Distortional buckling of compressed cold-formed lipped C channels via buckling mode shapes

Distortional buckling is a characteristic failure mode of compressed C channels with an intermediate unbraced length in the range of 4 to 7 times the web depth. The usual codified design method to calculate the buckling resistance by Eurocode EN 1993-1-3 proceeds through the determination of an effective cross-section with reduced thicknesses of the stiffener zones working against distortional buckling. To calculate the slenderness of the compressed stiffener zones Eurocode gives simple formulas, applicable separately for end stiffeners and intermediate stiffeners. The identification of the part of the cross-section to be used in these formulas can be doubtful. It can be demonstrated that these formulas will predict unreliable critical stresses being often on the unsafe side. EN 1993-1-3 allows to use more reliable values based on FEA or other approaches without giving a detailed, applicable design methodology. This paper presents a method which is able to assign cross-section buckling modes calculated with the help of finite strip analysis to compressed stiffener zones of a compressed general C section. Assignment is based on the determination of deformation energy balance. The proposed approach finds the zones of the section which tend to buckle during distortional buckling and the zones which are responsible to restrain this buckling. Additionally, it gives guidance on how to determine the exact area where the calculated thickness reduction shall be applied around the stiffener. For the thickness reduction the buckling curve given in the actual EN 1993-1-3 will be used. The proposed method is part of a larger research project which aims to provide a complete Eurocode compatible design methodology considering local, distortional and global buckling of general shaped cold-formed members subject to 3D loading and support condition, to be implemented in computer software programs working with 7DOF beam finite elements. © 2021 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.