Stressed-skin Design - Structural Forms.

In addition to providing the weathertight membrane, the steel sheeting can also be utilized as a structural element itself. Correct detailing of connections between individual cladding sheets (and their connections to the support steelwork) will induce a stressed-skin effect which can offer both resistance to transverse wind loads, and restraint to the compression flanges of the main frame elements.

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 dis- placement 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.

Stressed-skin action
Fig. 1.27 Stressed-skin action

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, ade- quate 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.

Stressed-skin action for gable-end bracing
Fig. 1.28 Stressed-skin action for gable-end bracing

Correct detailing and correct fixing is essential to the integrity of stressed-skin applications:

(1) Suitable bracing members must be used to allow the applied loads from diaphragm action to be transmitted to the foundations.

(2) At the position of laps in the sheeting, suitable mechanical  fasteners must be present to allow continuity of load between sheets. These fasteners can be screws or rivets, which must be capable of withstanding both shear and pull out due to the stressed-skin effect and applied loads respectively.

(3) The ends of the sheet must similarly be connected to the supporting members (i.e. purlins).

(4) All stressed-skin panels must be connected to adequately designed edge members, which must be capable of transmitting the axial forces  induced by bending, in addition to the forces induced by imposed and wind load effects.

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