J. Basic. Appl. Sci. Res., 3(5)415-419, 2013 © 2013, TextRoad Publication ISSN 2090-4304 Journal of Basic and Applied Scientific Research www.textroad.com *Corresponding Author: Nasim Irani Sarand (M.Sc. Student), Department of Civil Engineering, University of Tabriz, Tabriz, Iran; Email: irani9@ms.tabrizu.ac.ir Seismic Behavior of Zipper Braced Frames; A Review Nasim Irani Sarand 1* , Abdolrahim Jalali 2 , Yousef Hosseinzadeh 3 1 M.Sc. Student, Department of Civil Engineering, University of Tabriz, Tabriz, Iran 2 Assistant Professor, Department of Civil Engineering, University of Tabriz 3 Assistant Professor, Department of Civil Engineering, University of Tabriz ABSTRACT Inverted-V-braced frames are one type of ordinary concentrically braced frame. The behavior of this system is controlled by the buckling of the first story braces in compression, resulting in localization of failure and loss of lateral resistance. The unexpected failure of steel structures during past strong seismic excitation led to full fill adequate strength for modern structures in seismic areas. Concentrically Braced Frames (CBF) shows a concentration of damage within a single floor and tendency of strong mechanism formation. The undesired effect of the unbalanced force can be reduced by adding zipper struts which is labeled zipper frames. As a consequence, the results showed that zipper struts can improve the seismic performance of CBF system. KEYWORDS: Concentrically braced frame, Seismic behavior, Unbalanced Force, Zipper struts. 1. INTRODUCTION Frequent damage was found in CBF buildings during Northridge earthquake on 1994, and Kobe earthquake [1, 2]. Concentrically Braced Frames (CBF) is an economical system for resisting lateral loads when lateral forces applied, brace elements initially provide both tensile and compressive resistance to balance lateral effect. In general for a brace element, the tensile capacity is greater than the compression capacity, when reaching its compressive capacity, the brace members buckle and a plastic hinge is developed at its mid length. As a result a big displacement occurred under strong seismic excitations; braces in compression have buckled, and in consequence lose their buckling resisting strength. After buckling of braces occurred, beams were deflected downward as a result of the combined action of the gravity loading and the unbalanced force developed at the braces to beam intersection point due to difference between tensile and post-buckling capacity of brace members. So, strong floor beams are required to stabilize the system when the unbalance vertical load transferred from braces to beams has increased due to the attaining the post-buckling strength in the compressive braces. Thus, because of this characteristic, CBF system has a limited efficiency in terms of distributing the lateral loads over the building height. In spite of this limitation, several studies have shown that the system is still prone to soft strong mechanism under seismic ground motions. Khatib et al. [3] proposed to link all beam-to-brace intersection points of adjacent floor and to transfer the unbalanced load to the vertical element called zipper struts [1, 2]. As a result, all compressive braces will be forced to buckle almost simultaneously while only a few tensile braces will yield. When ground motion reversed, braces that acted previously in tension buckle in compression, while the zipper strut transfers the unbalanced vertical load upwards, and downwards depending on ground motion signature. Previous studies on zipper frame: Khatib et al. [3] proposed zipper braced frames. Sabelli [4] suggested design method for concentrically brace frame with weak zipper strut. In this case, zipper struts are allowed to buckle and to yield, while braces behave in inelastic range. Tremblay and Trica [5] developed design method for CBF system with strong zipper strut, in this design method, zipper struts were designed to behave in elastic range [2, 6, 7, 8]. Leon and Yang [2003] and Yang et al. [2008] proposed to add a truss system at the top floor while top floor braces were designed to respond inelastic range, labeled CBF with suspended zipper strut. Chen [1] and Tirca and Tremblay [10] developed and found method, called CBF system with elastic zipper strut. The research conducted in the field of zipper braced frame is focused on low and mid-rise building. Along with the increase in building height, adverse effect, such as large lateral deformation due to the activation of higher modes could drive the building near collapse [1, 2]. Zipper braced frame Typically, in the CBF system, large story drifts is concentrated within a few stories and large ductility demand is required. Khatib et al. [3] developed a modified CBF system called zipper braced frame. Zipper strut is a vertical element added to a CBF system, to link together all brace-to-beam intersecting points. In this case, the zipper strut can behave either in tension or in compression. The vertical unbalanced force transferred to 415