Cold formed light gauge purlins – Behaviour and design

Thin, cold-formed elements are regularly used in purlin, beam or floor joist applications.

To highlight the special features of these products, this article will focus on the "roofing purlin" function. Of course, all the considerations developed in relation to this role are transposable to other uses.

After clarifying the notion of "thin element" and the distinction to be made with the notion of class 4 section, the article briefly recalls the origin of these products and how they are produced.

We then examine the impact of the forming operation, on the one hand, on the constituent material by introducing the notion of apparent yield strength, and on the other, on the characteristics of the profile itself, as a result of the bending radii.

The following paragraph is devoted to clarifying the various instabilities likely to affect this type of profile: local buckling of the walls, buckling due to distortion of stiffened areas of the section and buckling of the purlin itself.

This is followed by a discussion of the design aspects of a purlin, in particular the interface between the purlin and the roof, with the notion of construction class. Common constructional provisions are also presented.

Finally, we present the philosophy and the various steps involved in calculating a purlin held in place continuously by the roof trough, which represents the most commonly encountered configuration.