THE TECHNICAL CHALLENGE
+ INTERVIEW WITH ARUP'S GLOBAL TIMBER SPECIALIST ANDREW LAWRENCE
The Smile may be a jaunty, cheerful installation but it is evident, even on a casual glance, that a lot of engineering thought and skill has gone into it. Just how much, however, may not be immediately obvious.
But it is to Andrew Lawrence, Arup’s global timber specialist. ‘This is by far the most complex CLT [cross-laminated timber] structure that has ever been made,’ he says. And he should know.
Lawrence has been involved with Arup's most complex timber structures, including the 18 storey tower currently being designed for Amsterdam, using glulam and CLT, and Crossrail Place, Canary Wharf with Foster and Partners, the largest timber roof in the UK. He has also worked on a number of earlier projects with AHEC (the American Hardwood Export Council) including the Endless Stair, the very first use of tulipwood CLT. Those projects required not only considerable engineering expertise but also research from first principles into the properties and behaviour of materials and components.
Every aspect of the Smile is working incredibly hard, Lawrence says. 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 allfuture 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.’
As Ishan Abeysekera, Arup’s engineer responsible for the detailed design of the Smile explains ‘We have reviewed every paper ever written on CLT, but many aspects of the Smile are simply so complicated that no one has had to consider them before.’
Tulipwood is in a real ‘sweet spot’ Lawrence believes ‘Denser wood is stronger, but if it’s too dense it’s really expensive to kiln dry and you can’t get the screws in.’ The attraction of tulipwood is that it is not very much denser than softwood but it is quite a lot stronger. Tulipwood from American forests also grows nearly twice as fast as softwood in Scandinavian forests.
Softwood will always be the workhorse of timber construction, but where improved strength and appearance are needed, then Lawrence sees great potential for tulipwood CLT. The time is right, he believes. ‘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.’
This allows the manufacturing to be incredibly accurate. Lawrence says that the Smile goes together ‘like a piece of IKEA furniture but on a much larger scale.’ With CNC cutting it is possible to achieve tolerances of just one mm. ‘If you want to prefabricate you want something that is strong and light, machines to high tolerances and is easy to connect,’ Lawrence says.
One of the first challenges for Lawrence was determining the properties of the tulipwood that the project is using. Since it is working so hard, this was essential. The strength of the CLT is derived from testing undertaken on 100 full size planks of tulipwood 15 years ago. In all the highly stressed areas the size of knots is limited to 25mm. Weaker boards with larger knots are positioned in the centre of the CLT ‘sandwich’ where they do not have to work so hard.
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. There will be a restriction on the total number of visitors at any one time – a figure of no more than 60. But the designers had to allow for the fact that all of them could be at one end of the structure at one time – an idea that is not fanciful if you imagine a group photo of a hen party or a (probably heavier) rugby club. Another design consideration is that everybody might stand to one side, tending to turn the Smile on its side.
As if people were not enough to contend with, there is also the wind. The structure has 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 are achieved with self-tapping screws, about 400 mm long and very carefully positioned. The first 40mm of each screw hole is CNC cut in the factory to ensure that the screws will go in in the right direction; this was also important for appearance to ensure all the screws were neatly in line.
The high level of stresses means that the window-like holes in the walls that were part of architect Alison Brooks’ original design are relatively restricted in number and in size – the structure is simply working too hard to allow many of them to go in. The holes are also carefully positioned relative to the direction of the wood fibres.
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 must not be a join in the centre, where the structure is most highly stressed. As a result, the modules measure up to 14m by 4.5m, larger than a standard truck load and can only travel by road between midnight and 6am on their journey from Germany.
Despite all the planning, the design has had to accommodate some unexpected elements, in particular the discovery that the tower of the old Millbank prison lies just below the surface in exactly the place where it was planned to put the foundations.
After all this work and effort, assembly will be relatively simple, largely carried out by a skilled team of six men wielding power screwdrivers. Once it is up, Lawrence will doubtless breathe a sigh of relief.
Asked why he enjoys working with wood so much, he said, ‘It’s really fun working out all the connection details and the great thing is you can still see them in the finished structure, which is one of the things that makes timber structures so appealing.’ That certainly applies to The Smile.