Numerical Study on Fire Resistance of Square UHPC Filled Steel Tubular Slender Columns Under Axial Compression

The mechanical response mechanisms of square ultra high performance concrete filled steel tubular (UHPCFST) columns under elevated temperatures differ significantly from those of conventional concrete filled steel tubular (CFST) columns; however, related investigations remain scarce. In this study, a validated thermo-mechanically coupled finite element model was developed to systematically evaluate the effects of section size, slenderness ratio, steel ratio, and load level on the fire resistance of square UHPCFST columns, and to elucidate the distinct load-bearing mechanisms of UHPCFST and CFST columns under fire conditions. The results demonstrate that section size, slenderness ratio, steel ratio, and load level significantly affect fire resistance. Specifically, increasing the section size from 200 to 1000 mm improves fire resistance by approximately 123.5%, whereas increasing the slenderness ratio from 30 to 70 decreases it by 57.4%. Conversely, the effect of material strength is relatively insignificant. At the same load level, UHPCFST columns exhibit 15%–45% lower fire resistance than CFST columns, but demonstrate enhanced load-sharing efficiency between the steel tube and core concrete. Based on the parametric study, a calculation method for predicting the fire resistance of rectangular UHPCFST columns is proposed.