Proceedings of the Cold-Formed Steel Research Consortium Colloquium 17-19 October, 2022 (cfsrc.org) Direct Strength Method Based Design of Cold-Formed Steel Built-Up Columns: Challenges and Solutions Akshay Mangal Mahar 1 , S. Arul Jayachandran 2 , Mahen Mahendran 3 Abstract The buckling behaviour and strength of cold-formed steel (CFS) built-up columns have been thoroughly explored in the literature. Additionally, different design procedures have been proposed for strength estimation as the current codes of practice suggest limited guidelines. In this paper, the proposed design procedures are compiled and evaluated against the ultimate strength results of experimental studies presented in the literature. The challenges and applicability of each design procedure are highlighted. As per the direct strength method (DSM), the nominal design strength is a function of yield stress and elastic buckling stress of the section. A wide discrepancy is observed while evaluating the strength of CFS built-up sections, as the proposed design procedures are based on the approximate elastic buckling stress calculation methods. Hence, the present study highlights the need for a unique design procedure and suggests design steps based on the existing solutions to obtain the elastic buckling stress of built-up section columns for better strength predictions. 1. Introduction Cold-formed steel (CFS) built-up columns are popular as wall studs in light gauge steel framing (LGSF) systems to resist high in-plane loading. Built-up sections formed by connect- ing two channels using self-drilling and self-tapping screws are popular as they are easy to assemble on-site, and the presence of bi-symmetry also helps in reducing torsional ef- fects. The popular choices are back-to-back connected built- up I-section and face-to-face connected built-up nested sec- tions. The CFS members under compression may fail in different buckling modes, i.e., local, distortional, global, or their interactions, before attaining the gross cross-sectional yield strength. For the design of CFS members, the direct strength method (DSM) is used, which provide simple de- sign equations based on a non-dimensional slenderness ra- tio (λ = p F y /F cr ). The DSM design equations are included in the North American Standard [1] and the Australian/New Zealand Standard [2] for local, distortional, global, and local- global interaction modes. For the design of built-up columns, current design provisions suggest using a modified slenderness ratio (MSR) for cal- culating elastic global buckling stress (F cre ). The modified 1 Post-doctoral researcher, Department of Civil Engineering, Indian Insti- tute of Technology Madras, India, [email protected] 2 Professor, Department of Civil Engineering, Indian Institute of Technol- ogy Madras, India, [email protected] 3 Professor, School of Civil and Environmental Engineering, Queensland University of Technology Australia, [email protected] slenderness ratio is given as, KL r MSR = s KL r 2 o + s r i 2 (1) where (KL/r) o is the slenderness of a fully-composite sec- tion, s is the fastener spacing, and r i is the minimum radius of gyration of an individual section. For elastic buckling stress calculation of a CFS section in lo- cal (F crl ) and distortional (F crd ) modes, the semi-analytical finite strip based software CUFSM [3] can be used. 2. Literature review Several studies have been performed in recent years to un- derstand the buckling behaviour and design of CFS built-up columns worldwide. As the current DSM design equations are limited to single CFS members, their applicability to pre- dict the strength of built-up sections has been investigated by various researchers. The major challenge was to obtain the elastic buckling stress of built-up columns to predict the strength using the DSM equations. Young and Chen [4] per- formed tests on built-up closed sections formed by two stiff- ened channels and suggested that the DSM design equa- tions were conservative when used with the elastic buck- ling stress solution of a single channel. Later, Zhang and Young [5] proposed a modified channel profile to investigate the strength of the built-up I-section column formed by two web-stiffened lipped channels using the existing DSM design equations. Li et al. [6] experimentally and numerically inves-
Direct Strength Method Based Design of Cold-Formed Steel Built-Up Columns: Challenges and Solutions
Jun 20, 2023
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