Theoretical and experimental research on flexural behavior of concrete-filled steel tubes with inner stiffening

The theoretical value of ultimate bending bearing capacity for the concrete filled steel tube（CFST） with inner stiffener was obtained through solving the balance equation of components under pure bending at limit conditions. A pure bending test was designed for three sets of nine concrete filled steel tube（CFST）specimens with inner stiffener, and the flexural bearing capacity of these specimens was obtained. The typical M-um、 M-ε、 M-Φ curves of these specimens were obtained. The curves can be divided into three stages, i.e.the elastic stage, the elasto-plastic stage and the reinforcement stage. The components had good ductility.The theoretical flexural bearing capacity was compared with test value, showing the average ratio is 0.9199, and the average relative error is 8%, which means that the theoretical value is reliable, and the theoretical values are all smaller than the test values, namely the theoretical solution is conservative. Taking the CFST with inner steel tube as example to study the effect of stiffening parts on flexural bearing capacity, the finite element model of CFST with inner steel tube under pure bending was established. It can be seen that the flexural bearing capacity of the CFST with inner steel tube is increased with the thickness and strength of the steel tube.