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Structural engineer's account

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SADLER'S WELLS, LONDON

As the visitor walks along Rosebery Avenue and enters Sadler's Wells, there are obvious elements of structural engineering interest - the expanse of masonry facade with its projecting prow and the grand foyer staircase - but the main elements are concealed within. The steel structures forming the two circles are retained from the previous 1930s theatre, with part of the supporting masonry dating back to even earlier constructions. Architectural studies showed that the retained part of the auditorium was in the optimum position in terms of leaving space in the footprint for the foyer and back-of-house facilities. With the required improvements to the sightlines being achieved by moving and enlarging the stage, there was no need to demolish the existing circles.

Structurally the circles are as their modern-day equivalents would be, with large girders spanning side to side supporting raking radial cantilever beams. Only riveted construction, instead of bolts or welds, gives the clue to their age.

The basements had to be extended, both in depth and in footprint, to encompass the existing retained structure. As there is a high water level within the gravels overlying the London clay, this necessitated a cut- off wall around the perimeter of the site. A hard/hard secant pile wall was provided for the temporary construction condition, with a reinforced- concrete lining wall containing a hydrophilic additive as the permanent means of water exclusion.

Abutting the retained masonry walls of the auditorium are the new slender reinforced concrete walls which form the remainder of the auditorium, the proscenium and the fly tower. The fly tower is 27m in height from stage to roof, with walls 300mm thick, which gain their stability from the grid, side galleries and adjacent staircases.

Sadler's Wells is on a confined site, and the height restrictions mean that all the available volume is used. This led to the need for close integration of the structure with the major equipment, a particular example being the flying system supported directly from the flytower roof beams to maximise the proscenium height.

Desmond Mairs - Whitby Bird & Partners

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