Steel Buildings in Europe

Part 8: Building Envelope 8 - 33 Rafter or column stays, as shown in Figure 4.11, may be used to provide torsional restraint to the rafter or column provided that they are connected to a suitably stiff purlin or cladding rail. Thin cold formed steel straps (working as ties) are often used, although angles may be used if the stay must work in compression (for example, if a stay can only be provided on one side of a member). 2 1 3 4 1 Built up or composite cladding 2 Cold-rolled eaves beam 3 Rafter stay 4 Column stay Figure 4.11 Details of column and rafter stay and connection In order to provide the required level of torsional restraint to the rafters or columns, the purlins or cladding rails must possess sufficient flexural stiffness. Otherwise, there is a risk that the restraining member will bend and allow the restrained members to rotate, as shown in Figure 4.12. As a rule of thumb, it is normally adequate to provide a purlin or cladding rail of at least 25% of the depth of the member being restrained. In practice, this generally means that the purlins and side rails will be sufficiently stiff for portal frames with spans up to 40 m and frame spacings of 6 to 8 m. However, as the span increases relative to the frame spacing (and the rafter size increases relative to that of the purlins), the purlin stiffness may become insufficient to provide adequate torsional restraint and should, therefore, be checked. Figure 4.12 The importance of adequate purlin stiffness 4.6 Restraint of purlins and cladding rails Cold formed steel purlins and cladding rails are extremely efficient at carrying loads by bending action, but they are susceptible to failure through lateral-torsional buckling unless they are adequately restrained. The economic