Load capacity of concrete-filled steel tubular members with multilayer confinement subjected to coupled axial compression-bending and fire

The demands to increase bearing capacity of concrete-filled steel tubular (CFST) members and, thus, cross-sectional size of the members, have caused a series of practical issues, such as the difficulties in manufacturing steel tubes and low quality of concrete pouring. As a solution for these, concrete-filled steel tubular (CFST) members with stiffeners can be used. There is no compression-bending interactive equation for CFST members with multilayer confinement at high temperatures. This paper is mainly based on the compression-bending interactive equations of traditional CFST members recommended by Chinese standards at room temperatures, introduces the reduction factors of material strength and elastic modulus at high temperatures, and further extends the compression-bending interactive equations at room temperatures to high temperatures. To verify the validity of the unified formulae, the calculation results of the theoretical formulae are compared with the numerical and experimental results in this paper. The results of numerical simulation show that the two sets of proposed compression-bending interactive equations of CFST members with multilayer confinement are conservative. In general, the equation computing coefficient is proportional to the fire resistance time. In addition, the experimental results are in good agreement with the compression-bending interactive equation.