Empirical fragility curves for masonry churches and their macro-elements using a large database: Proposal of a new likelihood function

Several earthquakes have highlighted over time the high vulnerability of masonry churches to horizontal actions, so that more and more research is being devoted to the assessment of their seismic response and to the design of strengthening interventions to guarantee their preservation. This paper analyses in detail the damage suffered by a large sample of 2870 masonry churches hit by the 2016–17 Central Italy seismic sequence, using the damage data extracted from the DaDO Web GIS. Fragility curves based on the empirical/observational approach are elaborated starting from the damage probability matrices (DPMs) in terms of global damage, both for the whole database and for some subsets of churches with homogeneous characteristics, in order to analyze the influence of some typological features. In particular, it is highlighted that the subset of small churches with rectangular plan and single nave show a generally lower vulnerability compared with those of the whole database. Moreover, the damage observed in the most common macro-elements detectable in masonry churches is examined by defining specific damage indices for macro-elements. The fragility curves for the macro-elements are, then, compared with those referring to the global damage, evidencing some interesting aspects useful both in the survey phase and for planning specific interventions. One of the main outcomes is that the seismic responses of the ‘Façade’ and ‘Principal nave’ macro-elements may closely approximate the global response of the church. Finally, the paper presents a new fitting procedure of the observational damage data aimed to build fragility curves for subsets of churches characterized by a reduced number of data falling in some PGA ranges. A ‘weighting’ procedure is, indeed, proposed in order to give less relevance to small groups of churches falling in the less populated PGA ranges.