On the strength and DSM design of cold-formed steel web/flange-stiffened lipped channel columns buckling and failing in distortional modes

Abstract

Results recently reported by Kumar and Kalyanaraman [1] suggest that the presence of web and flange V-shaped intermediate stiffeners in cold-formed steel lipped channel columns may alter considerably their distortional buckling, post-buckling and collapse behaviours. The objective of this work is to assess the validity of this assertion, constituting a significant break from the existing knowledge, which is widely accepted by the technical/scientific community working with cold-formed steel structures. In particular, it may even imply that the application of the Direct Strength Method (DSM) to web/flange-stiffened lipped channel columns requires the consideration of a different (higher) distortional strength curve. In order to carry out the above assessment, this work reports a thorough numerical investigation aimed at studying the peculiarities of the distortional buckling, post-buckling and collapse behaviours of (web/flange) stiffened cold-formed steel lipped channel columns. The first step consists of selecting column geometries associated with “pure” distortional buckling and failure modes, which is ensured by having local and global critical buckling loads much higher than their distortional counterparts – this task is carried out by means of sequences of “trial-and-error” buckling analyses, performed with the GBTul code. Then, it is necessary to address the mechanical characterisation of the distortional critical buckling modes in the stiffened columns, a task much more involved than in plain lipped channel columns (and by no means trivial). This is done through the in-depth inspection of the results provided by the aforementioned GBT buckling analyses. Next, an Ansys shell finite element model is employed to perform geometrically and materially non-linear analyses of the selected stiffened lipped channel columns. The numerical results obtained (equilibrium paths, failure loads and deformed configurations) are presented and discussed, in order to assess the influence of the V-shaped intermediate stiffeners. Finally, on the basis of the ultimate strength data obtained and also the experimental and numerical failure loads reported in [1], the paper presents some considerations concerning the application of the current DSM strength curve to the design of (web/flange) stiffened lipped channel columns failing in distortional modes – in particular, some conclusions are drawn concerning the need to develop a new DSM distortional design curve to handle columns with this cross-section shape.