Steel Buildings in Europe

Part 8: Building Envelope 8 - 29 gravity loading in the finished condition, when continuous restraint is provided to the compression flange of the purlin by the cladding. 4.1.4 The use of side rail supports for wall cladding Support for wall cladding is provided by a framework of horizontal cladding side rails that span between the columns of the building’s primary steelwork. Vertical restraints are connected to the side rails at discrete locations (similar to the anti-sag rods in roofs). These restraints prevent the occurrence of lateral-torsional buckling (due to bending of the side rails under wind suction loading) and also prevent the side rails from sagging under the weight of the cladding and its supporting steelwork. These vertical restraints are typically light gauge steel sections (tubes, angles or channels) or steel bars/rods. In order to channel the forces generated in the side rail supports efficiently to the primary structure (columns) and to prevent the side rails from sagging prior to the installation of the cladding, it is customary to provide a vertical braced bay arrangement between the lowest two side rails, as shown in Figure 4.10. These bracing members operate in tension, so it is common to use steel wires rather than cold formed light gauge steel sections. To restrict the forces in the tie wires, it is common practice to restrict the angle of the tie wire to the cladding rail to a minimum of 25° or 30° (refer to the manufacturers’ recommendations). With this restriction imposed on the diagonal tie wires, the number of side rail supports is predetermined, based on the spacing of the side rails and the spacing of the columns. For column spacings up to 6 m with a typical side rail spacing of 1,8 m, a single central vertical restraint will normally be sufficient (see Figure 4.10). However, for greater column spacings, two or even three vertical restraints may be required. In many cases, the uppermost side rail is connected to the eaves beam. This arrangement will reduce the forces in the tie wires, but the additional force in the eaves beam will need to be considered when this member is sized. It is also worth noting that, once installed, the cladding will stiffen up the wall substructure and transfer a significant proportion of the vertical load to the columns by diaphragm action. The cladding will also fully restrain the side rails against lateral-torsional buckling in the sagging case and will provide partial restraint in the hogging case. 4.1.5 Cleats Purlins are attached to rafters using cleats that are usually welded to the rafter in the shop before delivery to site. However, the use of bolted cleats (see Figure 4.9) is becoming popular due to savings in transportation (as the rafters stack more compactly) and the opportunity they present to adjust the alignment of the purlins on site (with beneficial consequences for the installation of the cladding). The cleats are often provided by the purlin manufacturer, in which case it is likely that they will have been designed specifically for that design of purlin. However, generic bolted cleats made from an angle section or simple flat plates welded to the rafter may also be used in many cases, either unstiffened or stiffened.