When a steel I-section beam that is curved in plan is subjected to vertical (or transverse) loading, its primary actions are bending about its major axis and torsion. Secondary torsion and minor axis bending actions are usually significant in steel I-section beams curved in plan under combined bending and torsion. However, the common circular interaction equation used in practice for rectangular solid or hollow section curved beams does not account for these significant effects and so may overestimate the interaction strength of steel I-section beams curved in plan under bending and torsion. This paper investigates the nonlinear elastic-plastic behavior of steel I-section beams curved in plan under vertical loading and proposes straightforward formulas for their design against combined bending and torsion actions. Comparisons with rational finite-element results and existing experimental results show that the proposed interaction equations provide a good lower bound for the design of steel I-section beams curved in plan against combined bending and torsion. The proposed interaction equations are simple and consistent with existing codified design rules.
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