Modal properties of existing building retrofitted by external bracings with viscous dampers

The paper deals with the seismic protection of existing buildings using external viscous damper systems to increase energy dissipation capacity. Dampers and bracings can be arranged in very different configurations and the possible solutions can be grouped into different categories, depending on the specific kinematic behavior. In this work, the formulation of the problem involving the coupling of the existing frame with an external damping system is presented in general terms and some issues concerning the influence of the bracing properties on the dynamic response of the coupled system are investigated. Presented results concern the so called "dissipative tower", a recent solution which exploits the rocking motion of a stiff steel truss hinged at the foundation level for the dampers activation; the influence of the external dissipative bracings on the most important modal properties of the system, such as the mode displacement profile, relevant internal action distribution and relations between damper dimensions and effective modal damping are shown. Finally the global effect on the seismic response, in terms of displacements and base shear, is presented by solving the dynamic problem with the modal decomposition method in order to investigate the contribution of the higher modes on the dynamic response.