عنوان مقاله [English]
Due to an increasing need for the restrengthening of structures, different methods have been proposed for existing moment resistant frames that do not satisfy current seismic demands. One method for the estrengtheing of moment frames is utilization of concentric braces. Concentric braces are known to have low ductility characteristics and, although suitable for strengthening, they reduce the structures overall ductility and, also, increase uplift in the existing foundations. In the last two decades, extensive research has been conducted on the ductility of concentric braces, and different methods are proposed for increasing ductility. One of the proposed methods for improving the performance of concentric braces is creating looseness in the connection of braces, which is investigated in this paper. The problem of concentric braces with initial looseness is the impact force that will occur in the brace. Based on this investigation, the most important factors affecting the magnitude of the impact force are: brace to frame stiffness ratio and the magnitude and period of the applied force, as well as the size of the gap used as initial looseness. It has also been observed that by limiting the brace/frame stiffness ratio, the impact force is minimized. The limitation of stiffness ratio, and the creation of looseness, reduces the force of the brace and uplift, and the full capacity of the frame can be used.Also, by increasing the period of applied force to the natural period of thestructure ratio up to 4, the impact force is reduced.Another effective parameter on the impact force was the size of initial looseness. In the frames studied in this paper, the impact force was increased up to 4mm looseness, but, for the greater size of looseness, the impact force was reduced.