ستون های فولادی در آتش و یخ زدگی Cold-formed steel columns at both ambient and fire conditions

1. Introduction Nowadays, in the light steel framing construction industry, individual profiles with different cross-section shapes (C, U, R, etc.) are used to fabricate built-up members. A built-up coldformed steel (CFS) structural member can span more distance and present higher load bearing capacity and torsional stiffness. Moreover, usually, built-up members are symmetric, eliminating eccentricities between shear and gravity centers, leading to improved member stability. Built-up cross-sections are built fastening individual profiles with self-drilling screws or alternatively using seam weld (Stone and Laboube, 2005; Li et al., 2014; Wilson and Guzmán, 2011). Research has been focused mainly on open sections such as plain and lipped channels, with simple and complex edge stiffeners, with and without holes and angles (Young and Rasmussen, 1998a; Young and Rasmussen, 1998b; Kesti and Davies, 1999); however, in the past few years, some research on the behaviour of built-up CFS members has been conducted (Georgieva et al., 2012; Yuanqui et al., 2014; Young and Chen, 2008). Currently, the effective width method (EWM) is used worldwide, and the direct strength method (DSM) (Schafer, 2008) is used in North America, for design of CFS structural elements. Design guidelines in EN 1993-1-3 (2004) for built-up members are still vague. For instance, the EN 1993-1-3 (2004) only predicts that the buckling resistance of closed built-up members should be determined using the buckling curve b in association with the basic yield strength fyb and buckling curve c in association with the average yield strength fya if Aeff = Ag. In fire situation, the design methods presented in the EN 1993-1-2 (2005) for hot-rolled steel members are also applicable to CFS members with Class 4 cross-sections, establishing the same reduction factors for the mechanical properties of hot-rolled and CFSs. Some studies show that the reduction factors for CFS (Outinen, 1999; Kankanamge and Mahendran, 2011; Ranawaka and Mahendran, 2009a) are different from those prescribed in EN 1993-1-2 (2005). Also, the EN 1993-1-2 (2005) for Class 4 cross-sections limits the critical temperature to 350°C, which may be overly conservative if the load ratio is not considered (Ranawaka and Mahendran, 2009b; Heva, 2009). Moreover, there are no specific design guidelines regarding the influence of restrained thermal elongation in the overall behavior of CFS columns in fire. Several studies on this matter were already conducted but for heavy hot-rolled steel columns (Franssen, 2000; Ali and O’Connor, 2001). The great majority of the conducted research on the fire behaviour of CFS columns has been focused on the local and distortional buckling phenomena using both experimental and numerical analysis (Feng et al., 2003a, 2003b; Lee, 2004; Ranawaka and Mahendran, 2006).