Pre-stressed vertical injected anchors on historical masonry: a case study, the Bell Tower of Botta di Sotto il Monte Papa Giovanni XXIII

The Bell Tower of the Church "Sacro Cuore Gesù e S. Egidio Abate" in Sotto il Monte (Bergamo, Italy), represents a case study of the research programme "ANIMUS", funded by the European Programme FESR-2007-2013. The necessity of building a steel belfry on top of the bell tower required a detailed study of its connection with the supporting masonry structure. This connection has been realized by means of deep vertical anchors, obtained by grouting steel bars by injecting the grout in special socks, such that any dispersion of the mortar in the masonry is avoided. By this technique, a "bulb" creating an effective interlock with the masonry structure is created. The behaviour of the system has been studied by means of a finite element model, allowing to assess the performance of the connection under wind and seismic loads both at service and ultimate limit states. The results have highlighted the opportunity to design vertical anchors with a double bulb of mortar. This way, by grouting the deeper bulb soon after installation, the anchor could be pre-stressed to avoid any decompression of the connection between the superstructure and the masonry tower under service loads. The upper bulb, grouted at a later time, has the role of transferring the loads exceeding serviceability limit state to the masonry structure. After the installation of vertical anchors, an experimental phase started with the execution of cyclic tests, thus allowing to evaluate the effectiveness of the intervention and to compare experimental and numerical results.