Collapse analysis of restrained concrete-filled steel tubular columns in fire conditions

This paper presents numerical analyses of the thermo-mechanical behavior of restrained square concrete-filled steel tubular (CFST) columns subjected to standard fire. It is found that the behavior is significantly influenced by the load ratio and axial restraint level. The collapse of restrained columns in fire conditions can be defined as the axial force in columns reaching 0 or the axial contraction deformation reaching a hundredth of column length. Three collapse modes are discovered, i.e., the expansion deformation collapse mode, contraction deformation collapse mode, and load-bearing capacity loss collapse mode. Under a low load ratio, three distinct modes of collapse are possible. Conversely, at a high load ratio, only the contraction deformation collapse mode and the load-bearing capacity loss collapse mode are observed. Through analytical and numerical approaches, a method for identifying the collapse mode based on geometric dimensions, axial restraint level, and load ratio has been established. This identification method can significantly contribute to optimizing fire protection strategies for CFST columns in structural designs.