Grimshaw’s first scheme, designed in 1997 to a budget of £106m, was for a sinuous tube of trussed arches, like an expanded version of its acclaimed Waterloo International Terminal. All-too-familiar problems with funding and site acquisition led to a scaling down of the project. Interestingly, it also led to a more ingenious and original design.
A site was eventually found in a still-active china clay pit. Not only was the bearing capacity of the clay subsoil at the bottom of the pit poor, but the ground level could not be determined, as the pit was still being mined during the design process. So, a lighter structure with a more flexible baseline was required. Whalley devised “the idea of a free form in plan and section that hugged the contours of the pit”.
The architect and structural engineer, Anthony Hunt Associates, then looked at geodesic shells as invented by the late US structural engineer Richard Buckminster Fuller, in which the curved surfaces were resolved into a series of flat facets bounded by repetitive straight members. Once the German subcontractor Mero was brought on board, the geodesic shells evolved into a more complex geometry of a double-layered spaceframe composed mainly of hexagons above and triangles below. The design and fabrication of the spaceframes was only possible with the use of advanced computer modelling.
Instead of conventional glazing, the design team chose transparent ETFE membrane, partly because it transmits more daylight and ultraviolet light essential to plant growth. By comparison, glass costs twice as much and weighs 100 times more. ETFE can span larger gaps without intermediate support and, if two or more layers are inflated to form “pillows”, it provides better thermal insulation than double glazing.
The adoption of the lightweight membrane had a reciprocal effect on the spaceframe structure. As they had less load to bear, the hexagons could be expanded to a maximum of 11 m in diameter, further reducing the quantity of steelwork and its weight and cost.
A sophisticated environmental system to heat and ventilate the biomes was devised by services engineer Ove Arup & Partners. This also involved advanced computer analysis in the form of dynamic thermal models and computational fluid dynamics. By these means, the environmental performance could be fine-tuned, and the air-handling units reduced in number.