Distortional Buckling of Cold-formed Steel Members in Bending

Laterally braced cold-formed steel beams generally fail due to local and/or distortional buckling in combination with yielding. For many cold-formed steel (CFS) studs, joists, purlins, or girts, distortional buckling may be the predominant buckling mode. However, distortional buckling of CFS beams remains a largely unaddressed problem in the current North American Specification for the Design of Cold-Formed Steel Structural Members (NAS). Further, adequate experimental data on unrestricted distortional buckling in bending is unavailable. Therefore, two series of bending tests on industry standard CFS C and Z-sections were performed and presented in this dissertation. The testing setup was carefully designed in the first series of tests (Phase 1) to allow local buckling failure to form while restricting distortional and lateral-torsional buckling. The second series of tests (Phase 2) used nominally identical specimens to Phase 1 tests, and a similar testing setup. However, the corrugated panel attached to the compression flange was removed in the constant moment region so that distortional buckling could occur. The experimental data was used to examine current specifications and new design methods. Finite element modeling in ABAQUS was developed and verified by the two series of bending tests and then applied to analyze more CFS beams. An analytical method was derived to determine the elastic buckling stress of thin plates under longitidunal stress gradient. And finite element analysis was used to study the stress gradient effect on the ultimate strength of thin plates. It was found that the stress gradient increases the buckling stress of both stiffened and unstiffened elements, and current design methods can include the stress gradient effect if an appropriate elastic buckling cofficient is used. ii i The moment gradient effect on the distortional buckling of CFS beams was also studied by the finite element analysis. The results show that the moment gradient increases both the elastic buckling moment and ultimate strength of distortional buckling of CFS beams. A draft design provision was proposed to account for the moment gradient effect. Research was conducted to explore the distortional buckling of CFS beams with partial restraint on the compression flange. A simple numerical model was proposed to calculate the elastic buckling moment of the CFS section-panel system. It was found that partial restraint has significant influence on distortional buckling, and that the influence could be considered by using a modified elastic buckling moment. For design purposes, simplified closed-form solutions for the elastic buckling moment of CFS C and Z-sections were proposed and verified. In the end, conclusions and recommendations for future research are presented