Tests of Cold-Formed Channels with Local and Distortional Buckling

Thin-walled channel sections may undergo a mode of buckling, called distortional, in which a lip-stiffened flange of the section rotates about the flange-web junction. If the sections are composed of high-strength steel, then there may be a significant postbuckling reserve of strength beyond the elastic distortional buckling stress in a similar manner to that which normally occurs for local buckling. The results of compression tests of thin-walled channel section columns formed by brake-processing are described. Two different section geometries, a simple lipped channel and a lipped channel with an intermediate stiffener in the web, were tested between fixed-end boundary conditions. The geometries and yield strength were chosen to ensure that a substantial postbuckling strength reserve occurred in the distortional mode and that local buckling might occur simultaneously at a shorter wavelength. Distortional or mixed local-distortional buckling stresses obtained from testing are compared with theoretical buckling stresses. Design curves for the maximum strength of the columns undergoing distortional or mixed modes of buckling are proposed and compared with the test results.