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.
CLT-plate with three layers made from spruce
A recent experiment by Skidmore, Owings and 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.
Advantages for wood skyscrapers
• Modern engineered wood can be as safe against fire
• Five times less material means less heavy equipment and fewer trucks to the work site
• Good delivery of airtight envelope
• Greater load distribution can reduce thickness of transfer slabs
• Light weight reduces load on foundations so less need for materials with high embodied energy (eg concrete)
• Need for robust upfront design may improve overall design and efficiency
• Robust finished wall will take sundry fixings
• Simple and fast onsite construction process
• Suitable for non-visible as well as exposed finishes
• Vapor-permeable wall construction
An architectural rendering of a 1,000-foot-tall wood skyscraper proposed for London. Source: PLP Architecture/University of Cambridge
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.
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.
• density: 480–500 kg/m3 (spruce)
• compressive strength:
• 2.7 N/mm2 (perpendicular to grain of boards)
• 24–30 N/mm2 (parallel to grain of boards)
• bending strength:
• 24 N/mm2 (parallel to grain of boards)
• elastic modulus:
• 370 N/mm2 (perpendicular to grain of boards)
• 12,000 N/mm2 (parallel to grain of boards)
SOURCES – Cross laminated timber wiki, Economist, TED, Bloomberg