Efficiency Assessment of Converting Single-Tube to Double-Tube CFST Columns in Fire Resistance Based on Functionally Graded Concrete

Conventional single-tube concrete-filled steel tubular (CFST) columns are susceptible to fire-induced failure due to rapid steel tube degradation at elevated temperatures. While external fireproofing measures are commonly used to improve fire resistance, they often increase construction costs and maintenance complexity. Addressing this issue, the present research introduces an innovative double-tube CFST column design incorporating functionally graded concrete between concentric steel tubes. This configuration maintains equivalent steel consumption and ambient-temperature structural capacity compared to traditional single-tube systems. Utilizing finite element modeling, the fire performance of axially loaded short and slender columns—both single-tube and double-tube variants—is systematically evaluated. Key parameters influencing fire resistance, including cross-sectional dimensions, outer/inner concrete compressive strengths, steel tube yield strength, applied load ratio, inter-tube gap distance, and column slenderness, are analyzed. Results indicate that the double-tube CFST columns exhibit substantially superior fire resilience compared to their single-tube counterparts, particularly in scenarios involving larger cross-sections and reduced load ratios, which collectively prolong structural integrity under fire exposure. The study concludes that optimized double-tube configurations incorporating functionally graded concrete can achieve fire resistance requirements without supplementary protective measures, offering a cost-effective and maintenance-efficient alternative for fire-prone environments.