Inelastic Bending Capacity of Cold-Formed Steel Members

Abstract

The objective of this paper is to provide and verify a general design method for prediction of inelastic bending capacity in cold-formed steel members potentially subject to local, distortional, and/or lateral-torsional buckling modes. An extensive experimental database of tested cold-formed steel beams is collected and indicates that inelastic reserve in the bending capacity of thin-walled cold-formed steel members is more common than typically assumed. Elementary mechanics for inelastic reserve are reviewed and simplified expressions provided for connecting the strain demand to the inelastic bending capacity in the range between the yield moment and the fully plastic moment. The strain capacity that can be sustained in inelastic local and inelastic distortional buckling is investigated through existing experiments coupled with nonlinear finite-element (FE) analysis. The nonlinear FE models provide a comprehensive means to investigate the relationship between cross-section slenderness, normalized strain capacity, and the resulting bending strength. A design approach for inelastic lateral-torsional buckling is provided on the basis of the hot-rolled steel AISC Specification. The resulting relationships for inelastic local, distortional, and lateral-torsional buckling are provided in a Direct Strength Method format for potential adoption in the cold-formed steel American Iron and Steel Institute (AISI) Specification. The provided design method is assessed against available data and shown to be a reliable predictor of inelastic bending capacity in cold-formed steel members.