Behaviour and DSM design of stiffened lipped channel columns undergoing local-distortional interaction

The paper reports the results of a numerical (ABAqus shell finite element analysis) investigation on the influence of local-distortional (L-D) interaction on the ultimate strength and design of fixed-ended cold-formed steel lipped channel columns with web and flange intermediate stiffeners. The columns analysed exhibit various geometries and yield stresses, ensuring several combinations of ratios between the (i) local and distortional critical buckling stresses, and (ii) yield stress and the highest of the above two critical buckling stresses (local or distortional). The objectives of this work are (i) to acquire in-depth understanding on the mechanics underlying the L-D interaction in the columns under consideration, and also (ii) to provide a contribution towards the development of an efficient Direct Strength Method (DSM) approach to design these structural elements. The results presented and discussed concern the post-buckling behaviour, ultimate strength and failure mode of fixed-ended web-flange-stiffened lipped channel columns previously selected to undergo more or less severe L-D interaction. Special attention is paid to the comparison between the ultimate strength erosion, due to L-D interaction, occurring in the columns analysed in this work and the (i) "plain" cross-section (i.e., without intermediate stiffeners) and (ii) web-stiffened lipped columns investigated earlier by the authors. Then, available experimental ultimate strengths of web-flange-stiffened lipped channel columns failing in L-D interactive modes are employed, together with the numerical failure loads obtained in this work, to assess the quality of the estimates provided by the existing DSM design approaches. The paper closes with (i) the reliability assessment of the DSM-based L-D approaches to predict the numerical and experimental failure loads, and (ii) some considerations about the impact of the findings reported on the design of cold-formed steel columns undergoing L-D interaction. (C) 2016 Elsevier Ltd. All rights reserved.