Study of U-shape steel-encased composite beams with variable sections under negative bending moments

To reduce steel consumption and improve the negative moment bearing capacity of composite beams, a new type of composite beam with an external U-shaped steel plate was proposed in this paper. Static load tests of three scaled simply supported composite beams were completed, and the failure patterns, load–displacement curves, strain distributions, and slip of the new composite beams under negative bending moments were researched. The results showed that the new composite beam under negative bending moments underwent typical flexural failure with excellent ductility (ductility coefficient of 6.2), wherein the embedded steel plate played a key role in the loading process of the composite beams, contributing more than 50% of the flexural load capacity. On the basis of the experiments, finite element models were established in ABAQUS to investigate the influence of different construction parameters, including the material strength, thicknesses of the external steel plate and the embedded steel plate, and reinforcement ratio. Finally, design formulas were derived according to the simplified plasticity theory to calculate the flexural capacity of the proposed composite beam for engineering practice, which achieved close agreement with the experiments, with an average relative error of 3.5%.