Seismic Retrofit of r.c. Buildings with Base Isolation

Seismic isolation is today largely used as a passive mitigation system for seismic actions. While this technique can be easily implemented in new structures, more difficult, whenever possible, is the implementation in existing buildings, which can become a real challenge especially in countries where maintenance of old structures, sometimes of historical importance, is an important issue, like Italy. The use of seismic isolation in existing buildings presents interesting features. Using a base isolation system, the period of vibration can be chosen in order to allow input of low spectral amplitude, so that the superstructure can substantially remain in the elastic range. This possibility allows for limiting or even avoiding the conventional retrofit intervention in the superstructure. In fact, the intervention can be concentrated at the lowest level, while no intervention is necessary in the superstructure, so it is not necessary to demolish non-structural elements. As a result, an "operational seismic retrofit" would be possible, that is the superstructure could remain operative during the retrofit works. In order to do that, at least a safety degree not lower than the existing one should be guaranteed. This can be obtained by using temporary fixing systems for the isolation devices or temporary structures for the columns. After a brief overview of the available research in this field, the paper proposes and discusses different solutions for "operational seismic retrofit", pointing out also the convenience in terms of costs. Furthermore, a case study is presented, in which some solutions are implemented.