Post-fire mechanical response of high strength steels

Abstract

A test programme looking into the residual mechanical properties of fire exposed high strength steels (HSS) has been presented in this study. Accurate predictions of the residual mechanical properties of fire exposed structures are necessary to develop a proper strategy for their restoration. A test programme was conducted to investigate the post-fire residual material properties of thermomechanical control processed cold-formed HSS tubular members. The nominal yield strengths of tubular members were 900 and 960 MPa. The tubular members investigated in this study were made up of square, rectangular and circular hollow sections. The tubular members were heated inside a gas furnace, where the fire exposure generally followed the heating part of the ISO-834 standard fire [1] curve. The tubular members were exposed to 4 fire exposure temperatures, i.e. 300°C, 550°C, 750°C and 900°C, followed by their natural air-cooling inside the furnace. Subsequently, at ambient temperature, a total of 80 post-fire coupons was extracted from these ISO-834 standard fire exposed tubular members. The post-fire coupons were extracted from the flat, corner and curved regions of the fire exposed tubular members. The post-fire material properties and their respective residual factors were determined using the static stress–strain curves, which were obtained from their corresponding test stress–strain curves. New prediction equations are proposed for the post-fire residual material properties using the post-fire residual factors obtained in this test programme as well as available data in the literature for steel grades more than or equal to S690. It should be noted that two sets of prediction equations are proposed in this study, first, for the flat region, and second, for the corner and curved regions. The proposed prediction equations for the post-fire residual material properties of the flat region are valid for both hot-rolled and cold-formed HSS with nominal yield strengths ranged from 690 to 1070 MPa. Whereas, the proposed prediction equations for the post-fire residual material properties of the corner and curved regions are valid for cold-formed HSS with yield strengths ranged from 900 to 1200 MPa.