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Engineering at the root of the Serpentine Pavilion

This year’s Serpentine Pavilion is reverting to an architecturally more simple concept, but the engineering behind the structure has remained just as challenging.

Consultant Aecom has once again designed the annual pavilion in conjunction with Francis Kéré, an African architect based in Berlin, Germany.

Harking back to Kéré’s roots, the structure’s roof has been inspired by a tree used as a meeting place in his native village of Gando in Burkina Faso. A series of cantilevering trusses ranging from 6m to 10m in length fan round to mirror the tree’s canopy, funnelling down to a central ellipse. Curved, 3m high timber walls constructed from a tessellating triangular pattern form four different spaces within the pavilion.

On the top of the roof, a simple polycarbonate covering fixed to the top chords of the trusses provides a rain cover to the structure, while on the underside the geometry of the engineering have been used to create a dramatic effect for the ceiling cladding.

“This was one of the things that we worked hard with the architects in the early days to minimise the secondary structure with this very defined feature of what the ceiling would look like.” said Aecom principal engineer Michael Orr.

To stabilise the series of planar trusses, ties run in rings between their node points. Diagonal K braces then fly from the bottom chord of each truss to the mid point of the ties connecting the top chords. The triangular surfaces created by this geometry has been clad in individual timber batons, allowing light to pass through the roof.

These trusses are supported by a central elliptical ring truss which in turn is supported by trussed columns which are 970mm at their deepest point. Due to the varying length cantilevers in the roof, the ring truss has to work hard to distribute the eccentric forces evenly to the foundations.

“For the ring truss we spent a lot of time refining a geometry and refining the size of the members,” said Orr. “We think we have a nice balance, keeping the members small but also providing a stiffness that the means the load was nicely distributed around the structure.”

The walls also provided an interesting design challenge for the engineers. To keep the heritage of the African theme, triangular modules were chosen as the repeating pattern from which the wall would be built. Each module is made out of 75mm thick, 175mm to 200mm wide timber planks stacked on top of each other, 700mm long at the base going to 100mm wide at the top, a chamfer on the front of each plank gives a non-flush finish. They are held together by 16mm diameter dowels glued 150mm into the timber and cross screws to give it robustness. Columns throughout the walls are effectively formed every half metre where the tips of the triangles meet.

Summer storm rainwater gathered on the roof canopy is funnelled off down to a central attenuation system formed from SDS geolight geocellular storage within the footprint of the pavilion. From here excess water is taken to another attenuation tank within the grounds of the gallery.

As with every year, the construction of the pavilion has to be rapid with only a few weeks allowed on site. To stay true to the simple methods of construction used on this type of structure in Africa, no cranes were used, only genie lifts and telehandlers. This had the additional benefit of reducing the effect of weather on the construction process.

To simplify the construction challenge, as much of the work as possible has been done off site, but the number of elements which have had to come together has presented the biggest test for the engineers and contractor Stage One.

“The challenges have come from the sheer scale of the construction,” said Orr. “From the outset it looks like a simple building, but there are around 1,500 individual elements which is around 2.5km when you put it end on end. In the ceiling there are around 13,000  which is over 3.5km end on end. There are 520 timber modules in the walls.”

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