WEB-FLANGE INTERACTION IN COLD-FORMED STEEL Z-SECTION COLUMNS

In the American Iron and Steel Institute (AISI) specification for the design of cold-formed steel structural members, the web of cold-formed steel sections under direct compression is generally considered to be fully stiffened and a buckling coefficient of 4.0 is used. This, however, may not be true if the flanges of the sections cannot provide sufficient restraint to the web. Also, if the web element is unstable, distortional failure of the flange/lip component may limit the ultimate capacity of these sections with very little postbuckling strength being realized. This paper presents results from an experimental program involving 85 Z-section columns tested under uniform compression. The test parameters included the length of specimens, the width of the flanges and the width of the lip stiffeners which were located transversely to the flanges. The experimental results indicate that the current AISI specification overestimates the capacity of Z-sections subject to web buckling by as much as 59%. A simple theoretical model is also presented in this paper for computing the ultimate compressive resistance of cold-formed Z-sections.