Study on nominal values of mechanical properties of high strength steel at elevated temperature and after fire exposure

The mechanical properties of high-strength steel at elevated temperature and after high temperature are important basis for fire resistance design and post-fire evaluation of high-strength steel structures. The calculation equations of mechanical properties at elevated temperature proposed by the current code GB 51249—2017 ‘Code for fire safety of steel structures in buildings’and Eurocode EC3 are not applicable to high-strength steel. A series of experimental studies on mechanical properties of high-strength steels at elevated temperature and after fire exposure have been studied world-widely. Due to the influences of steel strength grade, test equipment, heating rate, and loading system, the test results are discrete and cannot be applied to actual engineering. At the same time, the predictive equations of mechanical properties proposed by different scholars are different, which are not universally applicable.The test data of mechanical properties of high-strength steels at elevated temperature and after fire exposure were analyzed by means of t distribution and confidence interval, and the nominal values of 95% guarantee rate of mechanical properties at different temperatures were obtained. The equations for calculating the mechanical properties of high-strength structural steels at elevated temperature and after fire exposure were fitted and compared with those predicted by GB 51249—2017 and Eurocode EC3. The results show that the turning points at which the strength of the steel decreases significantly after air and water cooling conditions are 600 ℃ and 500 ℃, respectively. The yield strength reduction factor of high strength steels at high temperature is lower than that of mild structural steel, the elastic modulus reduction factor is lower than that of common mild steel under 600 ℃ but higher than that of mild steel beyond 600 ℃.