Behaviour and design of composite beams subjected to negative bending and axial compression

This paper investigates the behaviour of steel-concrete composite beams subjected to the combined effects of negative bending and axial compression. Six full-scale tests were conducted on composite beams subjected to negative moment while compression was applied simultaneously. Following the tests, a nonlinear finite element model was developed and calibrated against the experimental results. The model was found to be capable of predicting the nonlinear response and the ultimate failure modes of the tested beams. The developed finite element model was further used to carry out a series of parametric analyses on a range of composite sections commonly used in practice. It was found that, when a compressive load acts in the composite section, the negative moment capacity of a composite beam is significantly reduced and local buckling in the steel beam is more pronounced, compromising the ductility of the section. Rigid plastic analysis based on sectional equilibrium can reasonably predict the combined strength of a composite section and, thus, can be used conservatively in the design practice. Based on the experimental outcomes and the finite element analyses a simplified design model is proposed for use in engineering practice.