Thermo-mechanical response of encased steel-tube prestressed concrete beams: influence of load ratio, cover thickness and partial prestressing ratio under fire

Purpose This study investigates the fire resistance of encased steel tube prestressed concrete (ESPC) beams, addressing the critical challenge of prestressed systems losing strength under elevated temperatures. Conventional prestressed beams are vulnerable to fire due to prestress loss, rapid thermal penetration and spalling. Limited research has explored how load ratio, tendon cover thickness and partial prestressing ratio (PPR) collectively influence thermo-mechanical performance. Design/methodology/approach A combined experimental and analytical program was conducted, including full-scale fire exposure tests and validated finite element simulations. The study systematically varied load ratios (η = 0.25–0.5), tendon cover thicknesses (45 mm and 70 mm) and PPR values (0.5–0.75). Key response parameters such as deflection, stiffness degradation, cracking patterns, torsional behavior and ultimate failure modes were examined up to 600°C. Findings Results revealed that a higher load ratio (η = 0.5) significantly enhanced fire resistance by sustaining internal force equilibrium and delaying failure. Thinner covers (45 mm) accelerated thermal ingress but mitigated explosive spalling, whereas thicker covers (70 mm) effectively protected tendons, extending beam stability. PPR exerted a strong influence, with higher ratios (0.75) accelerating cracking and reducing ductility, while optimized combinations of η, tendon cover and PPR improved residual strength and energy absorption. Balanced prestressing and confinement were shown to enhance torsional stiffness and ductility, yielding superior fire resilience. Originality/value This work provides the first integrated evaluation of ESPC beams under fire, establishing clear correlations between thermal degradation and mechanical response. By offering design-oriented recommendations on prestressing configuration, tendon cover and PPR optimization, the study advances structural fire engineering and guides the development of fire-safe prestressed systems.