Empirical seismic fragility of masonry buildings in historical centres accounting for structural interventions

Fragility models are widely employed to forecast seismic damage in built environments. However, the actual conditions of buildings, depending on their construction systems and transformations, are crucial for model reliability, especially in historical centres. In this paper, an empirical fragility model for residential masonry buildings from data collected within 19 historical centres struck by the 2016 Central Italy earthquake is proposed. The dataset includes both traditional unreinforced and hybrid buildings, where reinforced concrete elements were applied in the 1980s according to seismic reinforcement prescriptions for existing structures. This study acknowledges the impact of such structural modifications on the seismic performance of buildings, which can alter their vulnerability classification either improving or worsening it. The intensity measure for the fragility functions is the peak ground acceleration obtained from actual records through interpolation for each centre. Damage data and vulnerability assessment of 2134 traditional and hybrid structural units are conducted in the framework of the European Macroseismic Scale (EMS-98). The fragility functions are plotted for both structural types and EMS-98 vulnerability classes, also considering the contribution of structural interventions, and then compared to literature empirical models. Hybrid buildings achieve a behaviour similar to traditional buildings when floors are strengthened but walls are not. However, when both masonry walls and floors are strengthened, their performance significantly improved, approaching that of modern clay block buildings with rigid floors. Compared to literature models, the proposed one showed good compatibility for the most vulnerable buildings, whereas less vulnerable ones appeared even stronger thanks to interventions.