A spectrum-based ductility demand approach for the design of Fluid Viscous Dampers (FVDs) for the improvement of hysteretic framed r.c. structures under seismic excitations

Fluid viscous dampers (FVDs) have been widely used due to their capacity to generate dissipative forces (velocity dependent) that are not in phase with the displacements, namely able to exhibit their maximum forces when internal restoring forces are minimum. The possibility of increasing the damping ratio of a structure without significantly altering the inherent stiffness is another reason for the advantaging use of FVDs. For these characteristics, fluid viscous dampers are often preferred over other types of dampers. However, the lack of specific code prescriptions and simple but sufficiently reliable design procedures for structures exhibiting a non-linear plastic behavior is an issue not definitively faced. Deepening this issue could make the use of viscous dampers more diffused than it is. In this frame, here, a novel design procedure for non-linear FVDs to apply to hysteretic r.c. framed structures is proposed and discussed in terms of reliability in practical applications. The novelty of the procedure is that the scope of limiting the structural response is searched considering the contribution of external viscous damping, inherent viscous damping and hysteretic damping that the structure is able to exhibit. Therefore, the dimensioning of the external viscous dampers is carried out taking into account the rate of energy that the structure can dissipate by hysteretic damping differently from the most diffused approaches based on the maintaining of a structural elastic behavior. To this scope, the hypothesis of a simplified dynamic structural response is assumed to be coupled to the equivalent linearization of FVDs. The suitability of this hypothesis is discussed by a comparison between the obtainable results and the design targets in the case of structures that do not satisfy the assumed hypothesis. The results obtainable are analyzed in a statistical sense. Time history analyses of FVDs-equipped (and non) structural non-linear models are performed under appropriate families of base accelerograms. The design procedure is tested on benchmark models and on a case study in order to assess the degree of success of the proposed approach in connection to the assumed target objectives.