What is a Steel Purlin?
Cold-Formed Steel Purlins are longitudinal beams installed to brace together trusses or rafters. Its primary function is to transfer external loads applied on the roof to the top chord of the truss. The design of purlins is idealized from a pin-connected structure where the distance of the trusses is treated as the span and its ends as its joints. Gravity and wind loads are commonly applied on the purlin and will cause the member to sag. In order to provide lateral support for the purlins, sag rods are added on discrete points.
Aside from being used in roofing systems, purlins can also be applied as girts for wall cladding. The image below shows a typical steel framing applied with purlins.