International Journal of Science and Engineering Applications Volume 7–Issue 12,483-488, 2018, ISSN:-2319–7560 www.ijsea.com 483 Seismic Performance Evaluation of Knee and EBF Braced Frames Using Nonlinear Static Analysis Amin Moshtagh Assistant Professor Civil Engineering Department Garmsar University Garmsar City, Semnan, Iran Vahid Saberi Assistant professor Civil Engineering Department Eyvanekey University Eyvanekey City, Semnan, Iran Hamid Saberi Assistant professor Civil Engineering Department Eyvanekey University Eyvanekey City, Semnan, Iran Abstract: Earthquake-resistant structure systems should be designed to stand large deformation to absorb and attenuate imposed energy due to an earthquake while providing sufficient stiffness to transfer the forces to the base without collapse. Knee Braced Frames (KBF), which involves added additional diagonal elements to a frame to increase its ability to withstand lateral loads, is suggested by several researches. In this study, the seismic performance of KBFs are evaluated and compared with Eccentric Braced Frames (EBF). Nonlinear static analyses were utilized for seismic evaluation and comparison between the mentioned frame systems. Three steel structures of 5, 10, and 15-story were numerically modeled, and the seismic parameters such as lateral stiffness, inter-story drift, ductility, and response modification factors were calculated for each structure system. It was observed that using KBF systems resulted in a reduction in intersotry drifts compared to EBFs. KBF systems show more stiff responses in comparison with EBFs and they presented much more stiff response by reducing the knee element length. The KBFs have more ductile behavior in comparison with EBFs, although base shear in KBFs is less than EBFs. Keywords: EBF Bracing, Knee Bracing, Nonlinear Analysis, Pushover, Seismic Parameters 1. INTRODUCTION Earthquake-resistant structures should be designed in a way that they are able to stand against large deformation due to the earthquake to absorb and attenuate imposed energy. On the other hand, they should have a sufficient stiffness for transferring the forces to base without collapse. To fulfill these goals, using bracing system which involves added diagonal elements to a frame to increase its ability to withstand lateral loads is an option. There are two major braced frame systems, Concentric Braced Frame (CBF) and Eccentric Braced Frame (EBF). CBFs consist of diagonal braces located in the plane of the frame where both ends of the brace connected to the ends of other framing members while for EBF system one or both ends of the brace do not connected to the ends of other framing members. In the CBF systems, members form a truss-like structure, creating a stiff frame while EBF combines the features of a moment frame and a concentrically braced frame and minimizing the disadvantages of each system resulting to improve in the system performance in the event of earthquakes. Although EBFs usually have appropriate behavior, after the failure of the link beam (the element between two ends of the brace in the floor), floor beam would be seriously damaged. Since this element is considered as one of the main structural components, structural rehabilitation would be difficult and sometimes impossible. Moreover, bracing elements and shear links dissipate energy when exposed to the strong earthquakes, but in the weak earthquake, link beam would stay in the elastic region. In addition, analysis and design of link beams are complex. Therefore, attempts for finding seismic resistant systems with large ductility and stiffness have been continued. These drawbacks are mitigated to some extent in the works of Aristizabel-Ochoa in 1986 by introducing Disposable Knee Bracing systems as a new alternative structural system for earthquake-resistant steel structures [1]. This system possesses an appropriate stiffness and absorbs earthquake energy through yielding of knee elements. In addition, the diagonal element provides lateral stiffness during moderate earthquakes. However, the knee element is designed to behave in nonlinearity range for dissipation of the energy under strong ground motions. Many researches have been performed to study the experimental and analytical performance of knee brace systems. Sam et al. 1995 carried out pseudo dynamic testing of 1-story and 2-story specimens using KBF system, which showed the system has enough capacity to reduce the earthquake damage effectively and economically [2]. Maheri et al. 2003 performed pushover testing of KBF and CBF systems mounted on concrete reinforced moment resisting frame structures. The response modification factors of the systems are evaluated and significant improvement in the ductile behavior was observed in the contract of unbraced reinforced concrete building [3]. In this study, the seismic performance of KBF systems is evaluated and compared with EBF systems. Nonlinear static analyses were utilized for seismic evaluation and comparison between the mentioned frame systems. Three steel structures of 5-story, 10-story, and 15-story were modeled numerically, and the seismic parameters such as lateral stiffness, ductility, and response modification factors were calculated for each structure system. 2. STRUCTURE DESIGN Seismic and gravity loads applied to the structures according to ASCE 7-10 [4]. For calculating static equivalent lateral load, it was assumed that the buildings were located in a high seismic region, and soil type C was selected. Response modification factors for EBF and KBF systems are assumed to be 7. Dead and live loads are 700 and 200 kg/m 2 , respectively. Design of the structures was performed