Seismic isolation of historic towers: Feasibility study on a simplified model of the Tower of Pisa

Historic masonry towers are among the most seismically vulnerable structures. The present work uses a simplified dynamic model of the leaning Tower of Pisa to investigate the feasibility of the design of a base isolation system for old masonry towers and evaluate its efficiency as a seismic retrofitting technique. The analysis focuses on a Limited Damage performance level, where the performance objectives are defined to ensure that the structure remains elastic for seismic events with a return period of 200 years. The seismic performance of the tower in its present state was first evaluated with response spectrum analysis, showing an excessive damage potential. Therefore, a hypothetical base isolation system consisting of elastomeric bearings inserted below the foundation level was designed. Response history analyses were then carried out to evaluate of the performance of the isolated tower showing that a Limited Damage performance objective can be fulfilled. Further analyses showed that both the vertical acceleration and the tilt angle of the base-isolated structure had a small influence on its seismic response. Seismic isolation is shown to have a real potential for application to monumental masonry towers. However, further investigations considering the effects of rare seismic events are still required to confirm these results.