Introduction and evaluation a combined system in order to a two-level performance

In this paper a new and innovative configuration of lateral bracing for steel frame construction is presented and discussed. Eccentrically Knee Bracing (EKB) frame, as it is called in this paper, is made up of four structural elements, the knee element, the diagonal brace element, the link element and the columns. The knee and link elements are a fuse-like element that dissipates energy by the formation of plastic shear hinges or flexural hinges when the building is subjected to moderate and severe lateral loads, respectively. The diagonal brace element, on the other hand, provides the required level of lateral stiffness and remains in the elastic range at all time. In order to examine this system behavior, ANSYS, which is a well-known finite element software is used. Also two half-scale EKB were tested using the SAC loading protocol and an innovative loading protocol. The experimental and numerical results indicated that in this system, ductility and cumulative dissipated energy can be significantly increased. Eccentrically braced frames (EBFs) have been indicated as the excellent elements of a structural typology suitable for satisfying the different design objectives of modern performance-based seismic engineering in medium or high-rise steel buildings. EBFs address the desire for a laterally stiff framing system with significant energy dissipation capability to accommodate large seismic forces. EBFs combine the high elastic stiffness of concentrically braced frames (CBFs) with the high ductility and energy dissipation capacity of moment resisting frames (MRFs). The excellent seismic behavior of EBFs reported in the first numerical studies on traditional eccentrically braced structures subjected to lateral static forces. To improve the seismic performance and behavior of the steel framed structures, further modification and innovation to enhance the structural performance is essential. For this purpose, a modified structural form that adopts eccentrically knee brace elements in the corner regions of the beams and columns, namely Eccentrically Knee Braced frame (EKB) is considered in this study. In an EKB, one end of the brace is connected to a short knee element instead of beam-column joint and other end of brace is connected to a short link element. The link element which is the short beam segment between the brace-beam and beam-column joint, is designed to dissipate energy through shear yielding during severe earthquakes. Thus, the link element acts as a ductile fuse to safeguard the brace against possible buckling. In this study, the knee element is designed in such a way that the knee yields in shear instead of flexure under moderate excitation and link element is designed to yield under severe excitation. The seismic performance and ductility of this proposed system is of great concern, which is covered in the current paper. All tests were of the cyclic quasi-static type for which similar displacement time histories were considered.