(refer to Figure 7-25). The ends of the purlins are joined to the nodes of the rafter (i.e. master-slave relationships between these nodes with the same coordinates in the structural analysis input file) in a way that they behave similarly to the fully fixed end supports shown in Figure 7-24 but with rotation about the vertical axis freed. In practice, the purlins will be bolted to steel cleats which are welded to the top flange of the steel rafter (Figure 7-26). Some degree of fixity will be provided by the bolts to resist twisting about the longitudinal axis and in-plane deflection of the purlin. An assumption is made in the model that the bolts are able to provide full restraint against twisting about the longitudinal axis and in-plane rotation of the purlin. In terms of the warping of the purlins, it is clear that warping is neither transmitted to the rafter nor to the adjacent purlin. A small gap usually exists between the purlins at the support due to geometrical tolerances. Figure 7-25 3D structural model of purlins and beams in SAFIR showing the element numbers Cleats at internal supports Figure 7-26 Connection between purlin and steel rafter (Dimond Industries, 1995) Rafter DHS purlin Fastening to cleat Translations slaved in x, y and z global axes Rotations slaved in x and y global axes
159 It should be noted that for simplicity, it has been assumed that the centreline (centrenode) of the purlin coincides with the centreline (centrenode) of the rafter. The connection detail between the brace channel and the purlin according to Dimond Industries (1995) is shown in Figure 7-27. The brace channel is bolted on to proprietary steel plates, which are in turn bolted onto the web of the purlin. Similarly, the nodes (i.e. with same coordinates) between the brace channel and the purlin share the same translations in the model (i.e. translation compatibility) and it is assumed that the connection is able to provide restraints against twisting about the longitudinal axis and in-plane rotation of the brace channel. Figure 7-27 Connection between brace channel and purlin (Dimond Industries, 1995) Analyses have been carried out for cold and hot conditions using both static and dynamic versions of SAFIR. For the hot analysis, all the members are exposed to the ISO 834 standard fire. The cold analysis has an increasing uniformly distributed load applied along the length of the beam elements and the hot analysis includes the self weight of the members and the dead load of the roof sheeting (refer to Table 7-2). The deflected shapes at the last time step are shown in Figure 7-28 (note: the load limit or the time limit was set to 14.4 kN/m or 14400 seconds, respectively, in SAFIR). The analyses have shown that the dynamic algorithm has successfully overcome numerical failures observed in the “static” analyses and predicted the behaviour of the structure until the load limit was reached under cold conditions, and up to the point where a snap through failure mechanism had occurred under hot conditions.
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