07.22: On the use of constrained finite element method in the design of cold-formed steel Z purlins

This paper discusses a potential simple new design approach for cold-formed steel Z-purlins. The new design approach tries to simplify the traditional design procedure based on effective widths and thicknesses, which is necessarily complicated due to the complex behaviour of the purlin. The complexity of the behaviour is due to the high slenderness of the purlin member, which makes it prone to various buckling behaviour, as well as due to the typically complicated supports conditions, including elastic restraints from the connecting structural elements. The direct strength method offers an attractive simple alternative for the design, requiring an elastic buckling analysis of the member and then the application of some simple formulae to predict the capacity. The elastic buckling analysis can easily be done by shell finite element method, the only real difficulty being the separation of the characteristic buckling modes such as local, distortional and global buckling. This separation of the modes, however, is now solved by the recently proposed constrained finite element method, which is able to decompose the complex behaviour of virtually any cold-formed beam or column member, under practically arbitrary loading and support conditions. In the frame of the actual research a potential new, direct-strength-based design approach is considered and tested by performing parametric studies on a wide range of Z purlins, by comparing the capacity prediction of the tested alternative method to that of the current Eurocodes. The first results are reported in this paper. © Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.