CLT: taking construction back to its roots

Wood has seen a slow-paced renaissance since the early 1990s, but now ambitious proposals for timber (CLT) structures seem to appear every few weeks. Technologist assesses how close these architectural dreams are to becoming reality.

Prof Kaufmann with CLT model skyscraper
CLT_Prof. Hermann Kaufmann; Professur fuer Entwerfen und Holzbau, Fakultaet fuer Architektur an der Technischen Universitaet Muenchen (TUM); Foto: © Astrid Eckert / Photographie;; +49 171 26 89 601

 Imagine the city of the future. What’s it built of – steel, glass, perhaps graphene? Actually, the answer may be good, old-fashioned wood. Right now there’s a race to build the world’s tallest cross-laminated timber (CLT) building, and Europe is winning.

Norway’s 14-storey Treet building holds the record for the highest inhabitable CLT structure. The torch will cross the Atlantic briefly when the University of British Columbia’s 18-storey Brock Commons student residence opens next year. But construction of Amsterdam’s 21-storey Haut building is due to start in 2017 and the 24-storey HoHo building in Vienna should open in 2018. On the drawing boards, meanwhile, are such ambitious projects as the 40-floor Trätoppen (“Treetop”) in Stockholm and the 80-storey Oakwood Timber Tower skyscraper (“the Toothpick”) proposed for London.

Societal solutions

The reasons for wood’s renaissance are not just aesthetic. Since the 2015 Paris Agreement on climate change, activists and decision-makers have shone the spotlight on the construction industry. In this context, concrete and steel suddenly look far from materials of the future.

Unlike concrete, which accounts for 5–8% of all global greenhouse-gas emissions, trees have the power to sequester carbon at a rate of roughly one tonne of carbon dioxide per cubic metre. Thus if you build a 20-storey high-rise of cement and concrete, the entire process would emit some 1,200 tonnes of carbon dioxide. Wood, in comparison, would lock up about 3,100 tonnes – a net difference of 4,300 tonnes. “In recent years, timber pivoted from a peripheral position among construction materials to a central one because of its low carbon footprint,” says Yves Weinand, Director of the IBOIS Laboratory at the École Polytechnique Fédérale de Lausanne.

Wood works

Concrete and steel will not disappear overnight, of course. In fact, many of the next-generation tall timber buildings include concrete or steel decks and façades, even if the main load-bearing structures are wooden. Consequently, some groups are debating which tall buildings should be classified as hybrid and which should be considered true timber structures.

Meanwhile, Weinand and his colleagues have been busy with far more radical, albeit less tall, structural experiments. Their current venture is a new €2.4 million building for the Vidy Theatre in Lausanne. “The building’s design is inspired by Max Bill, who built the main theatre in 1964,” explains Weinand. When it is completed in 2017, visitors will see what appears to be an intricate piece of origami on steroids. The ribbed structure shuns beams and columns; instead it is built with “surface-active” timber-folded plates. “Each panel connects to the next using mechanical, integrated wood-wood connections.” This allows for quick and precise assembly and, more importantly, exact transmission of forces through each plate. The achievement should not be underestimated: “Not a single metallic fastener will be used; it’s the first structure of its kind.”

CLT: An Austrian innovation

At the centre of this wooden revolution is engineered timber. “It’s very different from what existed before developments in machining and adhesives,” says Richard Harris, a structural engineer and Honorary Professor of Timber Engineering at the University of Bath.

The poster child for engineered wood is CLT, which rose to prominence in Austria in the early 1990s. CLT is produced by orienting and gluing planks (typically spruce or pine) perpendicular to one another. The cross gluing reduces swelling and shrinkage to an insignificant minimum and considerably increases load-carrying capacity and dimensional stability. Also, because CLT is engineered to precise tolerances and cheaper to transport than steel or concrete, it reduces onsite waste and disruption to nearby residents.

So wood is no longer a raw material; it’s a product. One of the flagship test cases was the Stadthaus project designed by Waugh Thistleton Architects and built at Murray Grove, London, in 2009. It was the first urban building to be constructed entirely from prefabricated solid timber; interestingly, the winning argument was not sustainability but the efficiency compared to concrete. For the same cost, construction was completed in 12 months, six months faster than for a traditional version. The building used only 300 tonnes of timber, while an equivalent concrete structure would have required 1,200 tonnes of material.

Tension and torsion

Kevin Flanagan is one of the architects helping push the limits of CLT construction in Europe. As a partner at PLP Architecture with international awards for tall (+200 metre) building design to his name, Flanagan was well-placed to respond to an invitation by Cambridge University’s Department of Architecture to collaborate on a CLT high-rise design, along with engineers Smith and Wallwork. The result is the spectacular, headline grabbing Oakwood Timber Tower proposal, which would create more than 1,000 new residential units in a 93,000 m2, mixed-use 80-storey tower in central London, integrated within the Barbican Centre for performing arts.

Of course, such concepts are not simple; so what are the challenges? “By the time you reach 200 metres, a building doesn’t react in the same way as its low-rise counterparts,” Flanagan explains. “For example, the dead load of a building is overtaken by loads like wind and torsion.” Typically, in these buildings an exterior loaded façade or bracing performs better than loading weight through an interior core.

Moisture matters

Another consideration is how buildings react when high winds combine with driving rain. The timber used in CLT is kiln dried with a moisture content of around 12%, low enough to preclude pest or fungal attack. But tall wooden buildings face other issues during construction. “With increasing height, the challenge is to provide dry conditions for the expected lifetime of the building,” explains Stephan Ott of the Technical University of Munich. “Additionally, large buildings require longer construction times during which the structural elements are especially exposed to moisture.” In some circumstances that could result in the wood taking on more than 30% surface moisture, which could affect performance over a building’s lifetime.

Ott works on the TallFacades project, funded by the EU to the tune of €1.8 million. With so little known about the impact moisture may have on lifetime performance, the project is creating a new model called RiFa (a contraction of ‘RiskFaçade’) to calculate risk of moisture damage, which he believes may be dramatically underestimated in planning and building processes. “If damage increases in future, it may put the image of timber buildings at risk,” says Ott. Therefore, safety concepts similar to those in static calculations are necessary to improve construction and prevent moisture damage.

The burning question

Of course, there’s an elephant in the room: fire. Even the temperature at which steel – which is not combustible – loses structural integrity is not fixed but varies according to such factors as load and temperature distribution. To take an extreme example: for small, fully loaded hot rolled sections, exposed on all four sides, the inherent fire resistance without added protection can be as little as 12 minutes.

By contrast, large timber panels and beams burn slowly and quite predictably. They resist heat penetration by the formation of self-insulating char. The char rate of CLT has been tested at a respectable 0.67 mm per minute. As such, these panels are capable of the 30-, 60- and 90-minute resistance times required by typical building regulations. These are not like-for-like comparisons, but serve to demonstrate how counter-intuitive fire performance can be in different contexts.

Even so, more data on the fire-performance of tall buildings made of engineered mass timber is clearly needed. “A fundamental expectation for the structural design of a tall building ought to be that the fuel burns, the fire goes out and the building is still standing,” says Luke Bisby, head of the Institute for Infrastructure and Environment at the University of Edinburgh. “This is manageable in CLT buildings where the timber elements are properly encapsulated by non-combustible material, but it may be a challenge if a lot of the timber is exposed, which is a growing trend driven by architectural aspirations.” A building made of combustible material has consequences on the design of fire suppression systems, the time for escape, the overall strategy for evacuation, and the duration a fire might burn, so regulations for concrete and steel buildings cannot necessarily be applied to timber without detailed consideration. Fortunately, top consultants such as Arup, who are showcasing the architectural possibilities of timber with structures like Haut and Sky’s Believe in Better building – the UK’s largest commercial timber structure, built in Osterley, West London in 2014 – recognise the uncertainties at play, so they seek advice from researchers. “The collaborations work well – the challenge is to keep up with new design proposals,” Bisby concludes.

The regeneration game

In other areas, regulation changes are already broadening the potential for regeneration in city centres. In the UK, when considering foundation size, architects used to have to calculate 50% of its load capacity. “Now, if a building has been standing for 10 years or so, the regulations account for natural compaction, meaning you can assume 100% capacity for the foundations,” Flanagan explains. That could prove particularly pertinent in London, where a major property crisis is unfolding: “In plain terms, every low-rise building over 10 years old in the City of London could be renovated to twice the size.” With timber panels established as an unobtrusive solution for urban retro-fitting, it’s easy to see a starring role for timber in revitalising inner-city communities across Europe for decades to come.