Elevated-temperature mechanical properties of high-strength structural steels over 500 MPa under transient state conditions

The mechanical properties of high-strength structural steel (HSSS) at elevated temperatures are critical for structural fire safety design and analysis. While most previous studies have used the steady-state test method because it requires less time and directly provides stress–strain curves, the transient-state method better simulates real fire scenarios where structural elements are preloaded before heating and creep effects may become significant. This paper presents an experimental investigation of Q550, Q690, and Q890 steels using transient-state test method. The mechanical properties of elastic modulus, yield strength, ultimate strength and stress-strain curve are measured and compared with steady-state test results. The results show notable differences between the two test methods: the reduction factors for mechanical properties obtained from steady-state tests are consistently higher than those from transient-state tests. Unified models for reduction factors of elastic modulus and yield strength of HSSSs are derived from curve fitting of the transient-state test data. Additionally, the applicability of existing models to predict the reduction factors has been investigated. Among the five existing models (EC3, ASCE Manual, NIST, AS 4100, and GB 51249), AS 4100 provides conservative predictions for yield strength, while GB 51249 offers conservative estimates for elastic modulus. The EC3 stress–strain model is shown to be applicable to HSSS at elevated temperatures.