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

This paper investigates the behaviour of steel–concrete composite beams subjected to the combined effects
of negative bending and axial compression. For this study, six full-scale tests were conducted on composite
beams subjected to negative moment while compression was applied simultaneously. The level of the applied
axial compression varied from low to high. Following the tests, a nonlinear finite elementmodel 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 beamis significantly reduced and local buckling in the steel beamismore 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. Detailingwith
longitudinal stiffeners in the web of the steel beam in the regions of negative bending eliminate web buckling
and increase the rotational capacity of the composite section. Based on the experimental outcomes and the finite
element analyses a simplified design model is proposed for use in engineering practice.