Seismic performance of irregular masonry aggregates: Effects of localized retrofitting on individual structural units

Historic masonry aggregates, commonly found in Italian historical centers, pose distinct challenges for seismic assessment due to their geometric irregularities, construction heterogeneity, and fragmented ownership. This study investigates the effectiveness of localized seismic strengthening applied to individual Structural Units (SUs) within an irregular masonry aggregate located in Arquata del Tronto (Italy). Nonlinear static (pushover) analyzes based on the Equivalent Frame Method were carried out to evaluate the impact of strengthening interventions, namely reinforced plaster and grout injection, on both the local response of individual structural units and the global behavior of the entire aggregate. The results indicate that while localized interventions can significantly enhance stiffness, strength, or ductility at the unit level, they do not consistently improve the global seismic performance of the aggregate. In several cases, performance indices on the aggregate scale remain unchanged or even deteriorate due to interaction effects and discontinuities between retrofitted and non-retrofitted units. These findings underscore the limitations of strategies based solely on isolated interventions and emphasize the importance of integrated assessments at the aggregate level. When full-scale modeling is not feasible, the priority should be given to mitigating local collapse mechanisms as a preliminary step to reduce seismic risk.