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STRUCTURE

The apparently complex form of the building is based on hyperbolic paraboloid forms defined by two sets of intersecting straight lines that may be laid across the warped surface. It is this that makes for a consistent and economic structural system for design and fabrication of the main steelwork and roofing sheeting.

The steel superstructure consists of a 25 x 135m braced-frame spine broken in the middle by a 75m-wide opening. Roof slopes vary from 20infinity with spans of 57m at the gables to the vertical at the spine. There are many other variable effects, but the most significant and difficult to assess is wind pressure. Anemos was commissioned to undertake a wind-tunnel test, which led to a refinement of the structure that more than offset the cost of the test.

Some 36 design-load cases were considered in the frame analysis: dead and live loads, snow, 20 different wind cases, a 40infinityC thermal range, the suspended internal bridge, restraining forces to the roof trusses, tensioning the gable-end fabric and suspension of aircraft.

The warped shape benefits the structural behaviour, enhancing stiffness by about 20 per cent, and allows a shallower depth and saving in material. The roof surface structure consists of deep-profiled steel sheets spanning between the rafters. These sheets support a layer of insulation and the standing-seam roof cladding. Most of the interior steelwork is unprotected - the environment designed to provide optimum conditions to conserve the exhibits also ensures a long maintenance-free life for the structure. The gable ends are removable to allow aircraft to be taken in and out - only under reasonable weather conditions. The PVC fabric is stretched against a series of triangulated posts designed by Base Structures that span from ground up to the gable rafters. The steelwork was fabricated by SH Structures in their North Yorkshire works, where the components were trial assembled. Having first erected the spine walls and the perimeter elbow pieces, the rafters, divided into up to three pieces, were lifted and supported in place by three mobile cranes, while erectors in cherry pickers completed the bolted flange-plate connections. The site is underlain by Bromsgrove Sandstone and foundations are spread concrete footings. The roof drains into perimeter trenches.

Michael Eatherley, Michael Barclay Partnership

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