Self-shape 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 principles behind the self-shape optimisation method are summarised herein and are applied to singly-symmetric (mono-symmetric) cold-formed steel columns. "Optimum" cross-sections for simply supported columns, 0.047 inch (1.2 mm) thick, free to warp and subjected to a compressive axial load of 11,167 lb (75 kN) are presented for column lengths ranging from 39.37 inches to 98.42 inches (1,000 mm to 2,500 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.