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Structural engineering: AKT’s role in Heatherwick Studio’s Shanghai Expo pavilion

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Heatherwick Studio’s Shanghai Expo pavilion - winner of the Lubetkin Prize - tasked AKT with the engineering of a metaphor

The competition to design the UK Pavilion at Expo 2010 Shanghai was for a building where the architecture and the exhibition were inextricably linked. The plan was to focus the budget on one extraordinary structure within a simple landscape. The final design is a metaphor for the movement of English grass, comprising cuboid construction supporting 60,000 7.5m-long acrylic spikes. Most people did not think this was buildable. The competition was initiated by the British Foreign & Commonwealth Office and the pavilion opened to the public in May.

Structural and civil engineering support, from concept to completion, was provided by my firm Adams Kara Taylor (AKT). Our role in this project was to enable the architect’s concept to be realised, using our expertise in construction as well as engineering. The whole process was supported, in design and construction, by the Architectural Design and Research Institute of Tongji University, a Shanghai-based design bureau.

Complex planes: the landscape
The landscape which surrounds the pavilion is approximately 86 x 68m on plan. Thomas Heatherwick proposed a 15 x 15m box surrounded by a landscaped ramp and accommodation space. The metaphor here is a present, with unwrapped paper forming the landscape. This concept required a unique structure for the pavilion itself, but also encouraged a frugal distribution of costs embedded within the concept, such that the landscaped areas are more economical than the pavilion, where more interest lies.

The ‘unwrapped paper’ that cradles the pavilion was manipulated into a complex shape of folding planes that rise to shelter the queuing public below and allow crowds to gather above. A breach in this folded canopy provides access to a bridge that connects to the pavilion.

Below the canopy, a ramp from site ground level spirals around the edge as it rises 4m to the pavilion. This change in the ramp’s level was designed to provide the necessary VIP rooms, plant rooms and staff offices below, where headroom permitted.

The entire structure of the folded surface is cantilevered reinforced concrete, surfaced with red and grey artificial grass, and stiffened on the underside by a set of tapering ribs that range from 2m to 2.5m in depth and from 7m to 17.4m in length at the corner. The area below the canopy provides circulation and a pleasing column-free edge. The cantilever ribs and folds support crowd loads, and many of the cantilevers are backfilled with soil to counterbalance the weight and prevent tipping. Public access to the 2m zone at the tip of the cantilevers is restricted.

The foundations became a major point of discussion between ourselves, Tongji Institute and contractor Mace. Although proposals for piled and raft foundations were developed in detail, settlement of the raft became a concern. The raft option was chosen because it was faster to build and, given that this is a temporary structure, it would be easy to break out and therefore would not leave behind obstructions to future use of the site.

Material considerations: acrylic spikes
The spikes on the outside of the pavilion take your breath away and the scale of these 60,000 members is phenomenal. They are 7.5m long and conceived by Heatherwick as ‘responsive blades of grass’ that move with the wind. Inside the pavilion, the spike tips encapsulate seeds (sourced in collaboration with the Millennium Seed Bank at Kew Gardens, London), revealing both the UK Pavilion’s message and Heatherwick’s concept to visitors. The choice of material for the spikes, along with their design, fabrication and assembly, provided the most exciting technological, design and construction challenges.

Design for the locality has to accommodate air temperatures of up to 40°C, monsoon wind gusts up to 120km/h and seismic activity. The concept also demanded fine-tuning of the slenderness of each spike, so it would move with mild winds but also resist typhoon loads. At one stage, spikes with automated movement were
considered. A detailed assessment by AKT and wind consultant BMT revealed that the effect of strong winds on the projecting surfaces of the spikes from the box, which range from 5m to 6.1m in length, affected the stability of the pavilion. Although the spikes would protect each other when the pavilion was complete, they would be susceptible to high wind loads during construction.

Each spike is identical, comprising an acrylic extrusion with two aluminium sleeves of different lengths, one inside the other. These reinforce the sections near their connections to the box, providing the required strength near the spikes’ roots and allowing flexibility for lateral movement. We worked with
Heatherwick and materials consultant Smithers Rapra to test various samples of acrylic in the UK. In parallel, the Tongji Institute team conducted load tests. The fatigue failure of the aluminium and the acrylic was carefully considered because of the anticipated cyclic movement, and we decided to avoid making holes
for bolt fixings in the aluminium near each spike’s root. The acrylic extrusions are 20 x 20mm and, at the support, the larger aluminium sleeve is 30 x 30mm. On the roof this is only attached to the outer skin of the primary structure.

Geometry and construction of box
The spikes are set out from their tips where, for visual reasons, they are 100mm apart. The 15 x 15 x 10m cuboid volume of the box, the setting-out and alignment of each spike, and technical problems such as site fixing, tolerances and deflections, demanded skilful processes which weren’t easy to automate or simulate.

We used in-house code while Heatherwick used RhinoScripts to develop the design and test geometries in three dimensions. Empirical testing of the spike fixings and geometry was carried out and the findings were integrated into a digital model which included all the spikes. We started with a total of 100,000 spikes, but this was reduced to 60,000 during design development.

Box construction and load management
The total weight of the spikes is 460 tonnes, with 100 tonnes on the roof alone - equivalent to 140 Smart cars – so the acrylic was by far the worst load case. The walls and roof, whichspans 14m, comprise two timber plates up to 28mm thick and 900mm apart, connected by laminated timber ribs, ie stressed skin construction, in the transverse and longitudinal directionsof the plate. The rib arrangement and connections were carefully coordinated to avoid clashes between them and the spikes that pierce the inner and outer skins at unique angles. Digital fabrication methods and automation enabled very precise setting out of the holes and ribs, to prevent clashes and achieve structural robustness. The structural analysis of the plates accounted for weaknesses due to the spike location holes. The outer plate of the roof is cambered to induce drainage and each penetration is waterproofed. The floor transfers the weight of the walls, roof and spikes to raking columns, supported by a raft foundation. A timber floor wouldn’t have been suitable due to the high loading from the spikes. Instead, we used 900mm-deep steel trusses, running in both directions, sandwiched between steel plates. This assembly acts as a deep continuous plate, the Tongji team then chose to design a set of primary trusses on the column lines. The raking columns are arranged in a circular formation on plan to provide central structural symmetry and lateral stability to the box in all directions.

Hanif Kara is founding partner of AKT. Contributors to this article include AKT founding partner Albert Williamson-Taylor, associate partner Reuben Brambleby and design engineer Marco Cerini



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