Steel Buildings in Europe

Part 8: Building Envelope 8 - 32 should note that in using the load/span tables they are automatically accepting the assumptions made by the purlin and cladding rail manufacturers, including assumptions regarding the level of restraint provided by the cladding to the supporting steelwork. If in doubt, the secondary steelwork specifiers should contact the manufacturers for advice on the suitability of the chosen section for the application in question, taking into account the proposed cladding type and any other circumstances likely to invalidate the manufacturer’s assumptions, e.g. heavy point loads. 4.5 Restraint provided to the rafters and columns The structural efficiency of any steel framed building depends not only on the selection of light and efficient sections, but also on the interaction between the frame members, the secondary steelwork and the cladding system. For this reason, it is common practice to use the secondary steelwork (the purlins and rails) to restrain the primary steelwork. It is generally accepted that purlins and rails need not be checked for forces arising from the lateral restraint of rafters in either roof trusses or portal frames provided that the following conditions are met:  The purlins are adequately restrained by sheeting  There is bracing of adequate stiffness in the plane of the rafters or alternatively the roof sheeting is capable of acting as a stressed-skin diaphragm  The rafters carry predominantly roof loads. In certain European countries, the assumption that the secondary members can restrain the primary frame is acceptable as long as the secondary member providing the restraint is connected to a node point of the bracing system. In other countries, it is presumed that the roof system supplies a sufficiently stiff diaphragm to relax the requirement. In this case, roof bracing is still required, but need not intersect with every secondary member providing restraint. If a purlin or side rail cannot be used with stays (as shown in Figure 4.11) as a torsional restraint, a hot rolled member may be provided to meet this requirement. Ideally, the compression flange of the rafter or column should be laterally restrained by direct attachment of the purlins or cladding rails. However, under the action of wind uplift, or close to the haunches of a portal frame under gravity loading, the inner flange of the member (i.e. the one to which the cladding is not attached) will be in compression and cannot be restrained directly by the purlins or cladding rails. In this situation, the frame designer can either introduce an additional hot-rolled steel member (often a structural hollow section) to laterally restrain the compression flange or, alternatively, the compression flange can be effectively held in position by a combination of lateral restraint to the tension flange (provided by the purlins or rails) and torsional restraint provided by rafter or column stays. Recommendations for the provision and design of restraints are given in EN 1993-1-1 [12] , § 6.3.5.2 and Annex BB.3.