While CLT is becoming a widely accepted means of building around the world, it has been done exclusively in softwood so far. In contrast to previous AHEC experiments with hardwood CLT, this installation would be made using a real manufacturing process rather than the hand-crafted approach. AHEC’s vision was to showcase a product that industry can embrace and present as a solution for architects and engineers: whether in public buildings or in lower cost affordable housing.
Exploiting the properties of tulipwood CLT, Architect Alison Brooks came up with the concept of a beam that curves up at both ends. The CLT tube is an inherently strong shape, in the same way that a steel tube is. The curve means that it cantilevers from, effectively, a single point in the centre.
Although she has worked with Glulam, Brooks has never had the opportunity to work with CLT before – she has tried to specify it but because for example of acoustic constraints it has never been possible to use it without a great number of additional layers.’ ‘Tulipwood itself is not heavy which is helpful; and it has character and a tight grain’, Brooks says.
Andrew Lawrence, Arup’s global timber specialist sees great potential for tulipwood CLT. ‘The reason that timber construction has really taken off, in addition to the sustainability argument, is down to three factors,’ Lawrence says. These are CNC fabrication, the invention of CLT and the introduction of cheap modern self-tapping screws.
‘The thing about this project,’ he says,’is that it actually combines all three innovations. It is all CLT. Nearly all the connections are modern self-tapping screws, and it completely relies on CNC manufacture.’ Softwood will always be the workhorse of timber construction, but where improved strength and appearance are needed, then he believes hardwood CLT is the answer.
The Smile is not just showcasing the use of hardwood CLT, but that 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.
Andrew Lawrence, Arup’s global timber specialist says ‘This is by far the most complex CLT structure that has ever been made’. It was 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’.
Not only does it have a double cantilever, but the entrance door in the centre of the structure is placed exactly where the stresses are highest. Solving these challenges has useful lessons not only for this structure but also for all future CLT buildings.
‘Although it is a sculpture,’ he says, ‘you are effectively looking at a 20m cantilever that is like the core we use to stabilise a building. The weight of 60 people at one end of the cantilever is equivalent to the wind load on a five-storey building. Nobody has done a core that slender in wood.’
One of the first challenges for Lawrence was determining the properties of the American tulipwood that the project is using. The strength of the CLT is derived from testing undertaken on 100 full size planks of tulipwood 15 years ago.
The design had to take some very demanding circumstances into account. The shape and the loadings mean that it is in tension at the top over the door and in compression through the floor under the door. In addition to the position of the door, there was the way that The Smile sits on its wooden foundation. This is partly buried in the ground, to minimise the visual impact and, for the same reason, is as small as possible, while not allowing it to be so small that the structure could roll over, lifting the foundation which is weighted by the use of steel weights within the wooded box.
Another consideration relates to the behaviour of visitors. The designers had to allow for the fact that all the visitors could be at one end of the structure at one time – another design consideration was that everybody might stand to one side, tending to turn The Smile on its side.
The structure also had to be strong enough to resist ‘lozenging’, the tendency to distort from a square cross section to a rhomboid when the wind blows from one side with a force equivalent to 10 tonnes. Therefore stiff moment connections have been introduced at the roof to wall junction to limit the amount of lozenging. All the connections were achieved with self-tapping screws, about 400 mm long and very carefully positioned.
Another challenge comes from the sheer size of the elements. CLT lends itself to being made in large pieces and in this case it is essential, as there could not be a join in the centre, where the structure is most highly stressed.
After all this work and effort, assembly was relatively simple, ‘like a piece of IKEA furniture but on a much larger scale,’ said Lawrence.