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Tensile struture for doubly curved fabric structures are PTFE-coated fiberglass and PVC-coated polyester. These are woven materials with different strengths in different directions. The warp fibers (those fibers which are originally straight—equivalent to the starting fibers on a loom) can carry greater load than the weft or fill fibers, which are woven between the warp fibers.Tensile structures make use of ETFE film, either as single layer or in cushion form (which can be inflated, to provide good insulation properties or for aesthetic effect—as on the Allianz Arena in Munich). ETFE cushions can also be etched with patterns in order to let different levels of light through when inflated to different levels.In daylight, fabric structure translucency offers soft diffused naturally lit spaces, while at night, artificial lighting can be used to create an ambient exterior luminescence They are most often supported by a structural frame as they cannot derive their strength from double curvature.Cables can be of mild steel, high strength steel, stainless steel, polyester or aramid fibres.Tensile structural are made of a series of small strands twisted or bound together to form a much larger.Steel are either spiral strand, where circular rods are twisted together and "glued" using a polymer, or locked coil strand, where individual interlocking strands form the structure.Spiral strand is slightly weaker than locked coil strand. Steel spiral strand cables have a Young's modulus,citation needed] Spiral strand suffers from construction stretch, where the strands compact when the cable is loaded. This is normally removed by pre-stretching the cable and cycling the load up and down to 45% of the ultimate tensile load.The behaviour of tensile structures which depend upon prestress to attain their strength is non-linear, so anything other than a very simple cable has,been very difficult to design. The most common way to design doubly curved fabric structures was to construct scale models of the final buildings in order to understand their behaviour and to conduct form-finding exercises.
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