This paper presents seismic retrofitting technique of existing mid-rise SRC buildings with newly added stories utilizing mass damper effect. This proposed retrofit method is summarized as follows. A number of the stories are reduced from the top of the building. A lightweight super structure of steel moment resisting frames is newly built on the rooftop through a mid-isolation system with lead rubber bearings (LRB). Total number of the stories is assumed to increase than ever as far as the total weight of the retrofitted building is below the former one. The downsizing of the building has effect on mitigation of inertia force induced by ground motion. On the other hand, the extension part is expected to work as a tuned mass damper having large mass. The objective of this study is to find optimum design parameters, which consist of required number of reduction stories, strength and stiffness of the mid-isolation.
Firstly, an equivalent two degrees of freedom (2DOF) system consisting of the existing part and the extension part is employed for computational efficiency of nonlinear time history analysis. As a result, design formulae of the optimum strength and stiffness are presented through a response surface method in terms of the maximum drift angle. By using the proposed optimal strength of the isolation, relationship between inter story drift of the isolation and the acceleration of the super structure are clarified in terms of the mass ratio and yield strength of the substructure. Thus, these numerical results are accurately estimated by introducing the deformation ratio of the substructure to the super structure without time history analysis, which ratios are derived based on the stochastic vibration theory.
Secondary, a performance curve is formulated based on the previous results. This diagram approach enables structural engineers to decide required number of reduction stories, stiffness and strength of the isolation in a similar manner to that of the other popular passive control systems. A retrofit procedure is presented with the formulae of the design parameters and the performance curve accordingly.
Finally, examples of the retrofit are shown for multistory buildings having seven or ten stories. The buildings are retrofitted in accordance with the proposed procedure. Three historical ground motions are used to verify improvement of seismic performance through time history analysis of stick models having all degree of freedom of the stories. Maximum inter story drift angles of the existing part are confirmed to be less than 1/200 radian, which requires no additional reinforcement for the column members. Both the maximum deformation of the isolation and the maximum acceleration of the super structure meet a specified seismic criteria and show good agreement with estimated values based on the performance curve.
