Numerical investigation of cold-formed lapped Z purlins under combined bending and shear

Abstract The plain Z-section is one of the most common cold-formed steel purlins in use for roof systems. Its lapping ability provides continuity, and double thickness material at the support regions results in greater performance and more economical designs. Design methods for this section are normally specified in the Australian/New Zealand Standard for Cold-Formed Steel Structures or the North American Specification for Cold-Formed Steel Structural Members. The newly developed Direct Strength Method (DSM) presented (Chapter 7 of AS/NZS 4600:2005, Appendix 1 of NAS S100-2007) is limited to pure compression and pure bending. Recently, design proposals for shear, and combined bending and shear for DSM have been added to the 2012 Edition of the North American Specification. The situation of combined bending and shear as occurs in a continuous purlin system is not considered in detail. Hence, this paper presents numerical simulations using the Finite Element Method (FEM) to determine the ultimate strength. The simulations are compared with and calibrated against tests at two lap lengths. For each lap length, tests were also conducted with and without straps screwed on top flanges. These straps provide torsion/distortion restraints which may enhance the capacity of the purlins. The accurate results from FEM allowed extension of test data by varying the lap lengths and section thicknesses. The results of both the experimental tests and FEM were used and plotted on the recently proposed DSM design interaction curves. Proposals for an extension to the DSM in combined bending and shear are given in the paper.