Post-fire residual yield strength and ultimate strength of structural steel SS275 considering load ratios

Fire is one of the most significant hazards to steel structures. For fire-exposed steel structures, assessing reusability and reliability before dismantling is crucial to determine the most cost-effective course of action—whether to reuse, repair, strengthen, or demolish and rebuild. This study establishes indicators of residual strength for fire-exposed steel, essential for evaluating structural conditions prior to dismantling. Fire tests were conducted on SS275 steel, widely used in South Korean construction, simulating real fire scenarios with constant loads applied during heating and cooling phases. Test temperatures were 540 °C, 650 °C, and 900 °C, with load ratios up to 30 %, considering allowable stress and safety factors. The residual strength of SS275 varied with exposure temperature and load ratio. At 540 °C, residual yield strength increased by up to 13 % as the load ratio increased from 0 % to 30 %, with minimal impact on residual ultimate tensile strength. However, at 650 °C, raising the load ratio from 0 % to 20 % reduced the residual yield strength by up to 20 % and ultimate tensile strength by 19 %. Results were compared with previous studies on structural steels, highlighting the influence of stress levels during fire exposure on post-fire residual strength. These indicators provide critical data for structural engineers to make safe and cost-effective decisions regarding the reuse, repair, or dismantling of fire-damaged steel structures.