VOL. 13, NO. 1, JANUARY 2018 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences ©2006-2018 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com 146 EXTENSION OF DIRECT STRENGTH METHOD TO PERFORATED STIFFENED COLD-FORMED STEEL COLUMN AND TWO DIMENSIONAL FRAME. P. W. Kubde and K. K. Sangle Department of Structural Engineering, Veermata Jijabai Technological Institute (VJTI), Mumbai, India E-Mail: pwkubde@st.vjti.ac.in ABSTRACT The use of cold-formed steel (CFS) structures is increasing due to the advances in manufacturing, construction technologies and relevant standards. CFS has many advantages. However the design of CFS structures is complex because of their thin walled open sections making them vulnerable to torsional-flexural buckling and local buckling. Direct strength method (DSM) is the method available for individual beam and column, with certain limitations. To overcome few such limitations this paper attempts to find some simplified formulae as an extension to DSM for stiffened perforated column and two dimensional frames. Already experimented column sections and frames were used for validation. Finite Element Method is used to analysis the column and frame for its load carrying capacity with various parameters. Based on FEM analysis simplified formulae are proposed as an extension to use of DSM for stiffened perforated columns and two dimensional frames. Keywords: cold formed steel, distortional buckling, global buckling, direct strength method, finite element analysis. 1. INTRODUCTION In India due to continuously increasing industrialization as well as heavy population, cold-formed steel structures like industrial storage rack structures and mass housing are the need of the hour, for which cold- formed steel frames can prove as very economical and efficient alternative. Column section with stiffeners can be the component of this frame to enhance its performance. Cold-formed steel has advantages of attractive appearance, fast construction, low maintenance, easy extension, lower long-term cost, non-shrinking and non-creeping at ambient temperatures, no requirement of formwork, termite-proof and rot proof nature , uniform quality, non combustibility. 1.1 Direct strength method-A brief The development of the DSM started at the University of Sydney by research in to distortional buckling of rack post sections (columns) and was further developed for method of beams.(Schafer B. W., 2006). Further Hancock (Hancock et al, 1994) showed that compressive strength in a distortional failure correlated well with the slenderness in the elastic distortional mode. Accurate member elastic stability is the fundamental to the DSM. The method is based on the idea that if all the three elastic instabilities viz , local, distortional and global buckling , along with load or moment causing yielding of the section can be found out , then the strength can be directly determined. The method uses column curves for global buckling with application to local and distortional buckling instabilities. The increased accuracy of the method occurs due to improvements in the local buckling prediction. The method also takes into consideration, deflection calculation (Serviceability). It is a reliable method, its reliability being established using limit state design format in use in the United States. (Schafer B. W., 2006) Appendix 1 of the North American Specification for the design of CFS structural Members, 2004, supplement to the 2001also includes a number of tables that provide the geometrical and material bounds members which passed in the verification of the direct strength approach, in the process of codification of the same. Pre- qualified is the apt name that has been given to theses sections. Although this method is mainly due to Schafer B. W. (Schafer B. W., 2000), Moen C. along with Schafer B. W., took this method further by developing DSM equations that are applicable to CFS structural members with perforations (Moen C.D. and Schafer B. W., 2006; 2008; 2010; 2011) In 2006 Schafer B. W. came out with a guide “Direct Strength Design method guide”. (Schafer B. W., 2006). 1.2 Literature Review A study on the flexural strength and deflections of discretely braced cold-formed steel C and Z sections was conducted at the University of Florida (Ellifrit D, 1991; Ellifrit D, 1992; Ellifrit D, 1997). In the research, typical C and Z sections were tested in flexure with various types of bracing. Researchers developed a finite element model for the nonlinear large-deflections and rotation analysis of beam-columns. (Pi YL, 1994; Pi YL, 1994). Researchers Performed lateral buckling tests on unbraced simply supported cold-formed lipped channel beams. A vertical load was applied at the shear centre of the section, or at a point below the shear centre. The beams were supported at the ends by connecting them to a steel block with two bolts at the web of the section (Bogdan M. Put, 1999; 1999). In 2003, Narayanan, S. and Mahendran, Mahen studied the distortional buckling behaviour of a series of innovative cold formed steel columns by performing more than 15 laboratory experiments on innovative steel columns of intermediate length under axial compression to