SEISMIC RETROFIT DESIGN OF EXISTING BUILDINGS WITH INTERSTOREY FLUID-VISCOUS DAMPERS

This paper provides simple indications for the seismic retrofit design of existing frame buildings through fluid-viscous dampers placed between consecutive storeys. The so called "direct five-step procedure" developed in 2018 for new structures is extended to the case of existing buildings, taking into account the hysteretic dissipation that occurs in the structural elements. Whilst for new structures 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 is easily achievable, for existing buildings designed for vertical loads only, the introduction of a damper system is not generally enough to keep the structure in the elastic range. Thus, it might be necessary to accept plastic hinge formation at the ends of beams and columns and to control that the ductility demand (plastic excursion under the design earthquake) is smaller than the ductility capacity (albeit probably limited) of the existing building. This revised design procedure starts from the target response reduction factor value defined as the global capacity/demand ratio, i.e. the maximum base shear that can be tolerated by the structure and the base shear that would be obtained in the elastic design of a new construction. The procedure identifies the mechanical characteristics of the commercial non-linear viscous dampers. An applicative example is presented for a 11-storey RC building designed for vertical loads only, before the enforcement of seismic codes, characterized by monodirectional frames (year of construction: 1980-1981) located in Bologna. Time-history analyses are carried out. The results obtained are compared with the target performances, showing the effectiveness of the simplified design procedure.