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Steel Designers' Manual - 6th Edition (2003)
30 Single-storey buildings
It is clear that, when vertical load is applied to a pitched roof portal frame,
the apex tends to deflect downwards, and the eaves spread horizontally. This displacement
cannot occur without some deformation of the cladding. Since the
cladding is fixed to the purlins, the behaviour of the cladding and purlins together
is analogous to that of a deep plate girder, i.e. the purlins resist the bending moment,
in the form of axial forces, and the steel sheeting resists the applied shear, as shown
in Fig. 1.27.
As a consequence of this action, the load applied over the ‘length’ of the ‘plate
girder’ has to be resisted at the ends of the span. In the case of stressed-skin action
being used to resist transverse wind loads on the gable end of the structure, adequate
connection must be present over the end bay to transmit the load from the
assumed ‘plate girder’ into the braced bay, and into the foundations, as shown in
Fig. 1.28.
Some degree of stressed-skin action is present in all portal frame structures where
cladding is fixed to the supporting members by mechanical fasteners. Claddings
which are either brittle (i.e. fibre cement) or are attached to the supporting
structure by clips (i.e. standing seam systems) are not suitable for stressed-skin
applications.
Fig. 1.27 Stressed-skin action
adequate
connections
required here
sheeting (web)
purlins (flange)
Fig. 1.28 Stressed-skin action for gable-end bracing
shear panel in sheeting