Seismic Assessment, Repair and Strengthening of a Medieval Masonry Tower in Southern Italy

Many historical masonry towers belong to architectural heritage. Their preservation is still a critical issue in several countries. In fact, the experience of past earthquakes has shown that even earthquakes of moderate intensity can cause the collapse of these historical structures due to their slenderness together with irregularities, openings, leaning phenomena and adjacent lower buildings. The damage and changes suffered over the centuries, and the complex behaviour of the materials makes the performance of this type of structures more uncertain, and the seismic analysis and the remedial measures to achieve their protection a very complex task. This paper presents a complete study (from the on-site diagnostic investigations to the seismic assessment and non-invasive strengthening) of an ancient masonry tower in Torre Orsaia (Southern Italy). The paper, through the discussion of the case study, shows effective procedures to assess the seismic safety of masonry towers and plan interventions without too time- and cost-consuming processes. The results have established that the deep and wide cracks on the belfry are due to the loss of tension of the ancient tie-rods. The seismic safety of the tower has been carried out using three levels of analysis: LV1 (evaluation with simplified models), LV2 (analysis of local collapse mechanisms) and LV3 (global analysis). The analyses have shown that the critical mechanism is the simple overturning of the masonry piers of the belfry due to the ineffective tie-rods. The corresponding safety index in terms of peak ground acceleration is 0.305. To avoid this undesired mechanism and, at the same time, fully close the cracks, a strengthening intervention based on stainless steel reinforcing rings has been developed. This chaining system avoids the local mechanisms of the piers of the belfry and allows developing global mechanisms. After retrofit intervention, the seismic analysis has shown that the safety index increases to 0.716 for the LV2 analysis and 0.793 for the LV3 analysis.