Experimental Investigation and Direct Strength Design of High-Strength, Complex C-Sections in Pure Bending

Plain C- or Z-sections are two of the most common cold-formed steel shapes in use throughout the world. Other shapes are high-strength SupaCee and SupaZed (Bluescope Steel Ltd., Melbourne, Australia) steel sections, which are widely used in Australia as purlins in roof and wall systems. They contain additional return lips and web stiffeners that enhance the bending capacity of the sections. Design methods for these sections are normally specified in the Australian/New Zealand Standard for cold-formed steel structures or the North American Specification for cold-formed steel structural members. In both standards, which include the newly developed direct strength method of design (DSM), the method presented is developed for beams and columns, including the reliability of the method. This paper presents two different test series on both plain C- and SupaCee sections in pure bending (constant moment). They were performed at the University of Sydney for the extension of the DSM to include channel section beams with complex stiffeners. Two different section depths and three different thicknesses of high-strength lipped channel sections were tested in pure bending. Tests with and without torsion/distortion restraint straps screwed on the top flanges in the pure bending region were also considered to allow local and distortional buckling to form in the sections, respectively. Test results and formulas developed from the DSM are summarized in the paper. Three different cases where moments are used in association with yield, inelastic, or plastic criteria in the DSM local and distortional strength equations are compared with the test data. By comparisons between cases, a proposed recommendation for DSM inelastic buckling strength design in pure bending with extended nondimensional slenderness limit for both local and distortional buckling is given in the paper.