Hardwood CLT: The next big innovation in structural timber?

AHEC, Alison Brooks Architects and Arup to collaborate on ‘The Smile’ for the London Design Festival 2016

The American Hardwood Export Council (AHEC), the leading international trade association for the American hardwood industry, has announced its participation at the London Design Festival (LDF) 2016 in collaboration with Alison Brooks Architects (ABA) and Arup. Brooks has designed ‘The Smile’, an urban installation that showcases the structural and spatial potential of cross-laminated timber (CLT) using American tulipwood. The Smile, which will be on display at the Rootstein Hopkins Parade Ground of the Chelsea College
of Arts from September 17 until October 12, is one of the Festival’s Landmark Projects; a timber structure that can be inhabited and explored by the public.

With expertise from top engineering firm Arup, and
 using construction sized panels
 of hardwood CLT for the first time, Alison Brooks’ concept is a spectacular 3.5m high, 4.5m wide and 34m long curved rectangular tube – the first ever hardwood ‘mega-tube’. Arup’s engineering team is working to derive the most efficient structural form, using only 60 cubic meters of wood to create a 150 square meter enclosed space. The forces of tension and compression working in the CLT walls will be expressed by perforations in its elevations. ABA has used these to generate patterns of light across The Smile’s interior spaces during the day, it will become an urban lantern at night.

“The Smile is a huge curved hollow tube made of cross- laminated tulipwood. It touches the ground at one point, like a wheel. Entering The Smile through an opening where the curved form meets the ground, the visitor
can walk from end to end of the 34-meter-long tube to discover a new kind of space that gradually rises toward light. All four sides of The Smile’s interior will be made
 of the same beautiful hardwood panels as the structure. It will offer a complete sensory experience of color, texture, scent and sound. The Smile’s two open ends will illuminate the funnel-like interior space and act as balconies to the city,” said Alison Brooks.

“Along the walls, perforations will allow sunlight to draw changing patterns on the 
floor throughout the day. The perforations will also give the visitor an understanding of how the structure performs as they’re located in positions where there are fewer structural stresses. At night the interior will be illuminated by linear light strips that trace its dynamic curving floor. This
 is the happy face of the timber revolution,” says Brooks, “a small building that performs big miracles by demonstrating how hardwood CLT can perform as a structural material.”

Brooks has designed the 34m pavilion to be entirely motionless, despite two swooping 12m cantilevered sections that appear hell-bent on teetering. The 12 industrial-sized tulipwood CLT panels, three of which are curved – supplied by German CLT pioneers, Züblin – run up to 14m long and 4.5m wide, some of the largest ever to make it into production. Even less plausible, they form an entirely self-balancing structure: no steel beams lie underground to support it. By fixing the CLT panels into a four-sided tube with 2,000 self-tapping screws, Brooks and engineers Arup, have effectively quadrupled its strength, enabling her to lift up the ends so they appear to defy gravity. To anchor it, the arc is bolted into a wood cradle loaded with concrete crane counterweights and buried a meter beneath the lawn.

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. For AHEC, The Smile is one of the most important developments in a decade of research and development into structural timber innovation with Arup, and one that could broaden the use of CLT in the construction industry. Andrew Lawrence, Associate Director, Arup says, “The Smile is the most challenging structure ever constructed in CLT. Every aspect is pushed to the absolute limit. It really shows the potential for hardwoods in construction.”

Running across the capital from September 17 – 25, this year’s London Design Festival brings together architects, designers and artists for over 400 events spread across the capital. The Smile is one of only four Landmark Projects, which are site specific and appear in some of London’s most prominent and covetable spaces. Ben Evans, Director of LDF comments: “The Landmark Projects are a key part of the Festival’s commissioning programme. They are at a scale that gets noticed and are always in major public places reaching a very wide audience. The choice of architect is key and Alison Brooks Architects are known for their innovative use of materials. Alongside a strong commitment to ambitious ideas they made an ideal choice for this year’s Landmark project with AHEC.”

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. The project builds on AHEC’s previous collaborations at LDF including the Timber Wave, Out of the Woods, Endless Stair and The Wish List, all of which have been significant projects. However, The Smile is the most significant advance because it
 will create the first-ever use of industrial-sized panels of hardwood CLT. These panels will be produced by Züblin Timber, who believe in the potential of tulipwood CLT as bringing a revolutionary new element to wood construction.

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. While CLT is becoming a widely accepted means of building around the world, it has been done exclusively in softwood so far. AHEC has been experimenting with tulipwood, most particularly in the design and making of the Endless Stair, a project for the London Design Festival two years ago, designed by architect dRMM with Arup as engineer. Whereas the Endless Stair used tulipwood CLT that had been prototyped for the occasion in a kind of hand-crafting approach, The Smile is being made by German company Züblin using a real manufacturing process that is an adaptation of the way that it makes softwood CLT commercially.

“The decision to use tulipwood CLT is vital because it is good 
both for the American hardwood producers and for the environment. In order to use timber in the most environmentally friendly way, it is vital to use as much as possible of what is grown and harvested and to throw away as little as possible. This is a double win for tulipwood CLT. First the timber is abundant, naturally representing a large proportion of hardwood forests. And secondly, CLT uses the lowest grades of the timber – grades that are no longer exported for furniture production and so would otherwise have a very restricted market. While using these previously unloved grades is evidently good for the environment, it is also good for the producers, who are seeing a potential new market,” concludes Roderick Wiles, AHEC Regional Director.

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