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Test Verification of the Effect of Stress Gradient on Webs of Cees and Zee Sections

C- and Z-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 AISI (1996) design provisions for stiffened elements under a stress gradient (i.e., the web of C- or Z-sections). New methods have been proposed for design, and an interim method has been adopted in the NAS (North American Specification 2001). However, existing tests on Cs and Zs do not provide a definitive evaluation of the design expressions, due primarily to incomplete restriction of the distortional buckling mode. Described in this paper are 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 (h/t) while varying other relevant non-dimensional parameters (i.e., h/b, b/t, d/t, d/b). Initial analysis of the completed testing indicates that overall test-to-predicted ratios for AISI (1996), S136 (1994), NAS (2001) and the Direct Strength Method (Schafer 2002) are all adequate, but systematic error is observed in AISI and S136 due to web/flange interaction.