Roofing

The Broadmead Centre in Bristol was a splendid example of post-war regeneration:a bleak collection of utilitarian concrete blocks bounded by arterial roads and a subterranean pedestrian roundabout cleverly designed to make street robbery quick and convenient.

So it was no surprise that when the Cribbs Causeway mall opened to the west of the city, much of its population went there to shop, eat and go tenpin bowling. As those who remained tended to head to the harbourside area around the Arnolfini of an evening, Broadmead became an unappealing area to do business.

The 5000 m² roof will span the shops and a new square, giving Broadmead a vibrant new identity

Now the area is to be given a chance to regain some of the status that its central location ought to bestow. The aim is to spend £500m to regenerate 95,000 m2 of property over 11.8 ha, to create a gateway into the city from the M32 (the spur that connects the city centre to the M4) and to create architecture that will make Broadmead a good neighbour to the beautifully preserved medieval city centre to its west.

Simon Scott, project architect at masterplanner Chapman Taylor, promises a clean break with the past. "It will be a completely different experience," he says.

The practice has drawn up a masterplan that includes 15 large stores, 100 smaller shops, a Multiplex cinema and more than 2600 parking spaces. Each of the buildings that will house these shops is being designed by a different architect, including Alec French Partnership, Wilkinson Eyre, Stanton Williams and Quakers Friars. When it opens in 2008, Broadmead might just be able to compete with Cribbs Causeway.

However the developer and masterplanner wanted to create more than a well-designed retail-led mixed-use scheme. They wanted to add something that would unite the buildings, announce the change in Broadmead's fortunes and unsettle their rivals at Cribbs Causeway. The solution: a spectacular 5000 m2 glass roof that billows over three new streets and a central square. This will give Broadmead a civic personality and will be a way of getting one over on Cribbs Causeway. The rival mall installed a comparatively ordinary glass roof in 1998.

Chapman Taylor wanted to move away from the artificially enclosed form that many glass roofs take. "People will be cool in winter and warm in summer," says Scott. It also wanted to produce a striking roofscape. "The roof had to be functional as a cover and create a strong design environment; it had to allow the continuity of the buildings on the streets and be one of the most striking features of the development," he adds.

Chapman Taylor worked with British artist Nayan Culkarni and Geman engineer Schlaich Bergermann und Partners to design a gridshell roof that is thought to be the first of its size in Europe. This kind of structure has been built in the UK before, most memorably at Foster and Partners' Great Court at the British Library in 2000, but that was small-scale by comparison.

The roof will be formed from interlocking shells and supported by the buildings

The roof, for which a subcontractor is about to be appointed, is made of a series of interlocking shells. It will be supported by the buildings and span between them on either side of the streets. The largest of the 10 individual glass sections will be for the central space. The gridshell roof enables the form to cover a wide area without columns, but it has curve to create the necessary stiffness in the structure. The height of the roof therefore varies from 20 to 30 m depending on whether it is attached to a three or four-storey building. The collateral benefit of this was that it allowed Culkarni to design-in a great deal of variation. This is just as well, as shoppers will have the opportunity to inspect the work at close quarters when they stand on high-level crossings between the buildings.

Cribbs Causeway: you have been warned.