A collaboration between the American Hardwood Export Council (AHEC), Alison Brooks Architects, Arup and the London Design Festival has resulted in a landmark urban installation that aims to change the way architects and engineers consider timber construction.
Recently opened on the Rootstein Hopkins Parade at Chelsea College of Arts, for the 2016 festival, the Smile is constructed in cross-laminated American hard tulipwood. It has been hailed as one of the most important developments in structural timber innovation – Andrew Lawrence of project engineer Arup called it “the most complex structure ever built in cross-laminated timber (CLT).”
The public are allowed to explore the inside of the showcase tulipwood structure. Although a spectacular 34 m in length it’s only 3.5 m high and 4.5 m wide, however the first ever hardwood mega-tube is surprisingly light and spacious inside.
Gracefully curved 12 m open-ended cantilevered arms rise gradually towards the light and touch the ground at the centre point like a wheel, providing both a visitor entrance and an engineering challenge. Walking from one end to another is an illuminating, sensory journey of texture, colour and scale.
Tulipwood has strong sustainability credentials, as every year twice as much tulipwood timber grows in the hardwood forests of America than is harvested. Growing naturally tall, straight and strong for its weight, it is easy to machine and an ideal material for innovative timber construction using CLT, which is usually made from softwood.
Alison Brooks commented: “Tulipwood can be selected clear and knot-free, offering a really clean-looking alternative to softwoods,” adding: “I wanted to create something that uses tulipwood CLT in its largest format possible (4.5 m x 20 m plates), and to express the additional strength hardwood CLT can offer. The best way to do this was to combine these plates into a four-sided CLT hollow tube. One of the most amazing qualities of the Smile is the thinness of the majority of its wall and floor panels – only 100 mm thick.”
The project’s 12 layered panels were manufactured by Züblin-Timber, each formed from short lengths of finger jointed timber, stacked and glued in alternating layers and vacuum pressed. During this process the Smile’s CLT panels were curved.
Andrew Lawrence of Arup commented on the engineering challenge: “not only does the Smile have a double cantilever, but the entrance is also placed right at the centre where the stresses are highest. If you turned the structure vertically and added the weight of 60 visitors one end, it’s equivalent to the core stabilising a five-storey building. Nobody has ever built a core that slender in timber.”
Arup applied the latest timber research combined with steel screws over a foot long to fasten the panels together. To prevent it rocking, the Smile is anchored to a wooden cradle filled with 20 tonnes of steel counterweights. Inches below the cradle are the remains of the 19th century Millbank prison. The structure is designed to resist about 10 tonnes of wind loading and glulam beams hidden in the roof give it rigidity.
David Venables, AHEC’s European director says, “The Smile is important. It’s effectively the latest stage in a 10 year project that challenges the way hardwood can be used structurally.”
The completed piece could transform the way timber is used to make space, form and structure.