Top of the tree
THE five-storey pagoda of the Temple of the Flourishing Law in the Nara prefecture of Japan is one of the world’s oldest wooden buildings. It has withstood wind, rain, fire and earthquakes for 1,400 years. Analysis of the rings in the central pillar supporting the 32-metre structure suggests the wood that it is made from was felled in 594, and construction is thought to have taken place soon after.
In an age of steel and concrete, the pagoda is a reminder of wood’s long history as a construction material. New techniques mean that wood can now be used for much taller buildings. A handful are already going up in cities around the world. The 14-storey Treet block of flats in Bergen, Norway, is currently the tallest. But Brock Commons, an 18-storey wooden dormitory at the University of British Columbia in Canada, is due to be completed in 2017. That is when construction is expected to begin on the 21-storey Haut building in Amsterdam. Arup, a firm of engineering consultants working on the project, says it will be built using sustainable European pine. Some architects have even started designing wooden skyscrapers, like the proposed Tratoppen (“the treetop” illustrated above), a 40-floor residential tower on the drawing-board in Stockholm.
Wood has many attractions as a construction material, apart from its aesthetic qualities. A wooden building is about a quarter of the weight of an equivalent reinforced-concrete structure, which means foundations can be smaller. Timber is a sustainable material and a natural “sink” for CO2, as trees lock in carbon from the atmosphere. Tall steel-and-concrete buildings tend to have a large carbon footprint, in part because of the amount of material required to support them. Using wood could reduce their carbon footprint by 60-75%, according to some studies.
There are two main concerns about using wood to build high. The first is whether wood is strong enough. In recent years there have been big advances in “engineered” wood, such as cross-laminated timber (CLT) made from layers of timber sections glued together with their grains at right angles to one another. In much the same way that aligning carbon-fibre composites creates stronger racing cars, aircraft and golf clubs, CLT imparts greater rigidity and strength to wooden structures.
A recent experiment by Skidmore, Owings & Merrill, a firm of architects, and Oregon State University, shows how strong engineered wood can be. The researchers used CLT in a hybrid form known as concrete-jointed timber. This featured an 11-metre wide CLT floor section with a thin layer of reinforced concrete spread across the surface. Thicker sections of concrete were added where the floor was supported by pillars. It was put into a giant test rig where a powerful hydraulic press pushed with increasing force onto the surface. The researchers wanted to see how the structure moved under load, but kept pressing in order to find its limits. The floor finally began to crack when the load reached a massive 82,000 pounds (37,200kg), around eight times what it was designed to support.
The concrete covering the floor was mainly for sound insulation, but it helps to deal with the second worry: fire. The concrete adds a layer of fire protection between floors. In general, a large mass of wood, such as a CLT floor, is difficult to burn without a sustained heat source—for the same reason that it is hard to light a camp fire when all you have is logs. Once the outside of the timber chars it can prevent the wood inside from igniting. The big urban fires of the past, such as the Great Fire of London, which occurred 350 years ago this month, were mostly fuelled by smaller sections of timber acting as kindling. Prospective tenants would doubtless need lots of reassurance. But with other fire-resistant layers and modern sprinkler systems, tall wooden buildings can exceed existing fire standards, reckons Benton Johnson, a project leader with Skidmore, Owings & Merrill.
He says the test showed that not only can wood be made strong enough for tall buildings but that “it makes sense to use it”. Although a cubic metre of concrete is cheaper than an equivalent volume of timber, wooden buildings can be built faster. Mr Johnson thinks the appeal of wood, both visually and as a sustainable material, will make it commercially attractive to property developers.
London’s tree house
What about woodworm and rot? “If you don’t look after it, steel and concrete will fail just as quickly as timber,” says Michael Ramage, head of the Centre for Natural Material Innovation at the University of Cambridge in Britain. Dr Ramage and his colleagues are also testing wooden materials for tall buildings, including for an 80-storey, 300-metre wooden skyscraper (see illustration) presented as a conceptual study to the City of London. Designed with PLP Architecture and Smith and Wallwork, an engineering company, it would, if built, become the second-highest building in London after the Shard.
For a busy city such as London, there are yet more advantages to building higher with timber, adds Dr Ramage. For a start, the construction site would be a lot quieter without the heavy plant required to pound deep foundations, pump concrete and install steel supports. There would also be less construction traffic. Dr Ramage calculates that for every lorry delivering timber for a wooden building, five lorries would be needed to deliver concrete and steel. All these things may mean that once the total construction costs are calculated, a wooden building can work out cheaper.
Anders Berensson, the Swedish architect who designed Tratoppen, believes engineered wood will become the cheapest way to construct tall buildings in the future. Another benefit of the material, he says, is the ability to carve the wood readily. In his current design the number of each floor is cut into the building’s exterior.
One big obstacle to this wooden renaissance is regulation. Building codes vary around the world. In America cities can restrict wooden buildings to five or six storeys (about the height of a fire engine’s ladder). Exemptions can be made, however, and proponents of wood are hoping that as taller timber buildings emerge, city planners will adjust the rules. If they do, an old-fashioned branch of architecture might enjoy a revival.