Vienna Concertina: An ingenious movable canopy over the main court of Vienna Town Hall enriches the place by allowing many more varied social occasions.

Vienna Town Hall is a splendid neo-Gothic affair with a large internal courtyard that is sometimes used for public events like concerts, theatrical performances and balls. On such occasions, the outdoor room has to be protected from the elements, and until recently a large marquee was temporarily erected for the purpose. There was a good deal wrong with this arrangement: the tent was clumsy to erect; its masts disturbed the space it enclosed; its rainwater was difficult to dispose of; and it had no relation to the surrounding building.

So Silja Tillner has produced a permanent fabric covering for the court that can be extended to cover a large part of it, or retracted to leave the whole space open to the sky. The folded fabric echoes the arches of the nineteenth-century arcade that surrounds the court, and movable drainpipes at each end of the valleys convey rainwater to drains at ground level. Acoustic performance is improved by the folded shape of the membrane.

The structure is made up of static and moving elements. Edge beams are mounted on the walls of each side of the court by steel brackets; the beams in turn support tracks along which the roof moves. The main structural elements of the moving part are four triangulated inverted bowstring trusses. Movement is achieved by using motors attached to each end of the front truss which either pulls or pushes the other elements into place.

Ridges in the fabric are carried on cables which are attached to movable fixings that bear onto the tracks, and are dragged into place by the front moving truss. Valleys are secured and tensioned at each side by chains which go inside the thin white downpipes to fixings over drainage holes in the paving. These tensile and drainage links obviously have to be attached or freed every time the roof is moved.

The trusses serve to resist horizontal tensile forces on the nineteenth-century masonry walls that would otherwise be exerted by the weight of the canopy tending to drag the tracks towards the middle of the court. While the old building can deal more than adequately with the new vertical loads, the horizontal ones would have destabilized it.

The jury was very impressed by the simplicity and grace of the basic idea, and by the thoughtful way in which the new work relates to the old. We made allowances for the fact that the photographs we saw (some of which are reproduced here) are of the canopy under construction: normally, there would be no vehicles in the court.