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In this article, one of the procedures proposed in literature for the design of viscous dampers to be inserted in existing buildings is examined and extended to 3D eccentric buildings. The proposed procedure has been verified through a case study characterized by a six-story RC building. Both plan-symmetric and plan-asymmetric configurations have been considered for comparisons. The effect of considering the plan-asymmetry in the design has been studied. Moreover, also the importance of considering the higher modes has been investigated. The effectiveness of the design procedure has been then evaluated through the comparison with nonlinear dynamic analyses.

The expressions proposed in literature for the supplemental damping ratio provided by nonlinear viscous dampers usually include a term related to the structural response, so that iterative procedures are required. This article proposes a simplified method for the direct assessment of the supplemental damping ratio on the basis of a new dimensionless parameter, called damper index. The methodology has been verified through numerical investigations, considering single- and multi-degree of freedom systems subjected to harmonic excitations and ground motions. A procedure is also proposed for obtaining design spectra of the damping ratio in terms of the damper index.