The Smile: Demonstrating the potential of American hardwood CLT

American hardwoods have made significant advancements in cross-laminated timber technology since the Endless Stair debuted at the 2013 London Design Festival. This year, two projects demonstrate the real potential of Tulipwood CLT, an abundant, versatile and structurally sound American hardwood.

The Smile

The American Hardwood Export Council (AHEC) and its partners Alison Brooks Architects and engineering firm Arup unveiled The Smile at this year’s London Design Festival, the most complex cross laminated timber (CLT) structure ever built.

At 3.5m high, 4.5m wide and 34m long, The Smile is the first ever ‘mega-tube’ made from construction-sized panels of Tulipwood CLT. Alison Brooks, who designed the organic-shaped timber, wanted to express the strength of Tulipwood CLT and the best way to do so was to present it in its largest format possible—4.5m by 20m.

“This is a beam profile that works very well in tension and compression to achieve long spans,” she said. “By making this CLT ‘tube’ into the shape of an arc at a huge scale, the plates form a dynamic, sensory space to inhabit.”

With its curved floor, ceiling and walls, Alison Brooks’ concept creates an interior space that bridges an undulating environment between landscape, adventure playground, bridge and diving board. The curved form cantilevers out from the centre point into space while hovering above the ground to provide sheltered outdoor spaces for visitors to explore and interact with.

The pavilion’s walls are perforated with oval holes, which allow daylight to filter in and track across the interior space throughout the day. After dark The Smile emits light from each end, elongating the structure.

“There’s a clever simplicity to her architecture, and a good bit of humility,” said AHEC European director David Venables. “Very quietly, she influences and changes thinking. She’s a strong advocate for timber in construction.”

More than art

The biggest engineering challenge of the project lay in the double cantilever and the entrance door right at the centre where the stresses are highest.

“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,” said timber specialist and Arup associate director Andrew Lawrence.

To prevent The Smile from tipping over, the structure is screwed down to a large wooden box hidden underground and filled with 20 tonnes of steel weights. Arup’s engineering team also worked to derive an efficient structural form using only 60m3 of wood to create a 150m2 space. 

However, the urban installation represents something far more important, said Mr Venables. Apart from showcasing the structural and spatial potential of hardwood CLT, “it is the creation of a brand new product and new use of hardwood that could transform the way architects and engineers approach timber construction,” he said. “It is 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 Arup to design the Endless Stair for the London Design Festival, pioneering the use of hardwood for CLT, an engineered wood product typically made from softwood. As a result, the debate about taking hardwood CLT to industrial production spread across Europe.

While CLT is usually made from softwoods, some public buildings require specialised, higher-quality products Mr Venables added. “The greater strength or improved appearance of hardwood CLT will make it the material of choice, and quite possibly the only solution.”

Furthermore Tulipwood CLT possesses great environmental credentials since the timber is abundant and CLT uses the lowest grades—those not exported for furniture production and would otherwise have a very restricted market.

“It is good both for the American hardwood producers and for the environment,” said Mr Venables. “A double win for Tulipwood CLT.”

A version of this article was first published in Panels & Furniture Asia (Nov/Dec).