Structural behaviour of cold-formed steel built-up closed section beam–columns

Abstract

In order to examine the beam–column behaviour of cold-formed steel (CFS) built-up closed sections, a numerical study was performed and is presented in this paper. The finite element model with careful considerations of material nonlinearity and initial geometric imperfections was established for the thin-walled section components under eccentric compression and verified against the existing experimental results. Then, the calibrated finite element model was adopted to obtain the numerical predictions of additional 280 beam–columns over an extensive scope of sectional geometries and initial loading eccentricities. Underpinned by the newly generated numerical predictions of 280 members and the existing test results of 30 specimens, the design approaches using codified interaction formulae and a bi-linear interaction expression proposed in the recent research for CFS built-up open section beam–columns together with the nominal compressive and flexural resistances determined by direct strength method (DSM) were assessed for thin-walled built-up closed section members under combined loading. Based on the nominal resistances determined by DSM, the interaction formulae in the ANSI/AISC 360 and EN 1993-1-3 as well as the available bi-linear interaction expression were overall unconservative for the design of built-up closed section beam–columns, while the interaction curve specified in the AISI S100 generally offered slightly conservative strength predictions. Furthermore, it was revealed that the interaction relationship in the AS/NZS 4600 and the bi-linear interaction expression modified in this study could provide accurate predictions for the CFS built-up closed section beam–columns, whereas the predictions obtained by the modified bi-linear interaction expression were overall less scattered.