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Raising the Roof
Abutments were built first, starting on site in October 1995. Attached to these were steel arms at 4m centres which bridge to the shell edge beam, so creating a daylight slot between.
O'Rourke won the structural package, using precast units from Malling Precast. It erected some 800 tonne of metal staging on its own foundations to support the precast concrete components. Each of the T-beams has two threaded sockets for hanging exhibits from the roof (supporting 13.5 tonne in any direction) and these were used to connect panels to the staging to hold them in position during construction. During design, construction inaccuracy was simulated at up to 100mm out; actual setting out was more accurate. Construction rate was up to eight T-beams or 40 upper panels per day.
Once the shell was complete, the staging was lowered. There were screw-jack connections to the panels and these were lowered 5mm over all the 600 support points, and this cycle repeated for 20 passes. The shell settles about 50mm at the apex of the arched opening as predicted. There is up to 50mm expected from shrinkage, creep and changing loads. Thermal and wind effects may produce up to ±20mm of movement. While the steel arms at the daylight slot are fixed down the sides of the shell, the four at the end are on sliding bearings.
There was significant making good to the fair face of the concrete shell, due to damage both during transport and erection and from weather staining before the joints were sealed. The surface is now good enough to be floodlit. It also provides reasonable acoustic. The roof was completed in September 1996.
Planes were moved in and the glazed wall assembled. The mullions, cut from 25mm steel plate, were pinned at the base and hinged up; they can be hinged down again when exhibits are to be changed. The transoms are loose-fit to cope with construction tolerances. The glass is 19mm single glazing.
The structure does work as a shell, though at the front it is much like an arch, and near the entrance like a beam. Once you know that it is a double-later shell, it form expresses much about how it works. This is a structural clarity it shares with the exhibits.
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