The paper deals with a hybrid procedure for seismic retrofit of an existing R.C. structure by using base isolation system. The case study is a high school building in Naples, built at the end of 1970s, without taking into account anti-seismic design criteria. This four-storeys building is made of three jointed bodies. "Building 1" and "Building 2" are characterized by a rectangular plant of 50m x 20m; the exception is given by the second floor, whose longest side is about 70m. "Building 3" corresponds to the gym, it has a square plan (25m x 25m), with a height of 6,70m.

All the buildings are made of R.C. framed structures. In particular, "Building 1" and "Building 2" are characterized by resisting frames only in transversal direction. These ones support the actions transferred by the floors, whose load-bearing elements are warped longitudinally. In the structure there are also some R.C. shear walls. "Building 1" has two transversal shear walls in correspondence of the inner staircase and a single longitudinal one for the outside staircase. Instead, "Building 2" is characterized by an only transversal shear walls in correspondence of the outside staircase. The buildings are separated by tight joints: the short clearance between adjacent portions is inappropriate to avoid pounding damages due to earthquake shaking.

Considering that the structure has no resisting frames in longitudinal direction, this school building has low seismic capacity longitudinally and a high lateral deformability, with a fundamental vibration period of T_AS-IS =1,37s. In this case, seismic retrofit by using base isolation system has been necessary to improve its seismic response. In fact, at the current state, the existing details cannot guarantee the structural components to face the effects of plastic range. On the other side, the main aspects of modern capacity design approach are not complied with between dissipative and no-dissipative components. For these reasons, it is assumed that the current structural system has no ductility reserves, on local and global scale.

For the seismic retrofit of the existing structure an "all rubber" base isolation system (BIS) has been assumed by using f600 HDRBs. To maximize the effectiveness of the BIS, the three bodies have been connected at each floor level and the upper structure has been strengthened previously. In fact, the use of the new shear walls leads to reduce the fundamental vibration period from T_AS-IS=1,37s to T_FB=0,30s, while the application of BIS allows to reach a T_ISO = 4,0s. This is a typical example of hybrid strategy for seismic retrofitting. It has been obtained by merging two conceptually opposite design strategies, i.e. a base isolation system applied to an existing structure, previously stiffened ad hoc by using shear walls.