Inner Courtyard, Vienna City Hall, Austria 

The inner courtyard of Vienna City Hall is frequently the venue for festive events. In order that these can take place regardless of the weather in the future, a folding roof has been built over the courtyard. With the roof retracted, visitors can still enjoy a clear view of the neo-Gothic façades, and once extended it protects part of the courtyard against sun and rain. The architect Silja Tillner, working in close cooperation with seele cover and consulting engineers Schlaich Bergermann & Partner, designed a folding membrane roof made from a translucent fabric. The loadbearing construction consists of 32.5 m long perimeter beams made from welded box sections. These transfer the vertical loads of the roof to steel brackets attached to the stone external walls of the building. The box sections serve as tracks for the trolleys carrying the ridge cables of the membrane roof and they also support the four fish-belly steel lattice girders that accommodate the horizontal forces of the structure, a solution that prevents large horizontal forces being transferred to the existing stone masonry.

Two of the four lattice girders are located at the ends of the membrane roof, the other two are space equally between these. The stainless steel ridge cables are positioned between and parallel to the girders at a spacing of 3.58 m. They are housed in membrane pockets beneath the roof surface of PVC-coated polyester fabric. There are also membrane pockets in the valleys in order to stabilise the roof, to guarantee accurate folding during opening and closing, and also to ensure adequate pretensioning. Those pockets are secured by internal steel cables which continue down to the ground, via membrane tubes, at both ends where they are connected to stainless steel chains to secure the roof against wind uplift. The roof is also drained via these membrane tubes. The high and low points of the roof match the neo-Gothic arches to the arcades of the City Hall and provide an appropriate enclosure for this prestigious courtyard. At night in particular, when the lamps beneath the valleys are switched on and the roof becomes a floating body of light, the inner courtyard radiates a festive atmosphere.