Assessment of Seismic Behavior of Eccentrically Braced Frame with Double Vertical Link (dv-ebf)

This paper is an experimental study on the behavior of vertical shear link in the single and double configurations. There have been three specimens selected for examining the effects of different details .The first specimen was an I section with no stiffener, while the second specimen composed of I section with stiffeners designed according to AISC 2005. Third specimens were composed of two I section with stiffeners. The tests carried out were of quasistatic kind and conducted on a full scale. The comments on double links are at first, explained in details and construction method, loading and test are then expressed. The details rendered for double vertical link have been in such a way that the results obtained show increase in its shear capacity and ductility. INTRODUCTION Nowadays, due to their appropriate seismic parameters, the use of Eccentrically Braced Frames in seismic design and seismic rehabilitation of structures in seismic areas is attendance. Thanks to its stiffness as high as that of CBFs and owing to its ductility compared to that of MRFs in severe earthquakes, the system shows appropriate behavior. Configuration of these frames is to such an extent that a horizontal or vertical eccentricity (e) forms at the end of brace members, Figure 1. This eccentricity is called link or fuse. These frames are designed so that the links are the weakest parts of the structure, and forming nonlinear deformations at the time of severe earthquakes in this region, dissipate energy resulted from earthquake and prevent any damages to other members of structure and in fact other members remain in elastic region (Popov, Ricles, and Kasai, 1992). Fig. 1. Types of eccentrically braced frames. Generally, advantages of vertical link to horizontal link consist of: • Transfer of nonlinear deformations to the outside of storey beam and only energy dissipation in vertical link and prevention of deformation in storey beam, Figure 2. 1 -Dept. of Civil Engineering, Iran University Science and Technology, Iran 2 Dept. of Civil Engineering, Iran University Science and Technology, Iran 2 • Simple replacement of vertical link after the earthquake because it is out of basic loading system. • Application in seismic rehabilitation of the existing buildings due to its low effect on basic lateral resisting system. Fig. 2. Display of deformation of eccentrically braced frames. Length of link is a considerable point in eccentric braces. The shorter the link, the higher the shear force as compared to bending moment and the frame's behavior is said to be shear. The effect of bending moment increases along the long length of the link, and the frame behavior is called bending. Shearing behavior is mostly emphasized in frames with vertical link. Dimensional proportionality between floor beams and link size, the use of doubler plates on storey beam web due to formation of concentrated moment and so on are the limitations of the use of vertical link in seismic rehabilitation of structures (Shayanfar, Rezaeian, Taherkhani 2008). The use of two vertical links is suggested to decrease the abovementioned limitations, Figure. 3.These eccentrically braced frame systems with double vertical link are called DV-EBF. The use of double vertical links has the following advantages over single vertical link: • Decrease in the number of braced bays and as a result decrease in the number of brace members and their connections. • Decrease in concentrated moment applied to the beam and control of panel zone adjacent to vertical link in storey beam. • Decrease in the design force of the connection details of vertical links to storey beam and brace members. Fig. 3. Display of eccentrically braced frame with double vertical link. Numerical studies concerning DV-EBF, including consideration of stability and their cyclic behavior, were conducted by the authors of the paper and the results imply general appropriate behavior (Shayanfar, Rezaeian, and Taherkhani 2009).