The mechanical properties of high strength steel will deteriorate under fire conditions, and this deterioration is consistently observed in fire accident of steel structures. The load induces stress in the steel member, which is significant, yet often neglected in current research on post-fire mechanical properties of steel. Thus, this paper offers a comprehensive analysis of the mechanical properties of LG700 steel under fire conditions. A series of tension tests were conducted on mechanical properties at elevated temperature and after fire exposure. The tensile stress was considered in the post-fire test and the stress ratios of 0, 0.2, 0.4 and 0.7 were applied in the steel coupons. The findings indicate that both the yield strength and ultimate strength are largely preserved, maintaining approximately 80.0 % of yield strength and 86.1 % of ultimate strength up to 400 °C. The post-fire tension tests reveal that tensile stress significantly affects the residual yield strength, ultimate strength, and elastic modulus of LG700 steel only when the temperature reaches 300 °C or exceeds 700 °C. By comparing the test results with current standards and available literatures, it was found that some of the recommended values in the standards are not suitable for LG700 steel. Furthermore, it has been observed that even when steels have similar yield strengths, their post-fire mechanical properties may not necessarily align. Consequently, a series of prediction equations based on the test results have been proposed to assess the mechanical properties of LG700 steel at elevated temperature and after fire exposure.
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