Multiobjective optimization of cold-formed steel columns

• Multiobjective optimization is used to maximize the strength of coldformed steel columns.
• Two objectives maximize separately local/global and distortional strengths
• Maximization of local/global strength leads to a decrease of maximum distortional strength (and vice-versa).
• Symmetric sections are better than anti-symmetric ones for distortional strength maximization
• Anti-symmetric sections are better than symmetric ones for local/global strength maximization

The optimal design of cold-formed steel columns is addressed in this paper, with two objectives: maximize the local-global buckling strength and maximize the distortional buckling strength. The design variables of the problem are the angles of orientation of cross-section wall elements—the thickness and width of the steel sheet that forms the cross-section are fixed. The elastic local, distortional and global buckling loads are determined using Finite Strip Method (CUFSM) and the strength of cold-formed steel columns (with given length) is calculated using the Direct Strength Method (DSM). The bi-objective optimization problem is solved using the Direct MultiSearch (DMS) method, which does not use any derivatives of the objective functions. Trade-off Pareto optimal fronts are obtained separately for symmetric and anti-symmetric cross-section shapes. The results are analyzed and further discussed, and some interesting conclusions about the individual strengths (local-global and distortional) are found.

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