This paper provides design indications for the seismic retrofitting of existing frame buildings by means of fluid viscous dampers. They are based on a simplified procedure developed in the last years for new buildings and called "direct five-step procedure".

This design procedure, which must be then followed by appropriate verification of the seismic behaviour through non-linear dynamic analyses, consists of 5 steps and is based on a prefixed seismic performance, such as a target damping ratio. The procedure aims at the full definition of the mechanical characteristics of the commercial non-linear viscous dampers, and at the estimation of the maximum forces both in the devices and in the structural elements (columns).

In the case of new buildings, the objective of sizing the viscous dampers in such a way as to keep the structural elements within the linear elastic range even for "rare" earthquakes of high intensity is easily achievable.

In the case of existing buildings designed for vertical loads only, the introduction of a damper system is not generally sufficient to keep the structural elements in the elastic range. Thus, it might be necessary to accept local plastic excursion of the structural elements, by taking into account the ductility capacity (albeit probably limited) of the existing building (hysteretic dissipations associated with damage in beams and columns).

This paper reports the extension of the so-called "direct five-step procedure" to the case of existing buildings and its first application to a 6-storey frame structure case study, which is representative of reinforced concrete buildings designed for vertical loads only, before the enforcement of seismic codes.