Test on post-fire residual mechanical properties of high strength Q690 steel considering tensile stress in fire

Abstract

As a steel building may not fail in a fire scenario, the residual mechanical properties of steel after fire exposure are essential in evaluating the post-fire performance of the steel structure. However, the current experimental research regarding the post-fire mechanical properties neglects the stress in steel during the heating and cooling phases, which violates the fact that a steel structure still carries load during a fire hazard. In order to quantitatively assess the influence of stress level in steel structures on post-fire residual mechanical properties of high strength steels, a series of tensile tests was carried out on a typical high strength Q690 steel specimen exposed to elevated temperatures under three stress ratios, namely 0.30, 0.55 and 0.80. During the heating and cooling phases, constant stress was applied on the specimen to simulate the possible stress in steel structures in fire hazards. After cooling, tensile tests were carried out to attain the post-fire stress-strain curves of Q690 steel, and accordingly determine the residual yield strength, ultimate strength and elastic modulus of Q690 steel with different temperature exposures and stress levels. The test results show that the tension stress in steel during heating and cooling can improve the post-fire yield strength and ultimate strength of Q690 steel with the maximum 8% when the exposed temperature is lower than 700 °C, and the improvement increases as the stress level increases. However, the tension stress has adverse effect on the post-fire strengths of Q690 steel when the exposed temperature reaches 800 °C. If the exposed temperature is below 700 °C, it is safe to take the original strengths of Q690 steel to evaluate the post-fire performance of Q690 steel structures. However, the stress effect on the residual strengths should be considered if the exposed temperature reaches 800 °C to avoid unconservative evaluation. The tension stress in steel has no significant influence on residual elastic modulus of Q690 steel. By comparing the obtained test data with other post-fire tests on similar steels, it is found that the post-fire residual mechanical properties of steels are different even if those steels have the same nominal yield strength. The maximum 40% dispersion for residual yield strength and 50% dispersion for ultimate strength are found when the exposed temperature surpasses 600 °C due to rolling, quenching and tempering for high strength steels.