Design of cold-formed steel columns subject to local buckling at elevated temperatures

Cold-formed steel (CFS) is one of the popular materials in low and mid-rise building construction. However, CFS member capacities reduce at elevated temperatures as their yield strength and Young's modulus decrease with increasing temperature. In addition, CFS exhibits nonlinear stress-strain characteristics at elevated temperatures, which is significantly different to the ambient temperature stress-strain characteristics. Although several studies have been conducted on the elevated temperature local buckling capacities of CFS columns, they have not considered the effects of nonlinear stress-strain characteristics such as the nonlinearity between proportional limit stress and yield strength, varying yield-strength to Young's modulus ratio and strain hardening between yield and ultimate strengths. Thus, this research study investigated their effects in detail, and proposed modified design equations for the elevated temperature local buckling capacities of CFS columns based on the effective width and direct strength methods given in AS/NZS 4600, since the current design equations give unsafe predictions in many cases. The modified design equations incorporating the effects of elevated temperature nonlinear stress-strain characteristics were developed using the local buckling capacities of CFS columns obtained from 740 finite element models of lipped channel sections. A simplified design method is proposed to determine the compression capacities of commonly used Australian open CFS sections at elevated temperatures.