Some aspects related to the 3-D base isolation of structures are presented in this paper through a numerical approach. A traditional horizontal base isolation system with the use of high damping rubber bearings (HDRB) is coupled with a tuned mass damper (TMD) in the vertical direction. Both lightweight (e.g. artworks or special equipment) and massive structures (a nuclear power plant building) have been considered and possible positive and negative aspects from the implementation of the proposed hybrid control strategy are investigated. It is found that the TMD is able to provide a reliable source of energy dissipation, and to control the vertical motion of the structure, only when this has a low value of damping. For damping values that approximately exceed the 5% limit, the positive effects of TMD are negligible, if not worsening
