A Hardwood 'Smile'

The use of glued laminated timber – aka timber plywood or CLT (Cross Laminated Timber) – in construction is being used more often by architects and structural engineers for the load-bearing construction of increasingly larger buildings. For ten years the American Hardwood Export Council (AHEC) has tested the use of hardwood CLT as an alternative to the prevailing softwoods, such as conifers. Three years ago at the London Design Festival, AHEC, together with dRMM Architects and Arup, presented the accessible sculpture "Endless Stair," a prototype made from cross-laminated tulipwood. The experience gained was incorporated into the industrially manufactured installation for this year's London Design Festival, which took place from mid-September to mid-October.

"The Smile" is a 34-meter-long, 3.5-meter-tall and 4.5-meter-wide curved tube also made of cross-laminated tulipwood. Located in the courtyard of the Chelsea College of Arts, the installation was designed by London's Alison Brooks Architects together with Arup's well-known engineers. With this design, both the client and the architect wanted to demonstrate the structural and spatial potential of CLT hardwood and test it on a 1: 1 scale. "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," said Brooks, who was influenced by the structural properties of the material in design. "The best way to express this strength was to combine these plates into a four-sided CLT hollow tube." Andrew Lawrence, associate director at Arup and a timber specialist, also expressed his enthusiasm: "The Smile is the most complex CLT structure that has ever been built. It’s equivalent to the core stabilizing a five-story building. Nobody has ever built a core that slender in timber. Where improved strength and appearance are needed, I can see great potential for tulipwood CLT."

​So why hardwood CLT? In terms of weight, hardwood CLT can be even more stable than concrete and can be machined to a high tolerance. With prefabrication and rapid assembly, construction times can be reduced. For "The Smile," only a few qualified specialists were needed to assemble the sculpture on site within a few days (see the construction process in the film at bottom). Additional wet work such as masonry, screed or plastering wasn't needed. According to David Venables, AHEC's European Director, there would be a need for specialized, high-quality solutions for hardwood CLT in public buildings or even in social housing. The higher strength and the higher quality appearance of hardwood CLT products are clearly an advantage here. However, from the outset Alison Brooks wanted to work with a cantilever to test the possibilities of the material. So "The Smile" touched the ground at only one point in the middle, where the entrance was found. In turn, a wooden box filled with 20 tons of steel hidden beneath the plaza was needed to stabilize the double-cantilever and stop it from tipping under the weight of people who walked to the ends to look out from within "The Smile."

A version of this post originally appeared as "Laubholzlächeln" on German-Architects.