Steel Buildings in Europe

Part 8: Building Envelope 8 - 34 and safe design of the cladding and its supporting steelwork relies on the interaction between the individual components that make up the whole system. Purlins and cladding rails are normally selected from manufacturer’s load/span tables, which are derived from analytical models supported by test data. In producing their design data, all purlin manufacturers have to make a judgement regarding the degree of restraint that is available from the cladding system under gravity and wind uplift conditions. These assumptions are central to the design model and can have a significant effect on the design resistance of the purlin or rail. It is therefore essential that an equal or greater level of restraint is achieved in practice. This will depend on the choice of sheeting and the spacing of the fasteners. In the gravity load case (or positive wind pressure in the case of a wall), restraint is provided directly to the top flange of the purlin (or side rail) by the liner sheet or insulated panel, as shown in Figure 4.13(a). Built-up cladding and insulated panels are generally capable of providing sufficient lateral restraint for the gravity loading case. In general, perforated liners are not considered to be restraining and the supporting purlins should, therefore, be designed as unrestrained members. C T (a) (b) T C 1 2 1 Lateral restraint provided to compression flange by cladding 2 Cladding provides lateral restraint to tension flange and partial torsional restraint Figure 4.13 Purlin restraint For wind uplift (or negative pressure on a wall), the cladding cannot provide lateral restraint directly to the compression flange. In this case, the purlin (or cladding rail) is restrained by a combination of lateral restraint to the tension flange and torsional restraint, as shown in Figure 4.13(b). The ability of the cladding to provide restraint is dependent not only on its in-plane shear stiffness (including the fasteners), but also its flexural stiffness. EN 1993-1-3 includes a method in Section 10 for assessing the degree of restraint provided by the cladding in this case. Unlike the gravity load case, the cladding only provides partial restraint to the purlin or rail. Consequently, the purlin manufacturers’ technical literature should always give a lower capacity for purlins subjected to wind uplift loading (or suction on cladding rails). EN 1993-1-3 [11] covers the design of purlins, liner trays and sheeting in Section 10.