Stress–Strain Relationship for Steel Bars at Elevated Temperatures

The stress–strain relationship of steel bars is fundamental for the fire safety assessment of reinforced concrete structures. This study investigated the mechanical properties of hot-rolled ribbed steel bars at elevated temperatures through tensile tests. The results show that the elastic modulus, yield strength, and ultimate strength decrease with increasing temperature, with a significant decline especially at 300–700 °C, and the yield plateau shortens or even disappears. The ultimate strain initially decreases and then increases when the temperature exceeds 400 °C. Relevant experimental data were collected and analyzed to discuss the temperature dependence of these properties. A simplified trilinear/bilinear constitutive model that accounts for the disappearing yield plateau with a defined critical temperature of 300 °C is proposed. The fracture mechanism was investigated through fractographic analysis. The experimental results were used to validate both the proposed model and the EC2 model, demonstrating that the former achieves higher accuracy for the steel bars studied.