Foster's bomb shell

Fitness for purpose *****
The success of the museum lies in the closeness with which visitors are brought up to the huge aircraft exhibits, which they can view from above and below. Ancillary spaces are neatly tucked out of the way below a perimeter earth bank.

Comfort **
Although the pre-cast concrete shell functions as a heat sink, the building with its south-east-facing window wall overheats in summer and gets cold in winter. The aircraft are brilliantly lit by daylight and by spotlights in the evening. Acoustics are poor.

Delight *****
In one of the most dramatic entrance sequences of all time, visitors pass through a narrow, unpromising passageway to emerge at the apex of vast light-filled hall and nose-to-nose with the giant B52 bomber.

Maintainability ****
The unadorned concrete shell of the building is nearly maintenance-free. But the surrounding grass mound is starting to disintegrate and is difficult to mow, and the building's attractiveness relies on intensive cleaning.

The American Air Museum was completed in January 1997 as part of the Imperial War Museum Duxford on a former military airfield near Cambridge. Designed by Foster and Partners, the building scooped both top architectural awards for 1998, the RIBA's Stirling Prize and the Royal Fine Art Commission building of the year award, along with three other awards. It boasts one of the largest pre-cast concrete vaults in Europe, spanning 90 m. The building was constructed for £8.4m, or £1300/m2, by John Sisk & Sons, with Arup as structural engineer, Roger Preston & Partners as services engineer and Davis Langdon & Everest as quantity surveyor.

Open display of aircraft
"The beauty of the building is that visitors can get right up to the planes," says Geoff Webb, contract manager at Jarvis Workspace FM, the museum's facilities management contractor. Twenty-one American warplanes of the Imperial War Museum's collection have been brought close together inside a large single-span hangar, with most of the aircraft standing on the floor slab and a few suspended overhead in dynamic flying poses by cables attached to the roof vault.

Visitors enter at the apex of the horseshoe-shaped building at mezzanine level, where they are brought nose-to-nose with the centrepiece of the collection, the vast B52 Stratofortress bomber. From there, the entire collection of aircraft is spread out in front of them. "You get the overall picture straight away because you can see everything all around you," comments visitor Philip Weatheragg. Open ramps curve round on either side down to the floor of the hangar, where visitors can wander around the planes and view them at closer quarters from below. Steve, a pupil from St Nicholas Special School in Southend, comments:

"I like getting up close to the planes. They're so small in the sky and so big on the earth."

The open-plan display and open ramps also make for easy supervision of visitors and security. "Usually we keep the children on a short leash, but in this building we can just let them loose," says teacher Andrew Clark, guiding a class from Colchester Royal Grammar School.

Ancillary spaces tucked below
The torus geometry of the vaulted building, which slopes down from the top of the window wall to a curving earth bank, enables all the ancillary spaces to be tucked neatly below the bank, out of the way of the exhibits.

The cafe and shop fill recesses on the mezzanine level on either side of the main entrance, where they open off the main visitor circulation route and command views over the main hall. The shop is closed off in the evening by drawing a set of glazed concertina doors.

Toilets, plant and storage are housed in spacious rooms on the ground floor beneath the earth bank.

Poky exhibition room
The only secondary accommodation that loses out is the display of airmen's memorabilia. It occupies a windowless alcove directly below the main entrance, where it is easily overlooked, gloomy and uninviting. The Imperial War Museum has considered extending the museum area back beneath the entrance tunnel but the project is currently on hold.

Confusing main entrance
Visitors gravitate towards the prominent window wall of the building. But to gain access, they must then walk round to what appears to be the back of the building.

Window wall opens up
For the display aircraft, there is only one way in and out of the museum building – through the window wall. After carefully removing the large panes of glass held in by neoprene gaskets, the entire wall can be folded outwards on hinges at the base of the steel mullions to lie flat on the external paving.

Ramps too long
Visitors in wheelchairs complain that the two ramps that curve down from the main entrance to the floor of the hall are too long and continuous. A horizontal landing would have given them a breather. There are plans to lower the handrails to suit disabled visitors.

Wow factor
"Everyone seems to gravitate towards this building, because it's so spectacular," says visitor Andrew Clark. The breathtaking nature of the huge light-filled vault is accentuated by one of the most dramatic entrance sequences of all time. Visitors enter through an unpromising concrete-lined cutting in the grassy bank. But just a few steps beyond the doors, they find themselves at the apex of the huge hall, with the entire space stretching out before them filled with aircraft above and below.

It is little wonder that groups of schoolchildren stand transfixed inside the main entrance. "From out there, the building looks quite small," says John, a pupil from St Nicholas Special School, Southend. "In here, it looks so open and big."

Simple building for a simple purpose
The building combines a basic form, a subtle torus geometry and an industrial character that perfectly matches the planes it houses. The concrete shell and ring beams are fair-faced and mostly unpainted, and the clerestory windows are set in utilitarian pot-riveted aluminium frames. Finer materials are used only where visitors come into contact with them, such as the stainless steel handrails on the ramp.

Boiling in summer; freezing in winter
"It gets stiflingly hot in summer and incredibly cold in winter," says Maggie, the shop assistant. The main hall has no heating or cooling installation, as the designers relied on the pre-cast concrete roof panels, concrete ground floor and the earth bank to act as heat sinks. In practice, these passive environmental controls are ineffective. In summer, temperatures can rise to 45°C; in winter, they can drop to 8°C. A contract has now been signed to introduce artificial cooling into the building. But the passive environmental controls would be more effective if shading had been provided to the window wall, or the building had a less southerly orientation. The cafe and shop are supplied with comfort cooling through intake grilles in the ceiling. But since these spaces open to the main hall, the effect is limited.

Good humidity control
Humidity control in the main hall protects the aircraft from condensation and corrosion. There have been no complaints.

High-quality lighting
The aircraft are all brilliantly lit by daylight streaming through the window wall, while the clerestory windows around the base of the concrete vault counteract glare and illuminate the ramps in the evening. Dramatic artificial lighting is provided by spotlights fitted directly below the clerestory windows.

Blurred acoustics
"The acoustics are terrible," says shop assistant Maggie. "When there are announcements during the air shows or when someone has got lost, you can't hear a thing." The loudspeakers are in metal bollards but the concrete ceiling blurs sound.

Low-maintenance building
"Of the 100-odd buildings on the site, this one is classed as one of the lowest in maintenance," says facilities contract manager Geoff Webb. "There have been the odd plumbing and electrical problems but nothing serious."

The concrete shell and ring beams and the glazing have so far been nearly maintenance-free, with no water leakage from either above or below, Webb claims. Slight cracking in the pre-cast concrete panels lining the entrance cutting has been caused by stress movement, and one of these shows unsightly deposits of salts leaching through. "It's not getting any worse, so I don't think it's critical," he comments.

Grassy banks an uphill task to mow
The grassy bank around the base of the building is suffering from landslips at its steepest points near the corners next to the window wall. Added to that, mowing the grass is a nightmare, according to Richard Munro, the museum's head of support services. The operative has to wear spiked shoes and a special harness attached to a purpose-made trolley that runs behind the concrete ring beam.

High-intensity cleaning
"It takes a lot of hard cleaning to keep it looking good," says Kathy Randle, supervisor of one of the museum's three integrated teams of cleaners and supervisors. "There's a lot of glass and stainless steel in the balustrading that has to be polished daily. The glass needs to be washed regularly to remove the grease from people's fingermarks."

Randle describes the epoxy-resin finish to the hall's concrete floor slab as a monster to clean. "We have to hand-wash it with a mop, which takes three of us all day once a fortnight. "