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Introducing the timber shell roof
Early in 1957 the design of a spectacular roof for a new weaving shed at the Royal Carpet Factory in Wilton was completed by Hugh Tottenham of the Timber Development Association (TDA): this was the first timber hyperbolic paraboloid (hp) shell roof to be built with a boarded membrane in Britain (Fig. I)1. In 1975 the last known timber hp roof was built for a theatre at Blackpool: coincidentally, Tottenham, by then a partner of Hume, Tottenham and Bennett, was again the structural engineer. The eighteen years between these two projects were the era of the timber hp shell roof in Britain.
A shell roof covered a building with a thin membrane that derived its strength and stiffness from its shape. Thus a 3 inch thick membrane of timber with the shape of a doubly curved hp and with stiffened edges was capable of covering a 60 ft square without any internal supports. The membrane was usually formed from layers of boards; glued laminated timber members were usually needed to stiffen the edges. Outward thrusts at the supports were normally resisted by steel ties or sometimes by reinforced concrete buttresses. Roofs of buildings were formed from either a single unit or by combining units.
The late 1950s and the following decade were the most exciting years of timber engineering in the UK since the early years of the Victorian railways; timber shell roofs, particularly hps, epitomised that excitement. One hundred and forty buildings (excluding farm buildings) are known to have been covered with timber hp shell roofs. Much theoretical work that was independent of the material in the hp was undertaken by the trade associations and by the universities on the stress distribution in the membrane; a detailed assessment of that work is outside the scope of this paper. After some general remarks on the geometry and the methods of construction employed during the era of timber hp roofs, this paper examines some aspects of the roof that have not been fully discussed in the past: particularly the search for a simple design method, the standardisation of roofs by one company, the publicising of the roof by TDA and industry, and the failure to penetrate the industrial building market...