Elastic bubble augmented spline finite strip method in analysis of continuous composite beams

摘要

Local and overall instabilities of the steel beam occur in the hogging-moment region in a continuous composite beam and these forms of buckling have been recognised to be highly interactive. The overall mode of buckling in composite steel-concrete tee-beams in regions of negative bending is what is herein termed restrained-distortional buckling (RDB). In negative bending the slab restrains the tension region of the steel and the neutral axis is not located at the mid-height of the web. The neutral axis is shifted towards the top flange, and in negative bending the steel region is subjected to predominantly compressive loading. In addition, the web usually carries proportionally higher shear loads than in ordinary steel beams. The lateral-distortional buckling resistance of the steel portion in continuous composite beams is therefore dependent on the extent to which the web can provide a restraining action to the unstable compression flange. This form of buckling is usually prevented in bridge girders by the cross bracing and the common view amongst engineers is that such bracing is excessive and uneconomical. Although buckling of plain steel beams in both the elastic and inelastic ranges of structural response has been studied extensively and a great deal of research work has been devoted to the understanding of their buckling modes, and codes of practice for the design of structural steelwork contain relevant clauses that presently are considered to be quite accurate, buckling of the steel component in composite beams still represents a grey area in structural engineering research and is much less well documented. The bubble augmented spline finite strip method of analysis, developed by the authors, is thus employed herein to study extensively the issue of RDB in composite tee-beams in hogging bending regions. The numerical investigations demonstrate that RDB mode is a governing mode for steel I-sections with one flange fully restrained, variations of the web slenderness parameter have a most pronounced influence on the elastic buckling capacity of composite T-section beams and the deformations can be reasonably large in the proximity of the internal supports and near the concentrated forces.