Base isolation strategy for seismic retrofit of water tanks with shaft staging

A base isolation study on the advanced seismic retrofit of a reinforced concrete elevated water storage tank with shaft staging is presented in this paper. The tank was built in the late 1920s and is still in service. In consideration of its historical significance, base isolation offers an optimal joined solution to the concurrent issues of a substantial seismic performance improvement and a virtually null architectural impact on the superstructure. The isolation system is constituted by a set of double curved sliding surface isolators placed below the cylindrical shaft staging wall. The incorporation of the protective system causes to modify the pressurized pipes delivering water to the tank and the discharge pipes too. Two different interventions are designed to this aim. The first one consists in installing an expansion joint with high lateral displacement capacity at the connection of the mobile (i.e. linked to the superstructure) and fixed (i.e. linked to the pump lying on the basement) branches of the pipes. In the second solution the pump is seismically isolated jointly with the superstructure, and is linked to the external distribution branch by means of the same type of expansion joint used in the first design. The results of the seismic assessment analyses in current state and retrofitted conditions are illustrated, along with the design details of the interventions on the structure and the plants.