Cold-Formed Steel Strength Predictions for Torsion

Locally slender open cross-section members are susceptible to significant twisting and high warping torsion stresses. Torsion considerations are complicated by whether it is derived as a first-order effect from loading or a second-order effect from instability. Previous direct torsion experiments on lipped channels have shown significant inelastic reserve in limited cases. The current design for combined bending and torsion interaction has some limitations, including only considering the first yield in torsion and ignoring the cross-section slenderness in torsion. A parametric study is conducted to predict the torsion capacity in locally slender cross-sections. Shell finite element models of lipped Cee and Zee section members are validated with existing experiments on combined bending and torsion. The validated models are utilized for a parametric study with applied torsion on a range of cross-sections, steel grades, and members lengths to cover the range of practically expected torsional slenderness. A set of bimoment parameters, including yield bimoment, buckling bimoment, and plastic bimoment, are calculated and the ultimate bimoment is determined by performing shell finite element collapse analyses. A simple uniform equation is adopted to predict the bimoment capacity and two bimoment strength curves under torsion only are proposed for local and distortional buckling controlled cases respectively