Mass Timber Architecture
Kay Hill looks at what's being done around the world with mass timber and asks whether changes to the regulations in Britain risk compromising the move towards a greener architecture
Words by Kay Hill
Concrete and Steel, the building blocks of conventional modern architecture, may be tried and tested, but they belong to an era untroubled by climate change. The manufacture of concrete, for example, was responsible for eight per cent of global CO2 emissions, according to a study by Chatham House reported in Architects Journal, while production of steel is one of the most energyconsuming and CO2-emitting industrial activities on the planet.
Carbon12 – an eight-storey, 26m-high residential tower in Portland, made from CLT and glulam with metal cladding and a steel core – was completed in 2018 and is currently the tallest mass timber building in the US. Architect: Path Architecture. Structural Engineer: Munzing. Construction: Kaiser Group. Engineered wood: Structurlam. Image credit: Andrew Pogue.
Wood, on the other hand, is a limitless, renewable resource that literally grows on trees, and stores rather than produces greenhouse gases. Natural timber, however, while a beautiful surface finish, has major limitations for structural building – its weight, sensitivity to moisture and, of course, the average size of a tree, means the concept of building a skyscraper out of natural beams would be laughable from an engineering point of view.
Carbon12 – an eight-storey, 26m-high residential tower in Portland, made from CLT and glulam with metal cladding and a steel core – was completed in 2018 and is currently the tallest mass timber building in the US. Architect: Path Architecture. Structural Engineer: Munzing. Construction: Kaiser Group. Engineered wood: Structurlam.
Not so with mass timber – engineered products such as CLT (cross-laminated timber) and glulam (glued-laminated timber) are proving to be the material of choice for architects around the world, and the buildings are becoming bigger and taller by the month. Anthony Thistleton is a partner at Waugh Thistleton Architects, which specialises in modern timber buildings: He says: ‘For the past 10 years Britain has been a global leader in the adoption of CLT. We build in the most suitable material for each job, but most of the time that is CLT because of the imminent climate breakdown and the need to be truly sustainable. We have been able to demonstrate that building using CLT is faster and cheaper. You can create amazing spaces and beautiful natural interiors, and we are only at the early stages of understanding the possibilities.’
Dalston Works, a 33.8m-tall, 10-storey mixed-use project in London, was completed in 2017 and was highly commended in the Architects Journal Sustainability Prize 2017. Its structure and core are CLT, with brick external cladding. Architect: Waugh Thistleton Architects. Client/contractor: Regal Homes. Construction cost: £24m. Structural engineers: Ramboll / PJCE. CLT: Binderholz, installed by B&K Structures. Image credit: Daniel Shearing.
Part of the reason why the UK was able to forge ahead with CLT was the nature of our regulatory environment, explains Thistleton: ‘UK building regulations are unique in that they are not prescriptive. They don’t limit anything, but place the burden of evidence on designers to prove they are complying with the basic tenet of the regulations. That’s what allowed us to build Murray Grove, the first tall timber building. Most other building codes have had limits on the height of timber buildings because of the limits of the technology at the time. The rest of the world has seen what’s happened here and has amended building regulations to allow building in this manner with engineered timber.’
But while governments around the world have been changing building codes to allow greater use of mass timber, the UK has done the opposite, with new restrictions following the tragic Grenfell Tower fire banning all combustible products on the exterior of buildings taller than 18m, regardless of their proven risk. ‘Having been a global leader in the world of CLT, this will push back the UK’s lead as innovators,’ warns Thistleton. Indeed, the government’s impact assessment of the policy noted: ‘It is likely to slow down the use of engineered timber in future development.’
Origine, a 13-storey, 41m mass timber luxury residential tower in Quebec was completed in 2017 and is the tallest all-wood building in Canada, part of a new eco-neighbourhood at Pointe-aux-Lievres. Architect: Yvan Blouin Architecte. Structure: Nordic Structures. Contractor: EBC. Image credit: Stéphane Groleau.
Kristin Slavin is an architect at Path Architecture, Kaiser Group, which has recently completed an eight-storey residential tower in Portland, called Carbon12, which has a CLT frame. The building regulations in Oregon were amended to allow its construction, and it is the tallest mass wood building in the US having overtaken T3 by Michael Green Architecture. ‘We were able to work with the jurisdictions to identify and address their specific concerns related to the mass timber through performance-based analysis and testing,’ explains Slavin.
Mjøstårnet in Brumunddal, Norway, which was completed in March 2019, now holds the title of the world’s tallest all-timber building, with even its lift shafts being made from CLT. It is 85.4m high and has 18 storeys, mainly apartments with some retail and office space. All the timber was sourced locally. Architect: Voll Arkitekter. Client: AB Invest. Timber structure: Moelven Limitre. Image credit: Moelven.
‘Mass timber provides an incredible opportunity to build more sustainable, safe, and beautiful buildings all over the world. The Grenfell Tower fire, while undeniably a tragedy, was completely unrelated to building with mass timber. Reacting to that tragedy by putting restrictions on unrelated building technologies is unfortunate, particularly when there is testing and science to support that mass timber is a safe and reliable building material.’
T3, an all-timber, seven-storey office building in Minneapolis, on a concrete podium, is designed with exposed mass timber columns and floor slabs to recall the heavy timber construction of the building’s predecessors. It was the largest mass timber building in the US until Carbon12 and won the 2017 Minnesota Construction Association Awards of Excellence, Green/ Sustainable Project. Architect: Michael Green. Architecture. Civil engineer: Loucks Associates. Contractor: Kraus-Anderson Construction Company. Image credit: Ema Peter.
Those working with mass timber are at pains to point out its safety, and to remind the public that even steel loses its structure and concrete can explode in the heat of a fire. Frédéric Verreault is executive director of corporate development at Canadian company Nordic Structures, which helped create Origine, a 13-storey, 41m mass timber residential tower in Quebec, designed by Yvan Blouin Architecte. In order to build Origine, the company had to convince the authorities that the construction was safe enough to proceed, despite the Quebec Building Code generally banning wood on buildings of more than four storeys.
Treet (which means The Tree) in Bergen, Norway, is a 14-storey, 49m-high residential tower made almost entirely in CLT, glulam and prefabricated wooden modules, with concrete slabs on the base for stability. It is clad in a mixture of glass and Corten steel. Architect: ARTEC. Project manager: BOB. Engineer: SWECO Norge AS. Main contractors: Kodumaja (modules), Moelven (CLT). Landscape architects: Smedsvig Landskapsarkitekter. Image credit: Morten Pedersen, Inviso.
‘We do not want any compromises in terms of safety,’ he says. ‘It’s important to differentiate between wood in general and mass wood – the latter has a behaviour and stability that can even be described as advantageous in the event of a fire. That’s why we’ve put a lot of effort into demonstrating the safety of our mass wood structures, including carrying out large-scale fire tests.’ When Origine was given the go-ahead, the all-wood structure was completed in just four months – a powerful demonstration of the speed and precision of timber construction.
The Arbour at George Brown College in Toronto, begins construction in 2021. The 10-storey, 16,250 sq m facility will be noteworthy for its large-span mass wood structure, but will also have a steel core, Architect: Acton Ostry
While mass timber buildings are springing up all over the world, they are not all created equal. Some, such as 14-storey The Tree by ARTEC and the 18-storey Mjøstårnet tower by Voll Arkitekter, both in Norway, are structurally almost entirely timber, while others, including many of those popularly described as ‘wooden’, have a conventional steel or concrete core, and sometimes stairwells, making them more accurately described as ‘hybrid’ buildings.
PLP Architecture, in conjunction with the Centre for Natural Material Innovation at Cambridge University, designed conceptual plans for Oakwood Timber Tower, an 80-storey, 300m-high mass wood skyscraper that could be integrated within London’s Barbican
Kevin Flanagan is a partner at PLP Architecture, which in conjunction with the Centre for Natural Material Innovation at Cambridge University, designed conceptual plans for Oakwood Timber Tower, a 300m-high mass wood skyscraper that could be integrated within London’s Barbican. He is concerned that some buildings can be 60 to 70 per cent concrete and still be presented as eco-friendly: ‘Many are simply about the media coverage – they are not really solving the existential problem. Real timber buildings need a different type of architecture. It is inevitable that engineered mass timber is broadly adopted as a 21st century building technology in keeping with the rising desire to live in cleaner and greener cities, as has begun in Scandinavia.’
25 King is a 10-storey, 45m all-timber commercial tower in Brisbane, with a verandah-style south façade referencing the wooden vernacular of Queensland. Completed in 2018, it’s the tallest engineered wood building in Australia. It was awarded a Commendation in the Queensland Regional Architecture Awards 2019. Architect: Bates Smart. Client: Lendlease. Engineers: Aurecon. Structural engineer (Timber): Design Make
But while purists might bemoan the compromise, retaining some conventional materials seems to keep planning authorities happy, especially when it comes to building at greater height. Russell Acton, principal of Acton Ostry Architects, designed the 18-storey Tallwood House at Brock Commons, student accommodation at The University of British Columbia in Vancouver, with a concrete core and stairwells. ‘That was a strategic decision to ensure that approval from the authorities to construct the mass wood building could be obtained quickly by means of a site-specific regulation,’ he explains. By using a technology the planners understood, the permissions took months rather than years.
Terrace House in Vancouver, when completed in 2021, will stand at 71m and overtake Tallwood House as the highest hybrid timber building in Canada. Designed by Pritzker Prize-winning Japanese architect Shigeru Ban, its use of wood is intended to blur the lines between indoor and outdoor spaces, and it will be constructed from local timber. An exposed mass timber (rather than encapsulated) structure of this scale had never been approved under existing building codes in North America. Architect: Shigeru Ban. Client: PortLiving. Landscape architect: Cornelia Oberlander. Structural engineer: Read Jones Christoffersen. Timber design engineer: Creation Holz AG. Image credit: Zwartlicht.
Another Acton Ostry project, The Arbour at George Brown College in Toronto, will begin construction in 2021. The 10-storey, 16,250 sq m facility will be noteworthy for its large-span mass wood structure, but will also have a steel core, which Acton says is necessary ‘to achieve speed of construction, required structural lateral loads, and distribution of mechanical systems, which could not be achieved with a CLT core’.
While acknowledging that hybrid buildings are an interim technology, Acton maintains it is a necessary stage: ‘One needs to crawl before they walk and to walk before they run. Concrete and steel are mature systems that have optimised and refined a whole host of design and construction innovation; there is little room left for improvement and reduction of costs. On the other hand, mass wood has only recently been introduced as an alternative to concrete and steel and is still in an innovation stage that will require time to achieve comparable resolution of design and construction methodologies, and optimisation of systems, standards and cost.
‘The future of tall wood is not simply about promoting a vision, but rather it is about fuelling an evolution, with the main objective being to take precise and consistent steps to nurture the future development of tall wood buildings. Unfortunately, it has been my observation that the design for many tall wood buildings seems to be more focused on presenting expensive, sensationalist spectacles, rather than on contributing to a meaningful process that will assist to optimise the cost, efficiency and environmental benefits of mass wood construction for the future – a future that must include humble, modest and economical mass wood buildings, as it is they that will be the foundation upon which tall wood buildings will make a genuine and meaningful contribution to the future sustainability of cities.’
In a way, what is happening at the moment with tall wood buildings mirrors the early part of the 20th century, when the race to the top that began with the Flatiron in New York, built in 1902 and reaching 87m, ended in 1931 with the construction of the 381m Empire State Building, unsurpassed for 40 years. Everyone, it seems, wants to know just how tall a wooden structure can go. At the moment, the tallest hybrid wooden tower that’s actually being built is HoHo in Vienna, by Rudiger Lainer + Partner. When completed it will have 24 storeys and stand 84m high. It has overtaken what had been the tallest hybrid in progress, HAUT in Amsterdam, by Team V Architectuur, which will stand 73m high when finished. But the race is on – Sumitomo Forestry has plans for a 350m hybrid wooden skyscraper in Tokyo, PLP has firm plans to build Oakwood Tower Three, although the details and location are firmly under wraps, and Perkins + Will hopes to build an 80-storey building in Chicago, called River Beech Tower.
However, according to Verreault, the race to the top is a distraction: ‘The idea is not to build as high as possible in wood but rather to build as much as we can using wood.’
Thistleton feels a more pertinent question is how high should we go: ‘Very tall buildings are part of the old order; they are not sustainable. An 80-floor wooden building might be possible, but is it desirable? No – it takes us away from the main message. Architects have to engage with the crises we have about housing and climate breakdown, and change the way we work.’
The one thing that everyone seems agreed on is that mass timber has exciting prospects. Philip Vivian, director at Australian architects Bates Smart, which has just completed 25 King, a 10-storey, 45m all-timber commercial tower in Brisbane, notes: ‘Each time an engineered timber project completes, architects learn more about CLT’s potential as a new building material and how we can work and innovate with it on all types of buildings. It’s exciting to see the ideas take hold and evolve across the globe. We know that people want to connect to nature, and using timber on the exterior and interior of buildings helps complete the connection, making people feel more at ease within the built environment.’
‘Wood is our only available renewable structural material,’ stresses Slavin. ‘Other structural materials rely on mining our earth’s minerals and energy-intensive manufacturing processes for production. In addition to being renewable, the trees used to make the mass timber consume carbon during their growth, which is then sequestered into the building for the life of that material. This effectively makes mass timber buildings carbon banks.’
Bill Jackson is director of architecture at PortLiving, which is working with Japanese architect Shigeru Ban on Terrace House in Vancouver, a 71m 19-storey exposed mass timber/concrete hybrid residential that is under construction after a lengthy process to achieve special planning consents. He sums up the benefits of using wood: ‘It functions as both a structural and architectural finish, it stores rather than emits carbon dioxide, and is a durable and sustainable resource that’s readily available and economically feasible. It’s strong in relation to its weight, can be formed and fabricated off-site and is quiet during construction. It has strong insulation qualities, carries zero emissions, uses less energy and water, mitigates climate change, benefits human health and enhances wellbeing, reduces waste, and has seismic resilience and fire resistance.’
‘We want to create greener cities to reduce CO2,’ says Flanagan. ‘Wood has all these psychological benefits and will save the planet. If I could build a city where every second or third building was made out of timber it would be saving enough carbon to be carbon neutral throughout London. And we could build all of that with the material from managed forests.’
