Blow-up beams carry the load

Tom Shelley reports on an ingenious structure that depends on pneumatics

Very lightweight beams can be set up quickly by inflating sausage-shaped balloons that support a compressive element – with two cables wound round the combination in a helix to provide tension and stiffen the structure. This makes it possible to transport the basic elements needed to support a bridge capable of being traversed by an SUV – in a box that could be carried in the back of the same vehicle. The basic idea is called the ‘Tensairity’ air beam and it is being developed by the Center for Synergetic Structures – a public-private partnership between The Swiss Federal Laboratory for Materials Testing and Research (EMPA ) in Dubendorf and Festo, under the direction of Dr Rolf Luchsinger. The principle combines the two basic concepts in structures: compression, as used in traditional buildings; and tension, as used in the guy ropes that hold up a tent. Its performance is not dissimilar to a truss girder, which also depends on a compressive element, and a cable that is spaced away from the compressive element by a series of vertical struts. The difference is that in the Tensairity, a bag full of air under slight pressure replaces the vertical struts. The sausage-shaped bag acts as a continuous support for the compressive element to prevent it buckling, while the tensioned cables prevent it bending. A miniature example was on show on the Biomimetics stand at this year’s Hannover Fair. Seeing that it weighed almost nothing, various visitors tried to collapse it by leaning on its centre, only to find it resisted all attempts. As a larger-scale demonstration, the developers built a small car bridge with a span of 8m, able to carry 3.5 tonnes. Two Tensairity beams, each 0.5m in diameter, provided the supporting structure of the bridge. The membrane of the beam was PVC-coated polyester fabric, while cables were 6mm diameter steel. Due to the moving load of the car, there was an extra set of cables wound twice round the beam to stiffen the structure in the first and last quarter of the bridge. The compression element was made of carbon sandwich, but the developers say that aluminium or steel would have worked equally well. Wooden plates for the car to drive on covered the two beams. The working pressure in the tubes was 400mbar and each beam weighed 98kg. The developers say the weight of a steel girder with the same load carrying capacity would have been 320kg. The bridge was modelled using Ansys 7.1. The compression elements can either be continuous lengths or made in sections fitting into each other. To assemble a structure, the membrane has to be rolled out, after which the compression elements are put together and attached to the membrane. Then the cables are positioned and attached, and air is pumped in. On tests, one of the beams for the vehicle bridge withstood 35kN before the compression element failed, owing to the shear forces at the ends of the wooden beam used to distribute the force from the hydraulic piston applying the load. A number of niche applications have been found for the idea, including an advertisement pillar, a 52m skiers’ bridge and a 28m span roof for a parking garage. However, at this point, it is still essentially a solution looking for serious problems to solve. Pointers * Structures can be rolled up/broken down for easy transport and then quickly deployed * Weight is less than one third that of a traditional steel girder supporting the same load