Local Buckling Tests on Cold-Formed Steel Beams

Abstract

\iC and \iZ sections are two of the most common cold-formed steel shapes in use today. Accurate prediction of the bending performance of these sections is important for reliable and efficient cold-formed steel structures. Recent analytical work has highlighted discontinuities and inconsistencies in the American Iron and Steel Institute (AISI) and Canadian Standards Association (S136) design provisions for stiffened elements under a stress gradient (i.e., the web of \iC or \iZ sections). New methods have been proposed for design, and an interim method has been adopted in the North American Specification (NAS). However, existing tests on \iC and \iZ sections do not provide a definitive evaluation of the design expressions, due primarily to incomplete restriction of the distortional buckling mode. Described in this paper is a series of flexural tests with details selected specifically to insure that local buckling is free to form, but distortional buckling and lateral-torsional buckling are restricted. The members selected for the tests provide systematic variation in the web slenderness (\Ih/t\N) while varying other relevant nondimensional parameters (i.e., \Ih/b, b/t, d/t, d/b\N). Initial analysis of the completed testing indicates that overall test-to-predicted ratios for AISI, S136, NAS, and the direct strength method are all adequate, but systematic differences are observed.