Utzon thought this ‘an elegant solution to a construction' which would otherwise have led to a lot of scaffolding and shuttering. The roof shells had to span large areas to accommodate the main hall and a smaller hall. The solution devised by Utzon and Ove Arup and Partners, the engineers, was to produce arched segments of varying curvature. The concrete shells were produced by cutting a three-sided segment out of a sphere. The roof shells and their vaulted concrete ribs were pre-cast and held together by pre-stressing steel tendons, an innovation at the time.