Material degradation of Q345 cold-formed steel under elevated temperatures

Material mechanical properties at elevated temperatures are important inputs for fire safety design and numerical analysis of multi-story cold-formed steel structures. This paper presents a detailed experimental investigation of Q345 cold-formed steel with a thickness of 1. 5 mm under steady-state thermal conditions. Both flat and corner parts of Q345 cold-formed steel sections were included. The results showed that the elastic modulus and ultimate strength of the corner part of Q345 steel were similar to those of the flat part at room and elevated temperatures. However, due to the cold-forming effects, the yield strength of the corner part of Q345 steel under elevated temperatures was much larger than that of the flat part. As a result, the current local buckling design specifications based on the reduced yield strength and elastic modulus of the flat part of Q345 steel may be conservative. In addition, the current BS5950, AS4100, and Eurocode 3 standards overestimated the mechanical properties of Q345 steel under elevated temperatures. An empirical equation was proposed to estimate the reduction factors for the yield and ultimate strengths and the elastic modulus of Q345 steel under elevated temperatures, where the necessary 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.