SEISMIC RETROFIT OF R.C. BUILDINGS IN USE THROUGH SEISMIC ISOLATION. THREE CASE STUDIES IN L'AQUILA, ITALY

Seismic isolation (SI) advantages for new buildings are well known: not only it allows to avoid damage of both structural and non structural elements under strong earthquake, but it mantains building functionality as well. This is possible thanks to strong reduction of accelerations and interstorey drift in the superstructure, i.e. the part of structure above the isolation layer. SI offers additional advantages for seismic retrofit of existing buildings. The main advantage is that the works can be limited at one floor (usually the basement, plus the foundation), without any strenghtening on the superstructure. Consequently, the building can be used during the retrofit works. The safety of the retrofitted building increases significantly. Reaching exactly the same safety level of a new building in the same site would be possible, but it would need some strenghtening in the superstructure, and thus is usually avoided in order to keep the building in function during the works. It is worth noting that for the seismic isolation system, the safety is the same than for a new building. The paper presents in detail three case studies of framed r.c. buildings built in the 1980s and now under retrofit with seismic isolation, that could be representative of many other buildings. During 2009 L'Aquila earthquake, those residential buildings were only slightly damaged, and immediately repaired but without any improvement of their seismic performance. Now the retrofit design is carried out for an earthquake stronger than the 2009 earthquake. Despite the buildings are in the same area (ag=0.261g for the Life Safety Limit State earthquake; ag=0.334g for the Collapse Limit State earthquake, used to design the seismic isolation system), the design spectrum is different because of different type of soil. The isolators are inserted in the basement or in the ground floor that host the garages, thus without affecting the apartments. The safety level reached in the three buildings was higher than 70% of that of new buildings in the same site, while before retrofit it was lower than 16%