Post-fire mechanical properties of base metal and welds of Q235 steel

Abstract

Fire hazard has a non-negligible impact on building structural safety. With reasonable fire resistance design, steel structures would not necessarily cause collapse when suffering fire and cooling down, but the reliability of structures after fire needs to be re-evaluated. Although there have been considerable investigations of the mechanical behaviors of structural steel in fire or at elevated temperatures, only limited study has focused on the post-fire mechanical properties of structural steel especially under different cooling conditions. In this paper, coupon tests of Chinese Q235 structural steel, including base metal and welds, were conducted and the post-fire performance, including appearance, stress-strain curve, elastic modulus, yield strength, tensile strength, percentage elongation after fracture, and percentage reduction of area, was obtained and analyzed. With the condition of cooling in air or in water, post-fire mechanical properties of base metal and welds cooling from different maximum temperatures were analyzed and compared. Based on the test results, it is concluded that cooling in air from temperatures above 600 °C increases the ductility of Q235 steel for both base metal and welds, whereas cooling in water causes a significant drop in ductility but clearly increases strength. The post-fire tensile strength of the base metal would be higher than that of welds after cooling in water from 600 °C or higher elevated temperatures. Meanwhile, welds would develop poorer ductility than base metal. Accordingly, a set of reasonable predictive formulas is proposed for the post-fire mechanical properties of Q235 steel base metal and welds.