24 WESTGATE SHOPPING CENTRE, OXFORD
designed so that the buildings either side follow an eight metre grid. This naturally led BDP to look at a covering that conformed to the same gridlines. “We started off trying to keep it as simple as possible so it was a flat, rectilinear roof with 8 m x 1.5 m wide panels,” says Downey.
They looked at using cold bent glass in order to allow the rainwater to run off either side and not just sit on the flat roof. However, as Downey explains, “there was only one specialist contractor that could do it so we were encouraged to design something that was more standard and practical.” The design therefore evolved so that the roof was vaulted slightly to create the curve.
© Gareth Gardner
attention to itself,” Downey adds. BDP designed three separate glass roofs to create the public spaces – Leiden Square, South Arcade and Middle Square. Each of these areas presented its own challenges, but arguably none more so than Leiden Square.
The curved roof that covers the square is a somewhat different design to BDP’s initial idea. Describing the approach, Hayward says: “There’s a hierarchy of elements and that really started with the building blocks themselves, and then the streets and spaces in between, then these covering elements just needed to be calm and quiet, aesthetically speaking.” This therefore led to the idea that they would use the biggest pieces of glass possible in order to reduce the number of supporting elements. BDP focus a lot of their work on glass and are always keen to push the boundaries and explore new technologies. Leiden Square, which is 25 metres wide, was
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Of course arching the roof increased its height and put it right at the maximum stipulated by ancient planning rules. This therefore left the roof with a relatively “flat” arch, which meant it “needed a tie in the form of a steel rod,” Downey says. “It’s so flat it almost wants to turn inside out so you’ve got to have that extra element.” The practicality issues surrounding such large pieces of glass also became a challenge. “In the end things like cost, how easy it would be to replace panels, and safety came into it,” Downey explains. They therefore divided each eight metre panel into three. “Within that there’s a hierarchy of structure as well – there’s the main primary beam and then the purlins which are a much smaller element,” says Downey. These box beams ended up deeper than a standard beam so were specially fabricated out of steel, as opposed to buying a standard rectangular beam and bending it. “They actually look better because a standard beam has got rounded edges whereas this is really crisp with sharp edges,” Downey explains.
The covering is raised above the top of the buildings either side by 1.5 metres. The glass is single glazed, but is strong enough for it to be walked on for cleaning and maintenance purposes.
As a general rule BDP incorporate as many safety features in their designs as possible. On Leiden Square this included handrails and walkways discreetly integrated into the gutters – everything was designed to have as little visual impact as possible.
The roof spans across and attaches two buildings, it’s actually only fixed on one side. “In terms of engineering, the buildings are moving differently so it’s fixed on one side and slides on the other,”
ADF MAY 2018
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