Architettura in legno
The American Hardwood Export Council (AHEC) has collaborated with Alison Brooks Architects, Arup and the London Design Festival to present a cross-laminated tulipwood structure, ‘The Smile’ at the Chelsea College of Art Rootstein Hopkins Parade Ground from 17 September until 12 October.
The Smile is one of the Festival’s Landmark Projects, which can be inhabited and explored by the public. The spectacular, curved, tubular timber structure measures 3.5m high, 4.5m wide and 34m long and is effectively a beam curving up at both ends. Showcasing the structural and spatial potential of cross-laminated American tulipwood, Alison Brooks' concept is the first ever ‘mega-tube’ made with construction-sized panels of hardwood CLT.
Q&A with Andrew Lawrence, Global Timber Specialist at Arup
Q: Why are we seeing more and more timber being used for buildings?
A.L.: Timber has many advantages, but I think the biggest is speed. Timber is lightweight and with computer fabrication it can now be machined to incredibly tight tolerances. This makes it ideal for prefabrication and rapid assembly. Assembling a timber building is like assembling a giant piece of flat pack furniture. The development of CLT has been a key part of the timber revolution as it gives us a way to create large timber panels which can be used for the walls and floors of entire buildings, without the need for any wet concrete trades.
Q: What makes The Smile such an important structure for timber?
A.L.: The Smile is the most complex CLT structure that has ever been built. Not only does it have a double cantilever, but the entrance door is placed right at the centre where the stresses are highest. You’re effectively looking at two 15m cantilevers. If you turned the structure vertically and added the weight of 60 visitors at one end, it’s equivalent to the core stabilising a five-storey building. Nobody has ever built a core that slender in timber.
Q: What makes tulipwood so suitable for CLT?
A.L.: Compared to other woods, tulipwood is surprisingly strong for its weight. It’s significantly stronger than spruce, but still low enough density to be easy to kiln dry, easy to machine, easy to transport and easy to screw into. Where improved strength and appearance are needed, I can see great potential for tulipwood CLT.
Q: What were the biggest engineering challenges of the project?
A.L.: The structure is designed to resist about 10 tonnes of wind loading that tends to want to distort the rectangular cross section of the tube into a lozenge shape. The obvious solution would have been to install internal cross-bracing along the length of The Smile but this would have ruined Alison’s concept for a clean interior, so instead we’ve hidden timber beams above the roof enabling us to rigidly connect the walls to the roof to prevent the lozenging action.
We’ve also screwed The Smile down to a large wooden box, hidden under the ground and filled with 20 tonnes of steel weights. This is what stops The Smile tipping under the weight of people at one end, or overturning under a strong wind. The need for 20 tonnes of weights and for the 6,000 long screws that hold the CLT panels together, really gives a feel for the huge forces which The Smile has to resist.
About cross-laminated timber
Cross-laminated timber (CLT) is an engineered timber that can be used to make the walls and floors of entire buildings. It has a layered construction with the wood fibres turned at right angles in each successive layer, creating a panel with equal strength in both directions, just like plywood. Weight for weight, CLT is stronger than concrete and it can also be machined to incredibly high tolerances. This makes it ideal for prefabrication and rapid assembly, reducing construction times by up to 30%.
CLT is usually made of a softwood called spruce, better known as the Christmas tree. Together with Arup, AHEC has started a process of experimenting with CLT made from fast-grown North American tulipwood. Testing has shown that the tulipwood is considerably stronger than spruce; it also has a superior appearance.
The Smile is the first project in the world to use large hardwood CLT panels, in fact the entire structure is made from just 12 huge tulipwood panels, each up to 14m long and 4.5m wide. Fabricating these panels in a real CLT production plant has been an important step forward, showcasing how the material can be used for commercial projects.