Elevated Temperature Material Degradation of Cold-Formed Steels under Steady- and Transient-State Conditions

Material properties at elevated temperatures are important factors in the fire safety design and numerical analysis of cold-formed steel structures. Most of the previous research on material properties at high temperatures has adopted the steady-state test method. However, the transient-state test method is more realistic for actual fire conditions. This paper presents a detailed experimental investigation of Q345 cold-formed steel with a nominal yield strength of 345MPa and a thickness of 1.5mm under transient- and steady-state conditions. Both the flat and corner parts of Q345 cold-formed steel sections are considered. The results showed that the steady-state method was not equivalent to the transient-state method for Q345 steel; in addition, current standards provided overestimations for the mechanical properties of Q345 steel under elevated temperatures. An empirical equation was proposed to estimate the reduction factors for the yield and ultimate strength and the elastic modulus of Q345 steel under elevated temperatures, where the essential parameters were determined through fitting. The stress-strain relationship of Q345 steel under elevated temperatures was further developed based on the Ramberg-Osgood model, which compared well with the experimental results. (C) 2013 American Society of Civil Engineers.