Behaviour of rectangular concrete-filled steel tube beams under monotonic and cyclic loadings

Concrete filled steel tube (CFST) has been extensively explored for columns but limitedly explored for beams. This current study investigates the behaviour of rectangular CFST beams under monotonic and cyclic loadings. Experiments were conducted for 12 CFST beams with depth-to-thickness (D/t) ratios of 66.7-111.1, followed by theoretical analyses. The experimental results indicated that tensile yielding and compressive local buckling of steel tubes governed the plastic behaviour of the tested CFST beams. The buckling significantly degraded the strength of CFST beams. When the D/t ratio decreased from 111.1 to 66.7, the stiffness, yield load, and ultimate load increased by 33.2%, 49.4%, and 40.0%, respectively, but the ultimate deflection was reduced by similar to 19.5%. The effect of cyclic loading induced a degradation of stiffness of 0.85-2.85%/cycle, depending on the increment of the peak cycle deflection. The buckling negatively affected the ultimate state, significantly reducing the ductility. Investigations into solutions to delay the buckling of steel tubes should be encouraged. Three approaches based on concepts of composite sections, fully plastic sections of steel tubes, and the proposed decoupling were applied to compute the ultimate strengths of the tested beams. The calculation results indicated that the model of fully plastic sections of steel tubes exhibited the best agreement with the test results. The fully plastic and proposed decoupling models provided reasonable results, while the composite model slightly overestimated the ultimate load of CFST beams. The results provide some technical information for structural engineers in designing rectangular CFST beams.