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Design for construction

technical

The grid shell integrates several structural ideas. It is close to funicular in shape - pure arch action - the sort of form found with hanging-chain models like that used by Gaudi for his Sagrada Familia, Barcelona. However streamlined the shape is, there will be some asymmetric loading from wind and snow inducing bending in the grid members. This will be countered by the stiffness of shell action. The geometry of the shell, with its saddles and thus double curvature, helps here; as will locking the rectangular grid of timber laths with stainless-steel cables running at 45degrees to the grid. The cables come into effect when the shell is distorted by wind and snow. They are niether prestressed nor need big tie-downs.

The result will be a lightweight shell, strong enough to withstand the weather, though not other imposed loads. There can be no turf roof or crane rails.

While the shell is structurally refined, it must also be straightforward to erect, drawing on the skills of the existing museum staff. They are used to working with big pieces of building in the form of frames - at 50m long, up to 15m wide and 9m high, this shell is around three times the size of any other building on the museum's site.

Under the supervision of an engineer permanently on site, the shell will start out as a flat, square, grid on the ground. It will be pin-jointed with well over 1,000 bolts. Then it will gradually be raised to become three dimensional through bending and twisting of the laths and lozenging of the grid squares to diamonds. If the raising is performed gradually enough, only a few people will be needed. A crane should not be required.

Once propping along the centre line has created some curvature, the shell can also be pulled in from the edges and the props raised until the grid perimeter meets the edge beams. The saddles in the structure will have to be held down in order to avoid their natural tendency to become vaults. Once the grid is in place, 6mm stainless-steel cables through every third node will be locked in place. This phase of the erection should take a week or so.

The cladding inside should be dramatic. Behind the 0.5m timber grid will be a sisal-based moisture barrier with an aluminium foil inner face providing Class O spread of flame. On that will be insulation, a breather membrane and timber boarding on battens, probably oak. The boards will sweep along the shell in a series of bands following the contours of the structure. At the ridge line where the shell is practically flat, a separate strip of flat roof will be built. Between this and the boarding will be a strip of rooflights. Foil cushions were an earlier idea, but were discarded as too costly. Translucent insulation materials are now being explored; these consist of mineral fibre between plastic sheets.

At the ends of the building, timber framing will provide openings large enough for a roller shutter.

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