This roof was a landmark in the transition from defi ned geometries to freeform shapes fabricated by automated industrial methods. The geometry is 'dome shaped' but is particularly optimized to fi t the constraints of the existing museum buildings from which it derives its support. The annular geometry fits a rectangular courtyard with a circular reading room that is not exactly in the centre. The surface geometry was generated mathematically using complex toroids to fit the boundary. The surface was discretized to triangles which were distributed over the surface using an algorithm that optimized the lengths of the sides of the triangles. The boundary is a ring beam that rests on the existing stone wall of the surrounding building but can only deliver vertical loads to it. Some arching / shell action occurs at the corners where the ring beam generated tie forces. In the centres of the sides there is little arching action and most of the load is taken in bending of the members. The individual rectangular members were fabricated so the depth could be varied in response to the local bending moments. Node and member geometries and lengths are non - regular, but a careful design of the nodes allowed accommodation of the varying geometry without varying design principles. The star shaped nodes were cut from plate up to 200 mm thick and the ends of the members were fabricated to fi t into the recesses of the nodes. The roof was assembled by welding up sections that could be lifted on to the scaffolding and then completed by welding on site.
Source: Liddell, Ian and McCormick, Fergus (2012) Special steel structures in Steel Designers' Manual: The Steel Construction Institute, Seventh Edition (eds B. Davison and G. W. Owens), Blackwell Publishing, Oxford, UK