Design Strengths of Cold-Formed Channels in Bending and Torsion

Channel sections in flexure, when not attached to deck or sheeting, are designed according to the American Iron and Steel Institute's equations for lateral-torsional buckling. These formulas assume an unbraced length with equal and opposite end moments, and only torsional restraints at the braces. These formulas tend to overestimate the buckling strength in some cases, especially where there are two or more intermediate braces within a span length. The reason for this is that the failure mode may be something other than lateral-torsional buckling. In fact, tests have shown that highly braced members usually fail by distortional buckling of the flange-lip intersection at a braced point. The present paper reports a series of experiments on channels with various bracing schemes and offers a mathematical prediction model for the ultimate strength of a channel in flexure.