Cold-formed steel strength predictions for torsion

Locally slender 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 only some cases. Additionally, previous experiments on steel beams under combined bending and torsion have found that the current interaction method to predict member capacity is too conservative. 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 improve the interaction prediction of bending and torsion in locally slender cross-sections. Shell finite element models of lipped channels and zee section members were created and validated with existing experiments on torsion only and combined bending and torsion. A range of cross-sections and members lengths in current use were investigated to cover the range of expected torsional slenderness and bracing conditions that could be practically experienced. Shell finite element buckling analyses were performed to determine the critical moment, torsion, and bimoment values, and shell finite element collapse analyses were performed to determine the ultimate moment, torsion, and bimoment values. For combined bending and torsion, the distance between the applied load to the shear center was varied to obtain a range of combined torsion and bending response in the member. Updated strength curves for torsion only and combined bending-torsion interaction are proposed.

Learning Objectives:
Describe the torsion behavior for thin-walled C-sections and Z-sections.

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