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A lightweight timber grid shell, complex to engineer but simple to erect, will provide more space for the Weald and Downland Museum Shell suits museum

technical

A large timber barn might be the obvious choice for a workshop and store at the Weald and Downland Open Air Museum of historic buildings in Sussex. Most of the buildings the museum has rescued from destruction and re-erected in its grounds have been of traditional oak frame. However, the design team of Edward Cullinan Architects and Buro Happold was given a free hand and could have proposed something in concrete. In fact, both practices have a strong interest in timber for environmental and structural reasons. Several Happold staff, then at Arup, worked with Frei Otto on the grid shell at Mannheim Garden Festival in the 1970s, the last major timber grid shell built. More recently, Cullinan and Happold produced a workshop building for John Makepeace's school for craftsmen in wood at Beaminster, where the vault framing was roundwood thinnings bent to form arches.

The new workshop for the museum is still at the design development stage, awaiting a decision from the Heritage Lottery Fund, hopefully in October. The workshop is unlikely to join the ranks of the great unbuilt: it was approved in principle in February 1997 and was the first building to be awarded a development grant - some £70,000.

Positioned on one of the terraces of the sloping car park, the base of the building will be a simple concrete box set into the earth to help provide climate stability. Only the parts projecting above ground will be insulated. Clay or concrete drainage pipes will be used to draw ventilation air through the earth to temper this. The box will provide archive storage for museum artefacts, that are not too sensitive to changing temperatures and humidity levels, so long as these changes are gradual. There will be underfloor heating and some air treatment for more extreme conditions such as very hot weather or the arrival of a busload of wet visitors on a rainy day.

The intermediate floor, which forms a roof over the archive store, will be insulated and waterproofed. Structurally, it will act as a diaphragm. Principal glulam beams at 2m centres, around 750mm deep, will cantilever over the edge of the box to follow the organic shape of the grid shell above, with 75mm softwood secondary members and a hardwood working surface on top. The workshop will be used for restoring timber frames, the members of which can weigh over a tonne. Visitors will also be able to see restoration work in progress, one of the most interesting parts of the museum.

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