Post-fire mechanical properties of cold-formed steel hollow sections

Cold-formed steel hollow sections are extensively used in building construction due to their potential benefits over hot-rolled and open cold-formed steel sections. Their performance in fire is important for successful structural applications while their post-fire residual capacities are also important to assess the extent of their fire damage and reusability with and without strength enhancing techniques. This paper presents the details of an experimental investigation on the post-fire mechanical properties of cold-formed steel hollow sections. Tensile tests were performed on coupons exposed to elevated temperatures varying from 100 to 800 °C and then cooled down to room temperature. These coupons were cut from cold-formed steel square and rectangular hollow sections with varying thicknesses and grades. The results from this investigation provided post-fire stress-strain curves, yield strengths, ultimate strengths and elastic modulus, and their reduction pattern. They show that the post-fire mechanical properties of cold-formed steel hollow sections are different to those of open cold-formed channel and hot-rolled sections. New predictive equations were proposed to determine the post-fire mechanical property reduction factors. Post-fire mechanical properties were also compared with their elevated temperature mechanical properties to evaluate their strength gain after cooling down. This paper also evaluates various strength enhancement techniques for use with fire damaged cold-formed steel hollow sections.