Mechanical properties of G550 cold-formed steel under transient and steady state conditions

Abstract

Material properties at elevated temperatures are the 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 G550 steel with a thickness of 1mm under both transient and steady state test methods. The test results obtained from transient and steady state methods are discussed, and the results show that the steady state method is not equivalent to the transient state method for G550 steel. The steady state test results of G550 may result in an overestimate of the fire resistance of cold-formed steel structures. In addition, the test results were also compared with those obtained from other researchers and the current design rules. The comparison shows that the yield strength predicted by BS5950 agrees well the transient state test results of G550 and is conservative for the steady state test results. However, BS5950, AS4100, and Eurocode 3 provide nonconservative predictions in other cases. Finally, a unified equation for the reduction factors, including the yield strength, elastic modulus, and ultimate strength of G550 at elevated temperatures, is proposed by the numerical fitting technique. A stress–strain expression of G550 at elevated temperatures is also given based on the Ramberg–Osgood model. The proposed equations are in good agreement with test results and meet the requirements for engineering.