Cold-formed steel strength predictions for combined bending and torsion

Locally slender cross-section members, such as cold-formed steel Cee and Zee sections, 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. 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. Previous work has derived a simple uniform equation to predict the bimoment capacity and two bimoment strength curves for local and distortional buckling under torsion only. This work is extended to consider combined bending and torsion for locally slender cross-sections. A parametric study is conducted to improve the interaction prediction of combined bending and torsion for a range of torsional slenderness. Shell fnite element analysis of lipped Cee and Zee section members with combined bending and torsion were created using a validated model. A practical range of cross-sections and bracing conditions were investigated with various ratios of applied torsion and bending. Shell finite element buckling and collapse analyses were performed to determine the critical and ultimate moments and bimoments. It was found that the current AISI standard is conservative under most scenarios. Updated torsion-bending interaction equations incorporating bimoment and bending moment are proposed. The interaction equations are dependent on the cross-section, the direction of the applied torsion, and the bracing condition.

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