Experimental Study of High-Strength Cold-Formed Stiffened-Web C-Sections in Compression

High-strength cold-formed steel sections are commonly used in a variety of applications including residential construction. These steel sections typically have a nominal yield stress of 550 MPa and the use of such high-strength material allows for a reduction in thicknesses. With this reduction in thickness, the high-strength steel can be manufactured into complex shapes including stiffeners. Such complex shapes exhibit structural instabilities such as local, distortional, and flexural-torsional buckling modes, and in some cases, interaction of the local and distortional buckling modes may occur. This paper describes the design and testing of web-stiffened high-strength steel cold-formed lipped channel columns. In order to be able to apply the direct strength method (DSM) in Section 7 of the Australian Standard AS/NZS 4600:2005 and Appendix 1 of the North American Specification to design, the steel sections must be prequalified as a compression member. The section chosen is prequalified and has nearly coincidental local buckling and distortional buckling loads as well as a flexural-torsional mode which varies with length. A series of compression tests was carried out in a 300-kN capacity SINTEC testing machine over a range of lengths with fixed-ended conditions. The varying lengths were chosen so as to observe the buckling modes and the possibility of interaction between them. The effect of the different types of failure modes is also discussed in this paper. The experimental results are then compared with design methods in the existing design standards. The methods include the effective width method and the DSM as described in the Australian Cold-Formed Steel Structures Standard and the North American Specification. It is shown that the existing standards are unconservative and new proposals for dealing with this are made.