axle load, while in the Gulf region rail networks are being considered for goods trains up to 2,000 m long as container-­carrying wagons with double-deck loading and up to 32.4 t axle load. Foot and cycle path traffic For pedestrian and cycle bridges (fig. 2), surface loads of approx. 3.5 to 5 kN/m2 are to be applied in accordance with the respective national standards, which in some countries can be reduced as the

Pedestrian 1.5 kN/m2

Pedestrian 2 kN/m2

Impact

2

59

LM 1 Load model LM1

2≈ 240 kN

2≈ 160 kN

9.0 kN/m2

2.5 kN/m2

3.00

3.00 FS 1

FS 2

Remaining area

a LM M Load model LMM

2≈ 240 kN

2.5 kN/m2

2≈ 200 kN

12.0 kN/m2

2≈ 100 kN

6.0 kN/m2

3.00

emaining area

FS 1

3.00 FS 2

3.00

FS 3 Remaining area

b Permissible axle load [t]

3.0 kN/m2

3 25

Railways

Road traffic

20 15 10 5 0 1830

1870

1910

1950

1990 Year 4

bridge length increases. A surface load of 5 kN/m2 corresponds to six people of average weight crowding onto an area slightly less than 1 m2. For cycle-only bridges, more reasonable assumptions should be found in the future in view of the highest possible load density (fig. 2, p. 59). As a rule, however, a service vehicle for clearance or rescue purposes must also be taken into account for footpath and cycle path bridges, which is relevant for local design situations, especially due to the axle loads.

60

LM M 3  Road traffic2≈ 240 kN Fictitious load models For road traffic, the bridge is divided into 2≈ 200 kN for road bridges with 2 12.0 kN/mlanes idealised and a vertical2≈ load distribution on the 100assump­ kN 3.0 kN/m2 lanes (FS) according 2 tion is made for each6.0 lane. loads are kN/mThe to Eurocode 1 DIN applied as fictitious load models with indi­ EN 1991-2:2010-12 vidual loads (axle loads) and area loads, a Load model LM1 b Load model LMM, which represent the current and predicted which is applied 3.00 3.00 life. In 3.00 over car and truck traffic the service in Germany in a 1 FS 2approaches FS 3 Remaining the Eurocode,FS these load can area ­forward-looking way 4  be adapted to the corresponding regional Development of the traffic by multiplication with the nationally railway and road traffic selectable adjustment factor. For heavyload models duty routes, there are special load models that allow vehicles up to 360 t. Horizontal loads from braking and starting play only a minor role for road bridges (fig. 3 and fig. 4).

Rail-bound traffic The design of a railway bridge is carried out with a general load model LM 71 [1], which represents the load from railway ­traffic (fig. 5). The load model can be adapted to lighter or heavier traffic by means of a classification factor 0.75 ≤ 1.33. In addition to the vertical vehicle loads according to the standard and the lateral forces from lateral impact and centrifugal force, the enormously high longitudinal forces from braking and starting are of ­particular importance for the dimensioning and thus for the design in railway bridge construction. In the case of long doubletrack railway bridges, braking forces of over 10,000 kN must be absorbed by the support structure. In comparison, according to the Eurocode in road bridge construction, the horizontal forces are ­limited to a maximum of 900 kN and play a subordinate role in the structural design (fig. 6). qvk = 80 kN/m

Qvk = 250 kN 250 kN 250 kN 250 kN

0.8 m 1.6 m

1.6 m

5  Load model LM 71 for railway loads according to Eurocode 1 DIN EN 1991-2:2010-12

qvk = 80 kN/m

1.6 m 0.8 m 5

6  Braking and starting forces on railway and road bridges

12,000

Horizontal loads [kN]

— Railway bridges Load case start-up LM 71 — Railway bridges Load case braking LM 71 — Double track railway bridges Load case braking on two tracks — Road bridge Load case braking in lane 1

10,000 8,000 6,000 4,000 2,000 0

10

60

110

120 210 310 Influence length [m] 6

Dynamic loads Traffic loads always act dynamically on the bridge structure and are therefore taken into account with a dynamic coefficient for normal requirements. In the case of light pedestrian bridges, resonance effects due to pedestrian or wind-induced vibrations can also occur if the natural frequency of the structure and the acting excitation fre­ quency are close to each other, with low structural stiffnesses. Especially in the case of pedestrian-induced horizontal vibrations, this can lead to rocking: as on a ship rock­ ing at sea, pedestrians try to stabilise their gait by stepping sideways and thus gener­ ate additional horizontal impulses which, synchronised in this way, further amplify the deflection of the vibrating system. This lock-in effect was, for example, the cause of vibration problems on the Millennium Bridge in London, which was therefore closed just two days after its opening in 2000 and could only be reopened after the installation of appropriately designed vibration damp­ ers and absorbers. In general, every construction dampens the amplitude (max. deflection) of the ­vibrating system through friction (energy dissipation), depending on the material and the connections used. When designing a bridge ­structure classified as vibrationsensitive, it is recommended that any ­necessary dampers or absorbers are

already taken into account in the design and included as a fallback position in the ­structural design. In the course of commis­ sioning, the actual vibration behaviour can then be specifically examined on the built object. In most cases, it becomes apparent that the ­system damping already effectively suppresses rocking. Railway bridges are exposed to high cyclic loads during train crossings due to the r­ egularly arranged bogies, which can lead to resonance phenomena on bridges, ­especially when part of high-speed networks. For this reason, the dynamic influences from train crossings must generally be investi­ gated and dynamic calculations carried out for railway bridges. In contrast to softer con­ structions, stiffer superstructures react less critically in dynamic terms. For single-span girders, higher stiffness can be achieved by increasing the construction height but this usually leads to massive and cumber­ some structures. A more efficient increase in stiffness can be achieved, for example, by system modifications, restraints, haunches and, in the case of multi-span structures, by changing the span ratios in the early design stage. Atmospheric effects A bridge is exposed to wind and weather. The effects from the immediate atmosphere

Impact

61

s­ imulate all essential effects sufficiently ­realistically. In addition, transient, i.e. timedependent controlled simulations still require very large computing capacities. Thus, for the majority of investigations of wind effects on bridges, wind tunnel testing is currently still considered the cheapest and most ­reliable method. 7

can be manifold and must be taken into account according to the regional require­ ments. Loads from wind Influences from wind can be design-­ determining, especially for long and exposed bridges. Depending on the geo­ graphical location, height above ground or water level and the aerodynamic shape, the wind loads to be applied can vary widely. The design loads recorded in the standards are necessarily on the safe side and can nevertheless represent specific situations only with difficulty. Thus, for bridge struc­ tures that are particularly exposed and ­sensitive to wind-induced vibration effects, it makes sense to determine the wind load effect more precisely. In wind tunnel tests, the structure location in the terrain and the turbulence-generating environment can be represented realistic­ ally in the physical models, and wind inflows from different directions can be simu­ lated (fig. 7). Sensitivity to wind-induced vibrations can also be easily identified, and thus become a design-determining parameter, especially for light, long-span bridges. Since the turn of the millennium, computa­ tional fluid dynamics (CFD) has been increasingly tested as an alternative to wind tunnel tests with physical models and corresponding measurement technology. However, experts are still debating whether commercially available CFD programs

62

7  Bridge model in wind tunnel

Temperature Bridge structures are exposed to sea­ sonal weather conditions and thus also to changing temperatures. Depending on the applicable standard, temperature ranges of up to 80 K must be taken into account. Temperature-related deformations essen­ tially depend on the structure lengths to be considered, the temperature differences to be applied and the material-specific ­temperature expansion coefficient (aT) of the superstructure material (fig. 8). For steel, the coefficient of thermal expansion can be assumed to be a constant due to its homogeneous, isotropic material ­properties; for wood, the values parallel to the grain are significantly smaller than those perpendicular to the grain. Concrete, due to its capillary structure, has a coeffi­ cient of thermal expansion that depends both on the type, composition and volume fractions of the raw ­materials (true thermal

Δ T = 80 K Beam length/material

ΔL = αw · L · ΔT Thermal expansion 100 [mm]

100 [m] Steel

96 mm

αw = 12 ·10-6

Concrete

80 mm

αw = 10 ·10-6

Wood (spruce, fibre-parallel)

28 mm

αw = 3.5 ·10-6 8

8  Unhindered thermal expansion of a 100 m long single-span girder made of steel, concrete and wood at a tem­ perature difference ∆ T of 80 K ∆ T = Temperature ­difference αw =  Coefficient of thermal ­expansion L = Length ∆ L = Change in length

Related amplitude y/D [-]

1 Rain / wind-induced vibrations Galloping 0.5

Vortex shedding Vortex stimulation (von Karman) n=1

0

Spiral cable flutter Gusts

n=5

n=2 n=3

0

10  Determination of earth pressure for integral structures according to RE-ING of BMVI a Distribution of the normalised earth pressure b Abutment movements due to length changes of the superstructure

20

30 Wind speed [m/s] 9

expansion) and on the moisture content, temperature, and age (apparent thermal expansion) [2]. In addition to expansion and contraction of the entire structure due to temperature, different temperatures in the cross-section or between individual structural compo­ nents can have an influence on the design of the structure. Thus, temperature differences are a signifi­ cant factor in the design of the structures and equipment parts such as joints and bearings (see “Bearings”, p. 69). The move­ ment capacity of the equipment parts of the bridges depends on the length of the structure, the building material and the choice of fixed points of the structure. Normalised depth [z/h of UE of abutment]

9  Schematic overview of excitation mechanisms in cables

10

a

Passive earth pressure Mobilised earth pressure Earth pressure at rest 0

Wind or rain /wind-induced vibrations Wind causes numerous different effects, such as galloping (cable vibrations trans­ verse to the direction of flow, triggered by wind) or rain /wind-induced vibrations on support structures are relevant for the design of cable-stayed bridges in ­particular (fig. 9). Rain /wind-induced vibrations result from the complex interaction of the cable ­structure with wind and rainwater rivulets that form on the cable surface and can have a strong impact on the serviceability, stability and service life of bridges [3]. The rainwater rivulets change the flow cross-section of the cable in such a way that strong vibrations occur in the affected stay cables. The characteristic feature of these vibrations is that they only occur when wind and rain act on the cable at the same time [4]. Terrestrial effects The subsoil as the footing of a bridge ­foundation acts on the structure through ­lateral earth pressure, its settlement proper­ ties and regionally very violent impactlike movements (earthquakes), and has a significant influence on the bridge structure.

sh(c), sh(s) sh(∆TN, con)

0.25

sh(∆TN, exp)

sh/h = 0.001 0.5 sh/h = 0.004

Winter position

Sommer position

0.75 1 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Normalised earth pressure [Eh/γh] eph, mob (z)/(y ∙z) = Kph, mob (z) ∙ z/h over the wall height h for a relative head ­displacement of sh = 0.001 ∙ h and sh = 0.004 ∙ h UE of abutment = Upper edge of abutment

Horizontal head point displacement: sh Displacements from: Creep, shrinkage: sh(c), sh(s) Temperature: sh(∆TN, con), sh(∆TN, exp) b

Impact

10

63

Number of bridges [%]

Light damage Medium damage

Severe damage Collapse

100 80 60 40 20 0 until 1971

1972 to 1980 ab 1981 Standard generation 11

Earth pressure Both for the verification of the internal ­stability of abutments and foundations (component design as a function of the ­acting loads) and the external stability, in which the influences from the interaction between structure and ground are taken into account, the earth pressure approaches according to the corresponding standardi­ sation are relevant (fig. 10 a). The earth pressure load approaches strongly depend on the boundary conditions such as the soil properties, the wall friction angle, and the deformation capacity of the abutments. As a rule, abutments are designed with the increased active earth pressure (lowest pressure of the soil), in case of very stiff foundation conditions and low deformation capacities with the earth pressure at rest. The passive earth pressure (earth resist­ ance, maximum earth pressure) is only used for abutments in individual cases, as very large deformations are required for its activation, which often exceed the limits of the serviceability checks for abutments and bridge piers. In the case of integral bridges, the passive earth pressure is usually only applied proportionally as mobilised earth pressure, which acts especially in the upper soil layers when the structure expands (fig. 10, p. 63).

64

12

11  Influence of earthquake standard generation on the extent of damageon 233 bridges in the main damage area of the 1994 Northridge earthquake near Los Angeles (US). 12  Catastrophic failure of a large concrete bridge pier

Settling In general, ground settling in foundations in the support axes of bridge structures must be taken into account. This can occur as displacements and as rotations. In statically determinate systems, set­ tling does not lead to additional stresses in the support structure. In statically ­indeterminate systems, however, it causes constraining stresses that must be taken into account in the design of the structure. In the case of settling-sensitive bridge structures, bridge engineers work closely with geotechnical engineers and carry out the settling calculations step by step. First, the probable and possible ­settling is determined by varying the ­stiffness ­moduli (Es, m and Es, u). In a first step, the settling fractions for the selfweight of the abutments and the piers ­(permanent impact) are then to be ­calculated. In a ­second step, the settling components from the superstructure (also permanent impact) are calculated. In the next step, the settling from the actual live loads is determined. In noncohesive and mixed-grained soils, settling from self-weight and traffic is largely ­completed before the structure is put into operation.

Notes: [1] DIN EN 19912:2010-12 ­Eurocode 1 [2] Taferner 2009 [3] Poston 1998, p. 58– 61 [4]  Nahrath 2004 [5]  ASTRA 2005 [6] Varela / Saiidi 2017, pp. 1751–1774

nd bottom rough the e top steel d bars proSMA bars d couplers also conl pipe that hrough the nsfer shear ot welding hrough the ity of the of pockets

Hand-tight nuts are then fastened to the top of the SMA bars and their corresponding pocket cans in the upper column are filled with thin steel shims. These are made of disk-sized plates. They are used to fill the gap between the top ends of the SMA bars and the small cans that are embedded into the columns and to allow the bars to engage in compression. The upper part of the column is subsequently lowered onto the EPH and the protruding steel bars are inserted through the holes in the top steel plate. Hand-tight nuts are subsequently fastened to the threaded bars completing the assembly. The disassembly procedure consists of following the same steps in reverse order. It should be noted that all the nuts that were used in this study were Disc-Lock,

Joint 13

13  Effects from earthquakes Joint for use on bridge Especially in earthquake-prone regions, piers made of a superbridges are of overriding importance as an ded view from above, (b) exploded elastic view fromshape below, and (c) assembled column. memory alloy essential part of the infrastructure, as they

are absolutely necessary to save human lives after seismic events. Today’s standard measurements assume that bridges should withstand normal earthquakes largely with­ out damage. For more extreme earthquake magnitudes, the bridge must be designed accordingly so that it does not collapse in an emergency. Since the 1980s, changes in standards have led to significant progress and visible success in the structural design and dimensioning of bridges with regard to their earthquake resistance (fig. 11). Bridge design with regard to seismic safety has undergone significant changes in the past decades. A seismic design concept that corresponds to the current state of development includes the application of the ductile behaviour of the structure. The consideration of ductile behaviour takes into account local deformations in the struc­ ture with the aim of specifically dissipating energy occurring there (energy dissipation). These areas are designed statically and structurally to absorb large deformation changes, while the other areas are designed elastically for the internal forces that occur in the structure when the plastic areas

reach their overstrength (capacity). This ensures that columns do not fail prema­ turely, e.g. due to a brittle shear failure (fig. 12), before the plastic bending areas have had a chance to develop their full energy dissipation capacity during cyclical plastic deformations. Strong differences in the horizontal stiffnesses of the columns and irregular spans have an unfavourable effect on an earthquake-resistant bridge design. Viewed in plan, the horizontal iner­ tial forces acting at the centre of mass should be transferred as symmetrically as possible. Long, jointless, continuous girders are generally favourable, because every intermediate joint forms a weak point (potential girder collapse) [5]. The subject of current research projects and developments are designs and mater­ ials with which the ductility of the columns can be increased by the structural design or by specific material combinations, for example in the use of the rocking ­behaviour of segmental piers as a mechanism for energy dissipation and seismic ­isolation in multi-span bridges. Alternatively, elasticity can be provided by super-elastic shape memory alloys that minimise damage through a cement-like composite while keeping the rest of the column elastic (fig. 13) [6].

Impact

65

Function Routing and bridge equipment

The primary function of a bridge construction is the load-bearing crossing of an obstacle to connect two places (see ­“Designs”, p. 94ff.). For the overall func­ tioning of a bridge as a convenient and safe utility structure, an appropriate routing and complementary components with desired modes of action are necessary.

cycle and footpath network with direct ­connections and compliance with clear passage heights. On the other hand, road bridges and railway bridges in particular, are s­ ubject to special constraints in terms of position and height for reasons of driving safety and comfort, which set decisive ­limits to the design.

Routing Traffic should be able to flow unhindered over a bridge. To achieve this, it is import­ ant to choose the right routing. Slow ­pedestrian and bicycle traffic tolerates tight radii in the ground plan and in the elevation, and often calls for places with a view to ­linger (fig. 1). Thus, cycle and pedestrian bridges can be designed more freely in geometrical terms to fit into the existing

Surface The Romans provided for a surface of loosely laid wooden planks on some bridges in order to make bridges impass­ able in case of danger of war or epidemics, and thus to be able to interrupt main traffic routes [1]. Today, a surface covering as a non-slip wearing course on the support structure, which enables unobstructed and safe movement on the bridge, is a

1

66

1  Passerelle Simone-deBeauvoir, Paris (FR), 2006, Dietmar Feichtinger Architectes

standard feature. Surfaces, including underlying waterproofing, primarily serve to provide upper protection of the support structure against the effects of weather and against the consequences of winter salt spreading. Footpath, cycle path, road Pedestrian bridges with reinforced concrete structures usually receive a bituminous seal with one or two asphalt layers. In the case of steel superstructures, an epoxy resinbonded thin layer is often used, which also serves as a waterproofing layer and ensures slip resistance by means of interspersed quartz sand. However, wood, glass, brick and natural stone as surface materials also offer a wide range of design options. For road bridges, a two or three-layer asphalt surface structure on a bituminous sealing layer has become established (fig. 8, p. 71). Efforts to dispense with the pavement and the waterproofing for cost reasons and to drive directly over the raw support structure were made early on. However, the assumption that prestressing adequately and reliably protects a concrete structure was not confirmed by early damage to some bridges from the ­initial period of prestressed concrete. ­Illinois, for example, relies on epoxy resincoated ­reinforcement in the deck as cor­ rosion ­protection. Accordingly, this construction method saves on a surface and a pro­tective seal (fig. 9 d, p. 71). In Europe,

2  Slab track with compensation plate (front) and rail extension (rear) on the Saale-Elster ­Valley Bridge, new ICE line Erfurt-Leipzig / Halle (DE), 2015

2

experiments are currently being carried out with layers of high-tech materials that would also make surfacing and sealing super­ fluous. Ultra high performance concrete with steel or carbon fibres prevents the penetration of moisture through high material density and thus saves weight, resources, as well as work steps during production (see project example “Stuttgart Wooden Bridge”, p. 104ff.). Rail The track made of rails with sleepers is the most important element of railway infrastructure due to its load-bearing and guiding function. Originally mounted on the bridge as an open track only on longitudinal girders for weight reasons, the ballast bed prevailed as superstructure construction in modern bridge construction and later, in highspeed lines, the so-called slab track (fig. 2; see also project example “Scherkondetal Bridge”, p. 142ff.). Transition zone to the open track Bridges expand and contract primarily due to the effects of temperature and due to material-specific properties such as creep, shrinkage and swelling. Originally, bridges were separated into single-span and c ­ ontinuous girders, which were easy to control in terms of calculation, by means of targeted expansion and bearing joints, largely free of constraints, and provided with bearing and transition structures. Today, movement gaps with special transition structures are state of the art for long bridges (fig. 3, p. 68). These are usually not maintenance-free and can cause cor­ respondingly high maintenance costs in case of heavy loads. Often profile constructions are used here, which elastically bridge the transverse gaps by means of dense rubber profiles (fig. 5 a, p. 69). In the case of larger expansion paths, additional steel girders ensure a distribution of the movement, which open

Function

67

o

o

o

Asphalt v

Asphalt construction v construction construction v v Asphalt

Bearing

Bearing Bearing

v Transition construction

v v

Bearing

v

v Asphalt

v

v

v

ε

v Asphalt vε a

ε

Lo

Asphalt

Transition construction

v

Asphalt Lo Lo

Lo

AsphaltL o v

Asphalt Asphalt v v

Lo

3 Asphalt Asphalt v v

Lo

v

v

ε

Asphalt ε vε

Lo

v

b

Anchor Anchor blockblock Anchor block v ε

Lo

Asphalt L o v

Lo

Lo

Prefabricated Prefabricated Prefabricated parts partsparts Fibreglass rods Fibreglass Fibreglass rods rods Anchor Anchor blockblock Anchor block

Perforated drag slab Perforated slab Perforated dragdrag slab

Sand bed

22 steel girders at the Queensferry Crossing v near Edinburgh allow an expansion path of 2,270 mm (fig. 5 b, see also project example ε p. 120ff.). For noise protection reasons, elastic pavement expansion joints and ­finger constructions are used. In the latter, two horizontal combs interlock and require a drainage channel due to the lack of impermeability. In railway bridge construction, closed elastomeric waterstops, open channels, abrasive plate constructions and mat transitions are used. However, due to the high maintenance costs and lack of robustness of individual products – also due to increasingly heavy traffic – many infrastructure operators today tend towards jointless or integral construction methods. Here, defined movement joints are dispensed with, as the structure can absorb the constraints due to tempera-

68

Drag slab

Drag slab

and close evenly by means of product-­ Lowered Sand bed specific control elements. As an example, drag slab v

o

Transition Transition Transition construction construction construction

b

Lowered LoweredSandSand bed Lowered bed drag slab dragdrag slab slab v

v

Drag slab

a Asphalt Asphalt v

v

v

o

o

Asphalt

v

DragDrag slab slab

v

v

Asphalt v Asphalt

Drag slab

DragDrag slab slab

Lo

v

v

v

Asphalt ε vε

ε

v

c Perforated drag slab material-specific effects. ture and building

The “jolt-free” driving over the bridge Anchor block ends also increases driving comfort. In v ­Switzerland and Austria, the lowered drag slab has proven itself up to bridge lengths ε of approx. 60 m (fig. 4 a). In this case, the movement v from the drag slab end to the asphalt is distributed accordingly over a length L0, so that the asphalt with its ­material-technical stretching capacity can absorb the movements (fig. 4). For longer bridges up to 200 m and more, there are currently several developments that are also based on the principle of activating the asphalt’s elongation capacity but with longer elongation lengths L0. Constructions with perforated drag slab in continu­ ation of the lowered drag slab (fig. 4 b) or of precast concrete elements, laid close to the surface one behind the other and ­connected with glass fibre rods (fig. 4 c and fig. 6), pull apart and together in an

Lo

4 Prefabricated parts Fibreglass rods 3  Anchor block Classical roadway ­transition construction (a) and roadway ­transition construction laid behind the drag slab (b) 4  Different variants of deformable roadway crossovers

5  Profile construction a Principle b Application with 22 steel girders and bolted-on noise protection elements, Queensferry Crossing, Edinburgh (GB), 2017, Jacobs Arup Joint Venture, Leonhardt, Andrä und Partner, Rambøll Group, Rambøll UK, Sweco UK

6  Deformable deck crossing at the Satzengraben Bridge on the Austrian A5 (AT), 2017, FCP ZT and TU Wien

a

b

5

accordion-like manner and ensure an even distribution of expansion in the asphalt. In the rail network, bridges represent an element of discontinuity, especially with regard to the dynamic properties of the track. With the introduction of the gapless track in the first half of the 20th century, the rail joints were eliminated so that a continuous and uninterrupted rail strip could be produced. On bridges in particular, this significantly improved the driving dynamics, while at the same time reducing the stress on the structure. However, the structural design of the superstructure creates a connection between the rail and the structure, which leads to additional forces in the rail, especially at the bridge joints, when the structure moves longitudinally. In order to ensure the serviceability and operational safety of the track, the effects of the additional stresses must be limited. Strain lengths of up to 90 m for solid and composite bridges and 60 m for steel bridges usually do not require any special measures. For larger

spans not examined in detail, s­ o-called rail extensions are to be arranged above the structure joints. In this construction, the rails are separated by an oblique cut into a stock rail and an internally abutting tongue rail (fig. 2, p. 67). The rails are positively connected to each other in the overlap area by support jaws. Since these components have proven to be very costly to install and maintain, long railway bridges are usually constructed without rail extensions or these are concentrated at only a few points. With a slab track, the deformations at the ­construction joints for the rail support points must be limited. This necessitates the installation of additional small bridges, ­so-called compensation plates (fig. 7, p. 70 and fig. 2, p. 67). Bearings Bearing structures connect the super­ structure to the substructure for concentrated force transmission with simultaneous

6

Function

69

Compensating slab Compensating slab Abutment

ϕ Superstructure

Abutment

Superstructure

a

Compensating slab δv

Compensating slab

δv Abutment Abutment

b

ϕ

Superstructure Superstructure

7

displacement and twisting possibilities. In this way, constraining stresses in the ­support structure can be reduced. In the past, simple steel rollers provided the corresponding movement possibilities, from which one could visually also clearly see how they functioned. Today, bearings in the form of elastomeric cushions for small and medium spans as well as cup and spherical bearings for large spans are in use. Due to the high pressures that occur, bearing constructions are considered to be susceptible to wear and maintenanceintensive. For this reason, so-called semiintegral and integral load-bearing systems are preferred today (see “Beams / Frames”, p. 94). Edge zone The edge zone with the lateral boundaries and accompanying noise barriers, railings etc. offers enormous design possibilities. However, appropriate planning and detailing of these components without decorative overload require a lot of experience and a comprehensive understanding of their individual functions (fig. 8). Bridge edge The bridge edge as a roadway-limiting ­component is one of the most controversially

70

discussed construction elements of a bridge and is subject to a great variety of design options and country-specific naming as cap, edge beam or bracket head (fig. 9). The original function of the edge construction is a lateral elevation called a kerb, which serves as a safety barrier for pedestrian traffic to rolling traffic and was already common in Roman road construction, such as in the Ponte Sant’Angelo in Rome [2]. The kerb collects ­surface water laterally and drains it off in ­concentrated form via bridge drains. The cap as edging of the ­laterally lowered s­ eal not only offers space for the attachment of the fall protection, a noise barrier and, in the case of road bridges, of the vehicle restraint system but also enables, through the cornice, an optical correction of unplanned deformations, which can occur especially in concrete bridges after stripping the formwork. The caps are anchored to the support structure laterally or from above. In the process, the fastening elements usually penetrate the seal and represent potential risk areas for leaks. There are developments in Sweden that ­dispense with a cap and provide the roadway closure with an edge plate (fig. 9 b). Here, as with the standard construction of the bracket head in Switzerland (fig. 9 e), it is necessary to carefully raise the waterproofing to prevent water from penetrating. Caps made of concrete are usually realised jointless or with defined movement joints in cast-in-place concrete or in pre­ fabricated construction. In some countries, it is also common for railway bridges to have precast concrete or in-situ concrete caps with integrated recesses for a cable duct and lateral ballast bed boundaries. For particularly high s­ uperstructures, a ­cornice extended ­downwards can optimise the proportions between the edge cap and the receding superstructure (fig. 8 b).

7  Compensating slab as a bridge on a bridge (according to Ril 804.5202) a Compensation of an end tangent angle ϕ b Compensation of an end tangent angle δv

Noise barrier Noise barrier Noise barrier Noise barrier Guardrail Splash guard Noise barrier Guardrail Guardrail Splash guard Splash guard Concrete base Guardrail Splash guard Ballast bed Guardrail Splash guard Cap Restraint Concrete system base Concrete base Ballast bedBallast bed Threshold Concrete base Ballast bed Concrete base Cap Restraint Cap system Restraint system Threshold Threshold Protective Cap Restraint system Ballast bed Threshold Cap Restraint system Threshold Kerb Cable trough Guardrail concreteGuardrail Protective Protective Cap Protective Guardrail Kerb Kerb Kerb Cap Cable trough Cable trough Protective concreteCap concrete Cable Seal trough concrete Seal Cap Kerb Cable trough Sealconcrete Seal Noise barrier Noise barrier Noise barrier Cap Seal Filling Filling Seal Seal Seal Seal Roadway Filling Seal Cornice Seal Seal Guardrail Splash guard Guardrail Splash Guardrail guard Splash guard Roadway Roadway Roadway Cornice Cornice Cornice Seal Roadway Concrete Concrete baseBallast base bed BallastBallast Concrete base Cornice bed bed Cap system Cap Restraint Cap Restraint systemsystem Restraint Threshold Threshold Threshold Cornice Guardrail Guardrail Guardrail Protective Protective ProtectiveCornice Cornice Cornice Drainage Kerb Kerb Kerb Cable Cable trough trough concrete CableCap trough Cornice concrete concrete Cap Cap Drainage Drainage Drainage Seal Seal Seal Filling Filling Filling Drainage Seal Seal Seal Seal Seal Seal Roadway Roadway Roadway Cornice Cornice Cornice Cornice Drainage

Cornice Cornice

Drainage Drainage a

b

Precast edge beam with integrated cable trough in Austria a

c

Termination with edge plate in Sweden b

8  Different designs of edge zones for a a road bridge b railway bridge c pedestrian bridge 9  Internationally different designs of the bridge edge zone

8

Control cap in Germany c

Bracket head with guide wall in Switzerland

Surface-free design in Illinois /USA d

e

9

Protective measures The protection of bridge users includes above all the guardrail as fall protection, which, depending on national regulations for pedestrians, is usually between 1.00 and 1.30 m high and must have standardcompliant filling with gaps ≤ 12.0 cm for public access. In addition, a solid splash guard is often required over traffic routes to prevent snow from falling during snow clearance. Bridges over railway lines require appropriate protection against c ­ ontact with the live overhead contact systems. With road traffic, vehicle restraint systems made of concrete crash barriers or steel

guard rails pursue two protective goals: on the one hand, a vehicle crash from the bridge is to be prevented by the restraint system deforming on impact and acting as a tension band – similar to a safety net. On the other hand, this system compliance ­protects the vehicle occupants from injuries in case of too great an impact. These requirements are regulated in Europe in the EN 1317 series of standards. The designs themselves vary from product to product. Testing is carried out in special large-scale impact tests with real vehicles, in which the effective range, i.e. the maximum geometric deflection of the system, as well as the

Function

71

Guardrail

Filling Sea

a

b

10

forces to be introduced into the bridge are determined. Noise protection is an increasingly important component of a bridge today and shapes its overall appearance. Depending on its height and the type of infill, a noise barrier has a decisive influence on the overall proportion and thus the ­external appearance of a bridge. Fig. 10 shows the different effects and proportion dependencies of a noise ­barrier. Opaque noise barrier panels, in contrast to transparent elements, make the bridge appear heavier, especially for structures of low slenderness. On the other hand, a noise barrier can also be integrated very effectively into the structure without significantly changing the overall appearance (fig. 11). Drainage A successful bridge design must include considerations for planned drainage from the very beginning. Purposeful drainage of surface water is one of the most import­ ant criteria for the durable functioning of a bridge. Bridge drainage prevents uncontrolled water penetration into the structure, which leads to frost and corrosion damage and can cause aquaplaning, especially on road bridges. The basic prerequisite for this is a sufficient transverse

72

11

and longitudinal slope of the pavement and road surface, sufficient bridge drains, and appropriate drainage of the waterproofing level through drip grommets. In addition, access points must be provided to allow cleaning and maintenance as well as partial or ­complete replacement of drainage pipes. Directly in the shadow area under the ­support structure, freely suspended ­collector pipes connected to the drainage pots via short component penetrations have proven to be a safe, durable and ­visually unobtrusive solution. Successful special solutions, as in the case of the Scherkondetal Bridge, are the hallmark of a good design (fig. 12).

12

10  Different effects of the noise barrier according to FSV, RVS 15.04.81 a slender superstructure without /with transparent /with transparent and ­concrete base board/with opaque noise barrier b as in “a” with squat superstructure 11  Integration of the noise barrier into the support structure, Stephanitor Bridge, Bremen (DE), 2006, schlaich bergermann partner

12  Centrally laid longitu­ dinal drainage with special cross-section design as a successful custom solution, Scherkondetal Bridge, new ICE line ErfurtLeipzig / Halle (DE), 2015

13  Integrated lighting in the restraint system of a motorway bridge

13

Masts and sign gantries Equipment such as lighting and catenary masts as well as support systems for signs and signals result from the route-related planning and require early coordination in the design and planning process. Vertical elements are often in visual conflict with the support structure and require special solutions that correspond to the overall design but also to the electro-mechanical requirements. In the case of pedestrian bridges, lighting is often placed in the ­handrail but this also reduces the sense of safety due to the lack of facial recognition. New possibilities arise with lighting systems that are integrated into the restraint system (fig. 13).

Notes: [1]  Merckel 1899 [2] ibid. [3] FSV: RVS 15.04.81, 2017 [4]  Eichwalder 2017

the overall appearance of a bridge, e.g. by providing parking and storage facilities in abutment and structure niches or by exploiting inspection spaces in hollow boxes. Alternative inspection variants, such as maintenance by industrial climbers or mobile inspection equipment, are also conceivable.

Inspection facilities Despite increasingly sophisticated digital sensor and drone technology, facilities for maintenance and inspection are still indispensable in order to be able to visually inspect a bridge at any time from close range and by hand, called “within touching distance” in technical jargon. This requires ladders, maintenance walkways and stationary or mobile maintenance platforms that allow easy and quick access to the main structural elements of a bridge. It is common to integrate these elements into

Function

73

Economic Efficiency Using financial resources responsibly

The question of costs has always been of importance when selecting a bridge type, since in modern times, as a structure of public interest, it is usually financed by public money. It is an unwritten law throughout history that the financial resources for infrastructure measures have always been limited, regardless of the form of government and therefore had to be used sparingly and appropriately. Even the Roman architect and engineer Vitruvius, in addition to his main requirements for architecture, namely firmitas, utilitas and venustas (see “Technical and design challenges”, p. 12), demanded that the architect should precisely allot and estimate the costs [1]. In

the 19th ­century, the German civil engineer, urban planner and university lecturer Reinhard Baumeister described the “money men” as important decision makers who influenced bridge building alongside the “utilitarians” as representatives of the exclusively utilitarian and the “art aristocrats” as guardians of the beautiful [2]. Not least because of their responsibility towards the public, many engineers such as Fritz Leonhardt, Pier Luigi Nervi or Eduardo Torroja, like Vitruvius and Reinhard Baumeister, combined attractive and functional construction with economic requirements [3]. ­Particularly in bridge construction and civil engineering, there is no way around

1

74

1  Simple elegance as an expression of efficiency and economy. One of the motorway bridges of the A11-Via Brugge infrastructure project, Bruges (BE), 2017, schlaich bergermann partner

Production costs [per m2]

Tensile construction

Haunched cantilever construction Beam bridge

0

100

2  Construction costs as a function of span according to Alfred Pauser

200

300

400

500 Span [m] 2

a comprehensible calculation of economic efficiency today, which certifies a moderate use of financial resources not only in the construction phase but also beyond. Considerations of economic efficiency must not be confused with cheap construction as a short-term effect, which characterised the so-called construction industry functionalism [4] of the post-war era with the aim of quick and cheap construction according to the formula “length times width times height times money” [5] and promoted faceless bridge construction (see “Motorway bridges”, p. 28). Bridges need quality according to Vitruvius’ requirements and quality demands its real price, especially if a positive image and sustainable use are to be taken into account. The question now arises as to how much the good design of a bridge should be worth to society? With the primary goal of con­ necting places (see “Designing Bridges”, p. 6ff.), a basic level of aesthetic quality is sufficient, which, if the formal logic is based on a unity of structure and design, entails no or only minor additional costs compared to the usual standard ­values. This applies

in particular to road and railway bridges, whose purpose-­determined connecting character is in the foreground (fig. 1). Christian Menn recommends an additional cost for the f­ine-tuning of the design of approx. 5 % for large bridges [6] and for smaller bridges even more; i.e. percentages that are in any case within the range of bid fluctuations when awarding the construction work. Especially in the case of railway bridges, additional costs for structural stages and auxiliary facilities to maintain railway traffic are possible, which often increase the construction costs by a factor of three to ten. Additional costs for a minimum design requirement are practically irrelevant here. In contrast to the additional operational costs, which are unquestionably accepted (by the company) for the shortterm nature of the construction work under railway operation, the added design value remains visible over the entire service life of the bridge as a tangible structure or architectural cultural asset. Guidelines For the construction phase, the costs per bridge area unit (bridge benchmark) are the most important economic benchmark. In the case of road and railway bridges, there is always a dependency on the span width if standard designs and construction methods are assumed. The bridge builder Alfred Pauser has attempted to illustrate the qualitative dependencies of the construction costs on the span width according to the type of construction in a graph (fig. 2). The larger the span, the higher the costs per m2 of bridge surface, and the more economical are haunched or dissolved constructions. This shows that the choice of an incorrect load-bearing system can lead to a failure to achieve the economic goal even in the design phase. However, fig. 2 shows, as do cost analyses of 2,000 road bridges and 150 railway

Economic Efficiency

75

Life cycle considerations In order to take sustainability effects into account, the tendency of economic efficiency calculations for public developers is to consider the entire life cycle from ­construction to the operating phase to the demolition of the bridge (see “Sustainability”, p. 78ff.), not least because the wave of rehabilitation and renewal of the rapid post-war bridge construction up to the 1980s has now caught up with infrastructure operators and put an extreme strain on ­public budgets. To be fair, however, it must be noted that durability issues and the technical knowledge of chloride effects from de-icing salts or seawater /air, creep

76

Construction costs [1,000Construction €/m2] costs [1,000 €/m2]

15

10 20 5 15 0 10

2—30

Cost basis: approx. 2,000 bridge structures since 2005 from the database of the BMVI (Federal Ministry of Transport and Digital Infrastructure), costs normalised incl. VAT, usually pure con30—100 > 100 struction costs of the structure Range of support width [m] without related measures

5

0

2—30

30—100

a Construction costs [1,000Construction €/m2] costs [1,000 €/m2]

bridges in Germany (fig. 3), a tendency for the costs per square metre to increase in the lower span ranges, as this is where the construction site set-up costs predominate and many bridges built during ongoing operation are included. For pedestrian bridges, on the other hand, a correlation between price per square metre and span length cannot be clearly established, as the latter depends on many factors such as the functional properties, the chosen construction material, as well as the integration into the site and the attraction character. The price ranges from 1,000 €/m2 for an ostensibly purposebuilt pedestrian bridge made of reinforced concrete with standard railings to the Gateshead Millennium Bridge (fig. 14, p. 19) at 45,600 €/m2 [7], which is a tilt bridge with a lifting mechanism and high-quality building materials such as stainless steel and aluminium decking. The example of the bridge over the Hoofdvaart Canal (fig. 4) shows that designated landmark projects that shape places and promise added value via a so-called indirect profitability in public perception may justify high construction costs. This is essentially also true for the project example “Footbridge at Tintagel Castle” (see p. 114ff.).

20

50

> 100 Range of support width [m]

Cost basis: approx. 150 bridge structures since 2000, costs normalised incl. VAT, usually specification of the total budget (construction and planning costs) incl. all related measures, mostly renewal measures in the existing network under operation

40 30 20 50 40 10 0 30

b 20

2—30

30—100

> 100 Range of support width [m] 3

3  and 10shrinkage of concrete and degra­d­ Cost evaluations of ation of building materials were not bridges in Germany, ­sufficiently known and researched at shown using extreme, 0 quartile the time. 2—30 30—100 > 100 and median values. Range of support width [m] A significant advance in current economic a Road bridges efficiency calculations is the separation b Railway bridges of construction costs and costs in the operation or utilisation phase. Whereas in the past, for the sake of simplicity, the maintenance of a bridge was often calculated as a percentage of the construction costs, something which stood in the way of innovative bridge construction, an economic solution for higher-quality construction can now be found using the life cycle approach. This makes it possible to use resistant building

4  Bridge over the ­Hoofdvaart Canal, Hoofddorp (NL), 2004, Santiago Calatrava

4

5 Scheme of the life cycle cost basic model for road bridges according to RVS 13.05.11 (LCC = life cycle costs)

Notes: [1] Vitruvius / Fensterbusch 1991, p. 43 [2] Baumeister 1866, p. 5 [3] Kleiser 2017, p. 19 [4] Klotz 1977, p. 4 [5] Pahl 1999, p. 105 [6] Menn 2015, p. 20 [7] Keil 2012, p. 81 [8] FSV: RVS 13.05.11 2017 [9] Highways England et al.: CD 355, 2019

Accumulated, area-related life cycle costs [€/m2]

mate­rials and construction methods, such as ultra high performance concrete (see “Resisting”, p. 29f.). Fig. 5 shows an example of the basic model for road bridges valid in Austria, RVS 13.05.11, in which the ­construction costs are added to the annual maintenance costs (represented by the increase), the cyclically required repair and upgrading measures, and finally the costs for demolition, while adding the inter-

est. [8] Similar life cycle costing methods, which also take into account costs from congestion, among other things, also exist in other countries. [9] An intersecting of this calculation method with relevant ­environmental and disposal costs is ­advisable and will certainly form a basis for variant decisions in the future.

4,000 3,500 3,000 2,500 Annual operational maintenance and appraisal costs

2,000 1,500 1,000 500 0 0

10

Year 0 Construction LCC = LCCC

+

20

Year 20 1st intervention

30

40

50

Year 40 2nd intervention

60

70

Year 60 3rd intervention

80

100 90 Service life [years]

Year 80 Year 100 4th intervention Demolition + LCCD

LCCO (t)

LCCC = Life cycle cost of construction, LCCO = Life cycle cost of operation, LCCD = Life cycle cost of demolition 5

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77

Sustainability Thinking about tomorrow today

“In essence, sustainable development is a process of change in which the exploitation of resources, the direction of investments, the orientation of technological development and institutional change are all in h ­ armony and enhance both current and future potential to meet human needs and aspirations.” [1] This concept of sustainable development, first defined by the Brundtland Commission in 1987 in its report “Our Common Future”, establishes our current under­standing of sustainability. The “Green Deal” proposed by the European Commission in 2019 aims to reduce net greenhouse gas emissions in the European Union to zero by 2050. This would make Europe the first climate-neutral continent. The societal debate today encompasses all areas in which man-made changes affect the ecosystem – and thus also construction.

In Europe, construction is responsible for more than 30 % of carbon dioxide emissions, for more than 40 % of primary energy consumption and for more than 50 % of the consumption of natural resources [2]. In this context, it is easy to understand that a structure like the Ponte Sant’Angelo in Rome, which was built from mineral material

1

78

and using only renewable energy almost 2,000 years ago and is still in use, can be described as sustainable (fig. 1). In addition to the economic and ecological criteria, the socio-functional aspects are obviously also fulfilled here. The planning and construction of bridges in today’s world requires a differentiated examination of all influencing factors in order to derive assessable sustainability ­criteria. Assessment methods Methods for assessing the sustainability of buildings have been developed by various organisations at national level since the early 2000s. For example, buildings are classified according to clearly defined ­criteria by various certification systems. However, the approaches of these systems cannot be transferred so easily to bridge structures, as the balancing is significantly different, in particular due to the inclusion of building-specific influencing variables over the entire useful life of the building. For example, the energy efficiency of buildings is decisive, whereas for bridges the type of construction and the choice of materials have a decisive impact on the durability, possible retrofitting potential and mainten­ ance costs. The embedding of the object

1 Ponte Sant‘Angelo (Pons Aelius), Rome (IT), ca. 134 AD

Global Warming Potential (GWP) The global warming potential includes all climate-impacting emissions that occur within the entire life cycle of a product. These emissions (carbon dioxide, methane, fluorocarbons) are referred to as greenhouse gases because their accumulation in the atmosphere leads to warming of the ­layers of air near the ground (greenhouse effect). The greenhouse potential of a substance is converted to the weight of the lead substance CO2 and given in kg CO2 eq. /reference size of the building substance.

Primary energy demand (PE) Primary energy demand refers to the amount of energy resources required to manufacture a product, including all processes of raw material extraction and processing. A distinction is made between renewable (PER) and non-renewable primary energy (PENR). The information is given in MJ/reference value of the building substance and is calculated from the lower calorific value of the energycontaining resources used. Acidification Potential (AP) The acidification potential of soil and water indicates the impact of acidifying emissions such as sulphur and nitrogen compounds. These are produced during the production and processing of materials, especially during combustion processes. The pollutants react with water in the air to form sulphuric or nitric acid, which then enters the soil and water as “acid rain”. For each acidforming substance, the potential is given relative to the acid-forming potential of sulphur dioxide in kg SO2 eq./ reference value of the building fabric.

in infrastructure networks also plays a much greater role. Thus, ecological, economic and social aspects can be of significant importance in a sustainability assessment due to construction or maintenance-related traffic impairment (see “Bridges and Traffic”, p. 44ff.). At the national level, based on the assessment systems for buildings, increased efforts are now being made to develop ­specific assessment systems with suitable criteria for the sustainability quality of infrastructural structures and to apply them in the early planning phases. Evaluation criteria In general, the technical quality of bridges is in the foreground: the goal is a bridge structure that is as robust and low-maintenance as possible, and can be dismantled in a recycling-friendly manner. Sociocultural and functional aspects such as user comfort and convertibility are equally important as economic and ecological qualities. Economic aspects can be precisely analysed using investment and life cycle cost approaches (see “Life cycle consider­ ations”, p. 76f.). Direct life cycle costs can be up to twice the investment costs [3]. Reliable data for life cycle costs can be obtained from the evaluation of bridges in operation. The assumed theoretical service life and the approach to maintenance costs are of decisive importance. A lack of basic data (number and service life of the objects) naturally makes the evaluation of new construction methods or materials more difficult. This can then lead to a delayed adoption of construction methods specifically developed for durability and sustainability, as is still the case in modern timber bridge construction in Germany, for example. In the ecological assessment, all effects of a bridge on the ecosystem are considered.

Structures consume resources in all life cycle phases, from production to use to end of service life. The resources required may include energy resources, non-renewable abiotic resources or water resources. In addition, emission-related environmental impact is considered, such as that caused by the climate-damaging greenhouse gas CO2. In addition, land use and the recyclability of building materials at the end of their service life are included in the assessment. Furthermore, the biocompatibility of the overall structure is considered, e.g. the emission behaviour of the building materials used. The quantitative evaluation of the environmental impacts is provided by the life cycle assessment (LCA). Principles and rules for carrying out life cycle assessments are laid down in national and international standards. In addition to the energy input for the use of a building, the life cycle assessment evaluates the materials used for construction and their subsequent disposal. Example of “cradle-to-gate” LCA Figs. 4 – 6 (p. 81) show an example of an environmental assessment based on three selected environmental indicators for three representatively selected bridge superstructures constructed of reinforced concrete, steel and wood (fig. 3, p. 81). Here, only the manufacture of the building products (“cradle to gate”) is accounted for, which consists of the provision of raw materials, transport and manufacture of the products. The consideration of further life cycle phases could well change the order of assessment of the environmental parameters listed here. However, the selected cradle-to-gate consideration can be reliably evaluated with regard to the available data sets, while the data collection for further life cycle phases must be further secured.

Sustainability

79

Building product

Designation

Reference quantity

Global Warm­ ing Potential (GWP)

Primary Energy Non-Renewable Total (PENRT)

Acidification Potential (AP)

Density

Manufacturing phase A1—3

Manufacturing phase A1—3

Manufacturing phase A1—3

[kg/m³]

[kg CO2 eq./ reference value]

[MJ/reference value]

[kg SO2 eq./ reference value]

Concrete C35/45

m³

2,400

244.0

1,200.0

0.348

Reinforcing steel

kg

7,850

0.7

8.9

0.001

Steel cables

kg

7,850

1.8

20.0

0.002

Structural steel (S355 + S235)

kg

7,850

1.1

11.5

0.004

Precast textile-reinforced concrete 1)

m²

–

103.0

1,230.0

0.164

Mastic asphalt

kg

2,400

0.1

4.6

0.000

Glue-laminated timber

m³

508

-643.3

2,263.5

0.644

Manufacturer’s data Source: ÖKOBAUDAT

2 Overview of the reference values of Global Warming Potential (GWP), Primary Energy Non-Renewable Total (PENRT) and Acidification Potential (AP) 3 Data on three pedestrian and cycle bridges made of different ­construction materials (considered superstructure length marked in grey) a Reinforced concrete b Steel c Wood

1)

Bridge 1 Bridge over the B 295, Weil der Stadt (DE), 2006, Leonhardt, Andrä und Partner Span length: 38 m Considered superstructure length: 38 m Clear width: 2.50 m Considered traffic area: 95 m2 Materials of the superstructure and mast: Slackly reinforced (not prestressed) FLC concrete C35/45, structural steel S355, cables: VVS, OSS a

b

c

80

Bridge 2 Campus bridge Opladen (single-span girder), Leverkusen (DE), 2014, Knippers Helbig, Knight Architects Span length: 23.58 m Considered superstructure length: 23.58 m Clear width: 3.00 m Considered traffic area: 70.4 m2 Materials of the superstructure: Weatherproof steel (S355), Pavement: Mastic asphalt Bridge 3 Integral solid timber bridge, Weinstadt an der ­Birkelspitze (DE), 2019, Knippers Helbig, Cheret Bozic Architekten Span length: 38.20 m Considered superstructure length: 30 m (wooden girder segment) Clear width: 3.25 m Considered traffic area: 97.50 m2 Materials superstructure: Glue-laminated timber, steel (S355, S235), Pavement: Textile concrete slabs (carbon fibre)

3

GWP per traffic area [kg CO2 eq./m2]

Concrete C35/45 Precast textilereinforced concrete Mastic asphalt Glue-laminated timber

accordingly increase the approach chosen here per m2. For the environmental indicators global warming potential (GWP), acidification potential (AP) and non-renewable ­primary energy demand (PENRT), the LCA data for the construction phase are based on the ÖKOBAUDAT material database (fig. 2).

400 200 0

Global warming potential

-200 -400

PENRT per traffic area [MJ/m2]

Steel S235+S355 Reinforcing steel FLC / Spiral cables Total

Bridge 1

Bridge 2

Bridge 3

4

Bridge 1

Bridge 2

Bridge 3

5

5,000 4,000 3,000 2,000 1,000

AP per traffic area [kg SO2 eq./m2]

0 1.60

Primary energy, non-renewable

1.20 0.80 0.40 0

4 Global warming ­potential (GWP) 5 Primary Energy NonRenewable Total (PENRT) 6 Acidification Potential (AP) Calculation of environmental impacts: Jana Nowak, M.Sc.

Fig. 4 shows that Bridge 3 (wooden superstructure) has a negative global warming potential of -125.7 kg CO2 eq. The negative potential of the wooden bridge is ­generated by the high proportion of wood (glue-laminated timber: -297 kg CO2 eq.), whereas the textile concrete slabs alone generate a positive potential of 103 kg CO2 eq. However, no environmental product declaration is currently available for textile concrete panels, so the values are taken from a publication by the manufacturer.

Bridge 1

Bridge 2

Bridge 3

6

The environmental indicators for evaluation are the non-renewable primary energy demand, the greenhouse potential, and the acidification potential. Foundations, bearings and finishing elements (drainage, lighting, etc.) are not included in the assessment, while the deck coverings and railings are. This simplification is intended to illustrate the influence of the choice of materials for the superstructure on the environmental indicators. The volumes of the substructures differ due to the different structural requirements for the foundations; reinforced concrete (abutment bodies, piles, etc.) is used as the material in all three projects. In an overall balance, these influences would

The large amounts of material and ­production-related energy inherent in steel are clearly evident in the case of the steel bridge (Bridge 2). However, the carbon fibre-reinforced fine-grain ­concrete slabs of the decking on Bridge 3 also make large contributions (fig. 5).

Acidification potential Bridge 2 (steel superstructure) has twice the acidification potential of the comparison bridges. The lower acidification potential of the concrete of Bridge 1 compared to Bridge 2 is striking (fig. 6). Example of “cradle-to-grave” LCA In addition to the consideration of the production phase A1– A3 (“cradle to gate”), a life cycle assessment of the entire life cycle from production to demolition (“cradle to grave”) is presented below, which also takes into account durability and disposal aspects inherent in the materials used. The

Sustainability

81

7 Comparison of a road bridge a Longitudinal section b Cross-section of composite construction c Cross-section of ­prestressed concrete construction

82

156.60 m

12.00 m

2.00 bis 3.50

a

0.55

balancing of the global warming potential (GWP) is done by comparing a road bridge in composite and one in prestressed concrete construction with background data from the ecoinvent database [5]. This comparison can be particularly useful in the design phase to determine environmentally relevant preferences of different construction methods at an early stage. The bridge under consideration extends over five spans with a total length of 156.50 m and a width of 12 m, and has a noise barrier on one side (fig. 7). The superstructure is alternatively designed with 1.60 m small hollow steel boxes using a composite slab and a 1.30 m high prestressed slab cross-section with lateral cantilevers. The substructure was pre-dimensioned according to the different dead weights. In order to also clearly highlight the potential use of recycled ­materials in the construction and operation phases as well as the provision of recyclable materials at the end of the service life, the “Circular Footprint Formula” (CFF) of the European Commission is applied [4]. The overall ­balance comparison in fig. 8, marked by the diamond sign, shows a slight advantage for the composite bridge, as larger negative shares, i.e. credits, arise specifically from steel recycling after the bridge is demolished [5].

156.60m

b

c

12.00 m

7

Greenhouse emissions [kg CO2 eq.]

8 Impact assessment of the GWP of a ­composite bridge and a prestressed concrete bridge ­without equipment according to material and service life phase (RC steel = recycled steel; ER concrete = emission-reduced concrete)

1,500,000

Overall balance of the individual construction methods

Concrete in construction Reinforcing steel in construction Steel in construction

Concrete in operation Reinforcing steel in operation Steel in operation

Concrete in demolition Reinforcing steel in demolition Steel in demolition

1,000,000 500,000

707.171

865.453 587.427

608.562

488.818

821.529

683.382

683.382

0 -500,000 -1,000,000

Composite bridge Composite Composite Composite Composite (RC steel) (ER concrete) (RC steel + ER concrete)

Prestressed concrete bridge Prestressed Prestressed Prestressed Prestressed concrete concrete concrete concrete (RC steel) (ER concrete) (RC steel + ER concrete)

8

Notes: [1] Brundtland Report, p. 38, para. 15. [2] Life Cycle in ­Practice [3] Geißler 2014 [4] European Commission 2010 [5] Van Eygen 2019 [6] FSV, RVS 13.05.11, 2017

The quantities of components required for this comparison for both bridges in the construction phase, as well as the quantities required in the course of repairs and upgrading in the operational phase, and the demolition quantities are taken from RVS 13.05.11. [6] In addition to the recycled proportion of steel, the type of concrete ­production was also varied with regard to possible CO2 reductions in order to obtain a qualitative influence on the ­overall balance. In the above-mentioned comparison, the term emission-reduced concrete in fig. 8 refers to the possible use of biogenic fuels in cement production and also to possible adjustments of the raw materials. Fig. 8 shows that the use of recycled steel and CO2 emissionreduced concrete can significantly reduce the global warming potential (GWP). As mentioned in the chapter “Economic Efficiency” (p. 74ff.), the preparation of life cycle assessments over the entire life cycle in combination with a life cycle cost analysis is also desirable. In this way, all costs from a technical-constructive, environmental and overall societal perspective for a variant decision can already be estimated in the design phase.

Outlook The life cycle assessment of the building materials used for bridge construction can be carried out with sufficient certainty based on the data currently available. However, the secondary effects caused by the construction, operation and dismantling of bridges, such as disruptions to traffic flow, which can significantly influence the overall balance, are currently difficult to represent, with the exception of rough estimates of congestion costs (see “Life cycle considerations”, p. 76f.). However, the evaluation of, for example, the environmental parameters of the greenhouse potential, primary energy and the acidifi­cation potential already provides a reliable basis for a sustainability assessment in early design phases.

Sustainability

83

Materials Properties, construction and gestalt

Through its material properties, the building material essentially determines the design of the structure and thus shapes the form and character of a bridge. A basic understanding of the material composition, the manufacturing process, the machinability and formability, the natural colouring and the haptic characteristics is just as indispensable in approaching bridge design as the knowledge of the strength and deformation properties as well as the resistance of the building material. Wood Wood and stone were the only bridge-­ building materials until the 19th century. The Chapel Bridge in Lucerne, built in the 14th century and the oldest preserved covered wooden bridge in Europe, as well as the 100 or so still-intact truss bridges in China, some of which were “woven” from logs more than 1,000 years ago (fig. 2), bear witness to the efficiency of this natural material and the crafts­ manship that has been further developed over generations. As the most important renewable material, wood is gaining in importance again today due to new technological developments and sustainability aspects taken into account in bridge construction.

84

Properties and use Wood is a fibrous material produced by ­oxygenic photosynthesis. In this process, the greenhouse gas carbon dioxide, which is harmful to the climate, is bound by absorbing water and light, releasing ­oxygen. The lignin cellulose (lat. lignum = wood) embedded in the fibres stiffens the cell walls. Wood is strongly anisotropic and inhomo­ geneous. In the direction of the fibres, the types of wood used can withstand tensile loads up to about a quarter of the values to be assumed for structural steel; in compression, values are achieved parallel to the fibres that roughly correspond to those of normal concrete. Perpendicular to the fibre, the load-bearing capacity of wood is greatly reduced and it is thus hardly ­usable for structural applications. Parallel to the fibre, wood behaves in a linear-elastic manner similar to steel, but only very slight plastic deformations occur before fracture (fig. 1). Wood is a capillary-porous system, water can be stored in micro and macro­pores. In the case of moisture changes, dimensional changes occur in the range between the kiln-dry state (wood moisture content 0 %) and fibre saturation (wood moisture content on average approx. 28 %): when

1 Average tensile strength of softwoods and hardwoods at a wood moisture content of 12 %. Material Tensile strength (parallel to the grain) [N/mm²] Spruce

95

Fir

95

Pine

100

Larch

107

Oak

110

Beech

135

Teak

115 1

2 Longest “woven” wooden bridge in China, Wan’am Bridge, Changquiao, Fujian Province (CN), first built 1648 3 Covered wooden bridges a with support ­structure below b with lateral support structure 4 Glued-in threaded rods for an integral solid timber bridge

a

30°

30° b

3

4

2

moisture increases, the wood swells; when moisture decreases, the wood shrinks. Swelling and shrinkage in the d ­ irection of the grain are very low, while swelling effects 10 – 20 times (radial) to 15 – 30 times (tangential) greater occur transversely to the grain. Conversely, the water or moisture absorption per unit of time is significantly smaller across the grain than in the direction of the grain. Thus, in large, solid crosssections (e.g. of block-glued glue-laminated or cross-laminated timber), the equilibrium moisture content is only achievable over the entire cross-section after very long ­storage. Therefore, the moisture there usually only fluctuates somewhat more in the edge zones. In the presence of free water in the cell structure (at an equilibrium moisture ­content above the fibre saturation level), fungi can settle that break down lignin, ­cellulose and other wood components. From a practical point of view, fungal attack can be ruled out at wood equilibrium moisture contents below 20 %; wood moisture contents of 30 to 60 % are required for fungal growth. To avoid damage caused by wood moisture, the water absorption capacity can be limited by using suitable resistant wood species, chemical wood preservation, ­protective coatings, or modification of the chemical /physical properties (e.g.,

reduced sorption capacity through acety­ lation). Effective constructive wood protection is also possible (fig. 3). For this purpose, the component must not be directly exposed to rain, there must be no standing moisture, all areas should be flushed with air and be able to dry off again soon after wetting. Chemical wood preservation is based on the use of biocides, i.e. substances that fight pests such as wood-destroying fungi and insects. Due to the potential danger to other living creatures, chemical wood preservation should be avoided. Wood is combustible. In areas exposed to fire, layers of charcoal with low thermal conductivity form after a short time, slowing down further burning and thus ­protecting the remaining core. The residual cross-section can be determined with ­sufficient certainty depending on the dur­ ation of the fire at a burn rate of approx. 0.5 to 0.7 mm /min. Solid, squat cross-­ sections with a low surface-to-volume ratio are therefore advantageous in terms of fire protection. Today, industrial processes have largely replaced the traditionally handcrafted woodworking. Through targeted gluing, dowelling or nailing of individual board lamellas to bar or panel-like elements, ­components are created that can be precisely classified in terms of load-bearing capacity. The easy mechanical machin­ ability of the dimensionally stable joined ­segments favours modern CNC-based ­fabrication processes and a high degree of prefabrication. Metallic connectors are used instead of handcrafted friction-locked and form-fitted joining elements. For the past 20 years, long fully threaded screws and, most recently, glued-in threaded rods have also been increasingly used in timber bridge construction (fig. 4). This makes new types of joints and construction forms possible, and thus, for the first time, integral timber bridges in which the timber

Materials

85

superstructure and reinforced concrete substructure are monolithically coupled. New types of material-hybrid construction methods (e.g. reinforced concrete-wood composite) lend themselves for use in road bridges, for example.

Construction and design Wooden bridges have to be constructed in a way that is appropriate for the material and maintenance. The trend towards robust, mass-intensive timber bridge structures that has been observed in recent years not only leads to greater durability but also points to a changed new view of the ancient building material. Unlike with all other materials, a more solid timber design is also more ­sustainable, since the higher CO2 storage in the large volume of timber and the lower technological processing depth lead to a comparatively low global warming potential, which is often negative when considering the superstructure alone. Compact, easy-to-manufacture block girder cross-­ sections can be designed with few or even no exposed connecting elements. A design aligned with the flow of forces can be achieved by d ­ ifferentiated shaping of the horizontally or vertically arranged, blockbonded, glue-laminated timber layers. Stone Extracted from the Earth’s crust, stone, due to is worldwide occurrence, mechanical properties, and great durability, has for many cultures always been an important bridge-building material (see “Stone bridges”, p. 16, and “Arch bridges”, p. 37). Natural stone bridges were built all over the world until the 20th century, and many historical structures testify to how durable they can be. The mighty piers of the Brooklyn Bridge were made of granite blocks, as the high-strength, very homogeneous and fine-pored deep rock is able to safely and permanently dissipate the high bearing forces of the main supporting cables.

86

Properties and use In contrast to the upwardly striving wood, stone is earthy, like clay and bricks, and is connected to the earth. Historically, stone was used for short slab bridges or for arch bridges. The arch composed of natural stones or bricks visualises the flow of forces in the support structure and at the same time reveals the very high ­compressive strength (fig. 9). The stones are usually staggered in the mortar bed, the joints giving the structures a structural grid. The mortar, which is matched to the strength of the stones, compensates for the unevenness of the stones and ensures that the forces are transmitted over the entire cross-section. Due to the very low tensile strength of the material, stone arches were designed in such a way that no tensile forces would occur in the support structure. Additions of steel fixtures for tensile transmission were not common. This is a major reason why arch and vault bridges are ­indestructible: only the stone can corrode – and these processes are not com­ parable in time to steel corrosion. The appeal of natural stone bridges is in their

5

6

5 One of four pedestrian bridges with prestressed granite superstructure and a span of 10.30 m, Kurpark Bad Herrenalb (DE), 2016, Execution: Kusser Granitwerke, Design: bbzl – böhm benfer zahiri landschaften städtebau 6 Tension band bridge with 40-m opening. The granite walkway slabs are prestressed via steel bands below. Pùnt da Suransuns, ­Viamala (CH), 1999, Conzett Bronzini Partner

a

b

7 The inner-city pedestrian bridge hangs from 16 tension rods that transmit their loads to a tree-like mast made of individual natural stone blocks. Bad Homburg (DE), 2002, schlaich bergermann partner 8 Bahrebachmühlen ­Viaduct, Chemnitz (DE), 1872, restored in 2010, Marx Krontal Partner 9 Compressive strengths of different building materials relevant for solid bridge construction

geometric shape and in the detail of the stone workmanship. Since natural stone is not a homogeneous building material, its strength and durability properties vary greatly depending on the type of stone and its occurrence. Numerous types of stone such as granite, porphyry, diorite, sandstone, basalt, shell limestone, marble and travertine are used in bridge construction as load-bearing components or as facing. As a rule, natural stone is a very weatherresistant building material, has high abrasion resistance to water and sand erosion, and was often used for the facing of river piers due to its durability.

Material

Com­ pressive strength [N/mm²]

Marble, ­granite

up to 300

Sandstone

up to 150

Limestone

up to 90

Today, the knowledge of engineers in solid construction is concentrated exclusively on industrially producible building materials, and experience in natural stone bridge ­construction is almost completely lost. In new construction, natural stone is currently used almost exclusively for veneering to enhance appearance and increase dura­ bility. This does not do justice to to it as a building material. The 3D-supported natural stone processing available today at many specialised companies is also economically feasible for modern bridge support structures with high sustainability and low CO2 emissions. In recent years, some pedestrian bridges have been built as

Solid clinker up to 80 Solid brick

up to 48

Concrete

> 20

Highstrength concrete

up to 150

Ultra high over 150 performance concrete 9

7

Natural stone in contemporary bridge construction

8

hybrid structures with natural stones and internal prestressing or external loadbearing elements with extremely high ­slenderness (fig. 5 and fig. 6). Today, however, the focus is rather on the preservation and rehabilitation of the many arch bridges from previous centuries, which are in great need of renovation due to weathering, traffic and alterations (fig. 8). Since these restorations require knowledge of old construction principles, materials and damage processes, planning and implementation represent a special engi­ neering and construction task. Iron and steel Processed iron ore has been used as a raw material for tools, jewellery and in weaponry for more than 3,000 years. Later, the Romans, for example, used iron for ­fittings and clamps as connecting elements of stone blocks, also in bridge construction [1]. The breakthrough of iron for structural bridge building and civil engineering did not occur until the 18th century, due to the improved opportunities for the large-scale production of pig iron with the use of coke instead of charcoal and the further development of the blast furnace. The superiority of iron structures over stone in their ability to withstand large forces compared to the weight of the structure used meant that costs could be reduced and construction time significantly shortened. Starting from

Materials

87

Character Even today, iron and steel are the epitome of industrial progress, not least due to technologically highly developed mater­ ials that are specifically differentiated in terms of their intended use. The advantageous properties, such as high tensile strength and mechanical strength, continue to inspire bridge builders in particular to this day, as do the material’s malleability and the possibility of precise, prefabricated production. Iron and steel express techni­ cally based logic and sobriety, lightness and the boundlessness of what is possible.

Mechanical properties Iron and steel are characterised by their excellent tensile and compressive strengths (fig. 10). In order to improve the plastic formability, iron as a chemical element (Fe) is alloyed with a maximum of 2 % by mass of carbon and other metallic and ­non-metallic elements to form steel. The stress-strain behaviour of steel is linearelastic over a wide range with a precisely determinable increase corresponding to the modulus of elasticity (fig. 11). The naturally hard, untreated structural and ­rein­forcing steel begins to deform plastically at a certain constant load level and hardens again slightly after the yield zone, whereby steel exhibits pronounced ductility. Due to the large deform­ations, a material failure can be registered in advance (prior warning of failure) and appropriate measures can be taken. This flowability, its toughness, gives steel an unsurpassable property that is very advantageous in terms of safety, not only in structural steel engineering but also in re­­inforced concrete construction. In addition, the plastic formability enables the production of sheets, rolled

88

and hollow sections with different crosssectional shapes, according to static and structural requirements. Initially known as malleable but contam­ inated wrought iron or brittle cast iron, the corresponding production technology developed rapidly, especially in the 20th century, through various types of alloys, post-hardening and post-treatment methods. Tension rods, wires and strands with strengths of up to 1,860 N/mm2 make hightensile elements possible that are used for prestressing in concrete construction and cable structures. A decisive criterion is the limited fatigue strength of steel, which can be many times lower than the tensile strength. ­Starting from unfavourable notches in the material, e.g. as a result of welds, material fatigue under cyclic stresses may lead to sudden brittle failure. Low-fatigue design is therefore particularly necessary for dynamically loaded structures, especially railway or highly loaded road bridges. Cast steel with modern processing methods enables fluid, low-stress and low-fatigue shapes adapted to the force paths and is suitable for geometrically complex node ­situations (fig. 12).

Stress σ [N/mm2]

Coalbrookdale in central England, iron shaped the industrial age like no other material, with the associated social up­­ heavals and achievements (fig. 7, p. 16).

1,600

Tension strands Y1860S7

1,860

VVS cable

1,440

Tensioning bar Y1050H

1,050

Reinforcing 540 steel B500(B) natural hard Structural steel S355

490

Cast steel G20

480 10

10 Tensile strengths of ­different steel grades 11 Stress-strain curves of structural, reinforcing, and prestressing steel

σ ε 1 A=1

Tension rod Cold-formed reinforcing steel

σ

Natural reinforcing steel

Construction steel

400 Construction -40 steel -400

Tensile strength [N/mm²]

Tension strand Tension wire

1,200 800

Material

40

80

Strain ε [‰]

Reinforcing steel 11

12 Casting node at the Seitenhafen Bridge, Vienna (AT), 2011, PCD, AGU, zeininger Architekten 13 Overpass bridge made of weatherproof steel in composite construction, Schörfling (AT), 2009, KMP ZT, Obholzer Baumann ZT

12

Corrosion The special property of iron to decompose by oxidation in the presence of oxygen and water becomes a burden on the material and requires extensive protective measures in the form of coatings or metallic overlays, such as galvanising, to prevent progressive damage. Alternatively, weather-resistant steels are sometimes used in bridge construction, where a natural protective rustbrown patina forms due to special alloys (fig. 13). For cost reasons, non-rusting steels with alloying elements such as chromium or nickel are used in structural bridge construction only in individual cases.

Construction and design Through the efficient use of materials, ­support structures made of iron and steel were broken down into fine-grained tension and compression elements and, among other things, truss constructions Clamp cover Cable hook

14 Cable node at the ­suspension bridge A 26, 4th Danube Bridge, Linz (AT), planned ­completion 2024, schlaich bergermann und partner, von ­Gerkan, Marg und ­Partner

Suspension cable package 2≈ 6 suspension cables

Securing bolt Clevis Suspension cable 14

13

were created in which the force transfer of the support structure can be clearly read (fig. 12, p. 96). The new material soon became apparent in bridge construction in the reversal of the load-bearing systems from pressure-dominated arch structures to tension-dominated suspension and cable-stayed structures. These only gained importance through high-strength tension elements in the form of rod, wire and strand bundles, as well as fully locked or spiralled cables, as this is what first made large spans possible at all. Sophisticated detachable joining techniques ensure that the prefabricated steel products and cables are joined together. In the past, this was done with detachable rivet joints but today it is mostly screw and eye-bar joints. The construction of node connections is one of the most demanding tasks in steel bridge construction and sometimes results in excellent works of engineering art (fig. 14). In steel bridge ­construction, however, welding as a nondetachable connection technique has almost completely replaced detachable connections for maintenance reasons. For some time now, the separation of the support structures into small tension and compression elements has given way in bridge construction to solid wall girders or closed, often tightly welded hollow boxes, since corrosion protection is becoming increasingly important. A trend reversal in the material expressiveness of steel is thus taking place, away from bar-shaped delicacy and material efficiency towards

Materials

89

a corporeal, voluminous design language. Whether this development can be successfully counteracted by new robot-controlled production techniques, as in the example of the MX3D bridge in 3D printing (fig. 15), and steel will regain its original fascination as a building material of absolute efficiency, remains an open question. Reinforced and prestressed concrete Due to its very good workability, fresh ­concrete can be poured as fresh concrete into almost any conceivable shape. Only in combination with ribbed reinforcing or ­prestressing steel are reinforced concrete cross-sections capable of absorbing tensile and compressive forces. In bridge construction, concrete has become established for numerous components, e.g. for superstructures, foundations, piers, abutments and extension parts, and is therefore the most frequently used building material in modern bridge construction.

Properties Concrete is an inhomogeneous composite building material made of cement, water and aggregates, which is supposed to fulfil predefined properties after hardening. These are derived from the external boundary conditions (the exposure), the required strength properties (compressive and tensile strengths) and the planned surface properties. In the reinforced or prestressed concrete used in bridge construction, concrete and steel enter into a bond in which each of the two components assumes part of the load-bearing behaviour of a bridge crosssection: the concrete carries the compressive forces, the steel the tensile forces. The effective bond is ensured by ribbed reinforcement. The design of the composite must be dimensioned and laid out in such a way that the compressive stresses on the concrete side and the tensile stresses can be absorbed by the steel cross-sections. In

90

15

the case of reinforced concrete, cracking is basically necessary in the ultimate limit state in order to be able to utilise the reinforcement up to the yield point. On the other hand, the prestressed concrete cross-section is over-pressed by prestressed tension members and thus cracking is prevented, especially in the service condition. Special concrete properties can be derived from the expected environmental effects such as natural weathering and the attack of, for example, de-icing salts on the basis of the exposure classes. The expected exposures result in requirements for the concrete with regard to the minimum compressive strength class, the concrete composition, the calculated values of the crack width, the minimum concrete cover of the reinforcement and the curing period. This is to achieve the planned service life of 100 years for reinforced concrete bridges under the given environmental influences. The initially high pH value of the pore water in the cement stone of the concrete protects the reinforcing steel from corrosion. External CO2 effects lead to the carbonation of the concrete in depth and the lowering of the pH value, which is associated with a reduction in the passive protection of the reinforcing steel.

Concrete processing in bridge con­ struction The properties of fresh and hardened ­concrete are decisive for today’s standard concrete in concrete bridge construction.

15 New 3D fabrication techniques and expressive opportunities as a future trend? MX3D bridge project, 2015 (project start)

16 Schanerloch Bridge, Dornbirn (AT), 2005, M+G INGENIEURE, Marte.Marte Architects 17 Schwandbach Bridge, Hinterfultigen (CH), 1933, Robert Maillart

The fresh concrete properties strongly influence the transport and workability of the concrete until it is placed in the formwork and begins to set. In bridge construction, very strict rules apply to the use, transport, placement and curing of concrete. To ensure the quality of the concrete, comprehensive tests are carried out both in the concrete plant and at the construction site. The quality and durability of concrete bridges is significantly influenced by the structural design, the reinforcement, the concrete cover and the execution. In the case of high stresses and multilayer, dense reinforcement concentrations, sufficient filling and vibration gaps must be ­provided in the reinforcement structures for the compaction of the concrete. Only with optimal compaction of the concrete can the planned properties of a very good bond between steel and concrete as well as homogeneous, dense concrete surfaces be achieved. After installation, the concrete components must be cured to ensure that cement hydration has occurred to the extent that the concrete has sufficient strength. Above all, the forced stresses caused by outflowing hydration heat can cause cracks if the strength development is low. These cracks can be avoided by curing, covering with foils, and by additional watering of the pavement surfaces. At low outside ­temperatures, insulation is required to keep the stresses from the temperature difference between the concrete surface and core low.

Construction and design Due to its malleability as a cast material, concrete offers bridge engineers great scope for design. Logically justified forms derived from static aspects and economic production have become established for concrete bridges (fig. 16 and fig. 17). ­Reinforced concrete and prestressed concrete bridges are characterised by a large proportion of visible concrete surfaces. The pure support structure as well as the structure and colour of the concrete ­surfaces give the bridges their own character. Formwork edges, construction and cycle joints show the process of manufacturing piers and superstructures, and form a logical unit with the support structure – the technology is allowed to be visible. In order to achieve homogeneous surfaces, the use of the same concretes with the same aggregates, even in several concreting sections, as well as the same processing and curing, is extremely important in concrete processing. High performance and textile concrete Ultra high performance concrete (UHPC) is the name for concretes with a special mix and grain composition. These are characterised by very high compressive strengths (> 150 MPa), a dense structure with high abrasion resistance and high durability. As reinforcement, micro-steel fibres can be added to the concrete in addition to steel inserts, which increases the tensile strengths and significantly improves the

16

17

Materials

91

ductility behaviour of a fibre-reinforced UHPC in the ultimate limit state (ULS). In recent years, pedestrian, road and railway bridges have been built worldwide from UHPC – some as pilot projects. Due to the material properties, extremely slender structures can be built that would not be possible with normal ­concrete (fig. 18). However, UHPC also has a significant range of applications in the repair and strengthening of bridges, e.g. in Switzerland and Austria (see “Resisting”, p. 29f.). Here, structural elements made of reinforced ­concrete are specifically strengthened with a slim UHPC layer and exposed surfaces are protected. By using non-metallic glass or basalt fibres and GRP fabrics in combination with finegrained concretes, textile concretes represent an alternative for applications in bridge construction. With a highly load-bearing reinforcement made of carbon, for example, structures or structural elements with reduced concrete cover, slender dimensions and high durability can be erected in combination with special fine-grain concretes.

3D printing in concrete Digital tools in planning and 3D printing ­production will change bridge construction dramatically in the next few years. In the future, it will be possible to produce complex shapes entirely without formwork or partially with formwork, thus making completely new load-bearing structures possible. Biometric structures enable complex static systems that are unthinkable with conventional formwork in concrete construction. Printed concrete elements can be prefabricated and assembled in the factory or produced directly at the construction site. Near Shanghai, for example, a 26 m long and 3.60 m wide bridge was constructed as a prototype using 3D printing. The structural elements, manufactured by industrial robots, were produced from fibre-reinforced concrete in an additive process and assem-

92

a

b

bled with the help of falsework. In addition to researching the performance of the print head, the recipe of the fibre-­reinforced concrete and the support structure were tested in a load test on a 1:4 scale model (fig. 19).

18 UHPC bridge. Pont de la République (also Pont André-Lévy), Montpellier (FR), 2014, Lamoureux & Ricciotti Ingénierie, Rudy Ricciotti Architecte a View b Due to the high mechanical resistance and watertightness, the organically shaped slender inclined columns were constructed of UHPC (compressive strength 150 MPa).

Fibre composites Since the 1940s, fibre-reinforced plastics (FRP) have been developed as a stable lightweight material for helicopters and ­aircraft to increase their transport capacity. Today, more than half of modern aircraft types are already made of high-performance fibre-reinforced composites (FRC). For use in bridge construction, in addition to the enormous mechanical strength with low dead weight, the high resistance to acting media, in particular the high freeze-thaw resistance, is of particular interest. In the Netherlands, bridges have been built with FRP components since 1995. In 2008, the United States Department of Agriculture provided a guideline to facilitate the use of fibre-reinforced composites for trail bridges in hard-to-reach places. As a result, there are already about 50 bridges with decks

a

18

19 Arch bridge near Shanghai (CN), 2019, Tsinghua University (School of Architecture, Zoina Land Joint Research Center for Digital Architecture – JCDA) a 3D print of a superstructure segment made of fibre-­ reinforced concrete b Bridge cross-section

b

19

Material Tensile strength [N/mm²] Glass fibre

1,500 — 3,500

Carbon fibre

2,700 — 6,400

Modulus of elasticity [N/mm2] Glass fibre

73,000 — 80,000

Carbon fibre

225,000 — 950,000 20

21 20 Tensile strength and modulus of elasticity of glass fibre and ­carbon fibre 21 Textile reinforcement: spatial carbon fibre fabrics

Note: [1]  Merckel 1899 22 Road bridge with GRP deck Friedberg / Hesse (DE), 2008, Knippers Helbig 23 The world’s first concrete bridge constructed exclusively with textile reinforcement (CFRP). Foot and cycle path bridge, Albstadt-­ Ebingen (DE), 2015, Knippers Helbig

made of glass fibre-reinforced plastic (GRP) in the USA, while only a few bridges of this type have been built in Germany to date (fig. 22).

Properties, manufacture and use Fibre-reinforced composites consist of the shaping plastic matrix, the reinforcing fibres and added fillers and additives to adjust additional, specific properties (e.g. UV, temperature and weather resistance). Inorganic fibres, mainly glass and carbon fibres, and to a lesser extent basalt fibres, are used as reinforcement in bridge construction. Glass fibres, which are spun from molten glass into 9 – 24 µm thin threads, have ­isotropic material properties and are nonflammable. Basalt fibres made from molten rock have comparable mechanical characteristics. Carbon fibres produced from carbonaceous raw materials by a stretching or spinning process in 7– 9 µm thin threads have strongly anisotropic properties and are combustible. The necessary multistage temperature treatment at up to 3,000 °C is very energy-intensive and thus costintensive. While the tensile strengths do not differ that much, the carbon fibre has a modulus of elasticity that is many times greater than that of the glass fibre (fig. 20). Both fibres are very corrosion-resistant, and carbon fibres are almost fatigue-resistant. The matrix consists of chemical compounds

based on carbon chains and is therefore combustible. Thermoset materials (epoxy, polyester resin) are mostly used in construction. Fibre-reinforced composites can be ­manufactured specifically with regard to mechanical properties and required resistance. Industrial processes (e.g. ­pultrusion for GRP standard profiles and reinforcement bars) are used, as are ­manual production techniques, such as hand lamination for geometrically complex shapes in small quantities. In bridge construction, suitable materials include moulded parts, sandwich elements and standard profiles as beams, bar or arch ­elements, bar and strip-like laminates as hangers, reinforcing bars or tensioning straps, as well as flat laminates as cladding elements, and spatially interwoven mats as textile reinforcement for concrete (textile concrete; fig. 21).

Construction and design Fibre-reinforced composites, still a relatively new material in bridge construction, do not yet have an independent design ­language that can be attributed to the ­material’s behaviour, nor have any materialspecific, readable joining principles been developed. Different fibre composite elem­ ents are used. However, the projects completed in the last three decades already show the variety of solutions made possible by this extremely durable and highly stressable family of materials (fig. 23).

22

23

Materials

93

h h h h h h

Designs

h h h

Catalogue of options hh h

h h

hh

h h h h

2  Single-span girder chain h h h h h h hh h h 3  Continuous beams h h h h

4  C  ontinuous beams with haunches and column tapers

h h h h h Bridges bear loads. A variety of possible structural typologieshcan be used for load transfer, but they differ in terms of functional requirements, suitable spans andhsensible choice of materials. Efficient h systems are based on a clear order load-bearing and uniformity. They prove to be coherent when nothing can be added or taken away. However, slenderness alone is not an indicator of efficiency. Much more important are logically arranged loadbearing structures that are based on the task of bridging an obstacle and the boundary conditions at the site.

Beams / Frames 1  Single-span girder

5  Integral construction

6  Frame construction

Return of horizontal forces into the structure

Direct transfer of horizontal forces

Single-hip cantilever bridge 7  Cantilevered stem bridges

94

Shaped according to the moment line, e.g. overpass near Kirchheim Teck (DE), 1992

Tree trunks or flat stone blocks for bridging smaller obstacles such as streams or bogs were the first examples of single-span beam structures (fig. 1). These beams support by bending, i.e. by compressing the top and stretching the bottom of the cross-section. The internal force state can be represented in a simplified way as a compression arch with tension band or, in the case of slender beams, as an idealised truss. The thickness of a rectangular beam under the load case of dead weight increases as the square of the span width. This means that beam bridges become disproportionately heavier and stronger the longer the span. For these reasons, beam bridges with short spans always appear much more slender than those with large spans [1]. Larger bridge lengths are achieved by single-span girder chains (fig. 2) or by means of continuous girders (fig. 3). In the case of the continuous girder, the stress alternates above the support: on the upper side, strain occurs (tension) and on the lower side, the beam cross-section is compressed (compression). In most cases, this negative bending moment above the support is decisive for the design. Parallel chord beam structures, which form the majority of existing bridges, have an unspectacular appearance and are usually made of reinforced concrete, prestressed concrete or as composite structures.

h

h

h 8  Trough bridge

h h

River bridge

Single supports

Valley crossing

Twin support 9  Haunched prestressed concrete bridges

V-shaped columns in unit with the superstructure, e.g. Filstal Bridge, new Wendlingen-Ulm line (DE), 2021

V columns as separate units 10  Tree support With centre reference 11  Inclined column variants

Shaping the supports or providing structural haunches according to the cross-sectional stresses creates greater expressiveness and makes the inner load-bearing behaviour tangible (fig. 4). Haunched girder bridges made of prestressed concrete hollow boxes transfer dead weight from the centre of the bridge to the supports and have therefore become established in the span range of approx. 80 – 200 m as river bridges or valley crossings (fig. 9). The same principle of favourable distribution of stiffness and mass can also be achieved with trough cross-sections for smaller bridge widths (fig. 8). In the case of frame structures, the superstructure must always be connected to the substructure in a flexurally rigid manner. In this case, horizontal support forces occur, which can, however, be influenced by an inclined position of the shafts (fig. 6). In the case of longer lengths, frame bridges are also referred to as integral constructions, the primary aim of which is to dispense with bearing and transition constructions, and which promise a high level of durability (see “Transition zone to the open track”, p. 67ff.). The especially temperature-related constraining forces are absorbed by a slender and flexible construction as well as supports with cor­ responding deformation capacity (fig. 5). A further development of the frame construction method is the inclined-beam bridge, whose horizontal forces are returned to the support structure by earth struts in softer soils or are absorbed directly by the ground (fig. 7). Other applications of inclined columns are continuous beams in symmetrical arrangement or asymmetrical position in bridges with a pronounced centre reference (fig. 11). In this case, columns can be designed as deliberately formally and materially separate units or coherent with the superstructure. The bridge with V columns as separate units in fig. 11 corresponds to the material-reduced resolution of the support area of a haunched continuous girder into a tension chord and a compression strut (fig. 9). Bridges with steel tree supports allow for a slender bridge deck due to the branched multiple support points and are often used for light pedestrian bridges (fig. 10).

Designs

95

1 2

2

3

1  Upper chord 2 Filler bars made of diagonal and vertical bars 3  Lower chord

12  Mode of action of a truss

tension-dominated

compression-dominated

13  Top steel truss, e.g. Ijssel Bridge near Zwolle (NL), 2011

Suspended structure

Underspanned structure

combined

Truss frame

Howe truss 14  Variations of overhead trusses

96

15  Vierendeel girder (bend-oriented)

16  Steel bridge over the Firth of Forth, Queensferry (GB), 1890

17  Haunched lower truss with composite slab

Trusses

18  Space truss with composite slab

Fish-bellied girder

Fink-type girder

Under-tensioned girder

19  V  ariations of lower truss systems or under-tensioned girders

The resolution of the inner lines of force of the beam into stable triangles leads to the truss, which provides for enormous material savings, as the forces are concentrated in individual tension and compression-dominated bars, while filler mass without a static function is omitted (fig. 12). This makes it possible to achieve larger spans with a lower dead weight, as already shown by the iconic bridge over the Firth of Forth (fig. 16) or a modern design in fig. 13. The massive continuous girder (fig. 9, p. 95) is resolved with inclined filler bars as a lower haunched steel truss with composite deck (fig. 17). In the same way, the girder can be designed as a parallel-girder space truss over several bays (fig. 18). The classic truss as a single-span girder is divided into horizontal chord members as well as diag­ onal and vertical members running between the chords (fig. 12). The compressive and tensile forces accumulate in the chord members at mid-span, while the transverse forces are absorbed by the filler members, which reach their maximum towards the supports. Timber and steel can be easily joined as prefabricated bar components to form a truss. In contrast to steel, the diagonals of timber trusses should be placed in the direction of compression in a way that is appropriate for the material and advantageous, as shown in fig. 14 by the different development paths of compression-dominated, tension-dominated and other truss variations. ­Variations of upper and lower trusses or of underspanned systems convey a variety of shapes and constructions in which the force path can be impressively understood by the observer (fig. 14 and fig. 19). The Vierendeel girder, the truss without diagonals, occupies a special position. Here, the shear force is not transmitted directly via the diagonals but via frame bending moments of the chord and vertical members to the supports (fig. 15).

Designs

97

5

1

4

2

3

1 Arch 2 Elevations 3 Abutment 4 End wall 5 Parapet

20  Stone arch bridge Anji Qiao, Zhao Xian (CN), 605 AD, Li Chun

21  Classical viaduct

Arches 22  Bar arch as continuous girder

23  Bar arch as support for the roadway

24  Arch with re-suspension of the tension force in the carriageway

25  Langer‘s girder

26  Network arch

98

The arch is the oldest load-bearing system after the beam. Early on, a benefit was recognised in stacking stone blocks radially and activating the arch thrust as a force transfer exclusively via compressive forces (fig. 20). Stone viaducts as railway bridges, which take up the design of the viaducts and aqueducts of the Roman Empire, are imposing contemporary witnesses of a robust but no longer modern bridge type (fig. 21). With uniform vertical loads, the arch unfolds its full effectiveness. Halfsided loads from traffic have to be transferred by bending moments or over-pressed by high dead weight. For this reason, the use of arched structures for larger spans only makes sense if the traffic loads are small in relation to the dead load and the building ground is very stiff (e.g. rock). Starting from the haunched beam (fig. 9, p. 95) via the resolved truss (fig. 17, p. 97), the haunch is further widened and articulated downwards in bar arch constructions so that the compressive force is not suspended high via filler bars but can be led directly to the support as a bar arch via arch action (fig. 22). The acting strut forces at the supports must either be directly introduced into the subsoil (fig. 23), ­suspended back into the bridge deck or short-­ circuited there, as shown by the example of the bar arch in fig. 22. The bar arch in fig. 23 acts as a support structure for the bridge deck, which is ­offset from the arch and absorbs the bending moments from half-sided loads. The load-bearing behaviour can be clearly understood on the basis of the component dimensions. For carriageways with a low height above the ground, arch-supported structures are suitable, which are arranged above the carriageway and return the horizontal force into the carriageway (fig. 24). In the case of the so-called Langer’s

girder, the arch runs entirely above the carriageway and its abutment forces are connected by the stretch girder integrated into the carriageway as a tension band (fig. 25). Due to the vertical hangers, it is predominantly this stretch girder that has to absorb the half-sided loads via bending and is usually dimensioned larger than the arch cross-section. In contrast, the network arch functions similarly to a bending girder due to the diagonally and crosswise arranged tension elements (fig. 26). The support structure here represents a symbiosis between the arch and the truss and is extremely stiff in relation to the pure bar arch. Asymmetrical live loads are effectively distributed between the arch and the stiffening girder by the hanger arrangement, which reduces the bending stresses there and allows the structure to be very slender. By analogy with the truss frame (fig. 11 at the top, p. 95) and the bar arch (fig. 22), fig. 27 and fig. 28 show two variants in which the arch apex merges with the deck. In contrast to the continuous effect (fig. 27), the arch in fig. 28 introduces the forces into the rock flanks. In doing so, a clearer centre reference to the apex axis can be observed, which characterises the arch as a suitable, dynamically acting support structure over obstacles such as deep ravines. Fig. 29 shows the classic, true arch with elevated carriageway, which, in contrast to the bar arch in fig. 23, appears as the dominant structural element. It braces itself powerfully against the rock, takes on the bending moments from half-sided loads, and its dimensions stand out accordingly. As pedestrian bridges, arch bridges can also be designed in different variations, such as with an overlying tension band that short-circuits the arch horizontal force again (fig. 30).

27  Arch with continuous beam effect

28  Arch with pronounced centre reference

29  Classic (true) arch bridge

Arch with counterweight (e.g. Dyckerhoft Bridge, Wiesbaden, 1967)

Arch with overlying tension bands (e.g. overpass near Olomouc, 2007) 30  Arch variations for pedestrian and cycle path bridges

Designs

99

1

2

3

1 Tension band 2 Abutment 3 Support

31  Tension band bridges as single-span and multi-span construction

32  Tension sails as support for the continuous beam effect (e.g. Neckar Bridge (DE), start of construction 2014, and Ganter Bridge, Ried-Brig (CH), 1980)

33  Extradosed bridge

34  Tension-chord bridge

Harp arrangement

(Semi) fan arrangement 35  Cable-stayed bridge

Supporting suspension cable 36  Combined suspension and cable-stayed bridge

100

Notes: [1]  Schlaich 2004

Cable structures Based on the principle of a Leonardo bridge

Curved cable bridge, e.g. pedestrian and cycle path bridge, Gelsenkirchen (DE), 2009 37  Further variations for pedestrian and cycle path bridges

e.g. Alamillo Bridge, Seville (ES), 1992

e.g. Erasmus Bridge, Rotterdam (NL), 1996

38  Single-span cable-stayed bridge with variations

39  Suspension bridge A 26, Linz (AT), approx. 2024

The tension band is the simplest form of bridging and thus the archetype of the suspension bridge. Tension members anchored on both sides can be guided over one or more spans (fig. 31). Due to the deformation capability, this type of bridge is usually only used for pedestrian bridges. As a reversal of the compression-dominated structures (fig. 11 at the top, p. 95; fig. 27, p. 99), fig. 32 shows two continuous girder variants with external tension reinforcement. These tension sails can be made of steel sheets or embedded cable elements. An extradosed bridge (fig. 33) functions in a similar way, where external tension cables support the continuous beam effect, with the tension cables acting like a dissolved haunch of a continuous beam lying above the support. Predecessors were the tension-chord constructions with individual tension elements, which are now only built in exceptional cases because redundant load transfer is not pos­ sible (fig. 34). In contrast to the extradosed bridge, the cable fan of a cable-stayed bridge takes over the dominant load-bearing function. The stiffening girders can be very slender due to the narrow cable routing (see “Long-spanning”, p. 26ff.). Fig. 35 shows two cable arrangements in harp and (semi) fan form; here, the cable forces are anchored in the bridge deck and generate compressive forces there. Fig. 38 shows designs of asymmetrical cable-stayed bridges with perpendicular, inclined and bent pylons of high expressiveness. In general, cablesupported bridges are also possible for pedestrians and cyclists in a wide range of variations from the principle of the Leonardo bridge to curved and spatially braced solutions (fig. 37). A spectacular sight are unilaterally suspended designs that absorb the torsional stress via tension and compression in the stiffening girder through circular ring action in the ground plan (fig. 37 below). As fig. 31 shows, curved tension elements as cables or ropes also form the basis of the mostly end-anchored suspension bridge (fig. 39), which also allows extreme spans (fig. 40). Self-anchored suspension bridges, in which the cable tension force is introduced into the superstructure as a compressive force, are less frequently built today due to their complex construction since the tension band must be functional before the cable structure. With the advantage of the stiffer cable-stayed solution but with the disadvantage of higher pylons compared to suspension bridges, combined designs are also being built (fig. 36).

40  Real suspension bridge

Designs

101

Bridges in Detail

Pedestrian and cycle path bridges “Stuttgart Wooden Bridge” in Weinstadt (DE) Knippers Helbig Advanced Engineering / Cheret Bozic Architects Chain Bridge in Weimar (DE) Marx Krontal Partner (renovation) Tintagel Castle Bridge (GB) Ney & Partners / William Matthews Associates

104 110 114

Road bridges Queensferry Crossing near Edinburgh (GB) Jacobs Arup Joint Venture / Leonhardt Andrä und Partner / Rambøll Group / Rambøll UK / Sweco UK Tamina Bridge in the Canton of St. Gallen (CH) Leonhardt Andrä und Partner / dsp Ingenieure + Planer / Smolczyk & Partner A5.Ü20 near Wilfersdorf (AT) Asfinag Bau Management / Öhlinger und Partner / Mayer Ingenieurleistungen Lower Hātea River Crossing in Whangarei, New Zealand (NZ) Knight Architects / Peters & Cheung (now Novare Design)

120 125 132 136

Railway bridges Scherkondetal Bridge near Krautheim (DE) DB ProjektBau / Steffen Marx, Ludolf Krontal Second Hinterrhein Bridge near Reichenau (CH) Dissing+Weitling, WaltGalmarini, Cowi UK Getwing Bridge in Zermatt (CH) schlaich bergermann partner / SRP Schneider & Partner Ingenieur-Consult / mls architekten

142 146 152

103

A New Type of Bridge Made of Wood Pedestrian and cycle bridge at the Birkelspitze in Weinstadt, Germany

“Solid, integral, and durable – this type of wooden bridge could help revive the use of this renewable building material in bridge construction. In addition to innovative static-constructive aspects, an independent design language is introduced into timber bridge construction, which is oriented towards manufacturing principles and load-bearing behaviour. A well-­proportioned, sculptural bridge that cuts a fine figure even from a worm’s-eye view.” Ludolf Krontal

On the occasion of the Remstal Garden Show 2019, the Rems Valley east of Stuttgart was transformed into a giant garden for 164 days. A total of 16 towns and municipalities along the Rems river organised this unique show and designed a landscape space with parks and green spaces as well as a new network of cycling and hiking paths over a length of 80 km. To connect the paths on both sides of the Rems, the municipalities of Weinstadt and Urbach were provided with three new pedestrian and cycle bridges based on the concept of the Stuttgart Wooden Bridge. Stuttgart Wooden Bridge The so-called Stuttgart Wooden Bridge is a new, durable type of bridge that requires as little maintenance as possible. This bridge

View Scale 1:250

104

type was developed by engineers, architects and timber construction experts in cooperation with the University of Stuttgart. As part of a research project launched in 2013, common causes of damage were first analysed on eleven existing wooden bridges from the 1980s and 1990s in the Stuttgart area that are still in use. The results of the investigation clearly show the reasons for the sometimes considerable damage. These include, among other things, accumulating moisture in the support area and waterproofing leaking underneath. Connecting structures that are exposed to weathering and do not dry sufficiently after moisture penetration shorten the service life of the bridges, in some cases considerably. The newly developed bridge type based on these findings is a covered bridge in which

Bending moment curve of a single-span girder clamped on both sides under equal load (top) Beam shape aligned with the bending moment curve (below)

the protruding walkway decking protects the girder made of block-glued glue-­ laminated timber from direct weathering. On the top side, the solid wood cross-­ section is also sealed with a breathable film. A distance of 15 cm to the walkway decking allows sufficient rear ventilation. Design and con­ struction: Knippers Helbig Advanced Engineering, DE-Stuttgart, Thorsten Helbig (project management) Cheret Bozic Architects, DE-Stuttgart, Peter Cheret (project management)

The first integral bridge with a wooden superstructure In contrast to historical wooden bridges, the new development is an integral bridge, i.e. without bearings or joints: the girder and the abutment are monolithically connected to each other. Glued-in threaded rods, which are integrated into the reinforcement of the abutment with a corresponding over-

lap length, transmit the bending tensile and normal forces between the timber superstructure and the reinforced concrete substructure. However, the direct coup­ ling between the solid, block-glued gluelaminated timber carcass and the reinforced concrete substructure carries the risk of cracking as swelling and shrinkage due to moisture-related changes in the wood are obstructed. To validate the connection concept, the planners developed a prototype on which measurements were taken and evaluated. Load tests on glulam and reinforced ­concrete test specimens, which were ­connected by reinforcing steel bars with a diameter of 16 mm and glued in with

“Stuttgart Wooden Bridge” in Weinstadt (DE)

105

Construction site sequence

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Views of bridge girders and abutments Scale 1:50 1 Reinforcing steel Ø B500B, 78 ≈ 20 mm 2 Superstructure, ­glulam 13 ≈ 20 cm, block-glued 3 Fully threaded screws Ø 8 mm, l = 260/280 mm, 45° inclined to the grain direction 4 Support angle, flat steel 15 mm 5 Fully threaded screws as transverse tension reinforcement Ø 8 mm, l = 640 6 Dowel connection for securing the position 7 Reinforced concrete abutment

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t­wo-component epoxy resin adhesive, ­confirmed the calculated load capacity and a very high residual load capacity. Staticconstructive aspects that had to be con­ sidered for the first adaptation of the “integral bridge” principle for wood were also analysed. Superstructure, substructure and subsoil interact with each other; the soil and the structure must be precisely recorded and sensitive structural components identified.

1

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Pedestrian and cycle bridge in WeinstadtBirkelspitze Opened in May 2019, the bridge at ­Birkelspitze in Weinstadt connects the ­Trappeler residential area north of the Rems with the Birkel area in Weinstadt-­ Endersbach. The shape of the superstructure follows the moment diagram of the bridge girder under equal load: the crosssection reduces from the widenings at the connection point to the abutment with a high restraining moment to a minimum at the zero moment point, and widens again in the middle of the span. The stepping ­indicates the production method: a total of 13 glulam segments, each 0.20 m wide, were block-glued in a horizontal pos­ition to form the 2.60 m wide and 0.93 m high bridge girder with a total volume of 45 m3.

“Stuttgart Wooden Bridge” in Weinstadt (DE)

107

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2%

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261 71 30

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This results in a moisture-sensitive fibre cut only on the upper side. This is p ­ rotected from accumulating moisture with a transverse and longitudinal slope and a breath­ able seal. Apart from a barrier coating for the end grain in the contact area with the reinforced concrete abutment, it is not necessary to preserve any other parts of the solid wood carcass. To ensure the durability of the bridge structure, per­manent moisture measurement and temperature sensors are installed at eight selected points. The data is read and evaluated at defined time intervals. Particularly at the neuralgic points near the contact joint between the wooden superstructure and the concrete abutment as well as underneath the film seal, a longer-lasting increase in the moisture content would thus be detectable. Carbon concrete and lead wool While still in the assembly hall, 78 concrete ribbed steels with a diameter of 20 mm and lengths of 2.30 to 3.00 m were glued into each end-grain area. The rebar is up to 1.20 m deep in the 30 m long, block-glued beams and ends in the reinforcement of the abutments. There they are firmly concreted

108

in. In order to guarantee smooth installation, the bridge body had to set inch-perfect and at the correct angle. After applying the breathable film seal and mounting the railing girders, the prefabricated superstructure was transported to the installation site and lifted into place by a mobile crane. The steel brackets bolted to the wooden body served to set down and align the segment on the two abutments. With the concreting of the tie-in areas, the force-fitted connection to the reinforced concrete abutments was established. The walking surface, which is subject to high mechanical loads, consists of prefabricated carbon fibre-reinforced fine-grain concrete slabs. To ensure slip resistance even in wet conditions, the top of the slabs, which are about 3 ≈ 3 m in size and 7 cm thick, is sandblasted; no further anti-slip and crack-bridging coating is required. The joints between the slabs are caulked with lead wool.

Cross-section Longitudinal section Scale 1:50 1 Substructure steel girder RHP 140 ≈ 80 ≈ 5 mm 2 Prefabricated slabs of textile concrete with 2 % transverse slope on both sides 3 Neoprene overlay 5 –10 mm 4 Breathable film seal 5 Superstructure, ­glulam, 13 ≈ 20 cm, block-glued 6 Handrail, glulam, larch 2≈ Ø 90 mm, screwed to U-steel profile 7 Railing, double posts of flat steel 10 mm 8 Rope net, stainless steel, mesh size 40 mm

“Stuttgart Wooden Bridge” in Weinstadt (DE)

109

Founded on History Restoration of the chain bridge in Park an der Ilm in Weimar, Germany

“How often do historic bridge structures disappear from our environment – partly out of ignorance, partly due to a lack of knowledge? This makes the fundamental restoration of the so-called Schaukelbrücke (lit. “rocking bridge”), an identity-defining part of the Ilm Park in Weimar, all the more gratifying. This structure was also threatened with total loss, as the steel tension members were initially to be reconstructed true to the original. However, through innovative concepts in material testing and an engineering approach, it was possible to prove the load-bearing capacity of the old chain links. And now the old bridge rocks again – as it has for almost 200 years.” Thorsten Helbig

The historic bridge structure On the edge of Weimar’s old town lies the 48-hectare Park an der Ilm. Duke Carl August and Johann Wolfgang von Goethe realised their horticultural ideas here and created a unique landscape garden. In the southern part of the park is the chain bridge, built by Karl Friedrich Christian

Ground plan • View Scale 1:200

110

Steiner in 1833 – one year after Goethe’s death. The suspended structure spans between two pylons and consists of three forged tension bands on each side of the bridge, which are made of puddle steel. An upper and lower tension band level is arranged to the side of the wooden tread. The tension bands, which are hinged

Design and con­ struction: Karl Friedrich Christian Steiner, DE-Weimar Renovation design, conception, compo­ nent tests: Marx Krontal Partner, DE-Weimar Oliver Hahn Checking engineer: Wolfgang Krüger, DE-Weimar Client: Weimar Classic Foundation, DE-Weimar

Chain bridge in Weimar (DE)

111

together with forged nodes via bolts, are guided over the pylons into the subsoil and back-anchored there to heavy-duty blocks and small bored piles. Hangers are attached to the joints of the tension bands, which hold the 15 wooden cross-beams. The running ­surface of the superstructure is designed as a girder grid with a plank covering. This construction, which is susceptible to vibrations, is why the structure has become known as “the rocking bridge”. The then new construction method for suspension bridges with iron chain links was used in England from about 1740 (Winch Bridge) and from the beginning of the 19th century also in North ­America. With its original suspension links, the chain bridge in Weimar is one of the ­oldest functioning suspension bridges in Europe. The initial planning situation Due to the worn-out pin connections of the suspension chains, a repair of the swing bridge was necessary. In addition, a flood in 2013 damaged the bridge to such an extent that replacement or upgrading became unavoidable. The planning of the repair initially provided for the dismantling of the suspension structure, the renewal of the decking and a ­subsequent restoration with restored chain links. Structural engineers then determined the maximum tensile forces for the chain links in pedestrian bridges and found that the permissible stress for puddle steel of 75 N/mm2 as applied to the full crosssection did not provide mathematical proof of the load-bearing capacity. As a result, plans were made to replace the historic, handcrafted chain links with new, externally similar steel eye bars. With a reconstruction, the suspension bridge would have looked like the historical model but this solution was not satisfactory in terms of heritage conservation.

112

Experimental set-up Scale 1:15 1 Press and load cell 2 Threaded rod M20 3 Connecting nut M20 4 Transport eye, rotatable 5 Chain shackle 6 Chain link L = 198 cm 7 Measuring points

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Procedure for alternative verification of the load-bearing capacity The client, the Klassik Stiftung Weimar, hoped to be able to avoid replacing the original chain links with reconstructed ones by means of alternative consider­ ations and verifications. To this end, the ­engineers implemented an experimental, step-by-step verification concept as a ­substitute for the unsuccessful mathemat­ ical verification. Consistent coding of all components ensured that the specified target loads could be checked after their removal and that they could be precisely reinstalled after completion of the test procedure. High-resolution measurements success-

fully secured the procedure against brittle failure of the pre-damaged chain eyes. The suitability of the test and measurement concept was confirmed using selected chain links in the first stage. After cleaning and inspection by the metal restorer, the engineers carried out individual tests on all 88 chain links and 176 bushings as the next step and documented this in a forceelongation diagram. In this way, compliance with the specified limit elongation of 0.2 % could be proven. With the step-by-step procedure and the experimental proof of the load-bearing capacity, it was possible to preserve the original building fabric against the background of new safety requirements.

Chain bridge in Weimar (DE)

113

Over the Abyss Footbridge at Tintagel Castle, United Kingdom

“The UK’s mainland and the historic Tintagel Island are, almost, reconnected. The elegant sweep of this ­pedestrian bridge’s lower chord suggests an arched structure but at the apex there is a 4 cm gap. The two cantilevers are coupled only by shear studs. A dramatic as well as poetic gesture; a successful staging of the meeting of mainland and island, of the present and history. The carefully developed details and the successful material concept make it a compelling design.” Thorsten Helbig

114

Tintagel Castle lies almost on an island on the north-west coast of Cornwall, not far from the village of Tintagel. Only a narrow isthmus connects the castle ruins, one of the most impressive historical sites in the United Kingdom, with the mainland. Perched on steep cliffs, it has defied the forces of nature for centuries. Legend of King Arthur According to the Arthurian legend written as early as the 1130s, King Arthur was ­conceived here when Uther Pendragon, king of Britain, allowed himself to be transformed by the magician Merlin into the form of Gorlois, Duke of Cornwall, for one night so that he could spend it with his wife Igraine. By the Middle Ages, however, the site was losing its status. Hardly any of the Earls of Cornwall regularly stayed in the castle, whose unstable location on the cliffs caused problems time and again. Nevertheless, finds from the Mediterranean indicate that Tintagel was frequented by merchant ships. By the end of the 14th century, the castle was being used to house high-ranking prisoners. By 1600, the castle had been abandoned and fell into disrepair. Today, the spectacular location and the mystical atmosphere that emanates from Tintagel attract many visitors.

A new ancient connection In the Middle Ages, the inhabitants of ­Tintagel crossed an isthmus from one side to the other. This narrow approach was so distinctive that it gave the fortress the name “Cornish Din Tagell” or “Fortress of the Narrow Entrance”. However, this connection has not existed since the 15th century, so visitors to the castle have to climb the cliffs via many steep steps to get from one side of the complex to the other. In order to re-establish a connection, the client, English Heritage, announced a competition in 2015, which Ney & Partners won in conjunction with ­William Matthews Associates.

Design and con­ struction: Ney & Partners – Laurent Ney, BE-Brussels William Matthews Associates, GB-London Team: Mathieu Mallié and

William Matthews (project management), Bart Bols, Karl ­Burgmann, Aline Roger, Taysir Ahmad, Aude Joannès Client: English Heritage, GB-Swindon

Tintagel Castle Bridge (GB)

115

116

View • Floor plan Scale 1:500

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Form finding and design idea A solution involving an overhead structure was out of the question right from the start. On the one hand, this would create a visual conflict with the ruins, and on the other, the future construction was intended to correspond with the steep rock faces and form one unit. A cable-stayed or suspension bridge was also ruled out because the anchoring forces in the uppermost soil layers would have been too high. In addition, it had to be taken into account already in the design phase that delivery and assembly would be extremely difficult at this exposed and remote location.

So the planners opted for a modified arched support structure. This type of structure has proven itself over centuries and seemed optimal for bridging the 190 m high precipice. In contrast to a classical arch bridge with gap closure in the middle, two cantilever arms with a gap of 40 mm in between were applied here. The advantage of this variant is that due to the lack of a gap closure, no constraining forces arise between the arch and the superstructure, while the top and bottom chords of the truss girders absorb the constraining loads. In the gap, only two small connectors transmit the shear force to ensure compatibility

Tintagel Castle Bridge (GB)

117

Sections Scale 1:50

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Detail sections Scale 1:20 1 Handrail, oak 20/45 mm 2 Railing support, stainless steel 15 mm 3 Box girder, flat steel ¡ 20 mm 4 Stainless steel profile | 30/30 mm 5 Floor covering, slate slabs vertically layered in stainless steel frame 75 mm 6 Stainless steel profile support } 60 ≈ 70 ≈ 10/15 mm 7 Steel profile ¡ 40 mm 8 Connection shear stud, stainless steel Ø 50 mm

7

of the deformation between the two canti­ lever girders. In addition, the gap also has a symbolic meaning: it is meant to embody both the tran­sition from the mainland to the island as well as from the present to the past. The support structure originates at the rock face with a height of 4.50 m and tapers towards the centre, where it measures just 17 cm. All 18 parts of the bridge were prefabricated in the factory and assembled on site from the mainland using a cable crane and helicopter.

3

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sense of security. The pavement is made of vertically laid slate slabs from the region, embedded in stainless steel boxes. This type of finish is often found in Tintagel. It provides a durable, non-slip surface while creating a link to the Middle Ages when this material was also used.

Materials The lower and upper chords are each made of steel with an anti-corrosion coating. The cross-shaped openings between the upper and lower chord are made of stainless steel. For the railing, square, 15 mm thick intersecting stainless steel filler rods were used. The inward-facing bars accommodate the wooden handrail, while the outward-sloping bars with a height of 120 cm give the user a

Tintagel Castle Bridge (GB)

119

260

Topography Leads to Function Queensferry Crossing near Edinburgh, United Kingdom

64

“The new crossing over the Firth of Forth completes a unique ensemble of bridges with support ­structures from different eras. The construction impresses with its central arrangement of slender pylons and cable levels, and the cantilevered directional carriageways on both sides. Due to the statically ­necessary overlaps of the three almost equally large, independent cable compartments, they combine to form an overall structure. A compelling example where a novel and expressive form is created through engineering skill.” Michael Kleiser

Opened in September 2017, the Queensferry Crossing road bridge spans the Firth of Forth and connects Edinburgh with northern Scotland. It supplements an overloaded suspension bridge from 1964, which is now only used by buses, bicycles and pedestrians. The world-famous Forth Bridge is also in the immediate vicinity. This red cantilever bridge is reserved for rail traffic and was the bridge with the largest span worldwide when it opened in 1890. A unique ensemble of three different bridge constructions from three centuries has now been created. The decision in favour of a cable-stayed bridge was based not only on economic advantages but also on functional requirements and topographical conditions: a rock jutting out of the water offered the ­possibility of load transfer in the middle of the Firth. Since the engineers were ­aiming for a symmetrical overall system – not only for aesthetic reasons – a span of twice 650 metres was specified with a view to the navigation openings required on both sides. This resulted in the location of the two outer pylons and the elevated foreshore area on the south side. With a total length of 2.64 km, the cable-stayed bridge is the longest in the world with three pylons.

120

Support structure with three pylons A characteristic feature of the construction are the two triangular fields of cables overlapping each other in a diamond shape. This detail answers a fundamental problem of multi-span cable-stayed bridges: in the case of one or two pylons, these can be back-anchored by restraining cables via the – usually landside – rigid side spans. With three pylons, this possibility of stabilisation is not available for the centre pylon. Pos­ sible solutions are a very rigid deck, very rigid pylons or different types of bracing. Here, the increased stiffness of the overall system is achieved by superimposing the stay cables in the centre of the main spans.

Design and con­ struction: Jacobs Arup Joint ­Venture, GB-Edinburgh. Leonhardt, Andrä und Partner, DE-Stuttgart Rambøll Group, DK-Copenhagen Rambøll UK, GB-Southampton Sweco UK, GB-Leeds Execution: Hochtief, DE-Essen

American Bridge ­International, US-Coraopolis Dragados, GB-London Morrison Construction, GB-Edinburgh Test engineer: URS, Aecom, GB-London Client: Transport Scotland, GB-Glasgow

80

90

87

87

87

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104

223

650 202.267 OD

650 210.717 OD

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202.267 OD

View Scale 1:10,000

Queensferry Crossing near Edinburgh (GB)

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101.5

SA S8

S7

S6

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S3

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S1

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NT

CT

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N1.1

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SA S8

S7

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NT

CT

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0,66%*

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SA S8

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S3

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S1.2

ST

ST.2

0,66%*

0,66%*

Bearing concept The centre pylon is monolithically connected to the superstructure and absorbs the entire loads from the roadway at this point. The outer pylons, on the other hand, are separated from the superstructure by a 70 cm wide joint to avoid constraining loads from temperature changes. The superstructure is supported here exclusively by the cables, while horizontal forces due to wind are introduced into the pylons by vertically aligned bearings. Due to this floating support, the torsion of the superstructure from eccentric traffic loads or wind must be absorbed by the v-shaped bridge piers S1 and N1. To counteract uplifting forces, the bridge body here is braced downwards with a prestress in the 51MN v-shaped piers. Expansion joints are only

122

provided at the two abutments, resulting in a total expansion distance of 2,270 mm at the southern abutment, which is 1,560 m from the fixed point. Structural elements The superstructure has a three-cell composite cross-section in the cable-stayed area with a 30 m wide, box-shaped steel girder and a concrete slab projecting approx. 5 m on both sides. Four longitu­ dinal webs form the central spine. The cable anchorages of the central pylon are fixed on the inside of the two inner webs, those of the outer pylons on their outside. This staggered arrangement allows the cables to overlap. The concrete slab of the carriageway is prestressed in the transverse 51MN direction to avoid cracking due to the central suspension and a resulting reduction in torsional stiffness. The 210 and 202 m high reinforced concrete pylons have an upwardly tapering cross-section with a wall thickness of max­ 51MN 51MN imum 2.40 m. In the upper area, a steel box girder is arranged for anchoring the cables via head bolt dowels. The total of 288 inclined cables are designed as seven-wire parallel strand ­bundles. The lowest number of strands (45) is found in the short cables at the

Monolithic connection middle pylon (top) Floating bearing at outer pylons (centre) System of v-shaped anchoring piers ­(bottom)

51MN

N2.2

CT

51MN

NA.4

Ground plan • View Static system SA S8

S7

S6

S5

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S2

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ST

CT

NT

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51NM

Queensferry Crossing near Edinburgh (GB)

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3.92

3.92

Section superstructure Scale 1:250

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1.62

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5.65

4.90

1.62

39.80

­ entral pylon, which dissipates the entire c vertical loads of the superstructure in its catchment area. In contrast, the short cables on the two outer pylons carry the greatest loads. With 109 strands, these are also the thickest cables. However, the stresses relevant for the design of pylons, cables and the superstructure often did not occur in the finished state but during the construction process. Construction While the foundation of the central pylon could be made on the rock protruding from the water, caissons with a diameter of up to 30 m were required for the outer pylons

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and the first southern anchorage pier. The pylons themselves were constructed using inner and outer climbing formwork. In the area of the cable anchorages, hollow steel boxes replaced the inner climbing formwork. The maximum 16.20 m long steel elements of the superstructure were prefabricated in China and brought to Scotland by sea. The concreting of the deck onto the upper chords was carried out in a purpose-built factory in the nearby port of Rosyth. A floating crane, more than 80 m tall, installed the first four segments of the superstructure on each of the three pylons together with a temporary working platform and the two

5.17

Horizontal section of pylon at a height of 120 m and 180 m 9,200 1,300

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950

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950

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950

Queensferry Crossing near Edinburgh (GB)

3,300

5,200

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3,300

5,200

derrick cranes required for the cantilever construction. After the deck of the launch segments had been concreted on site as a special case, they could be suspended from the first cables and lifted from the working platform. Then the cyc­ lical process of cantilevering began: the superstructure segments were shipped to below their location on pontoons, lifted to assembly level with the derrick cranes, aligned, fixed with bolted lug connections, welded and, after concreting the joint between the segments, finally suspended on the cables. Now the derrick crane could be moved to the new leading edge for lifting the next segment. For closing the gaps in the main spans, both cantilever ends had to be brought to the correct height. The final gap closure took place in February 2017 and the bridge was opened by Queen Elizabeth II on 4 September 2017.

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In a High Arch Over the Gorge 14.20

Tamina Bridge between Pfäfers and Valens, Switzerland

“The asymmetry of the arch results from considerations of a simple positioning of the abutments in the ­existing terrain from a manufacturing point of view and thus creates the unmistakable character of the bridge. The radial arrangement of the elevations additionally contributes to its dynamic appearance. Due to the heel-less fusion of the arch with the bridge deck and the statically determined haunches, the bridge appears as a monolithic overall unit.” Michael Kleiser

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16.75

53.50

51.55

44.55

42.69 2

45.3

57.15

56.77

44.60

49.20

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60.45

15.10 12.55

42.7

2 43.

05

43

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Section Scale 1:2,000

Design and con­ struction: Leonhardt, Andrä und Partner, DE-Stuttgart dsp Ingenieure + Planer, CH-Greifensee Smoltczyk & Partner, DE-Stuttgart Test engineers: Thomas Vogel, CH-Zurich Pascal Klein, CH-Zurich Geotechnical ­engineers: Dr. von Moos, CH-Zurich Client: Tiefbauamt Kanton St. Gallen, CH-St. Gallen

To go right or left? Until recently, this is the question that visitors and residents of the Tamina valley in the Swiss canton of St. Gallen had to answer as soon as they reached the spa town of Bad Ragaz at the mouth of the valley. Two roads lead up into the Tamina valley, one to the left to Pfäfers, one to the right to Valens. Only far beyond, at the Mapragg reservoir, does the road cross the valley. Since the summer of 2017, the Tamina Bridge with a total length of 472.60 m swings over the gorge. The ­villages of Pfäfers and Valens are suddenly only 1 km apart. Integration into the landscape In May 2007, the canton of St. Gallen invited tenders for an international competition, which was won by the Stuttgart-based en­­ gineering firm Leonhardt, Andrä und Partner. Particular attention was paid to the sensitive integration of the new bridge construction into the landscape conservation area. The asymmetrical construction follows the different heights of the valley flanks and incorporates the angle of inclination of the rock formations through the slanted position of the abutments. The generous openings between the arch and the superstructure create a filigree and transparent appearance.

Static system The static system of the bridge is composed of the arch structure and a continuous girder monolithically connected to it via uprights. The reinforced concrete arch has a span of 259.36 m and is clamped into the ground at the abutments, which transfer the loads from the arch into the unweathered rock. As a result of the clamping, the arch has its greatest overall height of 4 m at the start of the abutments, while this decreases to 2 m towards the apex. In order to save weight, the arch is designed as a hollow cross-section from the abutments to the uprights; in between – in the apex area – the planners chose a solid cross-section due to the low construction height. The inclined abutment piers are ­connected to the superstructure to form a

Tamina Bridge in the canton of St. Gallen

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Construction site sequence A Concreting of the abutments and abutment piers B Construction of the side spans C Construction of the arch using the cantilever method D Arch closure E Construction of the superstructure and dismantling of the auxiliary pylons F Construction of the superstructure above the arch using falsework G Completion

rigid frame construction, thus enabling the side spans to be bridged without ­supports. The superstructure is designed as a box girder cross-section with a floor slab width of 5 m and a standard construction height of 2.75 m. It has a length of 417 m. At the ends of the bridge, it rests on longitudinally movable spherical bearings, one of each two being transversely fixed. Construction The development of the bearing and ­abutment construction sites on the steep slopes as well as the entire construction process of the bridge posed a special logistical challenge for the planners and contractors. For the construction of the abutments, the abutment piers, the side spans and also for the delivery of the first arch segments, the contractors decided on a combin­ ation of tower crane assembly and erection by means of a cable crane. The highest self-supporting tower cranes ever erected in Europe were used, with a jib length of 75 m and a hook height of 115 m. Outside the slewing range of the tower cranes, con-

struction had to continue with a cable crane. The arch was erected in cantilever construction with temporary bracing. The steel auxiliary pylons required for this stood laterally on the abutment foundations. The height of the pylons was approx. 107 m on the Pfäfers side and approx. 78 m on the Valens side. The forward-facing retaining cables absorbed the weight of the arch and transfer it via the pylon to the rear-­ facing restraining cables. After the arch was closed, the retaining and restraining cables as well as the auxiliary pylons were dismantled again. The superstructure in the side spans was constructed on a falsework. Construction of the superstructure in the arch area took place in sections using ­shoring supported on the arch. Concreting was carried out in sections from the apex of the arch outwards towards the abutments. Parallel to this, the massive radial arch supports were constructed with floor formwork. After four years of construction, the residents of Pfäfers and Valens celebrated the opening of the new Tamina valley landmark in the summer of 2017.

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Forces Form a Bridge Bridge over the A5 motorway near Wilfersdorf, Austria

“The A5.Ü20 bridge is a welcome change to the often monotonous series of overpasses and acts as a landmark with a high recognition value. The visualisation of the inner load-bearing condition in the outer form through the expression of the compression arch and the tension element in the middle of the bridge, as well as oblique, intersecting lines, convey a sense of excitement and conciseness. The s­ tatics, the loadbearing structure, and the design are interdependent, and can be clearly read from the building design.” Ludolf Krontal

In the course of the new construction of the A5 Weinviertel motorway north of Vienna, the Austrian motorway operator Asfinag had a new bridge built near Wilfersdorf, which was opened to traffic in 2017 and has become a landmark visible from afar thanks to its design and shape. The planners and executors succeeded here in using the engineer’s range of activities beyond pure calculation and dimensioning, and in exploring and implementing the design potential that even this seemingly mundane construction task holds. Design and shaping The new route near the municipality of ­Wilfersdorf runs in an approx. 11.50 m deep cut in the terrain. The slope flanks with an inclination of 45 degrees are stabilised with a cement-bound slope material. Due to this deep cut in the terrain, the ­planners decided on a bridge spanning both sides of the motorway, whose inclined struts take up the angle of inclination of the steep lateral flanks. Another decisive factor for the design was that, due to the soft bedding properties of the subsoil, not all of the horizontal force can be transferred into the ground. A corresponding proportion must be returned to the ledger and short-circuited via struts close to the

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ground. The dissipation of tensile forces at the abutment generates a frame effect in addition to the truss effect. Simulations and working models were used to investigate different strut inclinations and their three-dimensional effect in space. The proportions of the bridge could also be adjusted in this way. The division of the struts in the transverse direction of the bridge loosens up the appearance and ­provides space for the concealed routing of the drainage pipe and the central maintenance staircase. The shape of the superstructure with its convex bulge in

Design and con­ struction: ASFINAG Bau Management, AT-Vienna Öhlinger und Partner, AT-Vienna Mayer Engineering Services, AT-Vienna Test engineer: ABES Wagner & ­Partner, AT-Graz Client: ASFINAG Construction Management, AT-Vienna

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the middle of the bridge also reflects the combined load transfer from the bending beam and the truss as shown in the static model. Construction and fabrication In order to design the required components as optimally as possible at all points, the entire system and the reinforcement routing were processed in 3D during the detailed planning. Special attention was paid to the design of the constantly changing curvature of the diagonal struts at the transition to the support structure. In this way, the shape adapts optimally to the local forces, and stress concentrations are avoided. The optimal curvature could finally be deter-

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mined by means of polygon points based on a clothoid. The concreting of the bridge was carried out in three sections, starting with the abutments and continuing through the struts to the support structure. All side formwork and clothoid transitions to the earth strut and abutment were delivered to the construction site prefabricated and assembled on the bridge falsework. This bridge shows that despite various requirements, standards and guidelines as well as time and cost pressures, innovative engineering can be achieved.

Geometry of the clothoid at the ­compression struts Scale 1:40

Vertical section Bridge centre Scale 1:50 1 Exposed concrete, strength class C 30/37 2 Surface and binder course 30 or 60 mm Protective layer 30 mm Waterproofing 2-layer 10 mm 3 Filter concrete, plastic-bonded 500/30 mm 4 Edge beam, precast concrete 250 mm 5 Guardrail, steel 6 Railing, steel with splash guard

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Emblematic Design Te Matau ā Pohe Bridge in Whangarei, New Zealand

“Building on sacred ground: The brief for this NZD 32 million project in New Zealand couldn’t have been clearer: it was to provide a bypass around the city of Whangarei in northern New Zealand, while expressing the art and culture of the local people. Except the central government funding excluded any extra provision for aesthetics and architecture.” Martin Knight

The city of Whangarei is located 160 km north of Auckland on the North Island of New Zealand. The Hātea River flows through the city, emptying into the Pacific Ocean at Whangarei Harbour. The Te Matau ā Pohe road bridge, which in the Māori language means “the fish hook of Pohe”, was opened in July 2013. It is named after the Māori chief Wiremu Pohe, who ­welcomed the first English settlers to the Whangarei region around 1830.

Design and con­ struction: Knight Architects, GB-High Wycombe, Buckinghamshire Sam White (project ­management) Peters & Cheung (now Novare Design), NZ-Auckland Duncan Peters (project management) Client:

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Whangarei District Council, NZ General contractor: McConnell Dowell Constructors, NZ-Auckland Transfield Services, NZ-Auckland Mechanical and electrical engineering: Eadon Consulting, GB-Rotherham Lighting design: Speirs + Major, GB-London

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Lower Hātea River Crossing in Whangarei (NZ)

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The bridge spans the Hātea River and connects Whangarei Airport with the eastern suburbs and the commercial area along Port Road via a bypass that was also newly built. It complements and relieves congestion on the Victoria Bridge further upriver in the city centre, where long traffic jams used to form daily. Emblematic design The decision to build a rolling bascule bridge was largely based on the client’s requirement that the new bridge would ­continue to provide access to the harbour basin for the many sailing vessels that ply the Hātea River. Unlike simple ­bascule bridges, where the bridge deck rotates point-like around the axis of the hinge, the rolling bascule bridge moves backwards at the same time as it opens by rolling over a support surface. This allows for shorter opening times so that motorists have as l­ittle waiting time as possible. In addition, the bridge structure with its curved arms tilted backwards along with the counterweights is reminiscent of the shape of traditional Māori fish hooks, thus creating a new identity-forming landmark for the city of Whangarei.

Rolling bascule bridge The bridge consists of three segments: the 120 m long eastern and western sections, and the 25 m wide opening in the middle. The two fixed bridge segments are designed as continuous girders with concrete piers and a steel superstructure. The latter is monolithically connected to the abutment. The movable part consists of two J-shaped hollow box girders made of steel with an orthotropic deck in between. On the outside of the J-girders, two cantilever arms made of steel accommodate the pedestrian and bicycle path. To open the bridge, the J-­girders roll along the main load-­ bearing direction onto the western bridge segment, thereby revealing a channel with a clearance width of approx. 20 m and unlimited height. Tooth-like counterweights at the ends of the cantilevered girders take the load off the folding section during the opening ­process, which reduces the amount of energy required. The bridge is lifted by two hydraulic cylinders on the underside of the structure. Parametric 3D CAD software was used to efficiently develop the geometry of the movable superstructure and to coordi­

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nate the position and size of the hydraulic cylinders. This made it possible to examine a large number of geometric variations within a short time and to find the best solution in terms of overall appearance and manufacturing costs.

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Railway Bridge on a Slender Footing Scherkondetal Bridge near Krautheim, Germany

“A linear and compelling bridge with a consistent style, which meets the purpose of a high-­performance line and bears the imprint of engineering excellence. With its joints and bearings reduced to an absolute minimum, low maintenance, and attention to detail from the overall composition to the d ­ rainage, the structure impresses with its compactness and the fine form accents of the haunches and piers.” Michael Kleiser

The Scherkondetal Bridge, commissioned in 2015, is located in the Weimarer Land district and is part of the Erfurt-Leipzig / Halle high-speed ICE line. North of Krautheim, it crosses the small river Scherkonde, which is dammed up to form a lake here. Design and load-bearing concept Compared to conventional road or railway bridges, high-speed railway bridges have to meet significantly higher requirements in terms of load-bearing capacity and serviceability. Ensuring dynamic sta­ bility and limiting structural deformations require extremely rigid structures, which is manifested by very massive component geometries. In contrast to the single-span girder chains commonly used in the past, the Scherkonde­tal Bridge breaks new ground with its semi-integral support structure: as is usual for semi-integral structures, the superstructure is monolithically connected

Design and con­ struction: DB ProjektBau, DE-Leipzig, Steffen Marx, Ludolf Krontal Detailed design: Büchting + Streit,

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DE-Munich Test engineer: Curbach Bösche ­Ingenieurpartner, DE-Dresden Client: DB Netz, DE-Leipzig

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to the piers and partly to the abutments. With this load-bearing concept, the bridge fulfils all static and dynamic requirements while being extremely slender and trans­ parent. Longitudinal force transfer The dissipation of the large horizontal forces resulting from the combination of braking, approach and constraining forces posed a particular challenge in the design process. The planners examined numerous variants with regard to the choice of fixed points and came to the conclusion that the superstructure would have to be monolithically connected to the western abutment and to eleven piers (from 01a to 10). The high forces are transferred with little deformation into the ground at the western abutment via two almost rigid, bored pile walls arranged in the longitudinal direction of the structure. Since the distance between the fixed point and the last connected pier is very large at 452 m, very high constraining stresses from temperature, creep and shrinkage occur in the monolithically connected piers. To min­ imise these, the piers and their foundations are designed to be very flexible and slender in the longitudinal direction of the structure.

The piers rest on single-row, vertically very stiff pile foundations, which also absorb constraining loads. Through the specific selection of the concrete’s aggregates, the modulus of elasticity was controlled so that the piers have softer stiffnesses and the superstructure has high, more effective stiffnesses. The reduction of bearings and joints allows for improved durability and thus a significant reduction in maintenance costs. Construction process The construction of the superstructure was carried out in sections, with the help of a scaffold mounted on brackets from the eastern abutment, where the temporary longitudinal fixed point was located for the construction of the superstructure. By changing the fixed point to the western abutment during the construction process, the constraints during construction could be minimised considerably. Each superstructure section was concreted in one go, without a construction joint. Other Deutsche Bahn high-speed railway bridges have since been built according to this innovative, semi-integral construction principle.

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View of movable ­scaffolding Scale 1:600 1 Scaffolding girder U 3000 2 Formwork girder HEB 550 3 Coupling joint 4 Suspension 5 Scaffolding on the pier

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The Younger Sister Second Hinterrhein Bridge near Reichenau, Switzerland

“Technology from two centuries coming together! The new construction shows how building tasks for railway bridges can be solved in a technically sophisticated way with a straightforward design. The development of bridges in terms of statics, construction and technology is on full display in this bridge ensemble.” Ludolf Krontal

In Reichenau in the Swiss canton of Graubünden, the Vorderrhein and Hinterrhein Rivers join to form the Alpine Rhine, which continues to make its way towards Lake Constance via the Chur Rhine Valley. For over 100 years, the Albula and Surselva lines of the Rhaetian Railway have also crossed here on the single-track steel truss bridge over the Hinterrhein. In order to renovate the historic bridge and at the same time ensure more time­ table stability and increase capacity, the Rhaetian Railway intended to remove this single-track bottleneck and build another single-track Rhine bridge. In addition to the new bridge over the Hinterrhein, the immediately adjacent crossing of the A13 national road – an existing prestressed

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reinforced concrete bridge – had to be replaced and redesigned to accommodate both sets of tracks as an integrated part of the overall ensemble. In February 2015, the operator launched an international competition, which was won by the Dissing+Weitling, Hager Partner, WaltGalmarini and Cowi UK consortium. Characteristics of the site Due to its location on two arms of the river, the topographically as well as culturally and ­historically significant town of Reichenau has always included numerous bridges. One of the most impressive is the existing listed railway bridge over the Hinterrhein dating from 1896. The three-span iron truss bridge is one of the last of its kind still in use for railway traffic. Two filigree ­lattice girders create a high degree of transparency. The new Hinterrhein bridge was to be a natural complement to this, as the two overpasses are always perceived as one and form an ensemble. It was also important to the awarding authorities that the new bridge construction should blend into the surrounding hilly landscape – the so-called Toma Hills – and that no further massive incisions in the form of retaining walls and rock removal had to be made.

Design and con­ struction: Dissing+Weitling, DK-Copenhagen WaltGalmarini, CH-Zurich COWI UK, GB-London Landscape architects: Hager Partner, CH-Zurich Client: Rhaetian Railway, CH-Chur

Second Hinterrhein Bridge near Reichenau (CH)

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Design of the new bridge The new bridge swings elegantly and ­simply over the Hinterrhein and the national road directly south of the historic railway bridge. This routing touches the river banks the least and leaves the ensemble of Reichenau Castle and the existing bridge structures spatially intact. The younger s­ ister – “Sora Giuvna” in RhaetoRomanic – thus blends harmoniously into its surroundings. The second Hinterrhein bridge consists of a slender steel trough that does not project over the parapet of the historic bridge. Two interlocking v-shaped steel struts, the so-called quadropods, support this steel trough construction. They in turn take up the height of the lower edge of the steel truss girder of the old bridge. The quadropods rest on concrete piers, which also correspond in position and orientation to the natural stone piers next to them. The concept was that – from whichever angle the two bridges are viewed – they should not interfere with each other but rather interact and form an ensemble that enters into a dialogue with each other thanks to the similar proportions and materials. Further on, the second Hinterrhein bridge also crosses the A13 national road. The

existing overpass had to be demolished due to adjustments to the clearance gauge and the changed track alignment. The new bridge accommodates both the old and the new track at this point. For this, the planners developed a second type of girder with the same external geometry as the river bridge. Due to the limited space available, this is designed as a V-shaped support. It is aligned parallel to the national road and is therefore at an angle to the superstructure. The V column limits the span width of the end span and thus enables a constant cross-section over the entire length of the bridge. Structural design The bridge is designed as a jointless, continuous steel girder with a u-shaped crosssection. The main support structure of the superstructure is formed by the two l­ateral box girders connected by a steel plate with transverse ribs. The quadropods support the main girder on both sides of the river bank. They were made of haunched steel box girders shaped like an inverted pyramid and welded to the box girders of the superstructure. The two almost identical river piers consist of a conical reinforced concrete

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wall, which transmits the forces from the superstructure via a pile head plate and two vertical large bore piles into the subsoil. Due to their shape, the river piers are soft enough to minimise constraints from the temperature-induced length change in the superstructure, but also stiff enough to limit the deflections of the ­carriageway. Two steel pin bearings are installed between each of the quadropods and the concrete piers.

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Railway Bridge With a View Getwing Bridge in Zermatt, Switzerland

“A distinct edge in front of the Matterhorn! The steel single-span girder – reduced to its constructional ­essentials – is robust and yet appears timelessly elegant. Despite the constraints of extremely tight space and very short assembly time, the Getwing Bridge succeeds very well in blending into its surroundings with a clear, striking form that makes the force flow tangible.” Thorsten Helbig

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In prominent proximity to the Matterhorn, the new Getwing Bridge is part of the ­narrow-gauge railway line between the ­Zermatt valley station and the Gornergrat railway station, located at an altitude of 3,089 m. The new bridge replaces an almost 120-year-old truss bridge that had dominated Zermatt’s townscape since the Gornergrat railway opened in 1898. However, the clearance height, which is too low according to today’s requirements, and the advanced corrosion of the steel parts made it necessary to rebuild the bridge. For this reason, the operators of the Gornergrat Railway decided to invite tenders in an international competition, which was won by the planning team of schlaich bergermann ­partner, SRP Schneider & Partner IngenieurConsult and mls architekten.

Initial situation and requirements For the planners, the local and time constraints posed a great challenge: due to its location in the narrow streets of ­Zermatt and the importance of the bridge for tourism as part of the important railway route to the Gornergrat ridge and the Matterhorn, construction could only take place within a limited time window. Therefore, the dismantling of the old truss bridge and the ­lifting of the new Getwing Bridge had to take place during a closure period of about 60 hours. Another requirement of the operators was to largely preserve and renovate the existing abutments made of natural stone. In addition, it was to be possible to extend this section of the line from a single track to a double track at a later date.

1.62 1.62

Design and con­ struction: schlaich bergermann partner, DE-Stuttgart SRP Schneider & ­Partner Ingenieur-­ Consult, DE-Kronach mls architekten, CH-Zermatt Client: Gornergrat Bahn, CH-Brig

24.00

Getwing Bridge in Zermatt (CH)

153

Ingenious single-span girder From a structural point of view, the 25 m long steel bridge is a single-span girder resting on four spherical bearings. The bridge body, which is made of twisted sheet metal, is characterised above all by its low overall height of a maximum of 1.80 m. The Getwing Bridge consists of two closed hollow steel boxes, which appear triangular in view and trapezoidal in cross-section. They are connected in the middle section by an 8.50 m long, pre-stressed tension band. At the deflection points, this undertension divides and transfers the tensile forces directly to the bearing points. Socalled compression strut plates in the centre of the bridge absorb the compressive forces resulting from the deflection of the tension band and simultaneously support the carriageway trough in the centre. At the deflection points, there is a three-sided frame in the box girder that additionally ­supports the carriageway trough at the ­tri-part points. To ensure a continuous ballast bed, a carriageway trough is located above the main support structure, which consists of two small hollow boxes with an orthotropic slab in between. The service walkways and cable systems are connected to the side. Calculation, fabrication, and assembly The bridge was modelled in 3D using the finite element method. The calculation of all necessary serviceability, load-bearing capacity, stability and earthquake verifi­ cations could thus be carried out on the model. The production process with the shortened installation of the tension band was carried out with great care in a hall, so that the bridge could be transported to Zermatt as a whole. The narrow serpentine roads presented a particular challenge. After excavating the old truss bridge and concreting the abutments, a truck-mounted crane lifted the new Getwing Bridge into its final position.

154

Compression strut plate Three-sided frame Milled tension band part

Web plate outside

00

1,0

Dissolved tension band

t=

Web plate inside

10

t=

15

Web plate outside

Dissolved tension band

Detail section of connection to tension band Scale 1:10 Vertical section Scale 1:20 1

5

60

3

150

2

127.5

4

345.5 473

  1 Web plate outside 15 mm   2 Web plate inside 10-15 mm   3 Steel frame, 3-sided 15 mm   4 Tension band, flat steel ¡ 60 mm   5 Compression strut, welded flat steel ¡ 20 mm   6 Hollow box girder, steel 15 mm Top flange, steel 30 mm   7 Flat steel ¡ 15 mm   8 Longitudinal stiffener, steel 15 mm   9 Cross member, steel section } 30/20 mm 10 Bracket to hold the service walkway 11 Railing, flat steel ¡ 10 mm

11

250 10

230

10

8

7

9

10

279

185

356

6

5

4

Getwing Bridge in Zermatt (CH)

155

Picture credits The authors and the publisher would like to express their sincere thanks to all those who have contributed to the production of this book by providing their images, by granting permission for reproduction and by providing information. All drawings in this work have been specially produced. Despite ­intensive efforts, we were unable to identify some of the authors of the illustrations but their copyrights are protected. We kindly ask you to inform us accordingly. Drawing on the cover: Tamina Bridge in the canton of St. Gallen (CH) Leonhardt Andrä und Partner, dsp Ingenieure + Planer , Smolczyk & Partner Designing Bridges 1  By Estec GmbH, cheap hotel in Prague – Own work, CC BY 3.0, https://commons.wikimedia.org/w/index. php?curid=7053219 2  form PxHere 3  Ludolf Krontal 4  Ralph Feiner 5  By Ezzeldin.Elbaksawy – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index. php?curid=92099689 6 https://pixabay.com/de/photos/parisbr%C3%BCcke-pont-neuf-seine-368384/ 7  A. Liebhart/pixelio.de 8  By Dinkum – Own work, CC0, https://commons.wikimedia.org/w/index. php?curid=20742587 9  By Behrad09 - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index. php?curid=52072141 10  Jean-Luc Deru/ Photo-daylight.com 11  By Mike Lehmann, Mike Switzerland – Own work, CC BY-SA 2.5, https://de.wiki­ pedia.org/w/index.php?curid=3891352 12 a  ETH Library Zurich, Picture Archive 12 b  ETH Library Zurich, Picture Archives / Photographer: Boissonnas, François-­ Frédéric / Hs_1085-1935-36-1-24/Public Domain Mark Bridges for Slow-Moving Traffic 1  By Axel Hindemith, CC BY-SA 3.0, https://commons.wikimedia.org/w/index. php?curid=80287773 2  By Vinayak Hegde - Flickr: A double decker living bridge, CC BY 2.0, https://commons.wikimedia.org/w/index. php?curid=18808200 3  Lothar Henke / pixelio.de 4  wikipedia Ochsenklavier_Pfrimmpark_CC BY-SA 4.0_Goldener Käfer.jpg 5  By Davepark – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.

156

php?curid=17874542 6  wikipedia CC BY-SA 3.0_Davepark 7 www.bernd-nebel.de 8  Petra Egloffstein 9  By MOSSOT – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index. php?curid=9403804 10  By Pufacz – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index. php?curid=21782053" 11  Junkyardsparkle Wikimedia Commons, Public Domain, https://upload.wikimedia. org/wikipedia/commons/ 8/8e/California_ Cycleway_1900.jpg 12  By Theo lauber – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/ index.php?curid=77913620 13  By Sunyiming – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index. php?curid=80878228 14  A.Windmüller / pixelio.de 15  Rasmus Hjortshøj – COAST 16  ipv Delft – Beeldtaal 17  Foster + Partners / Exterior Architecture 18  visualization Dissing+Weitling / POSCO, Engineering Division Road Bridges 1  iStock photo, Photo: Lukas Bischoff 2 a  from: Merckel, Curt: Die Ingenieurtechnik Im Alterthum. Berlin 1899, p. 301, fig. 108 2 b  agefotostock /Alamy Stock photo, Photo: Tono Balaguer 3 https://structurae.net/de/fotos/76196seinebruecke-neuilly 4  Tak /Adobe Stock Photo 5 a  Gesellschaft für Ingenieurbaukunst / Clementine Hegner-van Rooden 5 b  Eugen Brühwiler 6  Michael Kleiser 7  from Pauser, Alfred: Entwicklungs­ geschichte des Massivbrückenbaues. ­Österreichischer Betonverein, 1987, p. 89, fig. 96; Drawing: Michael Kleiser 8  ASFINAG / Drawing: Michael Kleiser 9  Data according to PIARC 2017 10  Ralph Feiner 11  By Michael from Germany – originally posted to Flickr as Skarnsundbrua, CC BY 2.0, https://commons.wikimedia.org/w/ index.php?curid=11460269 12  Photo by Maarten de Vries from ­FreeImages 13  By HK Arun - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index. php?curid=14202705 14  Michael Kleiser 15  By Storebæltsbroen.jpg: Alan Francisderivative work: pro2 - Storebæltsbroen. jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15320759

16  PantherMedia photo agency / iofoto 17  Michael Kleiser 18  Lutz Sparowitz 19 Walo 20  Michael Kleiser 21, 22  Sparowitz, Lutz; Freytag, Bernhard; Oppeneder, Johannes; Tue, Viet Nguyen: Quickway – smart mobility for the liveable city of the future. Proceedings, 25th Czech Concrete Days. 2018 Bridges for Rail Traffic 1  form PxHere, https://pxhere.com/de/ photo/663341 2 a, b www.letchworthparkhistory.com 2 c  By Andre Carrotflower - Own work, CC BY-SA 4.0, https://commons.wikimedia. org/w/index.php?curid= 82896860 3  Nicolas Janberg 4  Vincent Le Quéré 5 a  By Andreas Passwirth, Own work, CC BY-SA 3.0, https://de.wikipedia.org/wiki/ Langwieser_Viadukt#/media/Datei: ­Langwieser_Viaduct_Underneath.jpg 5 b  By a, https://commons.wikimedia.org/w/ index.php?curid=22012846 6  Marx Krontal Partner 7  Steffen Marx 8  According to DB Netz AG database 9 HyperloopTT 10  Max Bögl Group 11  Knight Architects 12  Hanno Thurnher Photography 13  Nicolas Janberg 14  Ludolf Krontal 15  SSF Engineers 16  Marx Krontal Partners 17  Nicolas Janberg 18  Marx Krontal Partner 19  SSF Ingenieur AG: Brücken mit VerbundFertigteil-Trägern. VFT-Bauweise. Project brochure 20  Marx Krontal Partner Bridges and Traffic 1  Lindsay Corporation 2  Markus Friedrich 3  (p. 45) various sources: – Network length: according to BMVI: Verkehr in Zahlen. Berlin 2018 and ­estimates by the author – Number of bridges: long-distance public transport: DB Netze 2018; cars on federal trunk roads: BASt 2019; cars on state, district and other roads, walking and cycling: Arndt 2013 and Difu 2015 (see Note 4; Bridges and Traffic, p. 159). – Passenger kilometres: author’s calculations based on BMVI: Mobilität in Deutsch­ land – MiD. Results Report. Bonn / Berlin 2019 and Bäumer, Marcus et al.: Fahr­

leistungserhebung 2014. Bundesanstalt für Straßenwesen. Bergisch Gladbach, 2017 4  FGSV No. 121 Forschungsgesellschaft für Straßen- und Verkehrswesen (ed.): Richt­ linien für integrierte Netzgestaltung (RIN). Cologne, 2008 5, 6  Markus Friedrich 7  ASFINAG, Statistics Austria Preservation and Evaluation of Bridges 1  According to BASt vol. B 68: Auswirkungen des Schwerlastverkehrs auf die Brücken der Bundesfernstraßen. p. 36 2  Marx Krontal Partner 3  According to Mark, Peter: Erhalt unserer Bausubstanz. In: Betonkalender 2015. Vol. 1. Berlin 2014, p. 8 4  Evaluation of data from Deutsche Bahn AG, research project: Digitale Instandhaltung von Eisenbahnbrücken - DiMaRB, Leibniz University, Hanover, 2019. 5 a  Ludolf Krontal 5 b ZKP 5 c  Laumer Bau 6 – 8, 10  Marx Krontal Partner 9  wtm-engineers, Marx Krontal Partner Impact 1  Michael Kleiser 2  Thorsten Helbig 3  According to Eurocode 1 DIN EN 19912:2010-12 4  According to historical sources and current standards, e.g. DIN EN 1991-2:2010 5  According to Eurocode 1 DIN EN 19912:2010-12 6  According to Krontal, Ludolf: Zum Entwurf von Eisenbahnbrücken. In: structurae ­Projektbeispiele Eisenbahnbrücken. Berlin 2014, p. 3 7 Wacker-Ingenieure.de 8  Thorsten Helbig 9  Nahrath, Niklas: Modellierung Regen-Windinduzierter Schwingungen. Dissertation. TU Braunschweig, 2004, p. 34 10  According to Richtlinien für den Entwurf, die konstruktive Ausbildung und Ausstattung von Ingenieurbauten (RE-ING) 2017. 11  According to Wenk, Thomas: Erdbebensicherung bestehender Bauwerke. Lecture notes. Zurich, 2000 12  Thomas L. Rewerts, of Thos. Rewerts & Co, LLC 13 a  From: Varela, Sebastian; Saiidi, Mehdi: Resilient deconstructible columns for accelerated bridge construction in seismically active areas. Journal of Intelligent Material Systems and Structures. 2017, vol. 28 (13), fig. 1 (C). Function 1  David Boureau 2  Marx Krontal Partner 3, 4  Essentially based on Eichwalder, ­Bernhard: Fugenlose Fahrbahnübergangs­ konstruktion für lange integrale Brücken. Dissertation. Institut für Tragkonstruktionen,

Forschungsbereich für Stahlbeton- und Massivbau. TU Wien, 2017, p. 27, drawing Michael Kleiser. 5 mageba 6 a ASFINAG 6 b  Michael Kleiser 7  According to Ril 804.5202 8, 9  Michael Kleiser according to RVS (AT), BASt Richtzeichnungen, Astra (CH), Deutsche Bahn, SSF, ÖBB, Kantbalk Typ 8 KTH, The Illinois State Toll Highway Authority 10  Michael Kleiser 11  schlaich bergermann partner / Michael Zimmermann 12  Marx Krontal Partner 13 Schréder Economic Efficiency 1  Kris Provoost 2  According to Pauser, Alfred: Eisenbeton in der ersten Jahrhunderthälfte. In: 100 Jahre Beton- und Bautechnik. Vom Beton-Eisen zum Spannbeton. Österreichische Vereinigung für Beton- und Bautechnik. Vienna, 2007, p. 127 3  According to Kessler, Anne; Marx, Steffen: Ingenieurwettbewerbe im Brückenbau. Eine Projektanalyse über Aufwand und Qualität. In: Deutsches Ingenieurblatt 10/2018, p. 36ff. 4  Alan Karchmer 5  According to FSV: RVS 13.05.11 – Lebens­ zykluskostenermittlung für Brücken. Directive. Österreichische Forschungsgesellschaft Straße – Schiene – Verkehr 2017 Sustainability 1  By AngMoKio – Own work, CC BY-SA 2.5, https://commons.wikimedia.org/w/index. php?curid=1253561 2  Data according to ÖKOBAUDAT 3 a  Hansjörg Lipp, CC BY-SA 2.0, http://geo.hlipp.de/photo/87371 3 b wilfried-dechau.de 3 c  Burkhard Walther 4 – 6  Data from Thorsten Helbig, Jana Nowak 7  From RVS 13.05.11: Lebenszykluskosten­ ermittlung für Brücken 8  From Van Eygen, Emile; Fellner, Johann: Ökobilanzierung im Brückenbau. Eine ­vergleichende Lebenszyklusanalyse einer Spannbeton- und einer Verbundbrücke. Study. TU Wien, 2019 Materials 1  According to Neuhaus, Helmuth: Engineered timber construction. Wiesbaden, 2017 2  Ronald Knapp 3  Thorsten Helbig 4  Schaffitzel Holzindustrie GmbH + Co. KG 5  Kusser Granitwerke GmbH 6  Conzett Bronzini Partner AG 7  schlaich bergermann partner 8  Marx Krontal Partner 9  According to Natursteindatenbanken, DIN EN 771-1, DIN EN 1992-1-1:2011, and DIN EN 206 10  Compilation Michael Kleiser

11  From Marti, Peter; Monsch, Orlando; Schilling, Birgit: Ingenieur-Betonbau. Gesellschaft für Ingenieurbaukunst. Zurich, 2005, illustration p. 47 12, 13  Michael Kleiser 14  schlaich bergermann partner 15  BridgeDesign2 by Joris Laarman Lab 16  Marc Lins, M+G INGENIEURE 17  ETH-Bibliothek Zurich, Image archive / Photographer: Unknown /Hs_1085-1933-218/Public Domain Mark 18  Lisa Ricciotti 19  Tsinghua University (School of Archi­ tecture)-Zoina Land Joint Research Center for Digital Architecture (JCDA) 20  According to Bau-Überwachungsverein – BÜV e. V. (ed.): Tragende Kunststoffbau­ teile. Heidelberg / Berlin 2014 21, 23 www.solidian.com 22 wilfried-dechau.de Designs 1– 40  Michael Kleiser Bridges in Detail p. 102 Hufton+Crow p. 105, 106 Burkhard Walther p. 107 top MPA Stuttgart p. 107 centre, bottom  Schaffitzel Holz­ industrie p. 109 top Wilfried Dechau p. 109 bottom  Burkhard Walther p. 111, 113 Alexander Burzik p. 112 Marx Krontal Partner p. 114 –115 Hufton+Crow p. 115 Ney & Partner, Laurent Ney p. 116, 118, 119 top Hufton+Crow p. 119 bottom  Ney & Partner, Laurent Ney p. 120 –122 top  Transport Scotland p. 122 bottom  Bastian Kratzke / LAP Consult p. 123, 124 left  PA Images p. 124 right, 125  Lukas Kohler / LAP Consult p. 126 Bastian Kratzke / LAP Consult p. 127, 128 top  Tiefbauamt Canton of St. Gallen p. 128 top centre, bottom centre, bottom Leonhardt, Andrä und Partner p. 129, 130 Tiefbauamt Canton of St. Gallen p. 131 Leonhardt, Andrä und Partner p. 132 –135 Michael Kleiser, Mayer ­Ingenieurleistungen p. 136 –137 Patrick Reynolds p. 138 top, centre  Knight Architects p. 138 bottom, 140  Patrick Reynolds p. 141 top Peters & Cheung Ltd. p. 141 bottom  Knight Architects p. 142 –143 Alexander Burzik p. 144, 145 bottom  Ludolf Krontal p. 145 top Alexander Burzik p. 146 Walt Galmarini p. 147 Stephane Braune p. 148 top, centre  Roman Sidler p. 148 bottom  Andreas Ludin p. 149, 151 Roman Sidler p. 150 Andreas Galmarini p. 152 David Hannes Bumann p. 154 –155 schlaich bergermann partner

157

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Authors Thorsten Helbig

Ludolf Krontal

Born 1967 in Nordhausen 1984 –1990  Bricklayer apprenticeship and assembly work in Erfurt 1990 –1994  Degree in Civil Engineering at FH Bielefeld / Minden campus 1994 –2001  Structural engineer at schlaich bergermann partner, Stuttgart since 2001  Founding partner of Knippers Helbig, with offices in Stuttgart, Berlin, and New York since 2018  Associate Professor at the Irwin S. Chanin School of Architecture, The Cooper Union, New York since 2020  American Society of Civil ­Engineers, Aesthetics in Design Com­ mittee

Born 1969 in Osterburg 1985 –1988  Apprenticeship as a carpen­ ter and work at VEB Denkmalpflege Magdeburg 1991–1998  Studies in Civil Engineering at Bauhaus-Universität Weimar and the Universitat Politècnica de València 1999 –2002  Office manager, Engineering office for bridge construction in Dessau 2002 – 2011  DB ProjektBau Leipzig, Head of the Structural Engineering Department since 2011  Managing Director and Partner of Marx Krontal Partner, based in Hanover and Weimar

Michael Kleiser Born 1967 in Vienna 1994  Degree in Civil Engineering at TU Wien 1994 –1997  Research activities at TU Wien, TU Aalborg and the University of California, San Diego 1998 – 2011  Structural engineering at schlaich bergermann partner, Stuttgart, and Ingenieurbüro Pauser / PCD-ZT, Vienna since 2011  Bridge expert at ASFINAG Bau Management GmbH, Vienna since 2014  Lectureship at FH Campus Wien, Department of Construction and Design 2017  Dissertation at TU Wien: Formlogik und Formdynamik am Beispiel von integralen Überführungsbrücken (Form logic and form dynamics exempli­ fied by integral overpass bridges) since 2017  Lectureship at TU Wien, ­Institute of Structural Engineering, with ­lecture on “Ingenieurformkunst” (“Struc­ tural Form Art”), among others.

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Markus Friedrich Born 1967 in Munich 1983 –1989  Studies in Civil Engineering at TU Munich 1994  Doctorate in Engineering Subject: Rechnergestütztes Entwurfsver­ fahren für den ÖPNV im ländlichen Raum (Computer-aided design procedure for local public transport in rural areas) 1989 –1995  Research assistant to the Chair of Transport and Urban Planning, TU Munich 1995 – 2003  Head of the Transport Planning Systems Division at Planung Transport Verkehr AG (PTV), Karlsruhe since 2003  Professor at the University of Stuttgart, Chair of Transport Planning and Traffic Control Engineering

Martin Knight Born 1967 since 2006  Director of Knight Architects, internationally renowned for bridge design Various teaching positions at schools and universities in the UK and Europe, including TU Delft 2017  Visiting professor at TU Graz Regularly holds lectures on bridge design