SEISMIC RETROFIT OF STRATEGIC MASONRY STRUCTURES WITH BASE ISOLATION TECHNIQUE: THE CASE STUDY OF “GIACOMO MATTEOTTI” SCHOOL BUILDING IN GUBBIO, ITALY

Seismic retrofit of existing structures is an extremely important topic in the field of earthquake engineering. For strategic structures such as school and military buildings, bridges and infrastructures in general, the expected performance level is further raised compared to conventional structures to guarantee two fundamental targets: to resist the ultimate design seismic action corresponding to the life safety limit state and to guarantee immediate occupancy after an earthquake event. This means no damage to the structural elements as well as protection of internal equipment from excessive accelerations. Structure functionality is therefore the minimum acceptable level of retrofit for strategic structures. In case of masonry buildings located in medium-high seismicity zones, conventional retrofit (i.e. wall-to-roof and wall-to-floor anchorage, out-of-plane wall bracing, diaphragm strengthening) does not allow to fully achieve the above objectives. Furthermore, the application of these techniques causes an important and invasive impact on the structure, distorting the building on both architectural and functional level. Base isolation is a technical solution that allows to drastically reduce the seismic demand on the structure using special anti-seismic devices characterized by high horizontal flexibility. This paper describes the application of seismic isolation technique to protect the “Giacomo Matteotti” school building, located in the city of Gubbio, Italy. The building was built in the 1940s, and it is made of cast stone and bricks and reinforced concrete-hollow tiles mixed floors. The building has three floors, a total area of 6.000 m2, an inter-storey height of 4 m and a total volume of about 23.750 m3. The City of Gubbio (Umbria region, Italy), is located in an area where strong earthquakes can occur, with expected peak ground acceleration on rock soil equal to 0.29g for 475 years return period (i.e., 10% exceedance probability in 50 years). For the base isolation system, Freyssinet has supplied 94 anti-seismic rubber isolators ISOSISM® type HDRB-H 550x155 and 93 flat sliders with confined elastomeric disc TETRON® type CD GL 3000.600.600 in order to reduce the eccentricity between centre of mass and centre of stiffness of the isolation system and consequently the torsional effects on the superstructure. This paper describes the performance characteristics of the isolators with particular attention to the experimental dynamic response. A special focus is given to all the construction phases and details necessary to isolate a masonry structure.