Travelling Membrane Roof in Vienna City Hall
This 34.21 x 32.52 m extendable roof structure provides protection against sun and rain and allows the courtyard to be used for balls, concerts and other events. The entire membrane, cable and cross-girder construction is assembled on roller bearings in tracks. The travelling roof is operated by motors attached to the cross-girders, with pinions engaging in toothed racks welded to the edge beams. Thecables and cross-girders are guided by combined radial and axial rollers, which also transmit all loads to the longitudinal beams. Atthe start of the closing operation, the end girder is set in motion. When the membrane behind it has unfolded, the next transverse girder is moved, and so on until the roof is fully extended. The opening operation follows the reverse procedure, until all the cross-girderscome to a halt and the membraneis folded into a four-metre-thick pack.
The edge beams are supported by brackets fixed on top of the stone walls along both sidesof the courtyard. “Crest cables” are spanned at 3.58 m centres between the two edge beams and sag to a depth of 1.5 m. The membrane is spanned between these cables ina single direction. Ballast was inserted in thevalleys between the crest cables in the form of 70 mm diameter steel tubes threaded on to valley cables and housed in membrane sheaths. The horizontal tensile forces from thecables are transmitted to four trussed compression girders above the surface of the membrane.
The roof is drained along the long edges. A membrane tube at the end of each valley extends down to the ground and discharges rainwater into a channel. The valley cables are also threaded through these tubes and fixed to a stainless-steel chain anchored in the channel. This form of construction serves as a means of preventing uplift.
The membrane consists of PVC-coated polyester fabric manufactured with high-strength, low-shrinkage polyester multi-filament thread. The soft PVC coating is applied asymmetrically to both faces, with the thicker layer on top as a protection against weathering. An additional protective fluoropolymer coating with a minimum thickness of 10 µm was also applied. The membrane has an overall light transmission rate of 15 per cent.