Self-shape optimisation application: Optimisation of cold-formed steel columns

Abstract

This paper presents the optimisation of cold-formed steel open columns using the recently developed self-shape optimisation method that aims to discover new profile shapes. The strength of the cold-formed steel sections is calculated using the Direct Strength Method, and the rules developed in the present work to automatically determine the local and distortional elastic buckling stresses from the Finite Strip and constrained Finite Strip Methods are discussed. The rules are verified against conventional and optimum sections yielded in this research, and found to accurately predict the elastic buckling stresses. The optimisation method is applied to singly-symmetric (mono-symmetric) cold-formed steel columns, and the operators behind the method for the special case of singly-symmetric open profiles are introduced in this paper. "Optimum" cross-sections for simply supported columns, 1.2 mm thick, free to warp and subjected to a compressive axial load of 75 kN are presented for column lengths ranging from 1000 to 2500 mm. Results show that the optimum cross-sections are found in a relatively low number of generations, and typically shape to non-conventional "bean", "oval" or rounded "S" sections. The algorithm optimises for distortional and global buckling, therefore likely subjecting the cross-sections to buckling interaction. A manual attempt to redraw the "optimum" cross-sections to include limitations of current manufacturing processes is made. Future developments of the method for practical applications are also discussed.