An external bracing system for the retrofit of r.c. frame buildings: modal properties and seismic response

The paper deals with the seismic protection of existing buildings using external viscous damper systems to increase energy dissipation capacity. Usually the addition of dissipative diagonals in existing frames has some drawbacks as increment of internal actions in the columns, need of intervention at foundation level, feasibility limits and indirect costs related to the interruption of the building utilization. These problems can be efficiently avoided by placing the dissipative bracings and the relevant foundations outside the existing building. 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, but all permitting the control of both the total amount of the dissipated energy and the frame deformation at the various storeys. 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 is employed to investigate both the influence of the external bracing properties on the overall dynamic properties of the coupled system (such as the mode displacement profile, the relevant internal action distribution and the modal damping ratios) and the global effect of the retrofitting on the seismic response. 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 are shown and it is observed that the bracing system notably influences the stiffness and damping properties while it modifies only marginally the mass properties of the existing frame. Finally the global effect on the seismic response, in terms of both displacements and base shear, is presented by solving the dynamic problem with the modal decomposition method by also investigating the contribution of the higher modes on the dynamic response.