‘The Smile’ by AHEC

Alison Brooks Architects and Arup paves the way forward for the use of hardwood CLT in the construction industry

‘The Smile’, a spectacular, curved, tubular timber structure measuring 3.5m high, 4.5m wide and 34m long, by the American Hardwood Export Council (AHEC), Alison Brooks Architects (ABA) and Arup has paved the way forward for the use of hardwood cross-laminated timber (CLT) in the construction industry. On display at the Chelsea College of Art Rootstein Hopkins Parade Ground until October 12 as part of London Design Festival 2016, the installation showcases the structural and spatial potential of CLT using American tulipwood. As one of the Festival's Landmark Projects, which can be inhabited and explored by the public, 'The Smile' is effectively a beam curving up at both ends and is the first ever 'mega-tube' made with construction-sized panels of hardwood CLT.
CLT is usually made of a softwood called spruce, better known as the Christmas tree. Together with Arup, AHEC started a process of experimenting with CLT made from fast-grown North American tulipwood a few years ago. Testing has shown that not only is tulipwood considerably stronger than spruce, but it also has a superior appearance. 'The Smile' is the first project in the world to use large hardwood CLT panels manufactured by Züblin Timber; in fact, the entire structure is made from just 12 huge tulipwood panels, each measuring up to 14m long and 4.5m wide. For AHEC, 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.
According to Andrew Lawrence, Global Timber Specialist, Arup, '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 center where the stresses are highest. In essence, it's 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 stabilizing a five-storey building. Nobody has ever built a core that slender in timber. However, compared to other woods, tulipwood is surprisingly strong for its weight. It's significantly stronger than spruce, but still low enough in density to be easy to kiln dry, easy to machine, easy to transport and easy to screw into, making it suitable for the project.
Commenting on the emergence of timber buildings and on the role of CLT, Lawrence says: “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.”
For AHEC, 'The' Smile is not an installation at all, but something far more important - it's effectively the latest stage in a 10-year project that challenges the way hardwood can be used structurally. Three years ago, AHEC worked with architects dRMM (de Rijke Marsh Morgan) and engineers at Arup to design The Endless Stair, the very first tulipwood CLT prototype, for the London Design Festival. As a result, debate about hardwood CLT spread and the learnings enabled manufacturing of hardwood CLT on an industrial scale. This project not only builds on AHEC's previous collaborations but represents the most significant advance because it has created the first-ever use of industrial-sized panels of hardwood CLT.
“I wanted to create something that uses tulipwood CLT in its largest format possible, which is 4.5m x 20m plates, and to express the additional strength CLT can offer when it's made of hardwood. The best way to express this strength was to combine these plates into a four-sided CLT hollow tube. This is a beam profile that works very well in tension and compression to achieve long spans. By making this CLT 'tube' into the shape of an arc at a huge scale, the plates form a dynamic, sensory space to inhabit. The result is a building that cantilevers from a single point in the center. One of the most amazing qualities of 'The Smile' is the thin-ness of the majority of its wall and floor panels - only 100mm thick. It's an autonomous and self-supporting piece of architecture that touches the ground lightly,” said Alison Brooks.
Life Cycle Assessment (LCA) of 'The Smile' revealed that it is better than carbon neutral at point of delivery to the site at the Chelsea College of Arts. 'The Smile' is made predominantly in tulipwood, one of the most abundant American hardwoods with forest volume of over 1,000 million m3, which represents 7 percent of the total U.S. hardwood resource. Every year, the volume of tulipwood in U.S. forests grows on average by 32 million m3, of which only 13 million m3 is harvested. This means the volume standing in U.S. hardwood forests expands by 19 million m3 every year. As such, it takes less than five minutes for natural forest growth to replace all of the tulipwood used to manufacture 'The Smile'.
“The Smile not only showcases the use of hardwood CLT, but it makes the elements work as hard as they possibly can. It is a massive challenge in terms of scale and engineering as well as a demonstration of just how exciting and beautiful a building using CLT can be. This creation of a brand-new product and a new use of hardwood will transform the way architects and engineers approach timber construction. The structure aims to prove that hardwoods have a role to play in the timber construction revolution. Tulipwood is an abundant, lightweight but strong hardwood, and 'The Smile' is the culmination of an effort by AHEC to show that it can have a structural use in buildings,” concluded Roderick Wiles, AHEC Regional Director.
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