Mechanical properties of cold-formed steel semi-oval hollow sections after exposure to ISO-834 fire

Due to modern architectural requirements, the applications of novel tubular sections are getting more and more popular. The semi-oval hollow section (SOHS) is one of the novels and recently developed tubular sections, and the post-fire mechanical properties of cold-formed steel semi-oval hollow section (CFSSOHS) members are not yet known. Thus, this paper presents a test programme to investigate the mechanical properties of CFSSOHS members after ISO-834 standard fire (ISO-834, 1999, [1]) exposures in a gas furnace. The CFSSOHS members were made of Q345 steel grade and obtained by cold-forming of hot-extruded seamless circular hollow section members. In total, 4 identical sets (i.e., set-1,2,3 and 4) of CFSSOHS members were prepared, and then heated up to different fire exposure temperatures of 300 degrees C, 550 degrees C, 750 degrees C, and 900 degrees C respectively, followed by their natural cooling inside the gas furnace. Once cooled down to ambient temperature, a total of 51 post-fire longitudinal tensile coupon specimens were fabricated. From each fire exposed CFSSOHS member, coupon specimens were extracted from the flat, curved, and corner portions of the sections. Using the static post-fire stress-strain curves, residual mechanical properties and corresponding residual factors were determined. Empirical equations were proposed to predict the post-fire residual mechanical properties of CFSSOHS members. In addition, using a scanning electron microscope, microstructures of a typical CFSSOHS member for all 4 fire exposure temperatures were compared with those without fire exposed specimens.