Buckling Mode Decomposition of Built-up Members via Modal Finite Strip Method (m-FSM)

This paper presents a method to perform the decomposition of the deformed shape of built-up members into the structurally meaningful "pure" modes: local, distortional and global modes. The method is based on the core concept of the authors' modal finite strip method (mFSM) for single sections, which utilizes normalized strain energies to identify different buckling mode classes. The attention to buckling mode decomposition of built-up sections is motivated by the requirements of the Direct Design Method (DSM) to identify and calculate the buckling loads of the pure local and distortional modes. The presence of discrete fasteners in built-up sections influences the overall buckling behavior and changes the buckling modes. In this study, the mFSM is further developed to achieve a complete decomposition technique that accounts for discrete fasteners. The proposed method is verified against finite element and finite strip solutions through numerical examples and shown to be accurate. The obtained results show that the proposed method can be a valuable tool for the assessment of the behavior of built-up members and their design.

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