Seismic retrofit of a rc building with a double-concave pendulum base isolation system

This paper presents a real practice project dealing with the retrofit of an existing residential reinforced concrete building using double-concave pendulum isolation bearings. The building, located in Siracusa, South of Italy, was designed in 1966 without taking into account any seismic input; it is composed of two reinforced concrete structures that are functionally linked but structurally separated to avoid pounding. For the retrofit, the design earthquake PGA is 0.26g for the Life Safety Limit State earthquake (712 years return period), and 0.37g for the Collapse Limit State earthquake (1462 years return period) used to design the seismic isolation system according to the Italian Seismic Code. The paper describes in detail the displacement-based design procedure of the double-concave pendulum isolation bearings and the performance evaluation of the retrofitted building. The effectiveness of the proposed retrofit strategy solution has been demonstrated by nonlinear time-history analyses under different sets of earthquake-strong ground motions. The results have been finally used to address some of the critical issues of the seismic response of the isolated building: pounding of adjacent structures, uplift or tension of isolators that may occur in slender structures, interferences with systems, stairs, and architectural finishes. The design has allowed to reach a capacity/demand ratio higher than unity. Minor strengthening interventions with C-FRP (Carbon Fiber Reinforced Polymers) and steel plates have been foreseen only on a few superstructure structural elements and, specifically, on seismically deficient RC beam-column joints to avoid their brittle failure.