Monotonic and Cyclic Loading Tests on the Structural Steel After an Exposure to High Temperature
Current available studies on postfire steel are primarily around the residual static properties,while the residual seismic performance of postfire steels under cyclic loads are rarely investigated.For Q235 and Q355 steel commonly used in building structures and bridges,a simulated fire heating treatment of 500-1 000 ℃ and natural cooling or immersion cooling was carried out.The experimental study including monotonic tensile tests and cyclic loading tests with different cyclic protocols was performed on Q235 and Q355 steel,in order to study the effect of peak temperature and cooling methods on the residual hysteretic properties of postfire steels.The results indicate that when the peak temperature is lower than 600 ℃,the naturally cooled and water-cooled postfire Q235 and Q355 specimens show similar strength and mechanical properties as the original specimens without any fire exposure.When the peak temperature is lower than 600 ℃,the initial yield strength of naturally cooled postfire Q235 and Q355 steels decreased as the increase of peak temperature.But under cyclic loads,the cyclic hardening behaviors can gradually restore the strength by experience of high temperature,and this effect will increase with the increase of peak temperature.Under cyclic loads,both types of steel show a reduced hysteretic stiffnesses,and the peak temperature and cooling methods can affect the extent of stiffness reduction.Under high temperature heating and water-cooling conditions,Q235 and Q355 steels exhibited a dramatic cyclic hardening behavior.Elastic strengths were significantly increased and the plastic energy dissipation ability of postfire steel are significantly reduced.
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