Experimental Investigation of Postfire Mechanical Properties of Q345 and G550 Cold-Formed Steel

Abstract

The postfire mechanical properties of cold-formed steel (CFS) are fundamental parameters for evaluating the residual performance of CFS structures after exposure to fire. In this paper, 230 tests were performed to obtain the postfire mechanical properties of commonly used Q345 and G550 CFS with thicknesses ranging from 0.75 to 1.9 mm. An electric furnace was used to heat the specimens in batches, and a shielding isolation was specially designed to avoid direct thermal radiation from the heating elements, which might cause nonuniform temperature distribution of the specimens. In addition, to accurately identify the critical exposure temperatures at which the postfire yield strength of CFS becomes less than its original value or nominal value, additional specimens were separately heated in a furnace. Subsequently, tensile tests of postfire specimens were conducted at ambient temperature. The critical exposure temperatures of Q345 CFS were found to be much higher than those of G550 CFS. In most cases, the effect of the present steel thickness on the reduction factors of postfire mechanical properties was insignificant and the corresponding difference between reduction factors was generally less than 0.1. Additionally, visual observations of postfire CFS specimens were presented, which can be used to judge the maximum temperatures reached by CFS in fire. The results showed that significant differences exist between the present and previous postfire experiments for CFS and hot-rolled steel. Hence, some conservative formulas for the postfire mechanical properties of Q345 and G550 CFS are proposed.