A direct method for the design of viscous dampers to be inserted in existing buildings

A direct procedure to determine the supplemental damping ratio required for the retrofit of existing buildings with viscous dampers is proposed here. The method has been developed following both analytical and graphical formulations. The graphical formulation is based on the construction of constant design acceleration or constant design displacement curves. These curves allow to estimate the required effective damping as a function of the effective period, associated to the secant stiffness at maximum displacement. Combining these curves with constant ductility curves, which provide a correlation between the effective damping and the supplemental damping for given available ductility and damper typology, it is possible to obtain the required supplemental damping and to design the damping system. The effectiveness of the proposed method has then been evaluated through the comparison with nonlinear dynamic analyses of selected RC frames and the application to real case studies. In these applications, nonlinear viscous dampers have been considered for the retrofit.