A design procedure for seismic upgrading of existing RC frames by BRBs

In the world, many existing buildings with RC framed structure were designed according to old seismic standards and, consequently, present structural deficiencies. Embedding Buckling Restrained Braces (BRBs) in the RC framed structure is a promising technique for seismic upgrading of existing buildings. In fact, BRBs, if properly sized, can improve many features of the seismic response of existing buildings. BRBs provide the RC frame with additional dissipation capacity, which reduces drifts. Furthermore, BRBs can avoid drift concentration at few stories, thus promoting a favorable and dissipative collapse mechanism. Finally, the heightwise distribution of the size of BRBs can be optimized, so that drift demand is tuned at every story with the drift capacity of the RC frame. In this paper, a design method for seismic upgrading of existing RC frames by BRBs is presented. According to this method, BRBs are designed to fulfill stiffness and strength requirements in order to achieve the target limit state. The parameters that rule the method are the design story drift Δd, and the behavior factor q. As an example, the proposed design method is applied to retrofit a RC frame which does not satisfy the minimum requirements stipulated by Eurocode 8 for RC framed structures in occurrence of strong ground motions. The case study frame is designed according to the provisions stipulated by the old Italian seismic code for structures located in low seismicity zones. The design procedure is applied considering different pairs of values of d and q, in order to investigate the influence of the parameters that control the outcome of the design on the seismic performance of the upgraded frame. The seismic response of the upgraded frame is evaluated by nonlinear dynamic analysis and its seismic performance is compared to that of the bare RC frame. The results shows the effectiveness of the design method and provide information on properly setting of the design parameters d and q.