Seismic behaviour of "all-steel" CFS structures: design criteria

The search for innovative building methods to ensure high structural, technological and environmental performance is promoting the development of light gauge steel structural systems. Among them, stick-built constructions realized with Cold-Formed Steel (CFS) profiles are attracting considerable interest in the construction sector and in the recent research studies. Nevertheless, the main European structural code for seismic design, the Eurocode 8 part 1 (EN 1998-1) [1], does not provide any prescription for the seismic design of CFS structures. Presently, the "North American Standard for Cold Formed Steel Framing - Lateral Design" AISI S213-07 [2] represents the only reference for the design of this structural typology under seismic actions. This document is developed by the American Iron and Steel Institute Committee on Framing Standards and it codifies the design of different seismic resistant CFS systems for Canada, Mexico and United States. As an effort to define the seismic design criteria for such structures, an extended theoretical and experimental study aimed to investigate the seismic behaviour of strap-braced stud shear walls has been carried out within RELUIS –DPC 2010-2013 research project. The research included a wide experimental campaign as well as theoretical analyses to define criteria for the seismic design of strap braced CFS structures. Among the different steel seismic-resistant systems regulated by the EN 1998-1, traditional concentrically braced frame (X diagonal) represents the closest system to the investigated one. In this paper a critical analysis of the current standards is illustrated, with particular reference to the analysis and comparison of the existing provisions for the two similar structural typologies (traditional concentrically braced frames and strap braced CFS system). Based on the results of that critical analysis, the design hypotheses for the definition of a case study have been defined. In addition, on the basis of the adopted design assumptions and the experimental results [3], guidelines for the seismic design of strap braced CFS structures are proposed.