Direct Strength Method for Cold-Formed Steel Unlipped Channel Columns Subject to Local Buckling

This paper evaluates the strength of unlipped channel columns using the Direct Strength Method (DSM), which applies to the sections with either intermediate or edge stiffeners. The local buckling behaviour of plain cold-formed steel sections (CFS) with fixed-ends is studied by investigating the results of (i) 25 experimental studies published in the literature and (ii) a numerical study of 105 CFS sections. This paper reveals that the aspect ratio (web height to flange width ratio) of the cross-section significantly affects the local buckling strength of plain unlipped channel sections. The DSM local buckling strength as specified in the codes of practice ignores the effect of this parameter, resulting in unconservative designs for such columns. The difference in strength prediction can be up to about 15% for columns with a web height to flange width ratio of 3.0. At this aspect ratio, the inter-element stiffness contribution in the post-buckling stage is the minimum, resulting in a lower strength estimation. To account for this inadequacy, this paper suggests a modified DSM design procedure that accounts for the effect of stiffness contribution from the aspect ratio of the cross-section in the post-buckling stage. The proposed modified DSM design equations agree well with the results of experimental and numerical studies and preserve the target reliability of AISI- S100 (2016) for LRFD