This paper presents the results of a case study carried out to verify the effectiveness, for the mitigation of the seismic effects, of placing viscous dampers so that they connect the corresponding storeys of two portions of the same building (i.e. the frame structure and the lateral-resisting elements).

The case study here developed regards an eighteen-storey building structure (about 2300m² of surface per floor and about 62 meters high) which represents the enlargement of the existing city Hospital "C.A. Pizzardi" in Bologna, Italy. The structure is composed of a central non-moment-resisting steel frame characterized by a small horizontal stiffness, which supports essentially the vertical actions, and two external lateral-resisting elements of reinforced concrete (concrete cores) which mainly support the horizontal loads.

Firstly, the linear elastic model of the reference structure as it is designed, where the link between steel frames and concrete cores is guaranteed through the realization of adequate rigid connections (fundamental period equal to 1.3 sec), is analysed as subjected to seven groups of artificial accelerograms (each group is characterised by two different dynamic input with PGA = 0.15g).

Secondly, the model of the reference structure as equipped with few viscous dampers connecting the steel frames and the concrete cores is analysed under the same seismic input. In this model, the rigid connection (at all floors) between the steel frames and the concrete cores is replaced with a flexible one (at selected floors in order to reduce the cost of the construction, and sized to provide a fundamental period equal to 3 sec) in parallel with viscous dampers. This system of flexible connection and viscous dampers (a viscoelastic device) is inserted into the structure so that it can perform along both axes in plan (four devices in the longitudinal direction and seven devices in the transversal direction). An additional vertical bracing system, realized through structural cross-like steel joined profiles UPN180 located at the corners of the central body, is introduced into the steel frames to take into account the effects due to the connection between frames and concrete cores which is realized only at selected floors, that is to avoid the occur of "weak floor" collapse mechanisms.

By comparison of the results obtained for the two models, it is deduced that positioning viscous dampers at the connection between the concrete cores and the steel frame allows to obtain substantial improvement in the seismic performances (roughly 50% reduction in the values of peak top-storey displacements, peak base shears and peak base bending moments).

Downstream of the results obtained by the performed simulations, an economic analysis has been developed, comparing the costs of construction for a structure realized without any damping system and the costs of construction introducing a damping system like the one described. This cost-accounting shows how the better seismic performances obtained by placing viscous dampers do not come at the expense of larger costs because the savings in the structural members compensate the additional costs of the dampers.

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