CYCLIC LOADING TEST AND CONSITUTIVE MODEL OF POSTFIRE HIGH STRENGTH Q690 STEEL

The structural steels will experience different levels of material degradation after high temperature heating and cooling processes in fire hazards. Available studies are primarily about the residual static properties of postfire steels. An experimental study was performed to investigate the residual static and seismic performance of high strength Q690 steel after high temperature fire exposure and the following cooling process. Different peak temperatures varying from 500 ℃~1000 ℃ and two cooling methods, namely natural cooling and water-cooling, were considered. Monotonic tensile tests and cyclic loading tests (with 4 different loading protocols) were performed on postfire Q690 steel specimens. Results indicate that postfire specimens with the experienced peak temperature being lower than 600 ℃ exhibit similar mechanical properties with intact specimens without any fire exposure. The normal Q690 steel has strain-hardening and cyclic-softening behaviors under cyclic loads. After experiencing the high temperature fire-heating (temperature higher than 600 ℃) and the following natural-cooling process, the postfire Q690 steel displays a decreased initial yield strength as the increase of peak temperature. While postfire specimens after water-cooling from high temperature (higher than 600 ℃) have increased initial yield strengths. Under cyclic loads, the postfire Q690 steel displays more significant strain-hardening and cyclic-hardening behaviors instead of the cyclic-softening characters in non-fire-exposure specimens. Cyclic constitutive models of postfire Q690 steel were calibrated based on experimental data, which can be used for residual seismic performance evaluations of Q690 steel structures after fire.