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

This paper deals with the seismic protection of existing buildings using external viscous damper systems to increase the 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. The study analyzes a recent solution called "dissipative tower", which, for the dampers activation, exploits the rocking motion of a stiff steel truss hinged at the foundation level. A state space formulation of the dynamic problem is presented in general terms and some issues concerning the influence of the bracing properties on the behavior of a case study consisting of a building coupled with a dissipative tower are investigated. The results presented concern both the influence of the external dissipative bracings on the most important modal properties of the system, and the global effect on the seismic response, evaluated via the modal decomposition method. It is shown that the addition of the towers yields a regularization of the drift demand along the building height, but it can also induce significant changes in the distribution of internal actions in the existing frame. Moreover, the contribution of higher order modes can be important for the internal actions evaluation, while it is negligible for the displacement estimation. The results obtained by considering the coupled system are compared with the corresponding results obtained by considering two limit cases: the bare frame, and the frame equipped with an infinitely stiff dissipative tower.