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Long-spans amplify the collaborative relationship between architects and engineers

By Sara Hart

Crystal palace

Wait 200 years to build another national botanic garden, then commission Foster and Partners to design the world’s largest single-span glass house as its focal point. That’ll bring the world’s attention to a country. Foster, who resurrected the British Museum with a spectacular Great Court and launched it into the 21st century [record, March 2001, page 114], has accomplished a similar feat with the Great Glass House at the National Botanic Garden of Wales in Carmarthenshire. The domed structure, 312 feet by 180 feet, will contain a Mediterranean climate for 100,000 plants.

The Great Glass House, National Botanic Garden of Wales Carmarthenshire, Wales Architect: Foster and Partners Structural engineer: Anthony Hunt Associates Date of completion: 2000 Roof span: 312 by 180 feet Consultants: Watson Steel (structural steelwork); Metallbau Fruh (glazing) The Great Glass House is a torus-shaped glass dome covering a single volume. The continuous arched structure, supported on a tilted compression-ring beam, has few dimensional repetitions. The primary arches are a composite steel profile fabricated from a circular hollow section (CHS) with a tee section welded to the top (opposite, bottom detail). The arches terminate in machined, solid, stainless-steel ball-and-socket joints (opposite, top detail).

The structure is reminiscent of Geiger’s U.S. Pavilion at Expo 70. Both roof systems are supported on a reinforced-concrete ring beam resting on an earth berm. Whereas Geiger’s roof was a vinyl-coated fiberglass fabric, Foster’s pushed the envelope with CAD tools unavailable in 1970, to create an oval, torus-shaped glass dome. Because the complex geometry of the dome (the design of which is detailed in Tech Brief, page 283) was difficult to describe in conventional working drawings, the architect and engineer used a 3D CAD model, which was distributed to the steel manufacturer and concrete and glazing contractors with a set of explanatory drawings. The house is inclined to conform to the contours of the rural landscape, and the structure is bermed so that the dome appears to emerge naturally from a hill.

Construction was not easy and required surgical precision from all the trades. Fortunately, much of the steel, precast concrete, and glazing fabrication took place in the controlled environment of a workshop. The ring beam, being poured in place, was a bigger challenge and eventually required a surveyor to locate the x, y, and z coordinates in the field for the contractor. The beam upon which the torodial, glazed roof rests is close to grade on the south side and rises 23 feet above floor level on the north. It tilts to the south to maximize daylight transmission and away from the north to resist cold winter winds.

Twenty-four tubular steel arches span up to 189 feet, giving a clear height of 49 feet over the general floor level. The arches vary in length due to the oval plan and terminate in a ball-and-socket joint at the ring beam (see detail, page 268). By using this kind of connection, the engineers avoided having to fabricate unique end plates for each arch.

The solid stainless-steel joint allows rotation, to prevent the transfer of moment stresses due to the thermal expansion of the steel members.

The glazing system is unique to this project. Trapezoidal panes of glass are bonded to an aluminum subframe, which sits over a fully drained framing system. The drains direct the rainwater, which is used to water the plants and flush the toilets, into two 18,500-gallon tanks. Natural ventilation is provided by 147 computer-controlled vents (see Tech Brief, page 283).