A Parametric Study of Impact Force in MRFS Retrofitted By Loose CBFS

Due to an increasing need for the retrofitting of structures, different methods have been proposed for existing moment resistant frames that do not satisfy current seismic demands. One of the methods for retrofitting of MRFs is utilization of concentric braces. Concentric braces are known to have low ductility characteristics and, although they are suitable for strengthening, they reduce the structures’ overall ductility and also increase the uplift in the existing foundations. 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 affects 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 to 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. It was also observed that by increasing the period of applied force to natural period of structure ratio up to 4, the impact force is reduced. Another effective parameter on the impact force is the size of initial looseness. The studies in this research showed that by increasing the gap up to 3mm the impact force increased, but for the greater sizes of looseness the impact force was reduced.