Lightweight structures, minimal enclosures.

Buro Happold's reputation was largely built up from its experience with lightweight fabric structures, in which new forms of computation were quickly developed. The expertise and innovation continues today.

Enclosure of large spaces and the maximization of structural efficiency are concepts naturally tantalizing to engineers and architects, demonstrating an aesthetic of economy of means. Nets and fabric are the lightest materials available yet Under tension -- which is the most efficient way of exploiting their strength -- they can satisfy demands for covering very large spaces.

Buro Happold's expertise in studying and explaining the behaviour of lightweight structures, and in developing current theory, has its beginnings in Riyadh in 1967, when Ted Happold met Frei Otto, inaugurating a lifelong friendship. Michael Dickson, Ted Happold and Ian Liddell had been working on tensioned fabric and cable technology and on the Mannheim gridshell while at Ove Arup & Partners. When Buro Happold was set up, they collaborated with Otto on the King Abdul Aziz University sports hall in Riyadh, Saudi Arabia. This lightweight pre-stressed cable net support structure, based on the minimal form of a soap bubble, covers 9500[m.sup.2] and is reminiscent of a huge Bedouin tent.

At that time, conventional structural calculations had difficulty in coping with the complexity of curved surfaces; forms and cloth patterns were generated by measuring physical models and photogrammetrical resolution. Buro Happold set itself a target -- to develop a computer program to do this. At the same time, 1978, a research department set up by Happold at the University of Bath, began to investigate properties and structural behaviour of lightweight and air-supported structures. The research, together with experience in applying ideas to actual buildings, led to TENSYL, a computer program which analyzed the geometry of lightweight structures, developed load analysis and produced cutting patterns. TENSYLs user-friendly graphics gave opportunities for interactive design -- architects and other designers could become involved in development of the forms. The first project to use TENSYL was the Baltimore Concert Pavilion, US (1980), a three-bay membrane structure supported by six masts which provided an open-air enclosure to a 3400 seat concert space on the harbour front. This was also the first collaboration with the New York practice, Future Tents Limited (FTL); the two practices subsequently combined their expertise in 1992 to form FTL Happold. Since then, the practice has worked on many structures worldwide including the demountable tapering barrel-shaped AT & T pavilion for the 1997 Olympic Games in Atlanta, and an aviary for Bronx Zoo in New York const ructed of a mesh of woven wire rope suspended from tubular steel arches. Essentially these projects are developments of anticlastic 'saddle' surfaces.

Three very large double-curved lightweight structures -- Skyline Pavilions -- at Minehead, Bognor Regis and Skegness Butlins' holiday camps in England, form covered public atria for performances and events. The largest, at Bognor Regis, covers 8000[m.sup.2]. Each structure consists of a polymer-coated polyester fabric supported by 45m high lattice-truss masts and held down at the perimeter by cable-stayed struts. Masts project above the fabric, giving additional uplift to a portion of roof immediately above a stage. An inner fabric lining prevents condensation and allows a warm air heating system to be used to achieve a naturally translucent ventilated environment.

The 1975 research into air supported structures led Buro Happold in several new directions. One was a feasibility study for a township in the Canadian Arctic (58 degrees N) which would be covered with a 35-acre air-supported roof of ETFE foil cushions to support the social requirements of permanent human occupation in a natural village environment (p21). The city did not materialize, but the foil cushion technology using aluminium extrusions was developed to roof the Chelsea and Westminster Hospital, London (architect Sheppard Robson), and the atrium of Michael Hopkins & Partners' Schlumberger headquarters in Cambridge, England.

Until recently a double-curved surface was the accepted form of tension structure: fabric is stiffened to resist down- and uploads by being tensioned in two directions, but the opposing direction of forces reduces its efficiency and creates greater geometrical volume. Ian Liddell and his team observed that marquee tents enclosed large volumes simply by using straight cables, tensioned to resist down-and uploads, with resultant forces working. in the same direction. Pretension stiffens cables against deflection, and fabric or foil, also pre-tensioned, can be used between the cables to create very large spans. This straight-cable concept was first used in 1994 in the roof of the Hampshire Tennis and Health Centre at Eastleigh, England. Pre-tensioned cables are supported by rows of external masts, with translucent ETFE cushions being supported from the cables. Overall stability is provided by external ties and ground anchors. The roof is very simple; the cables are straight, allowing the cushions to slide into grooved aluminium extrusions, achieving a wide span with a superb quality of light inside.