Sustainable Smile

‘The Smile’

The most complex hardwood cross-laminated timber (CLT – remember this acronym, we will be using it a lot!) structure ever built. This massive engineering marvel was presented at the  Chelsea College of Art Rootstein Hopkins Parade Ground from September 17 until October 12, 2016 and is one of four Landmark projects, which can be inhabited and explored by the public, at the London Design Festival this year.

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 and designed by  Alison Brooks Architects the  concept is the first ever ‘mega-tube’ made with construction-sized panels CLT.

In a testament to sustainable design, ‘The Smile’ uses American Tulipwood in it’s CLT which is one of the most abundant American Hardwoods. As such natural forest growth replaces the tulipwood used to manufacture ‘The Smile’ in under five minutes!

Engineering Challenges

“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,” concludes Roderick Wiles, AHEC Regional Director. 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 density to be easy to kiln dry, easy to machine, easy to transport and easy to screw into, making it suitable for the project.

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 Arup installed hidden timber beams above the roof enabling them to rigidly connect the walls to the roof to prevent the lozenging action. Arup 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 it from 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.

Sustainable Hardwood Smile

On the Design Concept

Alison Brooks, said: “It’s a very interesting wood – it has wonderful grain and colour variation. Tulipwood also has a kind of sheen, so the grain is tight and smooth. Tulipwood can be selected to be clear and knot-free, offering a really clean-looking alternative to softwoods. I love the way the strips of wood in cross-laminated timber have a direction that expresses how the structure is working.”

“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,”

The Smile’s form implies that it will rock. So the form itself is an invitation to test whether the pavilion moves, and how it feels to walk in on a curved floor. A single door and ramp from the square invites visitors to enter – something like our archetypal image of Noah’s Ark. Inside the door light spilling from the ends of the arc will invite you to walk up the slope of the curve to balconies at either end, rather like looking out from the rail of a ship. In addition, the pavilion’s walls are perforated with oval holes, measuring between 12cm and 20cm. These allow daylight to filter in and track across the interior space throughout the day. The holes are placed in the parts of the CLT walls under the least amount of strain, a kind of structurally expressive ornament. After dark, The Smile will emit beams of light from each end, elongating the structure.

With inputs from American Hardwood Export Council