Building with Hardwood 9783955535605, 9783955535599

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 Building with

Hardwood Konrad Merz Anne Niemann Stefan Torno

∂ Practice

Authors Konrad Merz Anne Niemann Stefan Torno With contributions from: Hermann Kaufmann (Preface) Markus Lechner (High-Performance Materials with Potential for the Future) Stefan Winter (High-Performance Materials with Potential for the Future)

Editorial Services Editing, layout and copy-editing: Steffi Lenzen (project management and Example Builds texts); Claudia Fuchs (Example Builds), Jana Rackwitz (theory chapters); Sandra Leitte (Proofreading German edition), Charlotte Petereit (Editorial Assistant) Cover design following a concept by Kai Meyer, Munich Drawings: Rana Aminian, Ralph Donhauser, Sandra Gunnermann, Martin Hämmel, Ursula Sparakowski Translation into English: Susanne Hauger, New York (US) Copy-editing (English edition): Stefan Widdess, Berlin (DE) Proofreading (English edition): Meriel Clemett, Bromborough (GB) © 2021 DETAIL Business Information GmbH, Munich ISBN 978-3-95553-559-9 (Print) ISBN 978-3-95553-560-5 (E-book) Printed on acid-free paper made from cellulose bleached without the use of chlorine. This work is subject to copyright. The rights arising from this copyright are reserved, especially the rights of translation, reprinting, presentation, extraction of illustrations and tables, b ­ roadcasting, microfilming or reproduction by any other means, and storage in data-­processing systems, either in whole or in part. Reproduction of this work, or of parts thereof, even on an individual basis, is permitted only under the provisions of the copyright law in its current version. This is always subject to remuneration. Infringements are subject to the legal sanctions of copyright law. This reference book takes into consideration the terms valid at the time of the editorial d ­ eadline and the state of the art at this point in time. Legal claims cannot be derived from the content of this book. We assume no ­liability for any errors or omissions. Typesetting & production: Simone Soesters Printed by: Grafisches Centrum Cuno GmbH & Co. KG, Calbe 1st edition, 2021 Paper: Peydur lissé (cover), Profibulk (content) A German edition of this book is also available (ISBN 978-3-95553-504-9). Bibliographical information of the German National Library The German National Library lists this publication in the German National Bibliography; d ­ etailed bibliographical data is available on the Internet at http://dnb.d-nb.de. DETAIL Business Information GmbH Messerschmittstr. 4, 80992 Munich, Germany Tel: +49 89 381620-0 detail-online.com

Contents

  4   6

Preface

 18

Forest Management Background

 20

 igh-performance Materials with Potential for the Future H New Developments and Improvements in Structural Hardwood Products  20 Bonding of Hardwoods  21 Connections 21 Durability and Moisture  22 Joinery and Processing  22 Regulations for Use  23

 24  30

Yesterday – Today – Tomorrow The Historical Use of Hardwood as a Building Material  7 Hardwood in Modern Timber Construction  11 Outlook and Relevance  15

Wood Species  ardwood Construction Products H Sawn Lumber and Finger-Jointed Solid Wood  30 Glued Laminated Timber and Hybrid Glulam  33 Beechwood Laminated Veneer Lumber  35 Engineered Timber Products of Hardwood  37 Information on Use  39 Durability 41 Emissions 43

 44 Building with Hardwood Reasons for Using Hardwood  45 Hardwood Products in Use  47 Connections 52   54

Example Builds Ten Real-life Examples

104

Appendix Authors 105 Approvals, Standards  106 Bibliography, Links  107 Picture Credits  108 Acknowledgements   109 Subject Index  110

Authors’ note for the international e ­ dition of the DETAIL Praxis publication Building with Hardwood:

This book deals mainly with regulations in Germany and Europe. There are several reasons for this. The existing high proportion of hardwood and its expected increase due to an active restructuring of the forests has reinforced the desire to put the material to sensible use. This goal has led to the relatively rapid development – compared to that in the case of softwood – of a research environment focusing on the potential suitability of hardwood for timber construction. Critical factors were the already historically documented high strength and stiffness properties of hardwood, and the fact that Europe’s construction sector was a large consumer of timber. In other words, there was also an economically attractive sales market. This development continues to this day. In addition, research is expected to significantly expand the application possibilities of wood in the construction sector as a whole – owing to the socially and culturally increasing significance of timber engineering. Thanks to the system of European standardisation and the building legislation in individual countries, research findings were put into practice relatively quickly and the first products and applications were developed. Although the field of structural hardwood construction is still of manageable size, it is steadily increasing, especially as efforts to improve and develop new products continue in research and development as well as in industry. From a global perspective, though countries with traditionally high timber construction rates such as Canada and the United States also use hardwood for ­construction, their applications, building methods and products differ from those in Europe. Wood is used for the most part in one and two-storey residential buildings of timber frame construction, so that on the one hand, the demands on the per­ formance of the products are not very 4

high, and on the other – especially for the framing timbers – the wood most commonly employed is softwood. In addition to the framing timbers, wood composite mate­rials in panel form (plywood, OSB) are used, some of which are made of hardwood. Overall, however, hardwoods are used mainly in interior finishing and rarely for the support structure. This book can be viewed as a “general case study”. The showcased products and key figures, and the possibilities that they represent, can often be transferred to other wood types, situations and countries. Individual adjustments will be required, such as the determination of characteristic values for new wood species, as well as the reconciliation among regionally varying rules and regulations. However, this in turn could give rise to further benefits, for example the use of even stronger timber varieties. So please consider this book as a welcome encouragement to you to become active in the “application of hardwoods in construction”. The Authors

Preface

Increasingly, construction is becoming a question of raw materials, because the demand for space and thus for building materials will continue to grow rapidly worldwide. The modern world has for­ gotten that the earth possesses a huge, constantly growing reservoir of raw materials that are also very suitable for building. For a hundred years, research and development focused mainly on modern

materials, while timber, once the most important building material, was virtually forgotten. Renewable materials represent a source of hope, as they symbolise a commitment to solving the pressing issues of the future having to do with essential decarbonisation and the growing shortage of resources in the construction sector. The increasing demand for timber

constructions is unmistakable evidence of a renaissance in this oldest of building materials. Coniferous or softwood is and will likely remain the primary material for building applications because it has superbly well-suited properties and is easy to work with. Compared to hardwood, it is more economical because the yield from the ­regular, straight-growing tree trunks is significantly greater. On the other hand, depending on the type of tree, valuable hardwood is available in large quantities, but is currently too often under-utilised as a material, too often burned or wasted in the manufacture of short-lived products. The reasons for this are alleged ­disadvantages in processing (due to the hardness of the material), greater drying costs as well as the lack of dimensional stability of the products. That must and will change. The outstanding properties of hardwood can lead to the optimisation of existing timber mate­ rials or to completely new products. Thanks to developments such as lami­ nated veneer lumber produced from beech, for the first time in history the ­second most common species of tree in Central Europe can now be used in ­timber construction. Hardwood allows for amazingly slender and aesthetic ­constructions that are very e ­ conomical. Material combinations of softwood and hardwood will also open up new structural and design options in architecture. This book presents the status quo for building with hardwood. It is intended to stimulate new ideas and contribute to a future in which the increasing demand for ecological and climate-neutral construction can rely on a broader palette of available tree species.

Hermann Kaufmann 5

6

Yesterday – Today – Tomorrow

Many people are under the impression that the use of hardwood in the construc­ tion sector is a relatively recent develop­ ment. It has only been a few years since modern hardwood products have been used in timber construction in Europe to a small but growing extent. The know­ ledge about their properties and possible applications is correspondingly small. The number one wood in construction is spruce, used as solid beams and boards, bonded to form glued laminated timber (glulam) or cross-laminated timber, or used in timber composite materials. From a historical perspective, however, hard­ wood has always been a part of timber construction. The associated practical experience has been lost over time.

The historical use of hardwood as a building material Wood is the oldest building material. ­Forests existed in most regions, and wood is relatively easy to work using ­simple tools. Naturally, the wood that was used was from native trees – not least for lack of transportation routes and methods. So people used whatever was available due to the climatic and geological conditions in their immediate vicinity. Even in primitive buildings it is clear that early builders used their experi­ ence to select mater­ials that exhibited the best-suited char­acteristics. Finds from an excavation of a Neolithic settlement yield evidence that 13 different types of wood were used to build the huts [1]. The main materials were willow, alder and hazelnut, which were easy to work with due to their flexibility, yet had sufficient structural strength. Over time, experience with the timber varieties and knowledge of their properties increased. In his Ten Books on Architecture, Vitruvius describes how the wood species that were com­

mon at the time were to be used: “Thus it is with the oak, elm, poplar, cypress, fir, and the others which are most suitable to use in buildings. The oak, for instance, has not the efficacy of the fir, nor the cypress that of the elm. Nor in the case of other trees, is it natural that they should be alike; but the individual kinds are effective in build­ ing, some in one way, some in another, owing to the different properties of their elements.” [2] In pre-industrial times, wood was an indispensable raw material, which had a lasting influence on culture as both a building material and a fuel. The soci­ ologist and economic historian Werner Sombart therefore spoke of an “age of timber”. Historically, knowledge about the special material properties of particular species of wood was critical. In 1918, during a ethnological trip through the Bohemian Forest, researcher of local ­history Josef Blau realised that, “of the 40 enumerated species of wood, 37 are known to the people and 33 of them are known to have a use which, based on the nature of the species in question, is usually a particular one to which that wood is especially well suited.” [3]

city surrounded by the water of a coastal lagoon. Oak piles were rammed into the ground in order to anchor the buildings in the marshy substrate. Today, there is little visible indication of any of this: Due to the accumulation of silt, the city centre is now located 9 km from the coast. Other examples are the cities of Amsterdam, Rotterdam, St. Petersburg

1 2

 imber piles are rammed deeply enough into T the ground to reach the clay or sand stratum. Engraving of a woodcutter, included in John ­Ogilby’s collection of Aesop’s Fables, 1664

1 Oak

Unquestionably, the most important tim­ ber species among the hardwood trees used for construction was oak. Cities on piles Since oak is very durable and is even considered “indestructible” under water (Fig. 4, p. 8), it is not surprising that entire cities were built on wooden piles and that these constructions survived the intervening centuries almost unscathed (Fig. 1). At the time of its foundation, ­during the Roman Empire, the northern Italian city of Ravenna was located on the Adriatic Sea. Like Venice, it was a

2

7

3

and Copenhagen, the foundations of which also rest on hardwood piles. To this day, half of Venice is supported on oak and half on alder piles. Half-timbered construction Half-timbered construction, known since ancient times, was widespread from the Middle Ages until well into the 19th cen­ tury. Experienced carpenters knew that among the available woods – mostly fir, spruce and oak, occasionally larch – the wood of the oak is the most resistant to moisture and should therefore be used for the purlins. High-load components were also made of oak, while softwoods were used for interior fittings, as ornamen­ tation and for cladding. A very impressive example of the deliberate selection of ­timber var­iety and of its application corre­ sponding to its properties is demonstrated by a residential building in Franconia built between 1421 and 1423. Oak was used for the outer frame, especially for supports and struts, while aspen was employed for smaller, less heavily stressed supports. Spruce and fir were used for the interior construction, for beams, purlins and the entire roof structure (Fig. 3). A particularly magnificent example of a half-timbered house is the Knochenhaueramtshaus Wood species

(Butchers’ Guild Hall) in Hildesheim, which was established as a meeting place for the guild in 1529. Elaborate carvings decorate the facade of the 26-metre high building illustrating the wealth of the guild at that time. Located in the centre of the city, the building ­survived for several centuries thanks to its solid oak construction. The stepped projections of the upper storeys protect the supporting structure below from rain. It was only in 1945 that it was completely destroyed by fire following a bombing raid. In the historically accurate recon­ struction of the hall built during the 1980s, 400 m3 of oak were joined to form 4,300 connections using 7,500 wooden nails (Fig. 5). Domes and roofs For large spans or in very specific, highly stressed applications, the hard oak helped early builders to realise their works. Brunelleschi built the huge dome of the Cathedral of Santa Maria del Fiore in Florence from two concentric domed shells, whose 4-m thick inner shell supports the enormous overall weight. The base of the dome is held in place in part by an anchor chain made of oak, which is stable and at the same time suffi­

ciently elastic. This detail, small in the scheme of the overall construction but nevertheless important, can also be found in other domes. In the United Kingdom, where, much like in Germany, oak was a national symbol due to its importance in ship and house building, there are beauti­ ful examples of oak roof constructions. Among them is the Great Hall of ­Stirling Castle near Edinburgh, built in the 13th century, whose roof structure was consid­ ered the largest in Scotland at the time (Fig. 7). Westminster Hall in London is similarly impressive in this regard. The 73-metre long assembly hall was roofed over in 1395 with a support-free hammer beam vault of oak. Numerous barges and carts were needed to transport the exten­ sive materials from the county of Surrey to Westminster. The roof structure of Notre-Dame in Paris, which burned down in 2019, was still the original from the 13th century. Its reconstruction is further complicated by the fact that oak beams of the length and quality required are no longer available (Fig. 6). Reuse The procurement of timber was an ardu­ ous task in pre-industrial times. Hard oak timber, in particular, was expensive and

Service life (in years) when stored outdoors

Service life (in years) when stored in dry ­conditions (constant moisture content)

unprotected, untreated

unprotected, impregnated with creosote

covered

under water

Spruce

10 ... 15 ... 30

20 ... 30 ... 50

50 ... 60 ... 75

60 ... 100

100 ... 900

Fir

5 ... 10 ... 20

10 ... 25 ... 40

15 ... 50 ... 70

30 ... 60 ... 100

100 ... 700

40 ... 80 ... 100

150 ... 200... 300

350 ... 600 ... 1,000

800 ... 1,200

Birch

European black pine

3 ... 8 ... 15

5 ... 20 ... 30

20 ... 40 ... 60

300 ... 500

Oak

40 ... 80 ... 120

100 ... 150 ... 200

300 ... 500 ... 800

600 ... 1,000

Alder

5 ... 15 ... 20

7 ... 20 ... 30

10 ... 30 ... 40

100 ... 400

Copper beech

10 ... 25 ... 40

20 ... 40 ... 80

30 ... 70 ... 120

200 ... 700

4 The middle value in each case is the most frequently measured value.

8

Yesterday – Today – Tomorrow

6

5

difficult to process. Another possible means of gaining access to timber was through the reuse of wood recovered from building demolitions; this wood also had considerable dimensional stability due to its extended drying time. Indica­ tions of such reuse can be found in the form of mortises and similar toolmarks on beams that evidently date from a ­former usage period. Even today, there is demand for historical oak beams. On the eBay internet platform, for exam­ ple, a 470-cm long beam, aged 250 to 350 years old, of “excellent quality, with ­mortises and notches” can be bought for the hefty price of €1,750. The reuse of building components is absolutely ­logical in the context of a circular econ­ omy and presents an obvious potential for long-lasting hardwood components. Ideally, its later reuse should already be incorporated in the current design of a building component. Some internet sales platforms, such as materialnomaden.at or salza.ch, deal in individual building components and materials whose origins can be traced, thus providing designers with information on material sources and history.

isms, however, did not even allow its use as plank flooring if it was going to be scrubbed with water – much less for any outdoor application. At most, its characteristic pronounced swelling was advantageous in quarry work: In the Carrara marble quarries, the Romans were already exploiting this fact by driv­ ing wooden wedges into existing cracks in the stone. When water was poured over these, the swelling wood developed such an explosive force that it could be used to detach stone blocks from the surrounding rock. 7 Fuel

The large supply of beech trees was used mainly for energy, since wood was the most important fuel until the emergence of industrial coal mining. Beech was of particular importance due to its high heating value. In the ­Middle Ages and in early modern times, its wood ash aided in the production of washing lye and glass, for which the existing beech stands were cut down on a huge scale. Until the 1950s, beech even powered cars in Germany: Numer­ ous vehicles, among others passen­ ger cars and pickup trucks, featured a wood gas carburettor. In these, wood

Beech

From a historical perspective, beech was not used as a structural building material. It was too difficult to process this heavy wood, which has a tendency to split and warp, and so is unevenly dis­ torted during drying. Common areas of application included, for example, stair stringers, handrails and furniture, since beech can be readily bent when exposed to steam. The best known example of the development of so-called bentwood is Thonet’s No. 14 chair from 1859, also known as the Viennese bistro chair and sold by the millions up to the present day [4] (Fig. 8). The susceptibility of beech to moisture and wood-destroying organ­

3

4 5 6 7 8

 ak (red-brown), spruce (olive green) and O ­aspen (yellow) wood in the original timber frame (southern gable and cross section) of the house at Konrad-Förster-Gasse 5 in Bad Windsheim (DE) Service life of different wood species (in years) under different climatic conditions Longitudinal and cross sections of the Butchers’ Guild Hall, Hildesheim (DE) 1529 Section through the timber work at the base of the steeple of Notre-Dame in Paris (FR) Roof structure of the Great Hall of Stirling Castle (GB) 13th century, recently renovated Beech can be easily shaped. The first Viennese bistro chairs were produced in the 1850s.

8

9

9

was carbonised in the absence of oxy­ gen, which produced wood gas. The gas produced from 3 kg of beech had about the same energy value as 1 litre of petrol. Even today, beech, much like other types of hardwood, is to a large extent still used directly to gen­ erate energy. Under certain conditions (within a regional framework), “modern” wood energy constitutes an important building block of energy policy, as no ­fossil carbon is released in this process. Nevertheless, for reasons of climate ­policy and sustainability, in future priority should be given to cascade usage in which the wood material is used for as long as possible through several stages before it is burned. Today, even high-­ quality wood which would be well-suited for use as construction timber is now often used “only” as an energy source (Fig. 10) [5]. Trunkwood

Railway sleepers

In the mid-19th century, the development of new preservation techniques allowed beech to be used for wood-block paving and, in particular, for railway sleepers. In this way, one of the most common tree species found in Central European forests, which in those days was also called the “mother of the forest”, gained economic importance for the first time. The expansion of the rail network required large quantities of railway sleepers. In Austria, in the years after 1945 almost 900,000 impregnated railway sleepers made of beech or oak were installed [6]. In the beginning, iron vitriol, wood tar or various coal products made the wood sleepers durable. Until recently, pressure impregnation with creosote – a com­ pound extracted from coal tar that pro­ tects wood from weathering as well as from pest and fungal infestations – was Sawn lumber

common worldwide. Wood treated in this way is recognisable by its tar-like smell and its black, sticky adhesions. In 2018, due to its skin-irritating and ­carcinogenic ingredients, creosote was clas­sified by the European Commission as a substance of particular concern to humans and the environment, and its use has since been banned. Although most sleepers are now made of concrete due to its greater longevity, the use of timber cannot be completely dispensed with. In specific applications such as track switches, in cases of tight turning radii or in shunting and high-load areas, concrete and steel sleepers are technically unsuit­ able for replacing timber sleepers. The elasticity of the wood pays off especially on hilly track segments where landslides are a risk. For these reasons, various products are being tested as pos­sible alternative protective agents for timber sleepers, such as salt and oil-based compounds (Fig. 9). Other hardwoods

47 % (34.2 M m3)

4 % (2.7 M m3)

Softwood 73 % Raw wood 100 % (73.3 M m3)

(53.3 M m3)

Pulpwood 14 % (10.6 M m3) 4 % (3.2 M m3) Firewood1)

Hardwood 27 % (19.9 M m3)

10

1) 

10

12 % (8.5 M m3) 19 % (14.0 M m3)

47 % (34.2 M m3)

4 % (2.6 M m3) Fibre and chipboard 7 % (5.3 M m3) 3 % (2.4 M m3)

Domestic heating 8 % (6.2 M m3) 15 % (11.3 M m3)

Veneer and plywood 0 % (0.0 M m3) 0 % (0.2 M m3) Mechanical and paper pulp 7 % (5.2 M m3) 1 % (0.7 M m3)

Thermal power plants 3 % (2.3 M m3) 4 % (2.7 M m3)

Other raw wood 0 % (0.1 M m3) 0 % (0.0 M m3)

firewood = raw wood used for energy purposes, including raw wood in wood pellets and charcoal

Types of hardwood other than oak histor­ ically played a minor role in the construc­ tion sector. Depending on regional avail­ ability, there are some examples that were probably used specifically because of their particular characteristics or for lack of the more appropriate oak. Exca­ vations at Lake Constance prove that, over 4,000 years ago, pile dwellings were founded on alder, a wood that is light but very resistant to submersion in water (Fig. 11). The famous Urnes Stave Church in Norway is considered the oldest of its

  9 Old railway sleepers on the track bed 10 Domestic use of raw wood in Germany in 2017 11 Replica of houses on alder piles at the Pile ­Dwelling Museum In Unteruhldingen (DE) 12 Urnes Stave Church (NO) 12th and 13th cen­ turies 13 Sommerfeld House, Berlin (DE) 1922, Walter ­Gropius and Adolf Meyer

Yesterday – Today – Tomorrow

kind in the world, with its origins dating back to the year 1100 CE (Fig. 12). The 12th and 13th-century building, which is still preserved today, consists partly of the elm typical of the region. There are also occasional reports of the use of ash. The wall planks and floorboards of Tävelsås Church in Sweden (1750) are supposed to have been made of this wood. The European chestnut origi­ nated in south-eastern Europe and ­Turkey and was brought north of the Alps by the Romans. In addition to its use in viniculture, chestnut wood was also used for the construction of resi­ dential houses and in some cases even churches. Even today, some examples of chestnut used in construction can be found in Spain, France and Italy in the form of ceiling and roof beams or terrace canopies.

atmosphere are gaining in importance. The clearly observable renaissance of timber construction is closely linked to the development of new technologies and construction products, but also to the rediscovery of old building traditions, which have since been further devel­ oped and put to use in new applications. The industrialisation of wood processing allows for the manufacture of composite timber products, with which it is possible to achieve larger dimensions and to homogenise undesirable properties. The result has been a rapid development in timber construction with multi-storey construction projects up to the high-rise building limit and beyond. Height records are now being broken in quick succes­ sion and standards and laws are being adapted accordingly (see the Suurstoffi office complex, p. 76ff.).

11

Potential and obstacles in the use of struc­tural

Hardwood in modern timber ­construction In the classical modern era, steel, con­ crete and glass were the most commonly used materials. Timber construction faded into obscurity, saddled with some­ thing of a “backward” reputation, since timber was not as high-performing as the new materials as well as being flamma­ ble. Once the primary building material, timber was now used only for small or temporary buildings. Sommerfeld House in Berlin, which was constructed in 1922 as a collective project of the Bauhaus School, is an exception. The construction material used here was teak wood from shipwrecks (Fig. 13). Only in recent dec­ ades has a reversion to the use of timber in building structures been taking place. In the context of climate change and the finite nature of fossil resources, renew­ able raw materials – especially wood – that absorb and sequester CO2 from the

hardwood

The use of hardwood in the building structure is part of this development. It offers much potential, but also faces some obstacles. Potential The high strength of hardwoods opens 12 up the potential for their use in new high-­ performance composite materials, which would represent an ecological alternative to steel and concrete in highly stressed components and significantly expand the current scope of timber construction. As in traditional applications, hardwoods are employed where specific properties such as load-bearing capacity and stiff­ ness matter. While the high-load joists in half-timbered construction were made of oak, in modern timber construction largespan beams, junction points (Fig. 14) and heavily stressed supports are now typical applications for hardwood products (see “Building with Hardwood”, p. 45ff.). Their 13 11

14 a

14 S  t. Josef Parish Church, Holzkirchen (DE) 2018, Eberhard Wimmer Architekten, structural design: Sailer Stepan & Partner, development of timber node connection and structural detail: sblumer ZT a The beech elements are located in the ­nodal points of the triangular spruce construc­ tion of the dome and are not visible from the outside. b  Timber node of beech plywood 15 Utilisation of ash, which was partially thermally modified for improved durability. Steampunk ­Pavilion, Tallinn Architecture Biennale (EE) 2019, Gwyllim Jahn & Cameron Newnham (Fologram), Soomeen Hahm Design, Igor Pantic

b

cross sections, which are relatively slen­ der compared to softwood equivalents, allow for the creation of architecturally sophisticated constructions. In add­ition, the exposed surface of hardwood fea­ tures a special aesthetic. Thanks to CNC-­ controlled production, even free-form and artistic objects are possible. Because of its strength and its associated slender profiles, hardwood is also well-suited for experimental buildings (Fig. 15). Obstacles Thus far, the industrial and manual machines and processes used in timber construction have been adapted mainly to the prevailing wood type, spruce. In this respect, the utilisation of hardwood represents a technical and economic challenge. While the properties of soft­ woods are very predictable and vary little between species, hardwood species

15

12

diverge considerably more in their char­ acteristics. Quality, density and appear­ ance differ significantly from tree to tree even within a hardwood species, so that the corresponding wood can only be worked with a lot of experience or through homogenising processes (e.g. in the pro­ duction of glued laminated timber and laminated veneer lumber) (Fig. 16). In addition, hardwood trees usually have a more voluminous branch structure and canopy, which means that their usable trunkwood share varies from 40 % to 50 % (Fig. 17). This yield is much higher for softwood trees, at up to 80 % [7]. Because of its hardness, its usually less straight stem growth and its significant branching tendency, hardwood is more difficult to process than softwood. It is thus almost impossible to use in larger cross-section formats, which neces­ sitates the further processing of small

connecting pieces into construction prod­ ucts. The high raw density of hardwood, which is an advantage from a structural perspective, proves to be a drawback in manufacture: Sawing or chipping the wood requires more machine power and more energy or, put another way, for a given engine output, the feed rate must be reduced. The drying of hardwood by technical means is likewise energy and time-consuming and thus expensive, and in addition often results in undesirable warping. In further processing, the wide range of different component materials often requires special adhesives. All of this complicates the development of standardised mass products. The development of hardwood products

Beginning in the 1960s, a study at the Materials Testing Institute in Stuttgart investigated the performance of glued beams and trusses of beech. In Switzer­ land in the 1980s, Prof. Ernst Gehri explored the topic in greater depth. An early example of the use of glued beech can be found in the roof struc­ ture of the Seeparksaal venue in Arbon, Switzerland. The space frame, devel­ oped by ETH Zurich in 1984, consists of glulam beams, primarily of pine. Par­ ticularly highly stressed members are made of beech glulam (Fig. 18). In the 1990s, Prof. Peter Glos at the Institute for Wood Research Munich explored glued laminated beech. However, the use of hardwood for load-bearing compo­ nents in building construction has only moved into focus in the last ten years. One of the main reasons for this is cli­ mate change: The average annual tem­ perature is rising steadily worldwide, and extreme weather phenomena such as strong storms and summer heat waves are increasing [8]. In Central Europe, this is spurring an accelerated conversion of forests away from susceptible spruce

Yesterday – Today – Tomorrow

55 %

45 % Hardwood trees

16 S  urface reconstructions of spruce (top) and beech (bottom) trunk segments by means of ­laser triangulation for the analysis of stem ­curvatures 17 Usable trunkwood share of hardwood (top) and softwood (bottom) trees 18 Seeparksaal, Arbon (CH) 1984, ABS working group with ETH Zurich/Institute of Structural and Steel Construction and engineering firm Wälli a Beech glulam was used for the tension and compression members of the support structure. b  Bottom view of a node 19 Industrial production of laminated veneer lumber from beech 16

monocultures to more robust mixed for­ ests with a higher proportion of hardwood trees (see “Forest Management Back­ ground”, p. 18f.). As a result, innovative companies and special interest groups are working intensively on expanding the material use of hardwoods. Since these products have become available on the market, more and more built projects are being created, promoting confidence in the constructive utilisation of hardwood (Fig. 24, p. 16f.). Whereas in Germany sawn lumber of beech and oak has had National Technical Approval since 2004, and ash, maple and poplar since 2008, beech g ­ lulam and beech glulam hybrid beams (composite beams with highstrength beech slat top layers and a spruce slat core) did not enter the market until 2009. In 2012, oak glulam from a manufacturer in northern Spain became available. Chestnut glulam, also produced in northern Spain, was approved in 2013, as was the post-and-beam oak glulam by a German company. Both birch glulam, which was developed in Austria and first used in 2015 for the construction of an industrial building, and cross-laminated timber (CLT) of birch, which was tested in the construction of a single-­family resi­ dence, do not yet hold National Technical

18 a

20 %

Softwood trees

17

Approvals. Up until now, however, the aforementioned products are more likely to be found in niche markets. Further research is needed in order to expand the range of applications and to improve economic viability (see “High-performance Materials with Potential for the Future”, p. 20ff.).

trunkwood share remainder (branches / leaves)

branches and other irregularities are homogenised in the composite product, its strength is significantly greater than for other manufacturing techniques. In addition, the processing of beech into laminated veneer ­lumber ensures that optimal material use is made of the uneven tree trunks. In the end, these building products are still more expensive than those made of spruce, but in their main applications, such as truss girders, the material savings due to their greater load-­ bearing capacity allow the overall costs to remain comparable (Fig. 19).

Example 1: Germany In Thuringia, beech laminated veneer lumber is produced in high-performance sawmills. Since its market launch in 2014, the product has developed into a popular and serious competitor of steel, as evidenced by the numerous build­ ings in which it has been successfully implemented since then. The company succeeded with much entrepreneurial skill in converting a previously unpopular raw material into a standardised product that is sold worldwide. Unlike what is required for glued laminated timber, the roundwood is not sawn, but rather sliced and peeled. Many layers of 3.5-mm thick veneers are glued together under high pressure to form high-strength laminated veneer lumber panels. These panels are then separated into slats and undergo a second bonding process to yield ­laminated veneer lumber beams. Since

b

80 %

Example 2: Switzerland In Switzerland, hardwood post-and-beam constructions have been used in facades for almost 20 years. A company there produces glulam from hardwoods such as beech, oak and black locust, but also combines other hardwood species such as ash with softwood glulam for partial reinforcement (see “Flexural members”, p. 48, and Fig. 10, p. 49). This was used in a t­argeted manner, for example, in the roof construction of the parking garage in Innerarosa (see Project Example p. 90ff.) and in the Sports Centre in Sargans (Fig. 21). However, the continued sur­ vival of ash is threatened by a severe

19

13

20

tree disease, the ash dieback epi­ demic. It is uncertain whether the tree species will still exist in our forests in the future. A company from the Canton of Jura has also specialised in the utilisation of regional hardwood, especially beech. These very high-quality glulam products are comparatively expensive and have so far been selected to a limited extent for special buildings and flagship projects. Here, too, a great deal of development work is still needed to optimise the manu­ facturing process so that the products become more broadly competitive. Example 3: North America In North America there are large swathes of deciduous forest that produce numer­ ous types of wood due to the diversity in growth environments. The first European settlers established a hardwood process­ ing trade that is still reflected today in a wide range of sawmills and wood pro­ cessing industries with different focal

21

14

points. According to the American Hard­ wood Export Council, which runs the international marketing of American hard­ woods, the USA is the world’s largest hardwood lumber producer. The majority of the timber industry, located primarily in the eastern USA, supplies the domestic market. In addition to being used as fire­ wood, the higher-quality material is mainly used there in furniture manufacture and interior construction. American hardwood timber has also been imported into Europe for over a century. This includes chiefly maple, American white oak and some cherry species. Products on offer include hardwood lumber, veneers, plywood, slats and floors. The use of hardwood in the supporting structure has so far played a minor role in North America. In 1991, Intrallam LSL (Laminated Strand Lumber) was launched on the US market. The idea was to manufacture a high-performing product from small-diameter trees “that are not strong or straight enough on

their own to be of structural value as ­conventional sawn lumber products.” [9]. The raw materials used for production are fast-growing hardwood species such as the quaking aspen and the big-tooth aspen. In Europe, laminated strip lumber could not establish itself in the long term and is no longer sold. For several years, the American timber industry has been trying to establish American hardwood as a construction timber in Europe. So far, however, the ­finished projects are few in number and are located exclusively in the UK, specif­ ically in London, because the material lacks approvals in other countries. Ameri­ can white oak was first employed for a support structure in 2001 by the archi­ tects Michael Hopkins & Partners in the construction of the atrium roof of Portcullis House. Extensive investigations by the structural planners of Arup confirmed its enormous load-bearing capacity and allowed the beams to be dimensioned at a mere 200 mm with a span of 20 m (Fig. 20). As a result, the American Hard­ wood Export Council had three other tim­ ber species – tulipwood, American ash and northern red oak – studied to deter­ mine their specific properties. The most recent example of the constructive use of white oak is the roof of the stand at Lord’s Cricket Ground (see Example Build p. 96ff.). In this instance, the critical factor was not only the strength, but above all the considerable durability of the wood, as its outdoor use class 3 designation requires it to withstand driving rain. For the 2013 Endless Stair sculpture, tulip­ wood cross-laminated timber (CLT) was used for the first time (Fig. 23). The tulip tree, which is widespread in the eastern USA, resembles the poplar and has a creamy-white wood that can be easily machined. The magnolia species owes its name to its tulip-like flowers. Origi­ nally native to Europe, the tree has not

Yesterday – Today – Tomorrow

22

existed there since the last Ice Age. Due to its high strength characteristics in ­relation to its low weight, the wood is wellsuited for use in construction. Arup, a globally active engineering firm, devel­ oped the CLT and calculated that the sculpture would be able to carry more than 100 people at a time. Most of the projects that have been realised were either directly commissioned or funded by the American hardwood industry. It has yet to be seen whether the efforts to permanently place American hardwoods on the European market will pay off. Con­ sidering the long transport routes, this is questionable, at least from an environ­ mental point of view. Example 4: Australia In Australia, solid eucalyptus wood has traditionally played an important role in single-family construction. For larger construction projects of up to eight floors, which a 2016 change in the laws has allowed to be built of timber, crosslamin­ated timber (CLT) of spruce is imported. In Tasmania, a hardwood CLT plant is to be built in order to develop wood products from Australian hardwood – mainly plantation-grown eucalyptus, which is otherwise only processed into high-quality chips for the pulp and paper industries. This measure is intended to support a sustainable timber industry and help Australia to become internationally competitive. In addition to serving the domestic markets, the aim is to export to North America and Europe.

plantations. The tree species, introduced around 1825, was first used commercially by Brazilian railway companies. Since then, eucalyptus wood has been used primarily in the production of charcoal for the national market and of pulp and paper for export. Its use as timber plays a minor role. One exception is the work of Brazilian engineer Hélio Olga. Olga has been building with eucalyptus wood for almost 40 years, since 2008 in the form of glued laminated timber (Fig. 22).

Outlook and relevance The process of developing hardwood products is only just beginning. Much research and development is still needed to fill gaps in knowledge, to generate experience and to regain confidence in the capabilities of hardwood – or to embrace these rediscovered abilities. A larger number of suppliers with an expanded product range would be advantageous for the market. This, and an increased use of hardwood in con­ struction projects, could cause it to lose its current exotic status and be accepted once more as a standard product by builders and designers.

Notes [1] Zwerger, Klaus: Das Holz und seine Verbindungen. Basel / Vienna / Berlin 2012 [2] Vitruvius: De architectura libri decem (Ten Books on Architecture) [3] Bedal, Konrad; Back, Michael: Unter Dach und Fach. Bad Windsheim 2002 [4] Niemz, Peter: “Von der Waschlauge bis zum Snowboard-Kern. Verwendungsmöglichkeiten von Buchenholz”. In: Wald und Holz 88, 10/2007, p. 35ff. [5] Krackler, Verena; Niemz, Peter: “Schwierigkeiten und Chancen in der Laubholzverarbeitung; Teil 1: Bestandssituation, Eigenschaften und Verarbei­ tung von Laubholz am Beispiel der Schweiz”. In: Holztechnologie 52, 2/2011 [6] Simmel, Christina: “Wegweisend – Bahnschwellen aus Holz”. In: Zuschnitt 64, 2016, p. 26 [7] see note [5] [8] The Intergovernmental Panel on Climate Change, ipcc.ch [9] weyerhaeuser.com/woodproducts/engineered-­ lumber/timberstrand-lsl/ (accessed on 23.09.2020)

20 O  ak roof structure of Portcullis House, London (GB) 2000, Hopkins Architects 21 Entrance hall with spruce glulam beams, re­ inforced at the supports by ash, Sports ­Centre Sargans (CH) 2012, HILDEBRAND (blue archi­ tects until 2018) & Ruprecht Architekten, struc­ tural design: WaltGalmarini 22 Hélio Olga House, São Paulo (BR) 1990, Marcos Acayaba, structural design: Hélio Olga 23 Endless Stair, tulipwood installation at the Tate Modern, London Design Festival 2013, London (GB), dRMM 24 The changes in weather caused by climate change affect the state and composition of our forests. This is driving the development of new hardwood products, which are used increasingly in the execution of construction projects.

Example 5: Brazil About 1 % of Brazil’s land area is cur­ rently covered by eucalyptus plantations. Because of their monocultural nature, these are classified by conservationists as being of environmental concern. In addi­ tion, rain forests are often illegally cleared to plant the economically more lucrative 23 15

2001

2002

Severe storms that caused damage to forests

Weather

Change in average annual temperatures in Germany [1]

2003

2004

2005

2006

Cyclone Jeanett

2007

2008

Cyclone Kyrill

Cyclone Emma

2) 2)

°C 11 11°C 11 °C 9.5°C 1)1) 9.5°C

10°C 10 °C 10 °C

9.4°C 9.4°C

9.0 °C 9.0 °C

9.0°C 9.0°C

8.9°C 8.9°C

9.9°C 9.9°C

9.5°C 9.5°C

9.5 °C 9.5 °C

9°C 9 °C 9 °C 8 °C 8°C 8 °C

Forested area in Germany by tree groups in hectares [2]

2nd Federal Forest ­Inventory 2001/02

larch larch

+2% +2%

pine pine

59% 59%

spruce spruce

+4% +4%

firfir

+ 11 % + 11 %

4) 4) OHL OHL

beech beech

3) 3) ODH ODH

Forest development

41% 41%

Change in forest area by tree groups between 2002 and 2012 [3]

oak oak

+7% 15 % + 7 % + 7 %+ + 15 % +7%

10.58 M 10.58 M

Douglas Douglasfirfir

+ 19 % + 19 %

-8 % -8 %

-3 % -3 %

Sawn timber beech, oak Glulam dark red Meranti

Hardwood glulam and sawn timber available / regulated in Germany and Europe with regulatory proof of suitability for construction

Sawn timber ash, maple, poplar Glulam scantlings for post-andbeam facade constructions oak

2001

2002

• Sclera Pavilion, London (GB) David Adjaye; tulipwood • Town Hall, Zalduondo (ES) Iruretagoiena Architects: oak glulam

• Downland Gridshell, Singleton (GB) Edward Cullinan Architects: solid oak

Projects with hardwood in the support structure

• Portcullis House, London (GB) Hopkins Architects: American white oak

Example projects

Product development

‡ Hardwood ‡ Softwood

2003

2004

2005

2006

2007

2008

10 of the 12 warmest years since 1881 have occurred during the years since 2000 (as per Statista) 2)  heat wave (three days in a row with a high of 30 °C or more, as per Statista) 3)  ODH: other deciduous trees with high longevity, e.g. maple, ash, chestnut, lime, whitebeam, service tree, locust, elm 4)  OHL: other deciduous trees with low longevity, e.g. birch, snowberry, alder, poplar, bird cherry, rowan, wild cherry, willow, wild fruit 24 1) 

16

• Vega Sicilia Winery, Valladolid (ES) Arkipolis Arquitectura & Urbanismo: oak glulam • Complete renovation of Schwanau Island, Lauerz (CH) Arde Architecture: solid oak and three-ply oak panels

• Holiday home, Büttenhardt (CH) Bernath+Widmer Architects: solid oak • Research pavilion, Stuttgart (DE) University of Stuttgart: birch CLT

• Timber Wave, London (GB) Amanda Levete Architects: northern red oak • Parking garage, Innerarosa (CH) Lutz Bus ­Architects: ash and ash / spruce glulam

• Sports Centre Sargans (CH) HILDEBRAND (formerly blue architects) & Ruprecht Architekten: ash / spruce glulam • Cathedral of Santa Maria de Vitoria, Vitoria-Gasteiz (ES) Leandro Cámara: oak glulam

• Tamedia, Zurich (CH) Shigeru Ban Architects: beech LVL • Neumattbrücke, Kirchdorf (CH) Ingenta, Arn + Partner: ash glulam

• Rotes Dach, Munich (DE) Rolf Enzel, Stefan Imhof, Florian Nagler Architekten: beech LVL • Raser single-family house, St. Magdalena (AT) Wolfgang Raser: birch CLT

• Industrial hall, Latzendorf (AT) Johann Plössnig ­Architekt: birch glulam • House of Natural Resources, Zurich (CH) Andrea Frangi, mml Architekten: ash / beech

• Production hall, Probstzella (DE) F64 Architekten: beech LVL • Canopy of the Eispavillon, St. Moritz (CH) Foster + Partners: ash glulam

• Art Museum, Aspen (US) Shigeru Ban Architects: birch CLT • Helicopter production hall, Mollis Airfield (CH) ­Leuzinger Architektur AG: beech LVL

• Training hall, Davos (CH) Fanzun: ash / spruce glulam • Peter Hall Performing Arts Centre, Cambridge (GB) Haworth Tompkins Architects: beech LVL

•  Bank, Stavanger (NO) Helen & Hard: beech LVL • High Altitude Olympic Training Centre, St. Moritz (CH) Krähenbühl Architekten: ash glulam

• SWG Schraubenwerk, Gaisbach (DE) Hermann Kaufmann + Partner: beech LVL

10.63 M 10.63 M

45 % 45 %

Cyclone Christian Cyclone Xaver Cyclone Ela Cyclone Niklas Cyclone Felix

10.3°C 10.3 °C

Predicted changes in ­forest area by tree groups between 2012 and 2052 according to the preferred conser­ vation scenario [4]

2016

8.7 °C 8.7 °C 9.9°C 9.9°C 9.5°C 9.5°C

+ 26 % + 20 %+ 26 % + 20 % + 13% + 13% + 10% + 10%

55 % 55 % + 17 % + 17 %

larch larch

2015

Douglasfirfir Douglas

2014

+ 0% + 0%

pine pine

9.1 °C 9.1°C

2013

firfir

Cyclone Xynthia

spruce spruce

9.5 °C 9.5 °C

2012

4) 4) OHL OHL

9.2°C 9.2 °C

2011

ODH3) 3) ODH

2010

beech beech

2009

oak oak

Yesterday – Today – Tomorrow

2017 2018

Cyclone Xavier Cyclone Friederike

9.5°C 9.5°C 10.4°C 10.4 °C

2019

3rd Federal Forest ­Inventory 2012

- 14 % - 14 %

2020

Cyclone ­Sabine

10.2°C 10.2 °C 11 °C 11°C 11 °C

linear trend linear trend

7.9 °C 7.9 °C

2009

2010

2011

2012

2013

2014

2015

2016

2017

2018

2019

2020

10°C 10 °C 10 °C

 9°C 9 °C 9 °C

8 °C  8°C 8 °C

Prognosis for 2052 [5]

10.63 M 10.63 M 52% 52%

-1 % -1 %

48% 48%

- 22 % - 22 %

Beech glulam Beech-hybrid glulam Oak glulam European chestnut glulam Oak post-and-beam glulam

Glulam from beech LVL

Sources: [1]  German Weather Service (DWD) [2]  Federal Forest Inventory 2012 [3]  ibid. [4]  WEHAM 2012 [5]  ibid.

17

Forest Management Background

In the absence of human influence, ­hardwood forests, especially mixed ­forests dominated by beech trees, would characterise the landscape of Central Europe (Fig. 1). But since at least the beginning of the Middle Ages, this pic­ ture has changed significantly. The onset of population growth led to the expan­ sion of settlement activity, with more and more arable land being set aside to ensure that nutritional needs were met. Forests had to be cleared [1] in order to free the area for these new uses on the one hand, and to obtain wood as a raw material on the other. This was mainly used as construction timber and firewood. The need for charcoal for the smelting of iron ore and the manufacture of glass also resulted in a high consump­ tion of wood, mainly hardwood – espe­ cially beech. As a result, by the end of the 18th century vast swathes of forest had been over­exploited. The onset of reforestation of the razed woodlands marks the starting point of modern, regulated and sustainable (for its time) forest management. It is thanks to this that the proportion of wooded land in Germany today has once again reached 32 % and continues to increase overall despite regular use [2]. In those early days, softwood trees were often preferred, as these usually grow faster than hardwood trees, were very prof­ itable and, due to their straight growth habits, were more advantageous in many applications, for example as load-bearing beams and planks in construction. How­ ever, from a long-term perspective, this practice also had some drawbacks. ­Softwood trees at potential “hardwood sites” tended to be more susceptible to natural influences, such as storms or ­diseases (insects, fungi). In addition, the establishment of purely softwood tree stands, which occurred in many places, led to a reduction in species diversity and 18

promoted the proliferation and spread of pests (e.g. bark beetles). For this reason, beginning in the early 1980s, Germany once again oriented itself increasingly toward forest management that closely (or more closely) mimics nature. After use or damage events, more attention was paid to the former existent location-­ adapted and natural composition of tree species. Above all, this meant the crea­ tion of species-rich mixed stocks with a high (and henceforth growing) proportion of hardwood trees. Because of the need to adapt to the increasingly apparent and rapidly ­developing change in climate, efforts to restructure forests have been and have been and are being intensified in recent decades, i.e. the rate of forest area trans­ formation is rising and the composition of tree species will shift further in favour of hardwood. Nevertheless, maintaining a high proportion of conifers in forests – in mixed stocks and at suitable locations – remains an important goal in silviculture. This is also achieved by the fact that, in addition to the previously dominant species spruce and pine, other soft­ woods such as fir, larch and Douglas spruce are again increasingly included in the composition. Aside from the cre­ ation of stable mixed stocks, an underlying reason for this is that the timber industry has specialised in this raw material and has thus far primarily developed soft­ wood processing technologies and cor­ responding products. Overall, however, the percentage of hardwood species – above all of beech – is growing, while the percentage of softwoods is falling. From today’s point of view, therefore, the conversion of the forests has been very successful. Logically, this development has led to a shift in the availability of raw timber from Central European forests: Softwood has decreased while the proportion of hard­

Forest Management Background

wood has increased. The result, unfortu­ nately, is a sales problem. For a long time, the timber industry, highly oriented toward softwood, and related industry branches such as the construction sector, had no or scant opportunity to make high-quality use of the available hard­ wood varieties. In many places, this long period of disuse led to an over-ageing of the stands with the associated problem of large and very large trunk diameters, for which no effective manufacturing and processing technologies exist. On the other hand, large quantities of small diameter, low-quality tree trunks acquired from the thinning of timber stands planted in recent decades are currently available. Here too, efficient and effective as well as cost-covering or even profitable pro­ cessing is still difficult. But timber research programmes and the timber industry are

working together on possible solutions (see “High-Performance Materials with Potential for the Future”, p. 20ff.). Right now it is difficult to predict the exact appearance of the ultimately sustainable forest, since particular developments, such as climate change, are progressing too quickly. What is certain, however, is that a “well-stocked warehouse” that includes both softwood and hardwood represents the best starting point for ­forest management and for the producers and consumers of the raw timber. Notes [1] Forest clearing (deforestation) describes a ­change in type of use, for example the conversion of forest into arable land. Clear cutting is a form of forest management. The affected area technically remains woodland and must be reforested. [2] BMEL publication: “Ergebnisse der dritten ­Bundeswaldinventur.” 2012; available online at: bwi.info (last accessed on 08.04.2020)

1

Mixed forest

1

19

High-Performance Materials with Potential for the Future Markus Lechner, Stefan Winter, Stefan Torno

Wood is the world’s leading biogenic building material and, from a current ­perspective, one of the key materials for developing sustainable solutions for building in the future. It can make a ­significant contribution to the necessary reduction in carbon emissions and the targeted use of renewable raw materials. The challenge of the future is to build faster, for more people, with fewer renewable resources, on less land and with lower emissions. This will require high-­ performance timber materials. Current research and development projects are often focused on increasing strength characteristics. High-performance mate­ rials with potential for the future, however, must have multidimensional properties. This includes, for example, a self-­ extinguishing feature in the event of fire or a reduction in VOC emissions into interior spaces. Compared to modern softwood timber products, those made of hardwood are still relatively new. Accordingly, based especially on experiences from early applications, there is still a significant need for research and development. This is the case even though hardwood has been used for a long time, in axe handles and oak beams, and of course for making furniture.

New developments and improvements in structural hardwood products Construction with softwood-derived glued laminated and cross-laminated ­timber has established itself in recent decades with designers and builders. The performance limits of the currently available timber building products are limited on the one hand by their strength, and on the other by their elastic modulus and its associated cross-sectional stiffness. Both properties can be improved on by using hardwood. The applicable 20

calculated strengths of timber building products in beech compared to those in spruce, for example, are two to three times as high. The use of beech can increase stiffness properties by up to 50 %. Two approaches to the development of hardwood-based structural products are possible. In one, the base material of a timber building product can be switched from softwood to hardwood, e.g. glulam, laminated veneer lumber (LVL) or cross-laminated timber (CLT) made of beech, oak and birch. At first glance, this approach is compelling. But the question is whether it is also resource-efficient, i.e. whether it realises the potential of hardwood ­efficiently. Alternatively, a targeted composition using hardwoods can improve the ­weaknesses of current softwood timber products. The composite beams in the International House Sydney (Fig. 2) are a good example of this. The maximum building height according to planning regulations and the number of floors desired by the client could not be built with conventional timber ­construction products. Therefore, a ­composite beam made of softwood ­glulam and beech LVL was developed and tested [1]. When cross-laminated timber is used as a panel, its load capacity is usually limited by rolling shear strength. In this case, replacing the transverse layer subject to rolling shear stress with hardwood is an excellent solution (Fig. 3) [2]. Another promising approach is the strengthening of spruce glulam with carefully placed hardwood veneers to create “timber-­reinforced timber” (Fig. 1) [3]. The highly anisotropic strength and stiffness properties of glued laminated timber can be homogenised by arranging the veneer layers between the glulam components at angles of 0° and 90°.

High-Performance Materials with Potential for the Future

glued laminated timber

1

The veneers can also be specifically modified, for example to produce self-­ extinguishing layers. The combination of hardwood with mineral building materials also offers great potential, for example for timber-concrete composite ceilings. Due to the higher ­tensile strength of hardwood, the wood layer can be made much thinner. Because this makes a thinner ceiling structure ­possible, it also enables a better utilisation of room height or an increase in the number of floors for a given building height [4].

hardwood veneers

v­ isible at its joints. In softwood con­ structions, the limiting conditions of the fastening technology are often ­decisive in determining member cross-­ sections. Initial investigations indicate that the use of hardwoods makes it ­possible to transfer greater forces within much smaller spaces. In the case of ­pin-shaped fasteners such as nails, ­dowels, screws and bolts, the reason for this are the higher hole-wall and ­transverse tensile strengths of hardwoods compared to those of softwoods. The load-bearing behaviour of estab-

lished timber fasteners must therefore be carefully researched for use with ­hardwoods, as in some cases failure mechanisms such as the shearing-off of the connecting means can dominate. Aside from the classic fasteners, new

1 2 3

 asic composition of timber-reinforced timber with B transverse veneer arrangements Composite beams, International House Sydney ­office building, Sydney (AU) 2016, Tzannes Architects, Structural Design: DesignMake Lendlease Test set-up for determining the rolling shear properties of a triple-ply CLT cross-section construction with a transverse layer of beech

Bonding of hardwoods Glued building components are of great importance in the use of hardwood because sections with growth-related reductions in stiffness and strength can be eliminated, leading to the homogeni­ sation of properties. It is therefore ­necessary to optimise bonding by re­­ developing the adhesives for the gluing of hardwoods as well as those for gluing hardwoods to softwoods in order to make the process faster, technically simpler and thus more cost-effective. Although the current use of phenolic resins (PF, PRF) already makes very reliable bonding possible, this group of adhesives places higher demands on occupational health and safety measures at the plant during product manufacture. Extended testing of other adhesive families, such as PUR, is currently being carried out in various projects [5].

2

Connections In the field of connection technology, hardwoods offer enormous potential due to their higher density and strength. The quality of a timber construction is

3

21

joining techniques are also possible when using hardwood. The hardwood plug-in connectors employed in an office building in Zurich offer a ground-­ breaking example (Fig. 5 and 6). In ­conjunction with the modern use of highly efficient and precise robotic joinery, traditional joining techniques such as step or dovetail joints can once again be ­profitably manufactured. Figure 4 shows the timber frame node of the new factory building of a fastener production plant in Waldenburg (see Example Build p. 56ff.).

Durability and moisture A particular challenge facing the use of hardwoods in structural timber construction is moisture stress. This can occur during manufacture, assembly and use and can result in water damage. Beech in particular has a significantly stronger shrinkage and swelling response than softwoods do. Aside from design-­

incorporated moisture management as well as wood protection (see “Wood protection”, p. 39f.), additional measures to improve durability are an important component of the overall concept – not least with a view to expanding the application range of hardwood ­products. For hardwoods in use class 3 applications, there are several possible types of chemical wood modification [6]. One cost-effective process involves impregnating the timber with synthetic resins, e.g. methanol-modified melamine-­ formaldehyde resins (MMF) or dimethyloldihydroxyethyleneurea (DMDHEU, also referred to as wood cross-linking). The treatment with DMDHEU causes the cell walls to swell permanently, leaving no space for water to penetrate. This increases dimensional stability and significantly inhibits the spread of wood-­ decomposing fungi. Currently, treatment with acetic anhy­ dride (acetylation) shows great promise for the use of hardwood in construc-

tion. Though the process is currently employed exclusively for softwood (radiata pine), it also works very well for different species of hardwood. In so-called furfurylation, on the other hand, the cell walls as well as the cell lumina (the interior spaces of the cells) of the wood are saturated with furfuryl alcohol, which then polymerises. Currently, furfurylated wood is primarily used for facades and terrace floors, but not in structural timber construction, for example. Both methods reduce the water absorption capacity of the wood. As a result, moisture content of the wood remains constantly low in a range that is unsuitable for harmful organisms (fungi, insects). In addition, the timber deforms less, as swelling is inhibited (increase of dimensional stability). Drawbacks with both methods are their high processing cost (acetylation, for example, costs more than 1,000 €/m3) and possible effects on adhesibility and the durability of adhesions. Research is still needed to improve economic viability and to ensure long-term product per­ formance. Further research approaches include wood impregnation with concrete or with the “rediscovered” polyethylene glycol (PEG).

Joinery and processing In recent decades, joinery technology and tools have been developed and ­optimised for the processing of softwood timber. Using these standard tools to cut and process hardwoods has proved to be a failure in practice, especially for bigger projects. Material waste is significantly greater. With the progressive development of new hardwood products, factories and tool manufac­ turers must also expand their product portfolios. Initial developments and

4

22

High-Performance Materials with Potential for the Future

5

­ roducts are already available on p the market, such as special air-cooled deep-hole drilling systems for hardwoods or appropriately adapted milling heads.

dependent on the future avail­ability of raw materials – and this in turn on the climatic conditions to which our forests will be exposed in the coming years. Robust hardwoods will therefore undoubtedly play an important role in the future supply of timber construction products.

Usability Since building with modern hardwood products is increasingly in line with the state of the art, the basics necessary for their use must also be made available to a broad public. Tasks to be completed include: • Development of the foundations and acceleration of the processes for the national technical regulation of hardwood construction products such as hardwood glued laminated timber in accordance with EN 14 080 • Adaptation of the Federal State building regulations to the existing Model Building Regulations. These steps would eliminate the general disadvantages affecting timber construction in comparison with other construction methods. The equalisation offers benefits in particular for multi-storey timber construction, for which hardwoods are particularly suited due to their high performance. Hardwood will not be able to replace softwood completely in construction, but will significantly expand the range of applications for wood components. It is important to bear in mind that the development of modern timber con­ struction founded on coniferous wood covers a period of well over 100 years. To emulate this development within a short time period with hardwoods is impossible, despite the ever-increasing rate of technological advances. Hardwoods will need a little more time: ­Associated developments are strongly

4 5 6

 tep joints. Production hall for a fastener pro­ S duction plant, Waldenburg (DE) 2020, Hermann ­Kaufmann + Partner Bearing of a node element, office building in ­Zurich (CH) 2013, Shigeru Ban Architects ­Europe Installed node element, office building in Zurich (CH) 2013, Shigeru Ban Architects Europe

Notes [1] Aicher, Simon; Tapia, Cristóbal: “Novel Internally LVL-Reinforced Glued Laminated Timber Beams with Large Holes”. In: Construction and Building Materials 169, April 2018, p. 662– 677 [2] ERA-WoodWisdom: “European hardwoods for the building sector (EU Hardwoods)”, Sub-project 1, 03/2014 – 06/2017 [3] Lechner, Markus; Dietsch, Philipp; Winter, Stefan: “Hybride Holzbauteile aus Laubholz-Furnieren und Brettschichtholz aus Nadelholz – Holz­ bewehrtes Holz”. Research project conducted from 1/19 –10/20 [4] Timber-concrete composite (TCC) slabs were ­installed in the ETH House of Natural Resources. [5] TUM.wood: “Entwicklung eines material- und ­energieeffizienten Holzbausystems aus Laubund Nadelholz (LaNaSYS)”, research project [6] Schwaner, Kurt (ed.): Zukunft Holz – Statusbericht zum aktuellen Stand der Verwendung von Holz und Holzprodukten im Bauwesen und Evaluierung künftiger Entwicklungspotentiale. Hochschule ­Biberach 2009

6

23

24

Wood Species

In the following pages, the wood species briefly characterised are European spe­ cies whose constructive use in Europe or Germany is either regulated by a ­European standard, a German approval (national technical approval (allgemeine bauaufsichtliche Zulassung – abZ) or ­general construction technique permit (allgemeine Bauartengenehmigung – aBG) or a European technical assessment (ETA), or will soon be regulated through the implementation of a standard or through the expected issue of an approval. For more detailed information, please refer to the series “Einheimische Nutz­ hölzer” (Native Timber) by the Holzabsatz­ fonds [1]. In general, the range of available hard­ wood species is larger than that of soft­ woods. On the one hand, this is an advan­ tage. On the other, however, on top of the fundamental differences between hardwood and softwood species, the associated high degree of variability in the individual hardwood species’ tech­ nical characteristics has a more pro­ nounced effect on the processability and thus ultimately on the uses of hard­ wood in construction than is the case for softwood. Even within a given species, the properties of hardwoods scatter more broadly than those of softwoods. All this means that known processing technologies cannot simply be adopted from softwoods or transferred from one species of hardwood to another; instead, new processes must be developed and simultaneously adapted to each individual hardwood type. This presents a major challenge for research and development as well as to industry.

Notes [1] Holzabsatzfonds (publ.): Einheimische Nutz­hölzer. Loose-leaf collection. Bonn 1998

25

Maple (Sycamore: Acer pseudoplatanus L.) Hard and medium weight Density 530 ... 630... 960 kg/m3 Wood is single-coloured light yellowish to almost white Formation of a grey-brown to brown core possible

Transverse section – Radial section – Tangential section

Birch (Betula pendula L.) Soft and medium weight Density 510 ... 650... 830 kg/m3 Wood is single-coloured yellowish-white or ­reddish-white to light brownish Formation of a yellowish-red to brown core possible

Transverse section – Radial section – Tangential section

26

Wood Species

Beech (Fagus sylvatica L.) Hard and heavy Density 540 ... 720... 910 kg/m3 Wood is single-coloured pale yellow to reddish-white Formation of a brownish-red to grey-brown core possible

Transverse section – Radial section – Tangential section

Chestnut (Castanea sativa Mill.) Hard and medium weight Density 570 ... 630... 660 kg/m3 Wood is multicoloured: Sapwood grey-white to ­yellowish-white, heartwood yellow-brown to dark brown

Transverse section – Radial section – Tangential section

27

Oak (English oak: Quercus Robur L.; sessile oak: Quercus petraea (Matt.) Liebl.) Hard and heavy Density 430 ... 690... 960 kg/m3 Wood is multicoloured: Sapwood yellowish-white, heartwood yellow-brown to grey-brown

Transverse section – Radial section – Tangential section

Ash (Fraxinus excelsior L.) Hard and heavy Density 450 ... 690... 860 kg/m3 Wood is single-coloured yellowish-light to reddish Formation of a brown to olive-coloured core possible

Transverse section – Radial section – Tangential section

28

Wood Species

Eucalyptus (Southern blue gum: Eucalyptus ­globulus Labill.) Hard and medium weight Density 480 ... 720... 980 kg/m3 Wood is multicoloured: Sapwood yellowish-grey, heartwood pale yellowish-grey to light olive or ­reddish-brown

Transverse section – Radial section – Tangential section

Poplar (Black poplar: Populus nigra L.) Soft and light-weight Density 410 ... 450... 560 kg/m3 Wood is multicoloured: Sapwood grey-white to yellowish-white, heartwood light brown to ­greenish-brown

Transverse section – Radial section – Tangential section

29

Hardwood Construction Products

An overview of the usability and availability of hardwood construction products is shown in Fig. 4. In general, a distinction must be drawn between the regulations governing the products and the rules for their application. Product regulations contain information about manufacture, quality control and labelling. Usage rules determine which classes of a product can be used in which applications.

Sawn lumber and finger-jointed solid wood 1

The term ‘sawn lumber’ is used as a ­synonym for the term ‘solid wood’. According to DIN 4074-5, sawn lumber ­describes a timber product of at least 6-mm thickness, which is produced by sawing or shaving roundwood along the length of the trunk (Fig. 1). Depending on the ratio of cross-section height to cross-section width, as well as the ­orientation of the cross section in its eventual application (vertical or horizontal), a distinction is made between squared timbers, planks and boards. Finger-jointed solid wood for load-bearing purposes consists of strength-graded timbers that are longitudinally friction-­ locked together with other timbers via ­finger (or comb) joints to form longer units (Fig. 2).

2

Product characteristics

1 2 3 4

30

Sawn lumber Finger joints Dimensional tolerance classes for lumber according to EN 336:2013-12 An overview of the regulations for use and ­availability of hardwood construction products in Germany

Sawn lumber is usually available in thicknesses ranging from 20 mm to 120 mm. The width depends largely on the diam­ eter of the roundwood used. The lengths vary mostly between 2.5 m and 6 m. Thicker squared timbers or beams with cross-sectional dimensions of > 120 mm up to 300 mm and greater lengths are usually only produced on request (primarily because of the difficult drying ­process and limited availability) and are

therefore only rarely in stock. Regardless of the cross section, the ratios of edge lengths specified in DIN 4074-5:2008-12 must be observed during planning (e.g. for squared timber: w > 40 mm and w ≤ h ≤ 3w), as otherwise a strength classification can lead to erroneous results. In general, hardwood is subject to the dimensional tolerance classes given in EN 336:2013-12 (Fig. 3). However, its applications must be individually defined. Based on DIN 68 365:2008-12, for example, class 1 can be defined for rough-sawn wood and class 2 for planed wood. Usability of sawn lumber

The European harmonised product standard for solid wood and sawn lumber is EN 14 081-1. It regulates structural timber with a rectangular cross ­section graded according to strength. Roundwood is not regulated by building authorities. The latest version of EN 14 081-1 included in the Official Journal of the European Union (OJEU) is from May 2011. Later versions from 2016 and 2019 are thus currently not ­officially ­binding. The OJEU is the Official Journal of the European Union. To be binding on construction authorities, a European har­ monised standard must be included and published in the OJEU. EN 14 081 addresses the visual and machine-executed strength grading of softwood and hardwood. Machine grading leads directly to classification into a strength class. Visual grading in the EU is carried out on the basis of national sorting standards. In Germany, the visual sorting standard for hardwood is DIN 4074-5. The national visual grading classes are assigned European strength classes via the “assignment

Hardwood Construction Products

Cross-section dimensions

Dimensional tolerance class 1 2

≤ 100 mm

+3 / -1 mm

± 1 mm

> 100 mm to ≤ 300 mm

+4 / -2 mm

± 1.5 mm

> 300 mm

+5 / -3 mm

± 2 mm

For dimensional tolerances in the longitudinal direction, the following applies: Negative deviations are not permitted, positive deviations must be limited as needed. The reference humidity is ≤ 20 %. In the event of changes in the wood moisture content, the dimensional changes in the transverse direction should be determined as 3 ­follows: 0.35 % per 1 % moisture change.

standard” EN 1912 or via assignment reports. Hardwood sawn lumber can ­currently be graded only visually. DIN 20 000-5 is the application standard associated with EN 14 081-1. There are different versions of DIN 20 000-5, as well. Since only DIN 20 000-5:2012-03 refers to EN 14 081-1:2011-05, which is cited in the OJEU, this is the version currently listed in the Model Administrative Rules on Technical Building Regu­ lations (Muster-Verwaltungsvorschrift Technische Baubestimmungen – MVV TB) for solid wood applications per EN 14 081-1. Consequently, these applications are currently limited to beech and oak. For other wood species, a ­general construction technique permit or a European technical assessment is required. Wood ­species

Sawn lumber / finger-jointed solid wood Regulations for use 1) 

Avail­ ability

Maple

EN 14 081-1 with ZiE 2) / –

– / –

Birch

EN 14 081-1 with ZiE 2) / –

Beech

Application standards The standard series DIN 20 000-x governs the application of European standards in Germany. These application standards spe­cify which regulations of a product can be used for which applications in Germany.

or a European tech­nical assessment. However, the non-­normative regulation of finger-jointed solid hardwood timber other than poplar would be a largely theoretical solution, which would be difficult to implement in the short term. Technical rules and labelling

Applicability of finger-jointed solid wood

The European harmonised standard for finger-jointed solid wood is EN 15 497: 2014-07 and the associated application standard is DIN 20 000-7:2015-08. In addition to softwoods, the only hardwood species both of these standards allow for is poplar. Thus, only poplar may be used for finger-jointed solid timber; other hardwood species require a German national technical approval with a general construction technique permit Glued laminated timber

Laminated veneer lumber Regulations for use

Structural plywood / OSB

Avail­ ability

Regulations for use

Avail­ ability

ZiE

–

ZiE

–

ZiE

–

EN 13 986 with DIN 20 000-1

– / –

x / –

ZiE

x

ZiE

x

ZiE

–

EN 13 986 with DIN 20 000-1

x / –

EN 14 081-1 with DIN 20 000-5

x / –

abZ / aBG Z-9.1-679

x

ZiE

–

aBG Z-9.1-838 ETA-14/0354 ETA-18/1018

x

EN 13 986 with DIN 20 000-1 abZ Z-9.1-841

x / –

European chestnut

EN 14 081-1 with ZiE 2) / –

x / –

ETA-13/0646

x

ZiE

–

ZiE

–

EN 13 986 with DIN 20 000-1

– / –

Oak

EN 14 081-1 with DIN 20 000-5

x / –

abZ Z-9.1-821 ETA-13/0642

x

ZiE

–

ZiE

–

EN 13 986 with DIN 20 000-1

– / –

Ash

EN 14 081-1 with ZiE 2) / –

x / –

ZiE

(x)

ZiE

–

ZiE

–

EN 13 986 with DIN 20 000 1

– / –

Eucalyptus

EN 14 081-1 with ZiE 2) / –

– / –

ZiE

(x)

ZiE

–

ZiE

–

EN 13 986 with DIN 20 000 1

– /

Poplar

EN 14 081-1 with ZiE 2) / EN 15 497 with DIN 20 000-7

x / –

EN 14 080 with DIN 20 000-3

(x)

ZiE

–

ZiE

–

EN 13 986 with DIN 20 000-1

x / x

1) 

Regulations for use

Cross-laminated timber

The technical regulations listed below are relevant to sawn lumber and finger-­ jointed solid timber. In each case in the following text, the valid (dated) version is given. • DIN 4074-5: Strength grading of wood – Part 5: Sawn hardwood • DIN 68 800-1: Wood preservation – Part 1: General • DIN 68 800-2: Wood preservation – Part 2: Preventive constructional measures in buildings

Avail­ ability

Regulations for use

Avail­ ability

In general, all timber species and the products manufactured from them can be regulated for structural purposes through an individual approval (ZiE). There are various versions of EN 14 081-1. In order to be binding on building authorities, a regulation must be included in the Official Journal of the EU (OJEU). ­Although EN 14 081-1 has been revised twice since, EN 14 081-1:2011 is still the version cited in the OJEU and is thus binding. There are also different v­ ersions of the associated application standard DIN 20 000-5. Since the later versions refer to versions of EN 14 081-1 not cited in the Official Journal of the EU, DIN 20 000-5:2012 ­remains valid for applications in Germany .

2) 

4

31

• EN 336: Structural timber – Sizes, ­permitted deviations • EN 338: Structural timber – Strength classes • EN 350: Durability of wood and woodbased products – Testing and classification of the durability to biological agents of wood and wood-based ­materials • EN 1912: Structural timber – Strength classes – Assignment of visual grades and species • EN 1995-1-1: Eurocode 5: Design of timber structures – Part 1-1: General – Common rules and rules for buildings in connection with EN 1995-1-1/NA: National Annex – Nationally determined parameters Sawn lumber is also subject to the ­following regulations: • EN 335: Durability of wood and wood based products – Service

classes: definitions, application to solid wood and wood-based ­products • EN 14 081-1: Timber structures – Strength graded structural timber with rectangular cross section – Part 1: General requirements in connection with DIN 20 000-5: Application of construction products in structures – Part 5: Strength graded structural timber with rectangular cross ­section Finger-jointed solid wood is also subject to the following: • DIN 15 497: Structural finger-jointed solid timber – Performance requirements and minimum production requirements in connection with DIN 20 000-7: Application of construction products in structures – Part 7: Structural finger-jointed solid timber according to EN 15 497

fm, k [N/mm2]

Em, 0, mean [N/mm2]

ρk [kg/m3]

C 18

18

9,000

C 22

22

C 24

Country establishing grading criteria

Grade 1)

320

France

10,000

340

24

11,000

C 27

27

D 24

D 30

Strength Class

D 35

D 40

1) 

In the European system of strength classification, the national sorting classes, wood species and regions of origin of timber with the same strength characteristics are pooled into groups within which the wood types are equivalent and thus interchangeable (EN 1912:2013-10). This allows the designer to specify a ­particular strength class and use the characteristic strength parameters of this class (EN 338:2016-07) as the basis for structural calculations. As an alternative to EN 1912, the appropriate Initial Type Testing reports can also be used to assign sorting classes to strength classes. Wood species

Origin

Botanical name

ST-III

Poplar

Populus spec.

France

Germany

LS10+ LS10K+

Poplar

Populus nigra

Germany

350

France

ST-II

Poplar

Populus

France

11,500

360

Germany

LS13 LS13K

Poplar

Populus nigra

Germany

24

10,000

485

Italy

S

European ­chestnut

Castanea sativa

Italy

30

11,000

530

Germany

LS10+ LS10K+

Oak

Quercus robur Quercus paetraea

Maple

Acer pseudoplatanus

LS10+ LS10K+

Beech

Fagus sylvatica

LS13 LS13K

Beech

Fagus sylvatica

LS10+ LS10K+

Ash

Fraxinus excelsior

MEF

Eucalyptus

Eucalyptus globulus

35

40

12,000

13,000

540

550

Germany

Germany

“+” means “or better”, “K” stands for board / plank cross sections graded as squared timber

32

Description

Common name

Spain 5

Solid timber and finger-jointed sawn ­lumber must have a CE mark. The user must also be provided with a Declar­ ation of Performance (DoP) containing a description of the product properties.

Germany

Germany

Germany

Spain

Hardwood Construction Products

The strength classes are named after the characteristic bending strength (edgewise) in N/mm2, where softwood and poplar designations are preceded by a C (for coniferous) and hardwood designations with a D (for deciduous). In the updated, but not yet binding, EN 338: 2016-07, there are also strength classes for softwood based on tensile tests (T classes). Hardwood sawn timber is divided into strength classes D 22 to D 70, softwood and poplar into strength classes C 14 to C 45 (see “Structural Relationships”, p. 46). Products

Figure 5 shows the European hardwood species enumerated in EN 1912:2013-10 and their characteristic strength properties according to EN 338:2016-07.

Glued laminated timber and hybrid ­glulam Glued laminated timber (glulam) of ­hardwood consists of at least two, hybrid glulam of at least three layers of dimensional timber whose faces are fullsurface-glued to one another with their grains running parallel (Figs. 7 and 8). The composition of hardwood glulam can be combined or homogeneous. Combined glulam consists of timber layers of different strength, whilst homoge­ neous glulam employs layers of the same strength. In the case of hybrid glulam, the outer layers of the beam are usually made of hardwood, the central layers of softwood. In addition to hardwood glulam and hybrid glulam, another glulam on offer is of softwood sheathed in a 5-mm to 10-mm thick hardwood layer. This product is treated like softwood glulam and will not be considered further in this publication.

Product characteristics

Glued laminated timber consists of dried, planed and strength-graded sawn lumber or laminated veneer lumber (see p. 35ff.). In most cases, boards are first friction-­ locked to each other longitudinally by finger joints to form the layers, whose faces are then glued together. Glulam subject to an aBG (general construction technique permit) or an ETA can only be used for the production of simple, straight building components – components with variable cross sections and /or single or multiple curves require an individual approval (Zulassung im Einzelfall, ZiE). According to information provided by the manufacturers, glulam made of European chestnut or oak is available in many standard cross sections and in standard lengths of 13.50 m and 4 to 12 m, respectively. Poplar, beech and hybrid glulam is produced exclusively to customer-­ specified dimensions. In general, the maximum permitted limits stated in the approval must be observed. EN 14 080:2013-09 gives the tolerances for the dimensional accuracy of poplar glulam (Fig. 6). Glulam products regulated by an approval (abZ/aBG, ETA) do not include any information on dimensional accuracy, but in some cases the manufacturer-provided technical data sheets refer to EN 14 080 (or to the now-withdrawn EN 390:1995-03).

Maximum permissible deviation Width Height

Length

6

± 2 mm ≤ 400 mm

+ 4 / -2 mm

> 400 mm

+ 1 / -0.5 %

≤2m

± 2 mm

2 m ≤ l ≤ 20 m

± 0.1 % ± 20 mm

7

Adhesives

Adhesives approved for bonding are filed with the applicable authority specified in national approvals (abZ /aBG) or listed in

5

6 7 8

 ssignment of national sorting classes, wood A ­species and origins to strength classes according to EN 1912:2013-10 and characteristic values ­according to EN 338:2016-07 Dimensional stability of poplar glulam according to EN 14 080:2013-09 Hybrid glulam Beech glulam 8

33

Manufacturer / Permit holder

Holz Schiller GmbH

Elaborados y Fabricados Gamiz S.A.

SIERO LAM S.A.

Studiengemeinschaft Holzleimbau e. V.

Product

Oak post-and-beam glulam

Oak glulam

Chestnut glulam

Beech glulam and beech-hybrid ­glulam beams abZ / aBG Z-9.1-679

Regulation /approval

abZ Z-9.1-821

ETA-13/0642

ETA-13/0646

Wood species 1

Oak

Oak 3)

European chestnut

Beech

Origin Grading   Visual (class)   Branch size limit   by machine (E-modulus)

Germany, Czech Republic DIN 4074-5 LS13 x –

France DIN 4074-5 LS10, LS13 – –

– DIN 4074-5 LS13 – –

– DIN 4074-5 + E-modulus LS10, LS13 x x

Wood species 2

Spruce, pine, fir

Origin Grading Class

– DIN 4074-1 or EN 14 081-1 S10 or C24

Glulam beam Construction

homogeneous

symmetric combined

homogeneous

Height [mm] Width [mm] Length [m]

76 – 280 50 – 70 ≤ 12 / ≤ 4 1)

80 – 400 50 –160 ≤ 12

80 – 400 70 – 220 ≤ 13.5

homogeneous or symmetric combined / hybrid ≤ 600 / ≤ 900 ≤ 160 –

19 – 23 50 – 70 – ≥ 300 4

20 ± 2 ≤ 160 – 300 –1,200 4

– – – – 2

≤ 30 / ≤ 42 4) ≤ 160 ≤ 4,000 / – 4) – 3 /4 + x 5)

31.5 / 59.0 1)

33

30.0

28.0 – 48.0 6)

28.5 / 29.4 1) 0.6

23.0 0.6

20.0 0.7

21.0 0.5

48.0 9.0 5.5

45.0 8.0 4.0

45.0 5.5 4.2

25.0 8.4 3.4 / 2.5

14,000 800

14,400 850

13,000 810

13,500 –15,100 /13,200 –14,700 6) 1,000

Laminates Thickness [mm] Width [mm] Cross section [mm2] Length [mm] Minimum number Strength [N/mm²] Bending fm, k Tensile   parallel ft, 0, k   orthogonal ft, 90, k Compressive   parallel fc, 0, k   orthogonal fc, 90, k Shear fv, k Stiffness [N/mm²] E0, mean Gmean Density [kg/m3] ρk

650

690

520

650 / 350

Use class

1, 2 2)

1, 2 2)

1, 2 2)

1

Standard /“premium” product with no finger joints in the outer laminates The compressive strength parallel to the fibre must be reduced by one third when used in use class 2. Also available in beech, chestnut and ash according to the manufacturer, but not approved in Germany 4)  Beech /softwood 5)  At least four beech laminates in the hybrid 9 6)  Lowest and highest class 1)  2)  3) 

34

Hardwood Construction Products

10 a

the technical documentation submitted for European approvals (ETA). In general, the bonding agents used are amino resin and phenolic resin adhesives (e.g. MUF, PRF) and polyurethane adhesives (PUR). The adhesive joints are dark if the glue contains resorcinol (PRF), transparent or pale otherwise. The amount of adhesive contained in the finished product is usually less than 2 %. Applications

The European harmonised standard for glued laminated timber is EN 14 080:2013-09, and DIN 20 000-3:2015-02 is the corresponding application standard. Apart from the softwood s­ pecies, the only hardwood species approved in EN 14 080:2013-09 for the production of glulam is poplar. In ­Germany, glulam made from other hardwoods can only be used with a national technical approval (abZ), a general construction technique permit (aBG), a European technical approval (ETA) or an individual approval (ZiE). Such documents are available for the beech, oak and European chestnut. For hybrid glulam according to the abZ / aBG, beech may be used in combination with spruce, pine and fir. Technical rules and labelling

For hardwood glulam, most of the general rules listed in the section “Sawn lumber and finger-jointed solid wood” (p. 30ff.) as well as the product-specific technical rules listed below apply. In the following text, the currently valid (dated) version for each case is given. • EN 14 080: Timber structures – Glued laminated timber and glued solid ­timber – Requirements, in connection with DIN 20 000-3: Application of ­construction products in structures – Part 3: Glued laminated timber and glued solid timber according to EN 14 080

b

• abZ/aBG Z-9.1-679: Beech glued ­laminated timber and hybrid beech ­glulam beams and associated constructions • abZ Z-9.1-821: Holz Schiller oak postand-beam glued laminated timber • ETA 13/0642: Solid hardwood glued laminated timber – VIGAM oak glulam • ETA 13/0646: Solid hardwood glued laminated timber – SIEROLAM chestnut glulam

Beech laminated veneer lumber Laminated veneer lumber (LVL) is composed of veneers that are principally arranged with their grains oriented in the same direction (Fig.10). Some products also contain transverse layers, which can account for 15 % to 30 % of the cross section. Beech laminated veneer lumber is thus far produced only in Germany. Product characteristics

For glulam in accordance with EN 14 080 or an ETA, a CE mark as well as a Declar­ ation of Performance (DOP) are required. For glulam in accordance with an abZ / aBG, the conformity of the product with the provisions of the approval must be certified through an attestation of conformity (Ü mark).

Products

Dried peeled beechveneers with a ­thickness of approx. 3 mm are glued together to form a continuous strand of laminated veneer lumber panel about 1.80 m wide. The individual veneer lamin­ ates are glued together in the direction of the grain by means of a scarf joint [1], with the layers arranged so that their joints are offset. The alignment of the veneers is either exclusively parallel to the wood fibre or primarily parallel with individual layers rotated by 90 °. The ­panels either form the end product as flat building components or are further processed into rod-shaped members. These are either beams composed of panel strips placed on edge or glulam, which consists of panel strip layers glued on top of one another. In this case, finger joints in the longitudinal direction of the layers are not allowed. After production, the moisture content of the final timber product is about 8 %. Beech laminated veneer lumber is avail­ able in a great variety of dimensions. For example, glulam made of laminated veneer lumber currently ranges in width

Fig. 9 provides a comparison of the glulam products that are certified for use in Germany through national technical approvals, along with their most important characteristics. The values for glulam of beech laminated veneer lumber are shown in Fig. 12 (p. 36).

 9 G  lued laminated timber (glulam) products made of hardwood in accordance with national tech­nical approval (abZ/aBG, ETA) and their properties 10 Laminated veneer lumber (LVL) made of beech a  beam with transverse layering b  beam with edgewise vertical layering

Designations

Glued laminated timber (glulam), just like sawn lumber, is divided into GL strength classes derived from charac­ teristic bending strengths (in N/mm2); for example, GL 24. An additional h refers to a homogeneous, c to a combined ­glulam. This designation also applies to beech laminated veneer lumber (see “Beech laminated veneer ­lumber”), since the final product is composed of individual veneer slats. It is not possible to distinguish between hardwood and softwood solely on the basis of the GL label – the same acronym is used for both.

35

11

from 50 to 300 mm and in height from 80 to 1360 mm. The maximum beam length is 18 m. The dimensional tolerances given by the manufacturers are ± 5 mm in length, ± 2 mm in width /height and ± 1 mm in thickness / width.    Adhesives

A phenol formaldehyde resin (PF) is used for the manufacture of laminated veneer lumber panels. For the subse-

quent production of glulam, a phenol-­ resorcinol-formaldehyde resin (PRF) is employed. Approved adhesives are listed in the technical documentation for approval (ETA) or filed with the DIBt (German Institute of Building Technology) (aBG). The adhesive joints are dark, though glulam is also available with lightcoloured scarf joints on both outer veneer layers. The amount of ­adhesive contained in the finished product is about 6 %.

Applications

The European harmonised standard for laminated veneer lumber is EN 14 374:2005-02. However, hardwood LVL currently still requires a national ­technical approval, since there is no application standard available in Germany. Beech laminated veneer lumber is currently regulated via a general construction technique permit (aBG) and European technical approvals (ETA).

Manufacturer /Permit holder

Pollmeier Furnierwerkstoffe GmbH

Hasslacher Holding GmbH

Pollmeier Furnierwerkstoffe GmbH

Product

BauBuche GL75 beam

HASSLACHER BauBuche

Laminated veneer lumber BauBuche S-board

Regulation /approval

ETA 14/0354

ETA 18/1018

Laminated veneer lumber BauBuche Q-board

aBG Z-9.1-838

Beam / panel Height [mm]

80 – 600 / 80 – 2,500 1)

21– 66 2)

Width [mm]

50 – 300 / 50 – 600

1,820 3)

Length [m]

≤ 18 / ≤ 36

≤ 35

40 ± 3

–

50 – 300

–

3

–

Laminates Thickness [mm] Width [mm] Minimum number Strength [N/mm2] Bending fm, k4)

75.0

80.0 5)

70.0 / 81.0 5), 6)

  parallel ft, 0, k

60

60.0 5)

46.0 / 49.0 5), 6)

  orthogonal ft, 90, k

0.6

1.5 5)

15.0 / 8.0 5), 6)

  parallel fc, 0, k

49.5

57.5 5)

57.0 / 62.0 5), 6)

  orthogonal fc, 90, k

12.3

14.0 5)

40.0 / 22.0 5), 6)

Shear fv, k

4.5

8.0

7.8 /7.8 5), 6)

E0, mean

16,800

16,800

11,800 /12,800 5), 6)

Gmean

850

760

820 / 820 5), 6)

Tensile

Compressive

 5)

Stiffness [N/mm ] 2

Density [kg/m ] 3

ρk

≥ 730

730

Use class

1, 2 7)

1, 2 7)

1)  regular / XXL 2)  width w, dimension in the direction of the panel thickness, independent of its orientation 3)  height h, dimension perpendicular to the fibre direction of the outer veneers 4)  values for h ≤ 300 mm. For 300 < h ≤ 1,000 mm, the characteristic strength value shall be multiplied by the coefficient kh = (300/h)0.12. h (in mm) is the ­dimension of the total cross section critical for bending stress. 5)  Values are valid for stresses on panel 6)  nominal thickness 21– 24 mm / nominal thickness 27– 66 mm 12 7)  the compressive strength may be increased by a factor of 1.2 for use in use class 1.

36

Hardwood Construction Products

13 Technical rules and labelling

For beech laminated veneer lumber, most of the general rules listed in the ­section “Sawn lumber and finger-jointed solid wood” (p. 30ff.) as well as the ­product-specific technical rules listed below apply. In the following text, the ­currently valid (dated) version for each case is given. • EN 14 374: Timber structures – ­Structural laminated veneer lumber – Requirements • aBG Z-9.1-838: Beech laminated veneer lumber for the construction of bar-shaped and flat structural elements “BauBuche S board” and “BauBuche Q board” • ETA 14/0354: Glued laminated timber made of hardwood – structural lami­ nated veneer lumber made of beech • ETA 18/1018: Glued laminated timber made of hardwood – structural lami­ nated veneer lumber made of beech For laminated veneer lumber in accordance with an ETA, a CE mark as well as a Declaration of Performance (DOP) are required. For laminated veneer lumber in accordance with an aBG, the confor­ mity of the product with the provisions of the approval must be certified through an attestation of conformity (Ü mark). Products

Fig. 12 gives an overview of the most important laminated veneer lumber products for use in construction, together with their technological characteristics.

Engineered timber products of ­hardwood Currently, structural engineered timber products made entirely from hardwood are available in the form of plywood and OSB (Oriented Strand Board) (Fig. 11 and

13). Chip or particle board is generally made of softwood, but can contain up to 40 % hardwood. It will not be considered further here. Engineered wood products as such are produced by chipping wood into par­ ticles of varying size and shape and subsequently joining these together, usually through the addition of bonding agents. The product does not retain the original wood fibre structure. In this publication, glued laminated timber, laminated veneer lumber and cross-laminated timber do not fall within the definition of “engineered wood product”. Plywood consists of a composite of bonded veneer layers, in which the fibre orientations of successive layers are ­usually perpendicular to each other. It is ­distinct from the cross-laminated timber (CLT or X-Lam) defined in EN 16 351, the individual layers of which are thicker (usually > 7 mm) and graded according to their strength. OSB consists of several flat layers of wood strands which are glued together using a bonding agent.

a directional dependence of the tech­ nical characteristics (strength, stiffness, swelling and shrinking behaviour) in the plane of the panel. Plywood can be upgraded for use in higher-level applications by the addition of flame retardants or fungicides. OSB is layered and consists of long broad strands more than 50 mm long (averaging between 75 mm and 130 mm), with a mean width of 35 mm and a thickness of usually less than 2 mm (aver­ aging 0.6 mm). The strands in the outer ­layers are mostly aligned parallel to the length or width of the panels; the strands of the middle layer(s) are either aligned randomly or (as a rule) perpendicular to the strands of the outer layers. As a result, OSBs have different properties along their longitudinal and transverse directions. Plywood and OSB panels are available in many different dimensions. Dimensional tolerances (dimensions, edge straightness, squareness) are given in EN 315:2000-10 and EN 300:2006-09. Adhesives

Product characteristics

Plywood is mainly composed of dried peeled veneers of 1.5 mm to 3 mm ­thickness and is therefore also referred to as veneer plywood. The manufacturing process is essentially the same as that for beech laminated veneer lumber. However, the composition of ­plywood deviates from that of LVL in that the ­successive layers are predom­inantly arranged perpendicular to one another. The number of layers is usually odd; the two outer layers always have the same fibre orientation and are made of the same material or are treated with the same coating to reduce warping. In ­contrast to laminated veneer lumber, the perpendicular orientation of the individual layers in plywood does not create

The adhesives used for plywood and OSB are mainly amino resin and phe­ nolic resin adhesives (e.g. MUF, PF, PRF); for OSB, polymeric diphenyl diiso­ cyanate (PMDI) is also used. The latter is formaldehyde-free and highly moisture-­ resistant. The adhesives for products in accordance with a general construction technique permit (aBG) are filed with the DIBt. The adhesive content of the finished product ranges from 7.5 % to 9.5 % for beech plywood, for example, and is approx. 2.5 % for poplar OSB.

11 B  eech plywood 12 B  eech laminated veneer lumber products in ­accordance with construction approval (aBG, ETA) and their properties 13 Poplar OSB panel with light-coloured surface

37

Manufacturer

Metsä Wood Deutschland GmbH

SWISS KRONO Kft.

Blomberger Holzindustrie GmbH

Product

Metsä Wood structural birch plywood

SWISS KRONO OSB/3 bright

Delignit BFU 100 plywood Class EN 636 2 S

Regulation / approval

EN 13 986

EN 13 986

EN 13 986

Wood species

Birch

Poplar

Beech

Adhesive

Phenol formaldehyde resin

Polymeric diphenyl diisocyanate

Phenol formaldehyde resin

Dimensions Thickness [mm]

12 – 50

8 – 25

15 – 60

Width [mm]

≤ 1,500

1,250 (2,800)

≤ 1,220

Length [mm]

≤ 3,660

2,070 – 5,000

≤ 2,500

9 – 35

3

7 – 22

  parallel fm, 0, k

42.9 – 36.8

18.0 –14.8

50.0 – 40.0

  orthogonal fm, 90, k

33.2 – 34.8

9.0 – 7.4

30.0 – 25.0

  parallel ft, 0, k

40.1– 37.0

9.9 – 9.0

20.0 –10.0

  orthogonal ft, 90, k

35.0 – 36.9

7.2 – 6.8

25.0 –12.5

Veneers or layers Strength [N/mm ] 2

Bending

Tensile

Compressive   parallel fc, 0, k

27.7 – 25.6

15.9 –14.8

20.0 –10.0

– / 24.3 – 26.4

12.9 –12.4

25.0 –12.5

  parallel fv, k

9.5

6.8

7.5

  orthogonal fr, k

9.5

1.0

1.2

  parallel Em, 0, mean

10,700 – 9,200

4,930

6,700 – 5,800

  orthogonal Em, 90, mean

6,700 – 8,300

1,980

3,300 – 5,800

  parallel Et /c, 0, mean

9,300 – 8,900

3,800

3,300 –1,700

  orthogonal Et /c, 90, mean

8,100 – 8,600

3,000

5,300 – 2,600

  normal to plane Gv

620 

1,080

550

  in the plane Gr

620

50

110

630

> 600

BFU: 750, FRCW: 840

1, 2, 3

1, 2

1, 2

  orthogonal fc, 90, k Shear fv, k

Stiffness [N/mm ] 2

Bending

Tension + compression

Shear Gmean

Density [kg/m ] 3

ρk 14

Use class

38

Hardwood Construction Products

Applications

The European harmonised standard for ­plywood and OSB is EN 13 986:2015-06. It applies in conjunction with the product-­ specific standards EN 636:2015-05 and EN 300:2006-09. The associated application standard is DIN 20 000-1: 2017-06. Plywood and OSB may categorically be manufactured from all species of hardwood. Currently, birch, beech and poplar are used most frequently. Plywood must have at least three layers for reinforcement purposes and at least five layers for all other structural components; the minimum thickness of structural panels is 6 mm. The minimum thickness of structural OSB panels is 8 mm. OSB planks used to reinforce wood panels for timber-frame timber houses must be 6 mm thick.

a Declaration of Performance (DOP) is required. Products

An overview of selected plywood and OSB panels for use in construction together with their technological characteristics is shown in Fig. 14.

Information on use The use of construction products made of hardwood follows the basic proced­ ures and processing techniques familiar from softwood products. In some cases, however, greater attention and adjustments are needed. The following summarises some important add­itional information not described in the preceding chapters.

Technical rules and labelling

Product characteristics

For hardwood plywood and OSB, most of the general rules listed in the section “Sawn lumber and finger-jointed solid wood” (p. 30ff.) as well as the product-­ specific technical rules listed below apply. In the following text, the currently valid (dated) version for each case is given. • EN 300: Oriented Strand Boards (OSB) – Definitions, classification and specifications • EN 315: Plywood – Tolerances for dimensions • EN 636: Plywood – Specifications • EN 13 986: Wood-based panels for use in construction – Characteristics, evaluation of conformity and marking / DIN 20 000 1: Application of construction products in structures – Part 1: Wood based panels

Wood features that have no bearing on strength (e.g. discolouration) are not taken into account in strength grading. Additional requirements regarding the visual quality of hardwood building products must therefore be agreed upon on a case-by-case basis. Lumber for construction purposes can be used in its rough-sawn state (satis­ fying the minimum requirements in DIN 18 334:2016-09), but is usually ground or planed before strength grading to allow for better identification of the timber’s characteristics. If the surface is subsequently processed, the original cross section must not be reduced too much (DIN 4074-5:2008-12) to ensure that the strength classification remains valid.

For plywood and OSB in accordance with EN 13 986, a CE mark as well as

Glued laminated timber is usually delivered having been planed and chamfered on all four sides. Other surface qualities can be individually agreed upon.

Wood protection

During the use phase, appropriate wood protection must be ensured. This includes all measures taken to prevent damage to or destruction of the timber by fungi and insects, but also damage to the building itself. The latter can be caused by excessive swelling and shrinkage of the wood, causing warping of the components and possible damage to neighbouring components. In protecting wood, construct­ ive measures should be prioritised – only in exceptional cases where protection against biological pests cannot be sufficiently guaranteed should chemical measures be taken [2]. Further explan­ ations can be found in the section “Dur­ ability” (p. 41f.). Wood protection measures must also be observed during the period between production and final assembly on the construction site. An interim increase in the wood moisture content of the ­components through weathering – for example during transport, during storage on the construction site or through the release of water from other building materials used in the same construction project, such as wet screeds – should be avoided. If there is a risk of weather exposure during the transport and assembly phases, for example, the use of a temporary protective coating is advisable. The manufacturers of hardwood products provide relevant information and recommend suitable products.

14 H  ardwood plywood and poplar OSB (in ­accordance with EN 13 986:2015-06) and their properties

39

Use class

Description

Wood moisture content

1

Wood moisture content corresponding to a temperature of 20 °C and a ­relative humidity of 65 %, exceeded for only a few weeks per year (e.g. building components in fully enclosed and heated buildings)

10 ± 5 %

2

Wood moisture content corresponding to a temperature of 20 °C and a ­relative humidity of 85 %, exceeded for only a few weeks per year (e.g. building components in covered, open structures)

15 ± 5 %

3

Climatic conditions that lead to greater wood moisture content (e.g. ­building components exposed to weather)

18 ± 6 %

15

Building material classification (DIN 4102-4:2016-5) / Fire classifi­ cation (EN 13 501-1:2010-01)

B2 (normally flammable) /  D-s2, d0 (1); glulam of beech laminated veneer lumber: E; structural plywood FRCW: B-s1-d0

Burn rate ß0 (EN 1995-1-2:2010-12 /  Approval)

Sawn lumber / finger-jointed solid wood Density ≥ 290 kg/m3: 0.65 mm/min (all) Density ≥ 450 kg/m3: 0.50 mm/min (all except beech) Glued laminated timber Chestnut, oak (Holz Schiller): 0.50 mm/min Oak (VIGAM): 0.55 mm/min Beech: 0.65 mm/min Laminated veneer lumber of beech /  glued laminated timber of beech LVL 0.65 mm/min Plywood Density 450 kg/m3 + nominal thickness 20 mm: 1.0 mm/min Other values: For calculations, see EN 1995-1-2:2010-12 Chapter 3.4.2 (9) OSB 0.9 mm/min

Differential shrinkage rate (EN 1995-1-1/NA:2013-08 /  Approval)

Sawn lumber / finger-jointed solid wood /glulam Transverse to fibre: 0.35 % per 1 % moisture change 2) Parallel to fibre: 0.01 % per 1 % moisture change Beech laminated veneer lumber Transverse to fibres of the outer veneers: 0.32 % and 0.03 % per 1 % change in wood moisture (without or with orthogonal layers, respectively) Parallel to fibre: 0.01 % per 1 % moisture change Glulam made of beech laminated veneer lumber Transverse to the fibre (along thickness /width directions): 0.45 % and 0.40 % for each 1 % change in wood moisture Parallel to fibre: 0.01 % per 1 % moisture change Plywood In the plane of the panel: 0.02 % per 1 % moisture change Orthogonal to the plane of the panel: 0.32 % per 1 % moisture change OSB 0.03 % per 1 % moisture change

Thermal conductivity λ (EN ISO 10 456:2010-05 /  Declaration of performance (DOP)

Sawn lumber / finger-jointed solid wood / glulam Density 450 kg/m3: 0.12 W/mK Density 500 kg/m3: 0.13 W/mK Density 700 kg/m3: 0.18 W/mK Laminated veneer lumber of beech / beech plywood Density 500 kg/m3: 0.13 W/mK Density 700 kg/m3: 0.17 W/mK Density 1,000 kg/m3: 0.24 W/mK Glulam of beech laminate veneer lumber /   birch plywood 0.17 W/mK OSB 0.13 W/mK

Water vapour diffusion resistance factor μ (EN ISO 10 456:2010-05 /  Declaration of performance (DOP) – dry / moist

1) 

16

2) 

Sawn lumber / finger-jointed solid wood / glulam Density 450 kg/m3: 50 / 20 Density 500 kg/m3: 50 / 20 Density 700 kg/m3: 200 / 50 Laminated veneer lumber of beech /  glulam of beech laminated veneer lumber / plywood Density 500 kg/m3: 200 /70 Density 700 kg/m3: 220 / 90 Density 1,000 kg/m3: 250 /110 OSB 300 / 200

Sawn lumber / finger-jointed solid wood: Density ≥ 350 kg/m3, thickness ≥ 22 mm According to the standard, the value does not explicitly apply to glulam.

40

Hardwood used in construction is dried by technical means to the wood moisture content that corresponds to the equilibrium moisture in its eventual application (Fig. 15). Finished buildings must therefore be sparingly air-conditioned, because the use of water-containing building materials – at least in the peripheral areas – initially increases the moisture content of timber in the component, and subsequent air-conditioning is associated with a decrease in the wood’s moisture. Because of the differences in properties between hardwoods and softwoods (including higher strength and rigidity, higher density, and in some cases greater degrees of swelling and shrinking, especially for beech), these moisture fluctuations could result in un­­ desirable warping or splitting. Since beech laminated veneer lumber products are delivered with a wood ­moisture content of only about 8 %, the swelling behaviour when exposed to higher equilibrium moisture levels must be carefully considered. Specific information on wood and moisture protection is usually included in the national technical approvals. In addition, manufacturers provide documentation with instructions on packaging, transport and storage. Applications

Hardwood timber may be employed in use classes 1 to 3, though with the exception of oak its utilisation in class 3 applications is not recommended. Finger-­ jointed solid poplar may only be used in classes 1 and 2. Oak glulam, which would be suitable for use in weathered components (use class 3) due to the durability of oak, is only approved for use in classes 1 and 2. Plywood can be used as a construction material in dry conditions (technical class

Hardwood Construction Products

17

EN 636-1: use class 1; non-weather-­ resistant bonding; former designation: BFU 20), in humid conditions (technical class EN 636-2: use class 2, moderately weather-resistant bonding; former designation: BFU 100) or outdoors (technical class EN 636–3: use class 3; weather-­ resistant or moderately water-resistant bonding). For OSB panels, only technical classes 2 to 4 as defined in EN 300 may be used for load-bearing purposes. OSB/2 may only be used in dry conditions (use class 1), whilst OSB/3 and OSB/4 may also be used in humid conditions (use class 2). When installing engineered wood products in use classes 2 and 3, it is import­ ant to remember, particularly for largearea installations, that fluctuations in humidity or direct moisture exposure can cause swelling. A separation must therefore always be maintained between neighbouring panels, or between panels and adjacent building components, during assembly. Engineered timber panels should be acclimated before installation; that is, they must be stored for a few days under the climatic conditions of the installation site. This acclimatisation to the ambient humidity of the eventual assembly area prevents excessive swelling and shrinkage. If tannin-rich timber species (e.g. European chestnut, oak, eucalyptus) are exposed to moisture (in use classes 2 and 3, or by temporary weather influences during construction), timber constituent substances (e.g. tannins) may leach out and stain the wood surface and adjacent building materials, such as exposed masonry and pale plaster or concrete surfaces. Therefore, an allaround treatment with a so-called tannin blocker (oil, varnish, paint) is recommended whenever such exposure is

likely. The strength of the timber is not affected by stain formation. The corrosive effects on fasteners must also be considered. The use of suitable corrosion-resistant steel (e.g. A4 stainless steel) is recommended (see also EN 1995-1-1/NA and publications by the Holzabsatzfonds [3]). Structural parameters

For documentation on moisture, wood and fire protection of hardwood con­ struction products, designers can refer to the characteristic values listed in Fig. 16.

Durability Wood is a natural material and, under certain environmental or installation conditions, it is vulnerable, if existing regulations are not observed, to degradation by fungi and insects. Important param­eters in this context are timber species, wood 15 U  se classes and associated wood moisture ­contents 16 Structural values for building products made of hardwood 17 Water on beech LVL with protective coating ­(hydrophobisation) 18 Temporary weather protection of a timber truss of beech LVL

18

41

moisture content, air humidity and air temperature. The terms service class, durability class and use class come into play in the determination of which wood species should be used in which part of a construction, and whether additional protective measures are required in addition to the structural measures. The section “Information on use” (p. 39ff.) contains further information about wood protection.

Service classes

Durability classes

Service classes in accordance with DIN 68 800-1:2011-10 with reference to EN 335:2013-06 are geared toward the wood moisture content of installed components. They help with the assessment of whether and what protective measures are required. The service classification of a component or a construction is done by the designer.

The durability classification in EN 350:2016-12 reflects the natural resistance of wood species to wood-­ destroying fungi, which represent the most important threat to timber in Central Europe. This classification only states reference values, which ­primarily serve to assess the expected usage l­ifetimes of different timber species relative to one another. The actual service lifetime depends critically on the installation situation and on the ­natural variations in wood characteristics. The designer selects the required durability class that corresponds with the service class of a building component.

Wood species

Durability class 1) DC 1 very durable

DC 2 durable

DC 4 slightly durable

DC 3 moderately durable

Maple

‡

Birch

‡

Beech

‡

European chestnut

‡

‡

Oak

‡

‡‡

‡

‡ ‡

Poplar

‡‡

Durability class ... is ... for service class ...

Service class 0 Indoors, always dry, insect damage precluded

satisfactory

1 Indoors, always dry

3.1 Outdoors, no ground contact, not covered, moisture exposure possible 3.2 Outdoors, no ground contact, not covered, moisture exposure with water accumulation possible

‡‡

Use classes

Eucalyptus

2 Outdoors, no ground contact, covered, occasional moisture ­exposure possible

DC 5 not durable

satisfactory

satisfactory

satisfactory

usually satisfactory 2)

usually satisfactory 2)

Correlations satisfactory

satisfactory

usually satisfactory 2)

usually satisfactory 3), 4)

usually satisfactory 3), 4)

The sapwood of all wood species is designated as durability class 5. Usually satisfactory, protective treatments recommended under certain conditions of use 3)  Usually satisfactory, protective treatment necessary under certain conditions of use 4)  The use of DC 4 and DC 5 classes of timber treated with protective agents is not recommended for SC 3.1 and SC 3.2. ‡  according to laboratory or field tests which simulate an installation into the ground ‡ according to laboratory tests to determine durability against wood-destroying basidiomycetes 19 (bracket fungi) 1)  2) 

42

Use classes according to EN 1995-1-1:2010-12 define the environ­ mental conditions for installed timber components. They are determined by the wood moisture content, which adjusts itself over long-time use in response to the ambient humidity and temperature. Individual timber load-­ bearing structures are assigned to use classes. This helps the planner in the design of timber building components. Fig. 15 (p. 40) provides an overview with more detailed descriptions of the use classes.

An overview of the assignment of service classes, durability classes and wood species is given in Fig. 19. It shows which minimum durability class is required for timber of a given service class if – as is usually the case – chem­ ical protective treatment of the wood is not desired. Service classes 4 and 5 are missing from the table, as these do not have any relevance for timber construction.

Hardwood Construction Products

Emissions Wood generally contains few natural ­volatile organic compounds (VOC), and hardwood usually contains significantly fewer than softwood. VOCs are composed of a large number of substances that belong to the classes of t­erpenes, aldehydes, ketones, alkanes, carboxylic acids and others. Terpene emissions (which smell of freshly cut pine) are fairly negligible in hardwood, with aldehydes (e.g. formaldehyde) and carboxylic acids (e.g. acetic acid) present in very small amounts. Most characteristic values are within a range that lies at the limits of measurability using routine analytical methods. Overall, based on the current state of knowledge, no damage or adverse effects to health are expected from hardwood timber building product emissions during proper use. With regard to the outgassing of formal­ dehyde, the following applies: Dried solid timber emits only very small quan­ tities. For example, oak and beech release 0.004 ppm and 0.003 ppm, respectively [4]. In glued hardwood ­products, amino resin (UF, MUF) and phenolic resin (PF, PRF) adhesives also release only very small amounts of formaldehyde. All products are allocated to emissions class E1 (0.1 ppm or 124 μg/m3) as described in DIBt ­directive 100 and the corresponding product standards, which likewise conforms with the stipulations of the Banned Chemicals Ordinance. In some cases, more specific values are also available from measurement or assay reports. According to the material parameters assay, VIGAM oak glulam (with a layer thickness of 20 mm) made with MUF adhesive has formaldehyde emissions of only 0.025 mg/m3 or 0.02 ppm. Products manufactured with PF adhesive have the following emission values: Beech

laminated veneer lumber