Analysis and mitigation of seismic pounding of a slender R/C bell tower

Pounding is one of the greatest sources of seismic vulnerability of slender R/C structures, including civic or bell towers. An emblematic case study falling in this class of structures, i.e. a modern heritage R/C bell tower constructed in the early 1960s to replace the former 19th century tower of the Chiesa del Sacro Cuore in Florence, is analyzed in this paper. In order to assess the effects of pounding, a special multi-link viscoelastic finite element contact model was devised and calibrated in this study to reproduce Jankowski’s non-linear viscoelastic analytical model. Indeed, the damping coefficient of the latter is defined as a non-linear function of time, and thus it cannot be directly implemented in commercial calculus programs, because the damping coefficient of the damper elements included in their basic libraries is assumed to be a constant. The non-linear dynamic enquiry carried out by the finite element model of the bell tower and the church incorporating the multi-link viscoelastic contact elements shows that pounding affects the seismic response of the two buildings as early as an input seismic action scaled at the amplitude of the normative basic design earthquake level. Furthermore, unsafe stress states are highlighted for the columns of the tower under seismic action scaled at the maximum considered earthquake level. A damped-interconnection retrofit solution consisting in linking the two structures by means of a pair of pressurized fluid-viscous dissipaters is proposed to prevent pounding. The technical implementation details of this rehabilitation strategy are illustrated, and the benefits induced are discussed by comparison with the response in original conditions and in the hypothesis of a conventional rigid-connection retrofit intervention between the bell tower and the church.