The extension of the San Mames football stadium roof in Bilbao is a triumph of structural engineering in several different ways. Arising from a need to improve spectator comfort on rainy days, a new lightweight, translucent 4,700m2 oculus of ETFE-clad roofing has been delicately poised within the existing stadium roof opening. The new structure extends the cantilevered distance of the original roof by between 13m and 23m, resulting in maximum projections of up to 75m.
The structure comprises an oval prismatic truss compression ring of structural steel linked by radial cables to a two-layer internal cable tension ring. The total weight of the roof extension structure, including the compression ring, purlins and reinforcement of the existing cantilevered roof, is around 500 tonnes, while the cable system and connections add a further 180 tonnes.
The entire roof extension structure was assembled, lifted into position and sequentially tensioned in a period of just two months during the stadium’s summer closure period.
The engineering design relied heavily upon the ability to accurately predict the complex non-linear interaction between the existing cantilevered roof and the new cable net. This required the development of a 3D BIM model and the non-linear computer analysis of the new and existing structures, supplemented by wind tunnel testing, CFD modelling and wind-driven rain simulation, and the use of a 1:30 scale operating mock-up of the roof to simulate and assess the complex processes of lifting and tensioning the new structure without overstressing the existing roof.