Investigation on the Effect of Near and Far Field on the Strongback Bracing System

This paper examines a newly developed seismic force-resisting system. To achieve improved seismic performance, the strongback system combines aspects of a traditional concentric braced frame with a mast to form a hybrid system. The mast acts like a strongback to help resist the tendency of concentric braced frames to concentrate damage in one or a few stories during severe seismic excitations. The purpose of the strongback system is to promote uniform story drifts over the height of a structure. For this purpose, in this paper, the design principles for achieving the desired goal, along with a new relationship for controlling the amount of story drift uniform index for this system is presented and the nonlinear dynamic behavior of structures 3, 6 and 12 stories with three seismic force-resisting system with different configuration at the intersection of the braces to the beam under seven acceleration of far and seven acceleration of near field for achieving the correctness of the design principles and comparing the effects of the near and far field earthquakes has been studied. The strongback bracing system is specifically designed to prevent the uniform story drift and alleviate the soft-story mechanism [2]. The design procedure is such to keep strong-back aspects elastic and stress ratio less than 0.5. Although the seismic codes specify limits for story stiffness and resistance ratio, this limitation does not guarantee to avoid damage concentration. It should be noted that the concept of uniformity is a relative concept and all stories can never have equal and uniform drift in a structure under earthquake. However, a structure can be designed such that the drifts are distributed uniformly in all stories approximately. For uniform story drift and prevention of soft-story in this system, it is not just enough to keep the stress ratio less than 0.5 so as to completely design this system, and the necessary examinations were carried out to reach a relation between conventional braces and aspects of strongback. In addition, studies by Lai and Mahin [2] show that the simple design strategy used does not result in adequate member sizes near the top of the strongback systems and limited yielding of the strongback spine occurs at these levels. According to the ratio of the braces length and Dimensions of their sections, when the stress ratio in strongback should be less than 0.5, this result was obtained some limitation must be considered about conventional braces stiffness ratio and the aspects of strongback. Therefore, by considering the issue of elastic state of the components of strongback in earthquakes, conventional braces were designed for code earthquakes force and stiffness ratio of strongback braces to conventional braces is in each story equal to or more than 2.5 and section of strongback zipper member was considered equal to section of strongback brace member in the story. During these tests, which were performed using the trial and error method, various earthquakes were used to perform the nonlinear time history analysis and various values were considered for the stiffness ratio of strongback braces to conventional braces and were evaluated, which revealed that the stiffness ratio of 2.5 would cause a more uniform distribution of drift along the height of the structure and was the basis for the design of the braces of this system. Results show that the proposed design principles, which are proportional to the stiffness ratio of bracing, are presented by providing an index of uniformity distribution error in structures 3, 6 and 12 stories, which occurred less than 6, 4.8 and 8.6 respectively. Also by creating a difference of 0.5% between the maximum and minimum of the story drift ratio in most of the structures studied, it has been able to hold the strongback truss in elastic to nonlinear analysis, reduce the concentration of deformations and prevent the creation of a soft story structure.