Performance of circular concrete-filled steel tube beams under monotonic and cyclic loadings

Abstract

This study explored the performance of concrete-filled steel tube (CFST) beams under monotonic and cyclic loadings. Experiments were performed on nine simply-supported 140-mm-diameter CFST beams, which were classified into three groups by the diameter-to-thickness (D/t) ratios of 56.0, 46.7, and 40.0, conforming to Eurocode 4. The experimental results indicated that CFST beams exhibited high ductility. The buckling of steel tubes governed the plastic behavior and the ultimate state of CFST beams. The deformation and curvature concentrated in the middle region of the beams, forming a plastic hinge. The yield load, ultimate load, and yield and plastic stiffness slightly increased when D/t decreased from 56.0 to 46.7. However, by further decreasing D/t to 40.0, the ultimate and yield loads were significantly increased by ∼100%, the elastic stiffness by up to 58.2%, and the plastic stiffness by at least 187.5%. With a decrease in D/t ratio, the yield deflections slightly increased, whereas the ultimate deflection significantly decreased by up to 56.3%. The strength degradation was more pronounced for CFST beams with a D/t ratio of 40.0. Decreasing D/t ratio increased the plastic-to-yield stiffness ratios. The absorbed energy increased linearly with the increase in deflection in the plastic range. The cyclic loading effect caused an average degradation of 1.7%/cycle for both loading and unloading stiffness, whereas it marginally impacted on strength degradation. A decoupling concept and simplifications were employed to propose a simple model for rapidly estimating the moment capacity of CFST beams, which can be useful in designing and evaluating CFST beams.