This paper presents an Effective Width concept based design method against the distortional buckling of cold-formed steel Z and C-section beams. The distortional buckling may be the predominant buckling mode for many cold-formed steel studs, joists, purlins, or girts, unless the compression flange is fully restrained by attachment to sheathing or paneling. However the distortional buckling remains a largely unaddressed problem in the main body of the current North American Specification for the Design of Cold-Formed Steel Structural Members (NAS, 2001) Edition. Experimental investigations have indicated that NAS provides unconservative predictions for the distortional buckling failures. It was also found that the Direct Strength Method and Australian/New Zealand code work well for distortional buckling, but they demonstrate limited applicability for today's industry due to the need of advanced computation tools to determine the elastic buckling behavior of sections. The proposed method in this paper was based on the current design procedure in NAS (2001), it enables engineers to predict the distortional buckling strength of cold-formed steel Z or C-section beams using the existing design method, Effective Width Method, with limited modifications. The new method shows good agreements with experimental results as well as the Direct Strength Method predictions.
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