Two different base isolation retrofit strategies for older elevated water tanks with reinforced concrete structure are presented in this paper. The mutual objective consists in minimizing the impact of the interventions, in view of the architectural and historical value of these plants. The two strategies incorporate double curved sliding surface isolators, and a combined system of high damping rubber bearings and steel-Teflon sliders, respectively. A representative case study is examined for a demonstrative application of the two base-isolation technologies, i.e. the water tower of S. Salvi ex-psychiatric hospital in Florence. The tower was erected in 1905 with a braced frame staging structure, characterized by a set of bracing trusses with equal inclination. This unconventional layout determines an asymmetrical behaviour, and a rather poor performance with respect to seismic action. The design criteria followed for the two retrofit interventions are presented by a step-by-step procedural description. An assessment analysis of their performance in comparison to the response evaluated in current conditions is offered, along with technical installation details of the two protective systems.
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