Effects of Material Properties and D/t Ratio on Performance of Square Concrete-Filled Steel Tube Beams

Abstract

This study investigates the effects of steel yield strength fy, concrete strength fc′, and ratio of diameter to thickness (D/t) of steel tubes on the behavior and properties of concrete filled steel tube (CFST) beams under bending. Finite element (FE) ABAQUS software was used to develop numerical models for simulating the behavior of CFST beams under bending. The FE model was validated by comparison with the outcomes of experiments, showing a good approximation. The results indicate that the effects of fc′ on the load–deflection behavior and mechanical properties of CFST beams are marginal and can be neglected. With the same fy of 430 MPa of steel, the elastic and plastic stiffnesses negligibly change when fc′ increases from 38 MPa to 80 MPa. Yield and ultimate loads also negligibly increase by less than 3%. This can be explained by the fact that, on cross section, steel governs the tension force and mainly contributes to the compression force, whereas only a narrow top region of concrete contributes to the compression force. In contrast, fy greatly affects and governs the bending behavior of CFST beams, extending the elastic branch and shifting the plastic branch upward. When fy increases from 235 MPa to 630 MPa, the yield and ultimate loads increase by 145.6% and 107.5%, respectively, whereas the steel strength did not exhibit a clear impact on the plastic stiffness. D/t ratio significantly affects the behavior and mechanical properties of CFST beams, whereas it marginally increases the plastic stiffness. The yield load, ultimate load, and elastic stiffness of CFST beams with a D/t of 25 increase by 127.0%, 104.3%, and 129.2%, respectively, compared with those of CFST beams with a D/t of 75. In the context of a lack of research devoted to CFST beams, these findings provide technical information for engineers in practice, e.g., confidently controlling the performance and failure of CFST beams by the governing parameters.