Steel Buildings in Europe

Part 8: Building Envelope 8 - 35 5 HOT-ROLLED SECONDARY STEELWORK As an alternative to cold formed steel, purlins and cladding rails may also be made from hot-rolled steel sections. At one time, this type of purlin was common in industrial buildings, often used in conjunction with steel roof trusses. The development of cold formed purlins (which are considerably lighter and cheaper) and the trend towards plastically designed portal frames with their onerous restraint requirements meant that the use of hot-rolled purlins became unusual in the UK and Ireland. However, hot-rolled purlins continue to be used in Continental Europe, often with long spanning cladding solutions such as deck and membrane or composite panels. They are particularly useful for providing an intermediate support to structural decking, where the decking by itself is incapable of spanning rafter to rafter. Hot-rolled purlins have a higher load-carrying capacity than all but the largest cold formed purlins. This means that they are generally used at much greater spacings than their cold formed counterparts, typically 3 m or more. This wide spacing makes them unsuitable for plastically designed portal frames, which commonly require restraint to the rafters at approximately 1,8 m intervals. However, they are suitable for elastic frames and also for spans beyond the range of standard cold formed purlins (above 8 m). Hot-rolled purlins could of course be used at closer centres, but this would be uneconomic in most circumstances. A considerable advantage of hot-rolled purlins over their cold formed rivals is their resistance to lateral-torsional bucking, especially where rectangular hollow sections are used. This property is essential if the cladding is unable to provide adequate restraint against lateral-torsional buckling. By contrast, cold formed purlins are only able to span as far as they do (typically 6 m to 8 m) because of the continuous restraint provided by the cladding. Similarly, where the local building regulations forbid using the cladding to restrain the structure, hot-rolled purlins are the only viable alternative to long spanning decks running rafter to rafter. Of course, apart from square hollow sections, hot-rolled purlins are not immune to lateral-torsional buckling and must, therefore, be designed with this mode of failure in mind. Unlike cold formed purlins, it is not common for the manufacturers to produce safe load tables for hot-rolled beams. Their capacities must, therefore, be calculated by a structural engineer according to the recommendations of EN 1993-1-1 [12] , taking account of the cross section resistance, lateral-torsional buckling and deflections. This process must be repeated for gravity and uplift load cases. If lateral-torsional buckling is the critical design criterion, the resistance of the member could be enhanced by the introduction of tubular restraints either at the mid-span or third points of the purlin. However, this will add cost to the structure in terms of additional steelwork and erection time. Hot-rolled purlins can be designed as single or double-span beams. The latter option will significantly increase the bending stiffness of the purlin and should be used where deflection is the governing criterion. However, the high reaction at the intermediate support (1,25  load in one span) can cause web crushing at this location. Sleeves are not generally used with hot-rolled purlins.