Investigation of the Residual Strength of Steel–Concrete Composite Columns after Exposure to Elevated Temperature

Encased steel–concrete composite columns are being widely used in high-rise structures, but the study of the behavior of such columns after exposure to elevated temperatures, such as fire, remains limited. Existing studies mainly focus on the fire resistance of columns during fire, and present design standards lack provisions for the postfire load-carrying capacity of these columns. Therefore, this study aims to investigate the residual capacity of concrete-encased steel composite columns after exposure to elevated temperature. Four sets of column specimens with varying dimensions and slenderness ratios were cast in two sets. The first set was tested under axial compressive load under ambient conditions. The other set of columns was exposed to the standard fire curve in the furnace for 180 min. Subsequently, these temperature-exposed specimens were also subjected to similar compressive loads. The 3D nonlinear finite-element numerical model was prepared to validate the experimental findings. The experimental results were also compared with the current design provisions of standard codes. The experimental testing of temperature-exposed columns revealed extreme spalling of concrete, followed by localized buckling of internal steel sections, resulting in a reduction in load-carrying capacity by 43 percent and a 140 percent increase in axial deformation. The developed numerical model predicted the peak load-carrying capacity, along with deflections at peak load, with high statistical accuracy, with an R2 value greater than 0.95 and ratios of PEXP/PFEA nearly equal to unity. On the other hand, the use of degraded material properties to determine the residual load-carrying capacity using standard code equation shows inconsistency in predicting the residual capacity, with ratios of PEXP/PCode varying from 0.77 to 1.49, necessitating the need for the proposed numerical model rather than relying on standard equations.