Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 50 Figure 9.15 Typical connection detail for circular hollow section bracing 9.3.3 Bracing using angle sections The use of angles is not common in modern structures, but cross-braced angles have an advantage in that the diagonal members are relatively small because they may be designed to resist tension only (Figure 9.16). Location of vertical bracing Position of gable posts Figure 9.16 Plan view showing both end bays braced using crossed angle sections 9.4 Restraint to inner flanges Restraint to the inner flanges of rafters or columns is often most conveniently formed by diagonal struts from the purlins or sheeting rails to small plates welded to the inner flange and web. Pressed steel ties are commonly used. As the ties act in tension only, angles must be substituted in locations where the restraint must be provided on one side only. The effectiveness of such restraint depends on the stiffness of the system, especially the stiffness of the purlins. The effect of purlin flexibility on the bracing is shown in Figure 9.17. Where the proportions of the members, purlins and spacings differ from proven previous practice, the effectiveness should be checked. This can be done using the formula given in Section 9.5, or other methods, such as may be found in bridge codes for U-frame action.