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working details

Bridge over shallow pool

The entrance lobby on the north side of the new office building takes the form of a glazed rotunda. Its curved walls are flanked by a shallow (450mm-deep) pool. The pool contributes to the natural ventilation system by cooling the incoming air when the doors are open. It is lined with cast in-situ waterproof concrete.

The lobby is reached by a bridge which spans 9.7m in a gentle curve over the pool. The bridge is designed to emphasise the entrance and, in the architect's words, 'create the act of entering the building into an event'.

A pair of curved 125mm-diameter chs strings, one at each side, form the bridge structure. They are supported at the entrance end of the pool by four concrete base posts which rise from the pool base to 200mm above the water line. At the other end the strings rest on the concrete walls of the pool.

At mid-point, four tapered universal beams extend diagonally from a 219mm-diameter chs stub column to the edges of the bridge to act as supporting arms. Decreasing perforations lighten their appearance. The CHS stub is capped top and bottom and bolted to a concrete base post. The beams support 200mm-diameter heavy-duty 'bosses', formed from a series of chs sections, to complement the tubular shape of the strings. Lugs welded to the tops of the bosses make a pin-jointed connection to the strings.

Square-edged 125 x 50mm oak boards are used for the bridge deck. They are fixed to ms flats welded to the strings, with 10mm gaps between them. To provide a non-slip surface, strips of 3mm chequer-plated aluminium were glued to grooves routed into the surface of the boards.

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