Strength and Stability of Point-Symmetric Cold-Formed Steel Members Undergoing Lateral-Torsional Buckling

The objective of this paper is to report on a series of numerical analyses performed to study the lateral-torsional buckling behavior of point-symmetric, typically Zee-shaped, cold-formed steel members. Zees have principal axes which are inclined from the geometric axes as defined by the web and flange. Further, loading and bracing is often in the geometric axes. As a result, stability and strength, even in the global lateral-torsional buckling mode is an unconventional coupling of torsion as well as major- and minor-principal axis bending. The governing design specification for these cold-formed steel members in North America (AISI S100) has long applied a conservative simplification for elastic lateral-torsional buckling of point-symmetric sections. Recently, the AISI subcommittee on Member Design, which the second author chairs, considered proposed changes for lateral-torsional buckling of point-symmetric and non-symmetric sections, but limited test data was available to support these changes. The purpose of this study was to provide additional exploration of the proposed design approach using numerical analysis. Using a set of 14 lipped Zee sections, numerical analysis was carried out using the finite strip method, elastic buckling finite element analysis, and inelastic finite element collapse analysis. The results are compared with proposed provisions for both stability and strength and recommendations provided for design.

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