A numerical study on seismic characteristics of knee-braced cold formed steel shear walls Mehran Zeynalian, H.R. Ronagh n a School of Civil Engineering, The University of Queensland, Brisbane, Australia article info Article history: Received 7 April 2011 Received in revised form 26 July 2011 Accepted 26 July 2011 Available online 30 August 2011 Keywords: Cold-formed steel Knee-braces Lateral performance Response modification factor Numerical analysis Finite element model Optimization Residual stress Initial imperfection abstract Non-linear finite element analyses were carried out to evaluate and optimize the seismic characteristics of knee-braced cold formed steel shear walls using software ANSYS. Different structural characteristics including: material nonlinearity, geometric imperfection, residual stresses and perforations are taken into account. The numerical models were verified based on experimental tests. Agreement of the numerical simulations and the test results showed that finite element analysis can be used effectively to predict the ultimate capacity of knee-braced CFS shear panels. A total of 12 models with a various ranges of knee-elements’ lengths were investigated. Of particular interests were the specimens’ maximum lateral load capacity and deformation behavior in addition to a rational estimation of the seismic response modification factor. Preliminary conclusions presented in this paper, refer to the optimum seismic characteristics of knee-braced CFS shear walls and the corresponding dimensions and configuration. & 2011 Published by Elsevier Ltd. 1. Introduction Cold formed steel (CFS) structures have been increasingly used as load bearing structural components in housing industry in recent times. CFS structures are often light, durable, cost effective and easy to work with. A proper design of any CFS lateral resistant system requires the prediction of its collapse load capacity and can take advantage of optimization of the associated configura- tion that presents the best performance. One potential lateral resistant system for CFS structures is the knee-braced system, which uses elements similar to studs or noggins at an angle that runs in between the studs to form a knee- brace. Due to complex performance of such system, a series of experimental studies were performed by the authors [1] on full- scale knee-braced CFS shear walls considering different potential configurations. One of the main outcomes of experimental studies was that using brackets at the four corners of the frames where the chords were connected to tracks would improve the walls’ lateral performance characteristics, such as strength, stiffness and ductility. Besides supporting the chords and the tracks against buckling by reducing the buckling length of the members, one great advantage of using brackets is to use the plastic bending capacity of the brackets as an additional energy dissipating mechanism in the frame. This study is a complement to the previous study of the authors with an intention to expand on that using numerical tools. ANSYS [2] finite element software is used here in the current study to simulate the geometric and material nonlinear behavior of CFS frames. The finite element models developed have been calibrated with the previous experimental results of the authors [1]. Hence, the main aim of the current research is to evaluate the lateral performance of knee-braced systems having various brack- ets’ lengths including an estimation of their seismic response modification factors followed by a comparison with the recom- mended code values for the R factor. It is necessary to mention that the walls studied here are unlined, and the positive effect of gypsum board on the lateral performance of the frame under cyclic loading is ignored. This is due to the fact that post-earthquake observations of timber frame structures in the Northridge earthquake have shown that many gypsum board shear walls failed under imposed dynamic load [3]. Also, some design codes [4] have recommended neglecting the gypsum board’s contribution and relying only on the bare steel frames. It is necessary to mention that using double stud sections for the second and forth stud members is essential to improve the lateral performance of the walls when brackets are incorporated as it increases the chord buckling capacity [1]. Fig. 1 shows the general configuration of knee-braced CFS shear wall. The parameter X varies from 125 to 400 mm with the step of 25 mm. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/tws Thin-Walled Structures 0263-8231/$ - see front matter & 2011 Published by Elsevier Ltd. doi:10.1016/j.tws.2011.07.012 n Corresponding author. E-mail address: h.ronagh@uq.edu.au (H.R. Ronagh). Thin-Walled Structures 49 (2011) 1517–1525