Local-distortional interaction behaviour and design of cold-formed steel built-up columns.

This research highlights the possibility of LD interaction in the built-up columns and provides a detailed design procedure to account for the strength erosion due to LD interaction. For this, a comprehensive numerical study using finite element (FE) method is performed where the FE models are validated using the test results in the literature. In this study, three types of built-up sections, i) back-to-back I section made of two lipped channels, ii) nested section made of two lipped channels, and iii) nested section made of one lipped and one unlipped channel, are selected. The effects of LD interaction, fastener spacing, and end fastener group (EFG) on the ultimate strength of built-up columns are investigated. The study reveals that the LD interaction behaviour of the built-up back-to-back I section in terms of failure mode and normalized ultimate strength result (P u / P y) is similar to that of a lipped channel. For the built-up nested section, improved strength is observed when the lipped channel fails in distortional buckling. No improvement is observed in the LD strength of built-up sections with reduced fastener spacing or by using EFG. The study shows that the traditional direct strength method (DSM) based design procedure for LD interaction is conservative for built-up sections. Hence, a reliable design procedure is proposed based on the modified DSM equations obtained from the literature. The strength and failure mode predictions of the proposed method match well with those of experimental studies in the literature and the numerical study of this paper. • Evidence of local-distortional (LD) interaction in built-up columns is presented based on a detailed numerical study • Investigated the behaviour of both back-to-back and nested sections made of lipped channels and unlipped channels. • Explored the effects of built-up formation type, fastener spacing and end fastener group on the LD interaction strength. • LD interaction is significant in local buckling dominant sections than distortional buckling dominant sections. • New direct strength method based design procedure is proposed which is more accurate than the traditional design approach. [ABSTRACT FROM AUTHOR]