Nature as model and mentor

We spent many days poring over past studies of minimal structures and began turning again to the book of nature for answers. Perhaps the two most important observations of natural systems to influence us were: how nature's designs are so elegant yet so sparing; and that in nature most solutions fulfill more than function.

From the outset of the project we had wanted to ensure that the architecture would contribute to the Eden Project story; it should be more than a container into which exhibits would be poured. Since the core message of the project is one of sustainability, it was important to develop a design that could, regardless of size, be considered sustainable. Studies of bubble and foam structures demonstrated to us the efficiency of spherical geometries. Minimum surface area for maximum volume sounded like an economic strategy that was going to be hard to beat. This benefit was compounded by the inherent efficiency of geodesic structural geometries.

These efficiencies extended beyond the structure and into the envelope and environmental systems. Spheres are minimal surfaces that have maximum volume. They also, unlike more common orthogonal glasshouse forms, allow direct sunlight to enter perpendicular to the surface at all times of the day, thus maximizing the free energy.

The ability of current desktop computers to carry out complex calculations enabled us to undertake solar animation studies for the pit. We studied 365 days to provide an accurate and essential solar profile. Maximizing solar penetration was a key target and knowing where this asset lay determined the optimal positions for the biome structures. The results of this mapping indicated that our design should be linear in profile with lean-to structures built against south-facing cliffs. Such a diagram refers back to the very earliest glasshouse structures, such as Bicton. (One can only wonder at what ephemeral designs would have been developed by these engineers had they had access to the sophisticated form-finding software we have now.) Not wanting to abandon the supreme efficiency of spherical structures, we replicated foam geometries by linking bubbles in three dimensions, carefully following our solar boundaries whilst ensuring that the brief areas could be sustained.

Having accepted that the ground profiles would be constantly changing we developed a three-dimensional computer model that could be continually modified. The geodesic spheres were 'pushed' into a ground profile model and the resulting intersection line extrapolated and approximated to the nearest hexagonal panel. As the ground profiles altered over the months this intersection line adjusted accordingly and panels were added or subtracted as required. Throughout this process the remaining 90 per cent of the building remained fixed and this could then be developed and resolved to a greater level of detail and cost certainty. Perhaps most important for us was the fact that the architecture could be established and was safe. No cost or programme penalties would emerge as the site continued to be mined.

From an architectural point of view one of our biggest concerns was the resolution of intersecting spherical geometries. The truss lines could not be made to have matching geometries on either side, and to an architect, particularly one educated within the Meisian system, this proves to be a very uncomfortable issue. Not wanting to compromise the efficiency of our structure, we again studied nature for possible solutions and eventually came across a dragonfly's wing as a model for how minimal surfaces (which when packed tend to form hexagons) intersect with straight edges such as support ribs. The geometries on either side of the ribs are not symmetrical yet the performance is not compromised and the wing remains

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aesthetically elegant. Such a model proved to be a lesson for us, demonstrating that we sometimes have a single-minded view of geometry and elegance.

Further efficiencies were gained when we began to assess the options available for transparent cladding systems. Upon analysis, double-glazing on this scale and in this form proved to be less than satisfactory and the design was developed using ETFE foil—a transparent Teflon foil system fabricated as triple-skin pillows inflated to 300 pounds per square inch. These pillows allow greater penetration of low-frequency ultraviolet light, are better thermal conductors and weigh less than 1 per cent of double-glazed glass panels. Maximum panel size on the biomes is 53 square metres, which greatly reduced the weight of primary steel structure and its subsequent shading effect. The downside of this system is lifespan, which is estimated at twenty-five years— fairly low relative to glass. However, it is commonly known that the weakest link in any double-glazed panels is the silicone seal, estimated at about twenty to twenty-five years, which should only be replaced under factory conditions. A quick calculation actually shows that the volume of foil used to enclose the biomes is almost that of the silicon joints that would be required for double-glazing, when flattened to an equivalent 0.3 millimetres. The foil panel, however, is designed to be replaced easily by two people and without expensive cranes.

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