Design and construction of long-span post-tensioned tubular steel structures

The design of long-span post-tensioned tubular steel structures in which steel cables are housed within the hollow profiles is outlined from fundamental principles. Treatment of the structural form as a planar catenary is discussed and then extended to three dimensional shell structures. The effect of post-tensioning on the behaviour of individual members is examined through numerical and analytical modelling. The influence of the bonding material between the post-tensioned cable and tubular casing is discussed, since this is crucial to the performance of these structures. The construction methodology is outlined and examples of implemented structures spanning up to120 m are presented. line shown in Figure 1(d). Since the horizontal reaction force can take any value, an infinite number of thrust lines can be generated for the applied loading (Heyman, 1982); the larger the horizontal reaction, the shallower the arch. Since an arch provides support to the applied loads through compression, masonry and concrete were the original materials of choice for the construction of arches. However, owing to the relative ease and reduced construction time associated with steel structures, growing interest is now placed on the use of steel in the construction of arches (Nazir, 2003). Thrust line analysis is a powerful tool that can be used to design a wide range of long-spanning steel structures. Figure 2 illustrates how thrust line analysis may be applied to the design of a portal frame for an aircraft hangar. The distributed uplift wind load in Figure 2 is applied to a weightless chain of a chosen length; using the funicular polygon method discussed earlier, the shape and tension within the chain is determined. The length of the chain is typically chosen in order to ensure that the bottom chord of the frame is always under tension during wind uplift. Inverting the resultant hanging chain gives the thrust line. It should be noted that consideration of other load cases would result in the formation of different thrust lines, from which an average thrust line can be chosen for design. If the structure were to be built along the thrust line, it would not be subjected to bending and thus a minimum amount of material would be required to support the design loads. However, structures designed in this way, while minimising the required material, may not provide the desired space. Instead, structures, such as the hangar in Figure 2, are built to satisfy the spatial requirements by designing against the bending moments that would arise due to the lever arm between the structure and the thrust line. F1 F2 F3 H H