SEISMIC MONITORING OF BASE ISOLATED BUILDINGS UNDER LOW INTENSITY EARTHQUAKES

Seismic monitoring of base isolated buildings in order to evaluate dynamic behavior during earthquakes and gain experience on the detailed seismic behavior of isolators in order to experience for future design and analysis. The results of previous seismic monitoring of isolated structures permitted to develope a database, used in the creation of new formulas and reference values for the estimation of the fundamental periods of the structures and the percentage of critical damping during dynamic analyses. The database is very well supplied with data on traditional buildings, but not enough for base isolated buildings. Some structures with seismic base isolation have been monitored during recent strong earthquakes in Italy, namely Amatrice earthquake (2016/08/24, Mw=6.0) and Norcia earthquake (2016/10/30, Mw=6.5) and aftershocks. For these structure, amplification phenomena have been observed up to about twice the accelerations on the superstructure, for very low energy value inputs. In these cases, however, monitoring revealed amplified accelerations extremely small, and very far from being able to damage the structure. Records from the ENEA permanent accelerometric network, installed on the structure under examination, and the tests carried out on the same isolators used for the qualification tests of elastomeric isolators positioned below the analyzed structure, describe the behavior of the isolators in terms of force and displacement defining two non-linear laws, derived from experimental data.