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Wave of the future

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MetalWorks Major Structures - An extension to Frankfurt's exhibition hall showcases an innovative folded roof structure

When Paxton's Crystal Palace was opened in 1851, not only was it the first exhibition hall ever built, its prefabricated and innovative structure of steel and glass launched a new movement in architecture. The imperatives of an exhibition hall have since been an inspiration to architects. Nicholas Grimshaw & Partners' new exhibition hall in Frankfurt is the latest response to this innovative tradition.

The Frankfurt trade fair centre - known as the Messe - is the third largest in the world. It is on an inner-city site that had already been developed when an opportunity came up in 1998 - an adjacent goods station became available, offering 16ha for expansion. The main halls are grouped around a large urban square, the Agora. To make the most of the potential for expansion, a new hall, four times larger, was proposed to take the place of the existing one on the Agora's south side.A competition was held and was won by Nicholas Grimshaw & Partners.

The brief was for a two-storey columnfree space of 220 x 120m with a gross exhibition area on upper and lower levels of about 40,000m 2, making it one of the largest halls in Europe. And it had to be built in a hurry;

there were only 18 months from the start to completion in June 2001, when it would be needed for the International Motor Show.

Nicholas Grimshaw & Partners has had long experience of large-span structures.

The roofs of the Oxford ice rink (1984) and the Stockbridge leisure centre (1988) are suspended from tall masts; the international rail terminal at Waterloo has a curved steel roof structure covered with frameless overlapping glass panels; and the Eden Project consists of a series of geodesic domes clad with inflated pillows of ETFE fabric.

The key element of the new hall is its roof.

It is an arch, but of a magnificent and innovative form: a folded plate structure that soars over the main space in a series of huge doublecurved waves. The waves comprise five tubular gridshells, which form the upper, concave portions of the roof, and six tensile nets of tubular steel, which form the lower convex portions. The waved shape spans 165m along the length rather than the width of the building, presenting an uninterrupted fully glazed gable wall to the Agora. Setting the longitudinal span along the main frontage facing the square means that the building can be extended to the rear by adding additional arches.

The roof is buttressed by 12 huge steel Aframes that rise between five-storey concrete 'box' structures at the short ends of the hall.

These structures contain conference and office spaces, catering facilities, secondary service equipment, and rest and information zones on all levels.

The roof is lower than a more conventional arched structure, so a smaller volume of air has to be treated. The total available area for acoustic absorption is maximised by the undulating soffit. As the structural tubes are partly covered by the inner skin of the deck, there are no upward-facing surfaces to gather dust. The arches of the roof act as smoke reservoirs. In case of fire, windows in the facade open automatically and smoke escapes through vents in the roof. As the roof is supported at the sides, it is flanked on its long sides by clerestory glazing that allows daylight to penetrate the building.

Plant and service ducts are accommodated in the intermediate floor. The floor is a 4m-high space frame that carries service trenches for ventilation and air conditioning, water, sewage, compressed air, sprinklers, high-voltage power, communications and data networks to both levels. It is deep enough to provide fire-escape tunnels and maintenance passages.

The intermediate floor accommodates a high-level access route, which serves existing halls, creating a loggia that overlooks the Agora. Now in use as part of the ensemble of exhibition buildings in Frankfurt, the hall has already realised its potential.

A folded plate roof of tubular steel gridshells

The roof structure, spanning 165m, consists of a single continuous folding grid of tubular steel members, which forms a wave-like sequence. The five upper, concave portions of the roof are tubular gridshells; the six lower convex portions are anticlastic tensile nets.

Both compression arches and tension nets are formed of steel tubes that vary in thickness to accommodate different stresses. They are formed of straight sections of tube welded in facets to achieve the curves.

The only curved element is the outer tube that defines the gable to the wall facing the Agora. In the original design, the tension nets had pivoted connections; subsequently, the whole structure was welded to achieve a fast building programme.

The gridshells and nets were assembled on site; they were divided into four segments, of which 80 per cent were prewelded on jigs on the ground before being lifted up to roof level, where the final 20 per cent of welding was carried out.

The roof deck is formed of prefabricated panels framed with 40x40mm steel angles. They consist of a liner with acoustic insulation and a profiled steel deck fixed to top-hat sections to Zshaped purlins. They are covered with a mineral-fibre insulation and have a Kalzip standing seam roof covering.

To accommodate the curve of the roof, the panels were manufactured on site with an in-built twist. In spite of its size, the repetitive profile of the roof required only 60 different panel types, reducing manufacturing costs.

The ends of the roof are supported by a series of huge Aframes of tubular steel. They lie between cast in-situ concrete 'box' structures, five storeys high, which contain ancillary accommodation; the concrete structures give lateral restraint to the A-frames.

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