FACTORS INFLUENCING THE STRENGTH OF CONTINUOUS COMPOSITE BEAMS IN NEGATIVE BENDING

Continuous composite beams are widely used in multi-storey buildings and bridges. For the purpose of flexural strength calculations, such beams may be divided into two distinct regions, namely regions of positive bending and negative bending. The flexural strength of composite beams in positive bending is governed by the strength of the concrete and the steel beam and simple calculations based upon fully plastic stress blocks provide acceptable accuracy in predicting ultimate bending moment capacities. In regions of negative bending moments, however, local and lateral instabilities of the steel section and cracking of the concrete influence not only the ultimate bending strength of the composite section, but also the ability of the section to redistribute bending moment by absorbing inelastic rotations. The factors influencing the behaviour of composite beams in negative bending and their relationship to the section geometry and span lay-outs are discussed in this paper. A theoretical model is presented which provides a basis for assessing the influence of the distortional restraint of the slab on the steel beam. The theoretical models are compared to experimental results obtained from tests on composite beams. The model is then used as a basis for an interactive design approach that also considers local flange and web buckling.