Strength of multi-span composite beams subjected to combined flexure and torsion

• Eight multi-span composite beams under combined flexure and torsion are tested.
• A FE model is validated and utilised for extensive parametric analyses.
• No increase in torsional strength within the presence of flexure is observed.
• Torsional strength is sensitive to shear connection, joist spacing and L/D ratio, with new design guidance being proposed.
• Flexure-torsion interaction is insensitive to parameters considered herein, with new equations being developed.

An improved understanding of the performance of composite beams subjected to combined flexure and torsion is essential for practical design as such loading states are very common in bridges for the main girder and in buildings for edge beams. Multi-span steel–concrete composite beams consisting of two steel joists and one concrete slab are concerned herein, on which an experimental study and finite element (FE) modelling analyses are presented. Eight reduced scale beams under combined flexure and torsion to varying degrees are tested, with both full and partial shear connection being incorporated. Either flexure-dominated or torsion-dominated failure modes are observed in each test depending on the ratio between the bending and torsional moments applied externally. Load-deformation responses, ultimate strengths as well as flexure–torsion interaction of all specimens are also reported and discussed. Based on the experimental outcomes, a three-dimensional FE model is developed and further validated against the test results, and an extensive range of parametric analyses are undertaken by using the FE model. It is indicated that in addition to well-known parameters relating to the concrete slab and stirrups, the torsional strength of multi-span composite beams is also sensitive to the degree of shear connection, spacing of the joists and span-to-depth ratio. Contributions of the steel joist and its constraint on the slab towards the torsional strength are increased. With respect to their flexure–torsion interaction relation, no increase in the torsional strength is observed within the presence of combined flexure, and vice versa. The interaction is insensitive generally to various parameters considered herein and the test results exhibit consistent phenomenon. Design guidance and new equations for predicting the torsional strength and the flexure–torsion interaction relation are proposed based on the test and FE analysis results as well as a comprehensive review of existing approaches in the literature.