Behavior of Ultrahigh Strength Concrete-Filled High-Strength Square Steel Tubular Columns Postfire

This study investigated the postfire load-carrying of ultrahigh-strength concrete–filled high-strength square steel tubular (UCFHST) columns subjected to the standard fire and sustained external load. Axial compression tests were conducted on eight UCFHST columns following fire exposure. The experimental program employed steel tubes with a fixed side length of 200 mm and concrete strengths ranging from 163 to 174 MPa, and the outer tube temperature varied between 339°C and 717°C. Comprehensive investigations were carried out on the residual load-carrying capacity, stiffness degradation, failure modes, and ductility of the fire-damaged specimens. The study provides a detailed description of the failure process, including the interaction between the external steel tube and the confined ultrahigh-strength concrete (UHSC). The test results revealed that, upon reaching the ultimate load, the axial deformation of the short columns continued to increase, accompanied by a rapid reduction in load-carrying capacity, reflecting brittle failure characteristics. Elevated temperatures were found to exert a significant influence on the residual load-carrying capacity of the columns. Furthermore, the mechanical properties of both the steel tube and the concrete core were analyzed, and the postfire structural performance of UCFHST columns was interpreted using finite-element simulations. Based on the combined experimental and numerical results, the influences of concrete strength, steel yield strength, steel ratio, side length, and fire duration on postfire behavior were evaluated systematically. Predictive formulas for calculating the residual stiffness and residual load-carrying capacity of UCFHST columns exposed to standard fire are proposed.