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A hexagonal grid structure clad with ETFE cushions

The 'biomes' - two climatecontrolled enclosures of 1.5 and 0.7ha respectively - are roofed with a series of interlocking spherical forms clad with lightweight ETFE cushions.

The roof structure comprises a grid of interconnecting hexagons which are formed from a series of 193.7mm diameter CHS tubes connected by cast steel nodes, each of which connects three CHS tubes to form the grid of hexagons.

Individual hexagonal frames are flat; the spherical shape of the roof is created by faceting them at the node connections.

A secondary triangulated structure of smaller tubular steel elements runs below the hexagon grid and is joined to the nodes by diagonal members; the two structures act as a three-dimensional space frame, allowing member sizes to be reduced.

The ETFE foil cushions, up to 2m deep at their centres, are formed from three layers of foil to create two cavities which are filled with air and lightly pressurised. The edges of the cushions are clipped to extruded aluminium restraints which are mounted on steel upstands, U-shaped in section, which in turn rest on the CHS tubes. Connections are sealed with neoprene gaskets. A series of Y-shaped brackets holding 2mm diameter stainless steel wires runs just above the edges of the cushions to deter birds from landing.

Each node is fitted at its centre with a 75mm diameter steel pin, projecting 400mm above it, which provides a secure fixing for scaffolding if access for maintenance is needed; the ETFE foil is selfcleaning. In some areas of the roof the hexagons are braced for snow loads with paired 10mm diameter stainless steel cables bolted to the spheres with forked connectors.

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