Pablo van der Lugt
TOMORROW'S TIMBER TOWARDS THE NEXT BUILDING REVOLUTION
TABLE OF CONTENT
Terminology Timber Myths Dispelled 1
6 7 8
TRANSITION TO A BIO-BASED ECONOMY Facing the Consequences of 1.1 a Linear Economy 1.1.1 #1 Global Warming: The Impact of Rising CO2 Emissions 1.1.2 #2 Materials Depletion: Reserves are Running Out Required Transition to 1.2 a Circular Economy 1.2.1 A New Economic Model: Designed for Circularity 1.2.2 Circularity in the Built Environment 1.2.3 Understanding the Bio-cycle 1.2.4 Bio-based Materials versus Techno-cycle Materials Back to a Bio-based Economy 1.3
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2
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2.1 2.2 2.3 2.4
3 3.1 3.2
FORESTS, FOREST PRODUCTS AND CLIMATE CHANGE Forests of the World Forest Products Industry Forests and CO2 at Global and Regional Level Forests and CO2 at Micro Level: Timber Application
WOOD BASICS Wood Anatomy: From Macro to Micro Scale General Properties
10 10 15 18 18 20 25 26
34 38 44 52
64 66
4 4.1 4.2 4.2.1 4.2.2 4.3 4.3.1 4.3.2 4.3.3 4.3.4 4.4
5 5.1 5.1.1 5.1.2 5.1.3 5.1.4 5.1.5 5.1.6 5.1.7 5.2 5.2.1 5.2.2 5.2.3 5.2.4 6
TIMBER’S EVOLUTION FROM PAST TO PRESENT TO FUTURE Historic Development Timber Optimization Engineered Wood – Mass Timber Modified Wood Construction Systems Light Timber Systems Mass Timber Systems Hybrid Systems Connections How It All Comes Together: Tall Timber Building
DESIGNING AND BUILDING WITH MASS TIMBER Material Performance Fire Safety Structural – Seismic Acoustics Vibrations Thermal Performance Durability Health and Well-being Building Process Prefabrication Building Site – Installation Economics Factors of Success and Failure
FUTURE OUTLOOK: TOMORROW’S TIMBER
74 76 82 82 92 98 98 103 108 113 116
124 126 126 131 132 135 135 140 144 152 152 160 165 174 178
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5
Wood Innovation and Design Center in Prince George, Canada, designed by Michael Green Architecture (see project case on page 106).
2
FOREWORD
From the earliest beginnings of humankind, we have crafted our homes and communities from the resources around us. From earth, clay and stone to timber, bamboo, thatch and straw, humans found harmony with nature as we sought to shelter from her forces. As time progressed, we mastered ingenuities of ever more complex materials, rooted in our ability to harness fossil energy and to slam the earth for resources once unreachable. We added chemicals, toxins and synthesized a stew of materials that erased the ‘imperfections’ of our natural materials heritage. But this ‘progress’ led to a new relationship with the earth and a distancing from a natural sensibility of architecture to one increasingly constructed of artificial manmade materials. This progress became habit and we began to assume that the original, natural options for building were less relevant, less modern, less sophisticated. But as nature does so often, humankind is being put in its place once again. Our dependence on fossil fuels and materials born from cheap massive energy has led to climate change, pollution and environmental degradation of our planet. We have surrounded ourselves with toxins. We are chipping away at our relationship with the natural world and our fundamental need to protect it. We have cut down old-growth trees and depleted our forests in unsustainable ways for various missions unrelated to building, as we clear them for cattle, crops, development and fuel. Our forests nevertheless still hold the answer. With more sophisticated engineering and material bonding techniques new forest products are opening an entirely new future, built on traditions of the past but imagining a healthier, happier civilization in a future more connected to our planet and managing our massive greenhouse gas output in much more sustainable ways. New wood products engineered from young-growth trees and sourced from truly sustainable forest practices are our path forward as city builders. Unlike the other major structural materials of steel, concrete and masonry, timber is grown by the power of the sun, stores carbon and is renewable.
With the invention of large engineered panels and elements called mass timber we can combine other engineered wood products into very large and tall buildings that are competitive in terms of performance and cost with concrete and steel. By leveraging the relatively light weight of mass timber and its robustness we are finally turning the corner to more affordable and capable off-site construction. By designing for end-of-life reuse of these large timber panels we are finally engaging a true circular economic solution. In Tomorrow’s Timber, new timber innovations are explored, including the materials, products, elements and complete building systems, providing context for this emerging shift in design and construction. Inspiring case studies worldwide show that the mass timber revolution is happening as we speak. Tomorrow’s Timber contextualizes the challenges and how forests and mass timber can help solve our global problems by mitigating climate change while supporting the move to a less resourcedependent, circular bio-based economy. Finally, the book tackles real mass timber design opportunities and challenges on building and site level, before providing a promising outlook towards the future. In many respects humankind has come full circle. We are returning to natural materials that build our cities to serve our modern lifestyles in harmony. We are embracing the benefits of timber that include measurably reduced stress levels and the corresponding improving health, learning, productivity and comfort levels. We are designing very large timber buildings in global cities with renewed optimism. Around the world building codes and public policy are evolving to increase the amount of timber buildings and innovations continue to emerge each year that push new boundaries of height, scale, typology and possibility for the cities of tomorrow. Tomorrow’s Timber is an informative and important resource for those committed to restoring our balance with the planet while serving the necessities of humankind.
Michael Green 3
1.
TRANSITION TO A BIO-BASED ECONOMY Because of the growing global population, overconsumption and a take-make-waste economy we are heading towards a potentially irreversible resource and climate problem. Only through a transition to a circular economy, with an important role for renewable, biobased materials (bio-based economy), will humankind be able to safeguard resources and a liveable habitat for future generations. This chapter explains the problem, but also (part of) the solution.
8
Hotel Jakarta in Amsterdam This hotel in Amsterdam (designed by SeARCH) combines a timber bearing structure with bamboo finishing materials (Moso), see project case on page 163.
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TRANSITION TO A BIO-BASED ECONOMY – FACING THE CONSEQUENCES OF A LINEAR ECONOMY
1.1
FACING THE CONSEQUENCES OF A LINEAR ECONOMY Addiction to Fossil-Based Resources and Non-Renewable Materials For thousands of years humankind lived in harmony with nature, sustaining a life built on renewable, bio-based resources supplied by nature (wood, crops, animals, etc.). Only in the past two centuries, since the Industrial Revolution, has humankind become dependent on fossil energy sources while also using them for the production of high-tech, non-renewable abiotic materials, such as plastics, metals and minerals, which have largely replaced the once dominant renewable bio-based materials. The growing human population in combination with an increase in consumption per capita has led to an enormous growth in demand for materials that are mostly abiotic, and energy sources that are mostly fossil-based. This trend is not expected to change: projections by the UN show an increase of the global population from 7 to 10 billion by 2050.5 This is further accelerated by increasing consumption per capita as a result of increasing wealth, which means we can expect a two- to three-fold increase in global resource demand by 2050. This unsustainable overconsumption – taking more from the earth than it can naturally reproduce – causes several interrelated global environmental problems, such as depletion of resources, deterioration of ecosystems and human health through global warming, toxic pollution, acidification, eutrophication, etc. For humankind the most prominent and urgent of these problems are global warming and resource depletion. The built environment has a huge impact on both.
1.1.1
#1 GLOBAL WARMING: THE IMPACT OF RISING CO2 EMISSIONS
Climate change is increasingly acknowledged as a threat to our environment and human society. Binding agreements have been made during COP 21 in Paris to try to prevent a temperature rise of 1.5 °C as a result of global warming, which means that global greenhouse gas (GHG) emissions need to be reduced to zero by around 2050. Global GHG emissions have increased by almost 50% since 1990, even though on a regional level considerable improvements have been made. For example, the EU has accomplished a 22% reduction in 2017 compared to the 1990 level. There is scientific consensus that GHG emissions are directly linked to temperature rise. At the moment (2020), the GHG emissions scenario seems to mostly align with the RCP4.5 scenario of the Intergovernmental Panel on Climate Change (IPCC), which forecasts a temperature raise of 1.7 to 3.2 °C by 2100.6
10
Figure 1.01 Global material extraction 1900 - 2050 (billion tons) 7 8 Biomass Fossil Metal
Minerals
Global population
Gt
2048 - 9 bln
200 180 160 140
2020 - 7.8 bln
120
1999 - 6 bln
100
1987 - 5 bln
60 40 20
1960 - 3 bln
1927 - 2 bln
1900 1904 1908 1912 1916 1920 1924 1928 1932 1936 1940 1944 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1992 1996 2000 2004 2008 2012 2016 2020 2024 2028 2032 2036 2040 2044 2048
0
Figure 1.02 Global GHG emissions (billion tons of CO2 eq per year) Division of GHG emissions over various sectors, expressed in Gt CO2 equivalent.9 10 11
0.4 Other (non-energy)
Transport
AFOLU (Agriculture, Forestry and Other Land Use)
International transport (aviation, ship) 1.5
6.7
11.9
Energy Buildings Energy use Materials
6.8 Gt 2.9 Gt
Concrete 1.6Gt (55%) Aluminium 0.2Gt (8%) Plastics 0.15Gt (5%) Steel 0.95 Gt (32%)
Total 49.4
20.2
10.2
6.8
2.9
Industry (material production)
Buildings (excl. transport)
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PROJECT CASE
Fully demountable mass timber construction
CIRCL PAVILION Year 2018
Location Amsterdam, the Netherlands Architect de Architecten Cie. Client ABN AMRO Bank Engineer timber structure Constructie Adviesbureau Geuijen Timber manufacturer / contractor Derix group Mass timber products used Glulam, CLT Amount of timber (CO2 benefit) 1000m3 (1537 tons CO2)
Circl is a Living Lab pavilion featuring a restaurant and exhibition and meeting space in the heart of the ‘Zuidas’, Amsterdam's Central Business District. Despite being only a small building (3000 m2) it has had a huge impact on circular building. The building deploys a maximum of recycled and renewable materials, including locally sourced timber (Dutch grown larch) for the bearing structure instead of concrete. Due to the large amount of prefabrication, the structure of Circl was put up in seven weeks by a team of only three workers of the timber manufacturer (Derix), who also acted as contractor for the timber structure.
structural beam after a shaving round at the end of life and are crossing on top of each other which besides circularity also facilitates higher structural integrity. All connections were designed for disassembly at the end of life, enabling the timber and steel material flows to return to the bio-cycle and the technocycle respectively for reuse. Circl was fully modelled in BIM and the various material components were registered in a material passport through Madaster.
The two-story building is based on a glulam post-and-beam structure with CLT floor- and wall panels (100-220 mm) on top of a concrete basement. The beams are kept intact as much as possible (sometimes over-dimensioned to facilitate high level re-use as
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PROJECT CASE
3XN HEAD OFFICE Year 2014
Location Copenhagen, Denmark Architect 3XN Client 3XN Timber products used Old timber construction is fully re-used
In 2014 architectural firm 3XN moved their head office to the old naval area of Holmen in Copenhagen. This redevelopment is based entirely on 3XN's own circular philosophy from vitalizing a dilapidated city district, extending a building's life cycle by the reuse of existing building materials and designing following the principles of disassembly enabling endless re-use of materials. Holmen served from 1690 to 1990 as the main base for the Danish Navy and featured several black-stained wooden Cannon Boat Sheds situated on the waterfront. In 2006, five of the sheds where ravaged by fire. The Danish heritage sheds were rebuilt using the same traditional wooden joints and brackets with bolts and carvings, already designed for disassembly during their first construction in 1860. The historic post-and-beam structure (2000 m2) with its typical stabilizing diagonals is revealed throughout the interior of the building, further emphasized by use of glass and neutral white finishing materials, resulting in a very light indoor environment taking maximum advantages of the well-being benefits of timber (see section 5.1.7).
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Designed for disassembly – already in 1860
2.
FORESTS, FOREST PRODUCTS AND CLIMATE CHANGE Forests and products derived from them play a crucial role in greenhouse gas emissions through deforestation and more importantly through their immense mitigation potential through sustainable forestry management, reforestation and CO2 locked in durable wood products. This chapter explains those mechanisms from macro to micro level and gives a thorough insight into the importance and versatility of the forest products industry.
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FORESTS, FOREST PRODUCTS AND CLIMATE CHANGE – FORESTS OF THE WORLD
2.1
FORESTS OF THE WORLD Assessing the Global Forest Balance Forests cover about one third of the global land area and hold many essential functions, providing ‘ecosystem services’ to mankind, such as provisioning food, fresh water and raw materials, regulating air quality, climate and water as well as providing cultural and recreational value.60 61 About 300 million people live in forests worldwide while 1.6 billion people depend on them for their livelihood. Of the 4000 million hectares of forest worldwide, around half (54%) is used for the manufacture of timber, food and other products. About a quarter (24%) is used for conservation purposes, of which half is grown on legally protected land. Although planted forests only account for 7% of the total forest area, they supply 35-40% of the industrial roundwood worldwide and may even supply up to 80% by 2030 if the current expansion continues. Ideally, these new forests have high species diversity and long rotations for higher biodiversity and better resilience against pests and fires.62 63 Deforestation versus Reforestation For the world as a whole, around 4.7 million ha were lost annually in the period 2010-2020 (almost 1% of the global forest coverage), which is a downward trend compared to previous periods (5.2 million ha annual loss in the period 2000-2010 and even 7.8 million ha annual loss in 1990-2000). Major forest losses occur in tropical regions. For example, according to FAO total forest coverage in Africa and South America combined was around 1550 million ha in 2020, with an annual forest loss of 6.5 million ha (2015-2020).23 64 If tropical deforestation continues at this rate, in approximately 240 years all forests on these continents will be lost. In more temperate regions the situation is reverse. Through active reforestation and afforestation policies, net forest areas are consistently increasing. This applies in particular to Europe, Oceania and Asia, with an annual increase of 0.3 million, 0.4 million and 1.2 million ha from 2010 to 2020 (this was 1.2 million, -0.2 million and 2.4 million from 2000-2010).64 It is noteworthy to mention that the two countries with the largest populations in the world, China and India, are strongly contributing to this increase in forest area through the implementation of active reforestation policies, although there are some doubts about the biodiversity of the newly planted forests. In general there is a strong relation between forest area and absolute carbon stock in forest biomass, showing that especially Europe has considerably increased the carbon stock in its forests by increasing reforestation and better forest management.65 As for Asia, the increased carbon stock through reforestation in China and India is nullified by tropical deforestation in South East Asia, with higher carbon stock per hectare than the recently established plantations in China and India.23
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Seedlings Douglas fir seedlings in the Saanich Forestry Center in Canada.
Forest certification In certified tropical forests only a small amount of trees is selected to be harvested after careful planning, here in a FSC certified forest in Borneo, Indonesia.
Figure 2.01 Global forest area (Mha)
1990
2000 2010
Forest area development over the various continents between 1990 and 2015.64
2015
Mha 1000 900 800 700 600 500 400 300 200 100 0
Africa
Asia
Europe (excl Russia)
Russia
North and Central America
Oceania
South America
35
Figure 2.04 Wood-processing flow in Sweden Almost every part of the original tree is used for a value-added application, with a highly developed pulp and paper industry in Sweden. Adapted from Swedish Wood. 75
energy production In some EU countries there are subsidies to stimulate the use of primary treebased biomass as ‘green’ energy (with many pellets being imported from North America) to meet short-term climate goals, thus missing substantially larger long-term carbon benefits that come with the higher value-added application of timber on a global scale.
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before they can finally be used for bio-energy production.76 Direct use of saw logs for energy production should therefore be avoided at all cost. As we saw in figure 1.05, the volume of timber in non-residential buildings and residential buildings in Europe and Asia is rather low. However, there are large regional differences in Europe. For example, in Germany and the UK about 15-20% of new houses are timber-based, while in Austria (35%) and Scandinavia (50%) this percentage is a lot higher.77 In potential there is enough capacity to raise this percentage substantially. In the next section we will see that this would also provide considerable climate benefits.
Box 2.1
HOUSING EACH EU CITIZEN IN A TIMBER HOUSE – FACT OR FICTION?
Projected roundwood volumes in Europe are set to increase with 44% to 750 Mm3 by 2030.62 81 If we assume that sawn timber volumes will increase with this percentage as well, this would mean that sawn timber production will be around 175 Mm3 in 2030. If we further assume that half of the sawn
timber production would be used for CLT production and that there is enough factory capacity, this will result in 62.5 Mm3 per year. A CLT-based family house (120-150 m2) requires about 60 m3 of CLT,82 which means that based on the predicted CLT production volume, 1.05 million CLT houses could be made in the EU each year. Assuming a lifespan of eighty years, this means that in total 83 million CLT houses could be erected over this eighty-year period. If we assume these houses each host three persons on average, this would mean that half of the EU population (approximately 500 million) could be housed in a CLT house by 2100 based on the expected future production figures.
Add to this the fact that CLT used in apartment buildings is more efficient. For example, the Dalston Works project (see the project case on page 169) in London features 4649 m3 of CLT. The building houses 121 apartments, i.e. 39 m3 per household (compared to 60 m3 for a house). This means that if smaller CLT houses were used and a larger part would live in apartment buildings, almost the full EU population could be housed in mass timber in the future.
CLT-based villa The project shown is a CLT house in Nederhorst den Berg, the Netherlands (designed by Schipperdouwes architects).
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3.
WOOD BASICS
Wood is a natural material and therefore can have a wide variety of properties – even for the same wood species – defined by multiple location-dependent growth factors. This variability also drives the need for more uniformity in the performance of engineered and modified wood. Before going into detail in chapter 4, it is essential to understand the basic characteristics of wood, which will be explained in this chapter.
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WOOD BASICS – GENERAL PROPERTIES
3.2
GENERAL PROPERTIES Water: Tree’s Friend, Wood’s Foe Water plays a central, dual role in the life of wood. Combined with oxygen, nutrients and sunlight, it allows the tree to grow and survive, but when present at too high levels in wood applications it becomes a natural enemy, setting the conditions for biological attacks and causing shrink-swell. As trees require water for their growth, they are able to store and transport water in their tissue. This also explains why wood is a hydroscopic material, i.e. it absorbs and releases water depending on the humidity of its surroundings. When harvested, softwood has a moisture content of up to 160% in the sapwood and 50% in the heartwood. After drying, this is reduced to about 8-16% moisture content, depending on the final application. Based on the humidity of the surrounding air, wood will either release or absorb water, initially in the cell wall (bound to hydroxyl groups in cellulose). When the moisture percentage reaches around 30%, the fibers get saturated and water will also be stored in the cell lumen. This point is known as the fiber saturation point. Up to the fiber saturation point there is a constant swelling of the wood cells, in particular in the tangential direction (approximately 8% from dry to fiber saturation) and the radial direction (approximately 4%), and only slightly in the grain direction (approximately 0.3%). Although there are small differences between various wood species, for typical European softwoods such as spruce and Scots pine this adds up to a volume change of about 0.26% per percent change in moisture content.115 There can be several kinds of distortion as a result of moisture content changes and axial shrink-swell, depending on the location in the log: cup, bow, spring and twist. Distortion can be controlled through the right sawing and drying regime. Shrink-swell in a construction can easily be solved by good detailing to provide sufficient ventilation possibilities, thus avoiding high moisture levels within the wood. Furthermore, presence of moisture in the cell walls can provide the right circumstances for fungi and insects, leading to wood degradation. The resistance of wood species against biological attacks – referred to as their biological durability – differs greatly per species, with most softwood species having low durability. Wood modification is one of the most effective methods to increase the dimensional stability and durability to a consistently high level (see section 4.2.2). Mechanical Properties In essence, wood consists of bundles of tubular cells set in the longitudinal direction within a lignin matrix. This results in different material properties in different directions (anisotropy). Because of the orientation of the fibers, wood in general is considerably stronger parallel to the grain than perpendicular to the grain (see figure 3.02).116
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Utilizing shrink-swell
Macro scale dowel This specially designed connection at Tamedia headquarters (see project case on page 115) smartly uses the varying shrink-swell behavior of timber to lock the members in place. Figure 3.02 Different properties in different directions As a rule of thumb in softwood used for CLT (strength class C24) allowable stresses parallel to the grain are 24 N/mm2. Compression perpendicular to the grain is around 10% of this value (2.4 N/mm2) while tension perpendicular to the grain is around 1% of the parallel value (0.24 N/mm2).117
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4.
TIMBER’S EVOLUTION FROM PAST TO PRESENT TO FUTURE While timber has been the most important building material in the past, it was overtaken during the industrial revolution by abiotic materials with more stable performance. For timber as a natural, non-homogeneous, hydroscopic material it has proven crucial to optimize and stabilize its technical performance in terms of mechanical properties, stability and durability. This is where engineered and modified wood come in, which will be introduced in this chapter, including related construction systems and the possibilities of tall timber building. 74
The International House Mass timber office building in Syndey, Australia, designed by Tzannes (see project case on page 151).
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TIMBER’S EVOLUTION FROM PAST TO PRESENT TO FUTURE – TIMBER OPTIMIZATION
glue laminated timber
cross laminated timber
Mass Timber Products based on Laminated Boards > Glue laminated timber (glulam) The first engineered wood product was glulam, first patented by Otto Hetzer in 1906. In a glulam beam a minimum of two dried, often finger-jointed softwood boards (thickness between 6 and 45 mm) are glued together in the longitudinal direction. Because of the finger-jointing, very long beams can be made (technically up to 80 meters). However, lengths are usually restricted by the factory hall dimensions, transport limitations and other logistic factors. Commonly used tree species in Europe for glulam production are spruce, pine, fir and larch. Glulam is strength-graded (EN 14080) and thus more consistent in strength than solid timber. It may be produced as a straight or curved beam and is particularly used in indoor and outdoor bearing structures featuring large spans and high stresses in which stability and visual appearance are also of importance. > Cross laminated timber (CLT) CLT has gained significant market share since its introduction in the 1990s.It consists of several layers of strength-graded softwood boards that are glued together perpendicularly (usually with MUF or PUR) to produce a strong and very stable structural panel. CLT always consists of an uneven number of layers (usually three to seven layers, but more is possible) to give the outer layers the same orientation for better strength. Thickness ranges between 60 and 500 mm, with a width of up to a maximum of 4.5 m and a maximum length of around 25 m. CLT is typically used for load-bearing floors, roofs or wall elements. The strength
Glulam beam production Here shown before and after planing, in a factory in Niederkrüchten, Germany (Derix).
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Figure 4.04 The production process for straight glulam beams Adapted from Swedish Wood 114
Box 4.1
A WORD ABOUT GLUE
Most mass timber products require some kind of adhesive or resin to bind the elements together. The most commonly used adhesives are: phenol (resorcinol) formaldehyde (usually for exterior-grade products because of its high bonding strength), polyurethane (PUR), melamine (urea) formaldehyde and isocyanate-based glues (EPI or MDI). The dry-weight percentage of the binders may differ per manufacturer but in general is about 10% for MDF, 3-6% for veneered products, and 1-2% for laminated products. Although the impact of the adhesives in terms of carbon footprint is relatively small because of the low volumes, and indoor emissions are usually very low (see section 5.1.7) in terms of the circular economy principles (enabling division of bio-cycle and techno-cycle components at the end-of-life) a
bio-based alternative is recommended. Whereas several soy-, lignin- and furfural-based adhesives are on the market, they have hardly been adopted, for several reasons (price, performance, availability, different processing requirements, etc.). Alternatively, some mass timber products can be produced without glue, see below. However, these alternatives have lower structural performance.
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TIMBER’S EVOLUTION FROM PAST TO PRESENT TO FUTURE – CONSTRUCTION SYSTEMS
4.3.3
HYBRID SYSTEMS
As we saw before (see figure 3.03), timber outperforms concrete and competes with steel in terms of strength and stiffness per weight unit, which explains its suitability for applications where the own weight of the structure plays an important role, such as flooring and roofing. However, in various other loadbearing applications, steel and reinforced concrete have higher strength, both in tension and compression. Furthermore, sometimes a combination of timber and concrete may be required to add mass to improve acoustic or fire safety performance. Therefore, even though from the perspective of sustainability an all-timber proposition sounds like the best solution, in several cases a combination of timber with steel and/or concrete may prove most efficient in terms of material usage. This particularly applies on the level of the building construction system, but it can also apply to the level of building components. Hybrid Flooring Structural flooring is one of the most challenging elements in a building, where many different requirements come together. This explains the popularity of composite flooring systems, usually featuring strong mass timber elements such as glulam or LVL (tension) supporting a thin concrete floor (compression) with a key role for the connectors and inserted nocks (shear). This solution is also often used in tall timber (apartment) buildings, as it also increases the mass and improves acoustic performance (airborne and contact). However, a connected hybrid floor decreases the circularity potential. If this is an important factor for a design team, they can choose a dry system, which often does require more material to meet the requirements. Hybrid Construction Whereas mid-rise residential buildings can very well be executed in an all-timber bearing structure, in many cases for (larger) non-residential buildings a hybrid solution could be more efficient. In general, reinforced concrete is not only an efficient and durable solution for the foundation but also for a stiff and firesafe central core. Furthermore, as the ground floor and first floor of commercial buildings usually require larger spans, here also reinforced concrete walls and/or steel columns could be a more efficient solution. The combination of timber (bending) and steel (tensile) is also a successful match in post-and-beam bearing structures as well as in connections. From a sustainability point of view it is important that these hybrid constructions are built following the circular economy principles, i.e. demountable and reusable or separable in clean material flows at the end of life (techno-cycle and bio-cycle materials).
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The Bullit Center This office building in Seattle (designed by Miller Hull Partnership) was built following the circular building principles, employing a postand-beam system with glulam beams in combination with a very visible steel bracing structure.
Hybrid floor Exploded view of a mass timber post-andbeam bearing structure supplemented with CLT flooring and a concrete floating floor and steel connectors in the projected Framework building in Portland, Oregon (designed by LEVER Architecture).
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TIMBER’S EVOLUTION FROM PAST TO PRESENT TO FUTURE – HOW IT ALL COMES TOGETHER: TALL TIMBER BUILDING
4.4 United Kingdom Other than in most countries, building regulations in the UK are descriptive rather than prescriptive, where specific parameters need to be met. This descriptive method has opened up the way for tall timber building in the UK, often based on CLT (the book 100 projects UK CLT gives a comprehensive overview).123
HOW IT ALL COMES TOGETHER: TALL TIMBER BUILDING Recent Upsurge in Tall Timber While in previous centuries tall timber building was commonly accepted, increased building legislation requirements lowered the application area of timber due to its nonconformity. With the rise of mass timber products and systems, North America and several European countries have now incorporated the possible use of timber for taller structures in their building codes.1 In some countries, such as New Zealand, the United Kingdom and Norway, there are no restrictions to the height of tall timber structures.2 It is therefore no surprise that we find many of the tallest timber buildings there. In the USA, the International Building Code allows timber structures of up to 80 meters tall since 2019.124 Figure 4.12 provides an overview of the various all-timber structural systems (excluding hybrid systems) described in the previous sections that are suitable for tall buildings. The choice for a certain system may be governed by different factors, including building type, aesthetics, services integration, flexibility and many others. In contrast to constructions based on concrete, timber buildings with more than ten stories require a bracing frame to ensure stability.
Stadthaus/Murray Grove This residential building in London (designed by Waugh Thistleton Architects) upon completion in 2009 was the first tall timber building (nine stories) of which the bearing structure was made fully out of CLT (including the central core).
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Mid-rise Timber Building Depending on the building height, various timber structures can be used, with light timber frame constructions applicable to a maximum of six stories and massive panel-based platform systems (often CLT) up to about a maximum of ten stories. Besides hybrid construction systems that combine mass timber with concrete or steel (see section 4.3.3) there are also suitable all-timber hybrid systems. These combine several different mass timber systems, such as a post-and-beam system with CLT and glulam bracing.
Figure 4.11 The evolution of tall timber building from past to present to future
Figure 4.12 Typical structural systems for tall timber buildings
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5.
DESIGNING AND BUILDING WITH MASS TIMBER One of the main differences between mass timber and abiotic alternatives commonly used in bearing structures is the relatively low weight. Although this provides many benefits for mass timber, in particular in the building process (see section 5.2), it also requires a completely different design approach to meet several performance requirements, including fire safety, acoustic performance and structural aspects (section 5.1). If this is done properly and the right prerequisites are met, mass timber is the best alternative material for new buildings in many cases, even from an economical point of view. 124
Swatch New headquarters of Swatch under construction in Biel, Switzerland (designed by Shigeru Ban Architects), see project case on page 102.
125
DESIGNING AND BUILDING WITH MASS TIMBER – MATERIAL PERFORMANCE
5.1
MATERIAL PERFORMANCE 5.1.1
FIRE SAFETY
The largest prejudice impeding timber building has always been fire safety, still causing a lot of fear with anyone who is new to timber building. However, with the right design and by taking appropriate measures it is very well possible to build very fire-safe timber buildings. For the information in this section, several sources were consulted.148 149 150 151 152 Reaction to Fire – Combustibility The combustibility of building materials and other fire classifications are covered under European Standard (Euroclass) EN 13501-1, ranging from A (non-combustible) to F (combustible and easily flammable, i.e. unacceptable for use). Combustibility is particularly important for escape routing (safeguarding accessibility for firemen at all times) and for facades (preventing the fire from spreading quickly). In these areas higher restrictions are often posed.
Most wood species with densities over 350 kg/m3 automatically fall under fire class D-s2-d0, which means they are a combustible material with medium contribution to fire (D), average smoke forming (s2) and no flaming droplet production (d0). Species with a very high density could fall under class C (limited contribution to fire) and through impregnation with fire retardants, class B is possible (very limited contribution to fire). For mass timber and modified wood in general, the same applies, for example, non-impregnated CLT typically falls under class D-s2-d0, while fire-impregnated LVL can reach B-s1-d0. As for the skin of a building, the requirements for facades vary per EU member state. In most countries, materials conforming to Euro-class D are allowed as cladding for low-rise buildings, but for high-rise buildings (typically above 20 meters) the minimum requirement is often Euro-class B. Fire Resistance The second crucial component of fire safety has to do with fire resistance (EN 13501-2), which is covered under the REI marking: the ability of a bearing structure to maintain its structural capacity in case of fire (resistance), while preventing the fire from spreading between compartments (‘etanchéité’ – partitioning) and reducing heat transfer for a certain period of time (insulation). Different ratings apply depending on the building type, the country and the function of the component, typically for 30, 60, or 90 minutes. Predictable Fire Behavior All building materials have their limitations related to fire safety and that in particular solid timber systems, if structurally designed well, behave in a more predictable way during fire than for example steel, which loses strength far quicker than timber, with a larger risk of immediate collapse of the full structure. 126
Figure 5.03 Designing for fire safety Vertical intersection of typical wall-floor connections in CLT based on a fully exposed and fully encapsulated system (detailed by LĂźning Structural Engineers). Note that these details also feature acoustic measures, see also figure 5.04.
CLT
Insulation
Fire rated gypsum board (suspended ceiling, facing wall) and fiber cement board (floating floor) Acoustic underlay Charring layer
Encapsulated
Fully exposed
Encapsulation In the 18-story Brock Commons building, designed by Acton Ostry architects, (see case study on page 164) encapsulation in three layers of fire-rated gypsum board in combination with sprinklers was chosen as fire prevention strategy to meet the 120 minutes fire resistance requirement.
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DESIGNING AND BUILDING WITH MASS TIMBER – MATERIAL PERFORMANCE
Nevertheless, there are also many bio-based insulation materials on the market (cellulose, hemp, wood fiber, etc.), which besides adding insulation capacity can have a positive effect on regulating indoor moisture levels, including a cooling effect in the summer through evaporation (endothermic reaction). The good thermal properties of timber also avoids problems with thermal bridges in the transition from inside to outside, which require additional and costly (covering) measures in concrete and steel constructions. For mass timber this also provides additional design opportunities, for example for protruding glulam beams, running from inside and cantilevering outside or a protruding CLT floor slab carrying an outdoor balcony. Furthermore, prefabricated mass timber elements can be produced so precisely that the number of connections can be lowered and high airtightness can be achieved. This makes mass timber systems suitable for buildings insulated according to the highest standards, including passive house requirements.1 It is recommended to build in a vapor-open manner, although this is not a necessity as long as condensation within the mass timber elements is prevented by making the right design decisions. Lightness = Low Thermal Mass = Different Design Approach The low weight of mass timber structures also means that in contrast with heavy concrete and masonry constructions it has a lower thermal mass, making it more difficult to buffer heat and cold within the construction. Although this can be Student housing at Dyson Institute The thermal conductivity of acetylated wood (Accoya) is 0.12 W/mK.164 Here featured in the window frames in the student housing pods made from prefabricated CLT modules at Dyson Institute of Technology in Malmesbury, UK, (designed by WilkinsonEyre).
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Figure 5.06 Thermal bridges Because of the inherent low thermal conductivity of timber, it by default avoids problems with thermal bridges in the transition from the indoor to the outdoor environment. Here, a fictive example is shown of isotherm development (>11.7 °C) throughout a protruding CLT versus a concrete flooring system. As a result of the thermal bridge, a concrete flooring system will need additional measures such as a thermal break. Adapted from Nieman Raadgevende Ingenieurs
Outdoor
Indoor
Continuing floor (e.g. balcony)
Thermal bridge
Insulated facade
Outdoor
Indoor
Continuing floor (e.g. balcony)
No thermal bridge
Insulated facade
Concrete
Timber
No thermal bridges Protruding beams Beams running from inside to outside in the Circl Pavilion of ABN AMRO bank (see the project case on page 23).
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6.
FUTURE OUTLOOK: TOMORROW’S TIMBER In the previous chapters we have covered the evolution of timber from a natural, non-homogenous, traditional building material to an engineered high-performance product and to complete mass timber-based construction systems even suitable for prefabricated highrise skyscrapers. The momentum for mass timber is growing as a result of some important societal developments, which will be covered below. Pushed by the right incentives and investments in the forestry and wood industry, this can pave the way for mass timber as main building material in the 21st century. 178
Quay Side Toronto Render of the projected Quay Side smart city development at Toronto’s waterfront, fully based on prefabricated mass timber solutions, including PMX (design by Michael Green Architecture).
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FUTURE OUTLOOK: TOMORROW’S TIMBER
6.
FUTURE OUTLOOK: TOMORROW’S TIMBER There are some important societal developments evolving which may further accelerate the adoption of mass timber for construction: #1 Sustainability and Health The increased focus on climate change and resource scarcity has brought timber back in the center of attention. An increasing amount of governments adopts climate-smart forestry strategies as a means of climate change mitigation, including the benefits of carbon storage and avoided CO2 emissions through building in mass timber. As one of the few abundantly available renewable, bio-based materials, timber also offers part of the solution for the transition to a circular economy, especially if timber buildings are designed for disassembly and reuse (see for example the Circl, 3XN and Triodos featured project cases on pages 23, 24 and 63). With an increasing number of buildings being financed following total cost of ownership (TCO) models, the residual value of building components will become increasingly important in circular business cases. The reworkability of mass timber increases options for reuse and thus economic value. The circular building development cannot and should not be a biobased-only story. Although by 2100 there should be enough timber available in the EU to house over half of the population (see page 43), it will always need to be partly complemented with abiotic materials (see figure 1.08), preferably in applications where these provide additional value to timber and other bio-based materials (e.g. concrete foundations, steel tensile rods, etc.), which should of course be designed for disassembly and reuse as well. Health and well-being are becoming increasingly important in today’s society, which sparks the interest for biophilic design, including the evolving evidence that timber exposure in the interior has direct health benefits. The low environmental impact of timber as well as the health effects are valued in various global green building schemes, including LEED, BREEAM and WELL, which helps inclusion of timber in new sustainable building developments. #2 Industrialization Simultaneously we see an upsurge in ICT use and digitalization, which will have an enormous impact on the built environment: integration of disciplines through BIM modeling, fully automated prefabrication possibilities based on CNC, increased structural calculation capacity enabling complex modeling including parametric design, new product developments including nanotechnologies, 3D printing, and many more. These developments are especially relevant for mass timber given its easy workability, where design-to-production connected to advanced, fully automated
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Hamburg, the German wood capital In the Wood Cube project in Hamburg (designed by architektur agentur), an alltimber solution was chosen, including the use of a dowel laminated timber system, without the use of glue.
City council with guts Figure 6.01 Building stock: China vs. Europe Compared to Europe, the building stock in China was over twice as high in 2015. By 2050 the Chinese building stock will be five times higher than in Europe showing the importance of mass timber also being adopted in countries such as China. Adapted from Circle Economy 16
Building stock China
2050 562 Gt
2015
Stock in use 239 GT
Building stock Europe 2050 107 Gt 2015
Stock in use 95 GT
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COLOPHON Lead Author Co-author (Project cases) Technical Supervision Editorial concept Text editing Graphic design Schemes Illustrations Picture management Printing Paper Paper (Cover) Project management Publisher
Pablo van der Lugt – Green Matters Atto Harsta – Aldus bouwinnovatie Gert-Jan Rozemeijer – Lüning Structural Engineers Atto Harsta – Aldus bouwinnovatie Pablo van der Lugt – Green Matters Sybrand Zijlstra Sigrid Lussenburg – MaterialDistrict vanRixtelvanderPut ontwerpers vanRixtelvanderPut ontwerpers Jakob Lagerweij – Broertjes Lagerweij Sigrid Lussenburg – MaterialDistrict Wilco Printing & Binding (PEFC™ and FSC® certified) Multidesign Rough (PEFC™ and FSC® certification available) – Papyrus Ensocoat 2s (PEFC™ and FSC® certification available) – Stora Enso Annemarie Kleve – Anders2 Jeroen van Oostveen – MaterialDistrict
This publication is printed on certified paper. Special thanks to the following parties for making this publication possible ACCSYS Technologies www.accsysplc.com PEFC Netherlands www.pefc.org LÜNING Structural Engineers www.luning.nl DERIX GmbH & Co. www.derix.de FSC Netherlands www.fsc.org Publisher MaterialDistrict, The Netherlands +31 (0)20 71 30 650 info@materialdistrict.com www.materialdistrict.com © 2020 MaterialDistrict No part of this publication may be reproduced or transmitted in any form or by any means without prior permission in writing from the publisher. In the selection of text fragments and illustrations, the publisher has tried to comply with and honour existing copyrights as much as possible. Persons who feel that they have copyright claims are requested to contact the publisher. All books published by MaterialDistrict are available at books.materialdistrict.com
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MORE INSPIRATION
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TOMORROW'S TIMBER
TOWARDS THE NEXT BUILDING REVOLUTION
Timber has evolved from a traditional building material to ‘mass timber’, an engineered high-performance product from which complete construction systems and even highrise buildings can be made. Mass timber has the potential to usher in a three-fold revolution in the way we build. • An ecological revolution because of the enormous sustainability benefits of mass timber over its non renewable, fossil-based counterparts. • An economical, functional and technical revolution through the integration of ICT-based production technologies to support prefabrication of complete building systems in factories, increasing quality, safety and building speed. • An aesthetic and well-being revolution made possible by the spectacular possibilities of parametric design and the substantial psychological and health benefits of mass timber. Abundantly illustrated, Tomorrow’s Timber covers all the above topics based on the latest scientific insights and statistics and lessons learned from practice. Also included are inspiring case studies from all over the world, which show that the mass timber revolution has already begun. About the authors: Lead Author Pablo van der Lugt (PhD MSc) is a senior sustainability consultant for the wood and bamboo industry as well as a lecturer in Biobased Building at Delft University of Technology (Environmental Technology & Design). Previously he wrote Booming Bamboo, also published by Material District. Co-author Atto Harsta (MSc) is an independent innovation consultant for the building industry and architecture with more than twenty years’ experience, focused on contributing to a healthy and circular built environment. Cover photo: ©ICD/ITKE University of Stuttgart