This study investigates the structural fire behaviour of square and rectangular concrete-filled steel tube (CFT) columns with different aspect ratios using a validated three-dimensional thermo-mechanical finite element model. The model was verified against experimental temperature and axial displacement data. A parametric study was conducted to examine the effects of load ratio and sectional geometry on fire resistance. The results indicate that load ratio is the dominant factor governing deformation behaviour and fire resistance. Increasing load ratio significantly shortens the concrete-dominated resisting stage and may lead to premature global instability. Rectangular sections exhibited lower fire resistance than square sections due to reduced concrete participation during the critical resisting stage. Comparison with the AISC fire resistance equation suggests that current design provisions do not explicitly account for load-ratio-dependent confinement development or geometry-related reductions in concrete contribution. The findings highlight the need to consider load ratio and aspect ratio effects in the fire design of rectangular CFT columns.