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The Shell is the Core
Foster's design encases its display of veteran USAF warplanes under a vast curving concrete canopy, 90m wide at the great glazed mouth which opens on to the airfield, 18.5m high and 100m deep.
The main pedestrian entrance, set at the back of the building, opens on to the exhibition space's mezzanine level, level with the cockpit of the centrepiece of the museum's collection, a Boeing B52 bomber. A perimeter ramp set below the line of the glazing slot, takes visitors down to ground level.
If the distinctive form of Foster's museum look familiar, it is because the first drawings, showing a concave structure with a roof curving in two directions, were produced in1986. The design then, as now, was for a great wall of glazing, displaying the contents of the hangar behind. Since then, the Bilbao Metro and his scheme for the Canary Wharf station on the Jubilee line extension have extended the principles, but Duxford remains the prototype and, with the latest revisions, the most advanced example of this new building form.
In fact, the torus (ring-shaped) geometry on which the hangar is based was generated by the peculiar and menacing dimensions of a Boeing B52 bomber, which has a low profile, 61m wing span and a 16m high tail fin. Around it, a series of other US military planes will be hung from the roof structure, arranged by 'theatres' of conflict.
To provide a 90m spanning structure capable of supporting the exhibits as well as live loads of snow, wind and rain, Foster has borrowed from the structural principles of aircraft design, using a double skin of concrete panels stitched together by struts.
The geometry is rationalised as a section from a torus 278m in radius, with an inner radius of 64m. the wide front end of the structure acts as a series of large spanning arches, becoming progressively more like a dome towards the rer. Fixing the front of the structure allows the structure to act as a shell, with loads at the front taken down to ground at either side and at the rear.
The initial proposal – a lightweight steel frame with an exposed internal structure and rooflights set between the arches – was not shelved until late in the design development. Despite the capital cost savings of a steel frame, there were drawbacks; a steel structure of this nature would have had huge thermal expansion problems and a lower thermal mass, compared with the higher density of concrete. The Gulf war released an unexpected 'source of income' – a £1m donation from Saudi Arabia – along with an A-10 Tankbuster and an F-111 fighter in return for a display on Desert Storm.
The roof design uses a series of precast, reinforced concrete elements. The torus geometry allows for an identical curvature in each element, resulting in the use of just five separate moulds for the precast roof structure elements.
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