Behaviour and Design of Cold-Formed Steel Members Subjected to Combined Axial Compressive Load and Bending Moment

Abstract

This study aims to provide an in-depth insight into the structural behaviour and design of the cold-formed steel (CFS) warping-restrained structural components subjected to combined actions sourced from gravity and lateral loads (i.e., beam-columns). A literature study exposes a lack of understanding of this subject and a need for the development of more precise design formulas for the design of CFS warping-restrained beam-columns. This study first evaluated the reliability of the Direct Strength Method (DSM) as specified in the American Iron and Steel Institute (AISI-S100) and Australian/New Zealand Standard (AS/NZS-4600), as well as the design methods proposed in the literature for estimating the load-carrying capacity of the CFS warping-restrained beam-columns. This investigation was conducted using a comprehensive dataset of beam-column capacities, generated based on detailed experimentally validated Finite Element (FE) models of CFS elements, considering material nonlinearity and initial geometric imperfections. The results indicated that the estimated capacity of beam-columns is significantly affected by the errors associated with (i) warping-restrained boundary condition effects, (ii) using equations for the calculations of buckling loads, and (iii) using the code-prescribed linear interaction equation. Subsequently, the structural performance and potential failure mechanisms of the CFS members subjected to various load combinations of compression and bending were assessed. The results were then used to improve the strength predictions of the beam-columns by proposing a general trinomial expansion representing curved surfaces for the interaction of axial compression and bending as a function of slenderness ratios. Next, to maximise the efficiency of the codeprescribed linear interaction equation, other new interaction expressions were developed using an Artificial Bee Colony (ABC) optimisation algorithm. Different exponent parameters have been proposed for minor- and major-axes bending, which result in a considerable improvement in the accuracy of the beam-column strength predictions compared to the existing methods. Lastly, the beneficial effects of additional restrained warping at the supports of CFS pinended columns with different lengths, cross-sectional dimensions, and shapes were evaluated using a comprehensive parametric study. Then, more precise and practical design equations aligned with the DSM have been developed for predicting the strength of these elements, considering various cross-sectional shapes.