Experimental study on the post-fire mechanical behaviours of structural steels

The high temperature of fire has an impact on the mechanical properties of structural steel, hindering its use. Q345, Q460, Q550 and Q690 structural streels were chosen to investigate their mechanical behaviours after fire treatment at temperatures from 300 °C to 800 °C and air cooling. The mechanical parameters (yield stress, elastic modulus, yield plateau size, ultimate strength and necking strain) were analysed by considering the fire temperature, and the residual factors of the parameters for the Q345, Q460, Q550 and Q690 steels were compared with the results obtained at an ambient temperature. The results showed that the mechanical parameters for each steel began to reduce when the target temperature exceeded 600 °C, and the residual factor was a descending exponential function or a linear function of the target temperature. The predictive equations of residual factors in the literature, about Q460 and Q690 steels, were collected and analysed, and the new equations were predicted depending on all available experimental data. Finally, a stress–strain model incorporating fire temperature was proposed by a piecewise function to describe the stages of the stress–strain curve (elasticity, yielding and plasticity), and the model can represent the experimental results accurately. •The mechanical parameters of the post-fire steels were analysed and its residual factors were given.•The true stress–strain curve was divided into three parts (elasticity, yielding and plasticity) to describe the whole curve accurately.•The residual factor predicted equations were analysed, and the exponential equation and power equation were better than the polynomial one.