Design Rules for the Sectional Buckling of Built-Up Cold-Formed Steel Members in Compression

The paper investigates the design rules for built-up cold-formed steel columns, focusing on local and distortional (sectional) buckling. The accuracy of current design methods is studied through a comprehensive comparison with both experimental and numerical data. The results of these comparisons highlight the strengths and limitations of current approaches when applied to the sectional buckling of built-up cold-formed steel columns. A detailed reliability analysis is then undertaken to assess the performance of the current design methods. The results suggest that the direct strength method (DSM), combined with accurate elastic buckling stress calculations that include semirigid interconnections between sections, performs better in predicting the distortional buckling strength while still providing safe and efficient predictions of the local buckling strength. In addition, adjustments are proposed to enhance the accuracy of the current direct strength equations for the ultimate capacity predictions of built-up compression members to comply with the required level of reliability in the Australian and North American provisions while conserving the currently prescribed resistance factor. The proposed adjustments are established upon a rational modification of the parameters of the direct strength equations for local and distortional buckling capacities based on the relative number of restrained components in the built-up configuration undergoing the sectional buckling mode of interest and the fastener spacing relative to the associated critical buckling half-wavelength. The proposed modifications are reasonably accurate and robust for different built-up cross section and fastener configurations.