Tom Webster, UK Associate Director, at AECOM describes the challenges behind realising SelgasCano’s colourful design
The 2015 Serpentine Pavilion hinges on extremes and opposites, requiring engineering innovation to be pushed its limits to meet the architects’ goal of movement and lightness. At its core are primary steel frames comprising curved and faceted steel arches that form the structural canvas for panels of translucent fabric.
The primary frames are 2-pin arches supported by purlins acting as struts, plus tension-only bracing elements. To provide a sense of light and openness, we created a column-free entertainment space in the centre of the pavilion, with a transfer frame supporting the primary frames.
The primary frames are fully welded with pinned bases. Each braced bay was welded off site then transported to London. The purlins employ semi-rigid connections to the primary frames as it was impractical to site-weld them due to time constraints, as well as the requirement for easy disassembly.
Finding a suitable fabric was especially challenging. The architects wanted highly translucent, fire-resistant material in clear, coloured and mirrored finishes. It had to be capable of spanning approximately 2.5m between frames, while resisting wind and prestress loads. Ethylene Tetrafluoroethylene (ETFE) was the logical choice.
Typically, ETFE is used to create inflatable pillows but due to the pavilion’s design we needed to use a single layer of unreinforced, prestressed fabric. Single-layer membranes are commonplace but are typically tied by cables. ETFE is not as strong as PVC, so it was important not to overstretch ETFE during prestressing.
The structural analysis for membranes is very different from rigid structures, where the form is essentially fixed. Instead, the final shape is unknown before the form-finding calculation. To achieve a minimal surface, we used the force density method as the mathematical basis for calculating static equilibrium; the calculation considering nonlinearity in both geometry and material behaviour.
Uniformity and regularity are typically key factors in this kind of structure – ideally, panels span between parallel members with only gentle curves. The pavilion, by contrast, has no two identical frames, non-parallel axes and sharp changes in angle.
We felt the geometry would lead to wrinkles and creases when the fabric was tensioned on site, and put this to selgascano. It was agreed that these imperfections would be a feature of the pavilion’s unique design.
Q and A with Tom Webster
What is your involvement in the project?
From Aecom’s perspective, we lead the technical design of the pavilion. We have delivered the past three pavilions, since Sou Fujimoto. Our role is twofold. One is to ensure that the architects vision is accomplished - we facilitate the delivery of the architecture and, secondly, we advise on local approvals. Working with architects who have never worked in the UK, they may not be familiar with the planning system and regulations. The most important one is the licensing. To get permission to serve alcohol at the pavilion is paramount - sometimes the process is more troublesome than obtaining planning permission and building regulations.
How has it been working with Selgascano?
What I loved about working with Selgascano, which is new for me, is to work with an architectural practice where colour is almost more important than the form. That for me was an usual experience, the palette they use is extraordinary and I haven’t seen anyone else who does this in the same way. As an engineer, we tend not to have to worry about colour.
What were the challenges you faced with the Pavilion design?
One of the biggest challenges we faced on this project was to find a material that was translucent as possible, so we get the feeling of movement of light and dark and colour in a way that can be procured. What I mean by that is: to buy ETFE, you can get it in any colour you want, but you are subject to minimum orders. So if you are going for an extruded, coloured or dyed ETFE, you are going to order 1000 sq m of each colour. If you have 20 colours, then you have 20,000 sq m when the whole pavilion is about 200 sq m in plan. So that causes a real cost issue. So one of the biggest innovations on this is that we used clear ETFE everywhere on the project and it is digitally printed with all the colour. This allowed use to procure the right amount of ETFE and we can use any colour we want. It allowed us to make this pavilion a reality.
How does the collaboration work with the architect?
We have to provide the assistance to make sure the vision is achievable in the time we have available. At a certain point, you have to say, if we progress down this route, we wont hit the deadline. The key, where we have been quite lucky, is that we have to innovate - as the forms and pieces of architecture couldn’t be realised otherwise. Knowing when to say we have gone far enough is critical. Selgascano came on board late December. The design went through a number of iterations. What you see is the end of an evolution. By the end of January the concept is a little more settled, we go for planning early to mid February, and then we are manufacturing in mid March/April, then we are on site early May.