Performance of Steel-Concrete Composite Beams under Combined Bending and Torsion

Eleven steel-concrete composite beams, four under pure torsion, and seven under combined bending and torsion were tested to study their torsional behaviors. Torsion-dominated and bending-dominated failure modes were observed, depending on the ratio between the applied bending and torsional moments. Testing results also showed that the reinforced concrete slab contributes mainly to the torsional resistance of composite beams and the contribution of steel joists to torsion is negligible. However, the steel joist plays a vital role in restraining the concrete slab from deforming longitudinally, which enhances the torsional strength of the concrete slab. Based on the experimental observations, a three-dimensional behavioral truss model capable of analyzing composite beam sections subjected to the combined bending and torsion was presented. In this model, the section is subjected to one- and two-dimensional stresses separately. The former resists the longitudinal stresses due to bending and torsion while the latter resists the shear stresses due to torsion. These two systems are related based on the compatibility of strains and the equilibrium of stresses in the longitudinal direction. The results predicted with this method are in good agreement with those obtained from the tests. Additionally, the interaction between bending and torsional strengths was discussed.