During the years, the observed behaviour of seismic isolated structures in different countries allowed to testify the suitability of this technique in protecting structures under strong earthquakes.

Otherwise, until now, in Italy, base isolated buildings have been subjected only to low energy earthquakes, leading to the opportunity of studying, thanks to the recorded data of structural health monitoring systems implemented in these structures, the behaviour of seismic isolation systems under earthquake actions much lower than the design ones.

In this paper, the observed behaviour under low energy earthquakes of different structures with different isolation systems is described and compared. In particular, three buildings are considered:

-        A strategic building seismically isolated by means of HDRBs

-        A residential building seismically isolated by CSSs

-        A strategic building seismically isolated by means of HDRBs and SDs.

In the first case the structure showed resonance frequencies much higher than the design ones but the decoupling was always guaranteed also thanks to the stiffness of the superstructure. In the second one the onset of motion was governed by the friction, which was much higher than the design one. Finally, in the last case the onset of motion was governed by the SDs and the successive motion by the HDRBs.

The study allowed pointing out the fundamental aspects of the structural behaviour of the isolation systems and the buildings and to get suitable information for the design.