Low-LOD fragility curves of structural units in masonry building clusters for territorial risk analysis

The constant yet irregular construction process taking place in historical towns during decades, if not centuries, is based on spontaneous, and often unlawful, aggregation, superposition, and juxtaposition of building units, realized in different times with different construction techniques and materials. This process produces the so-called masonry building clusters, which constitute the fabric of most historical towns in Southern Europe. Though usually well integrated from the functional standpoint, their structural functioning is often hardly interpretable, especially under seismic actions. Since 2008, the Italian technical code, recognizing the complexity of analyzing masonry building clusters, allows subdividing them into distinct structural units and then by per -forming separate non-linear analyses on each structural unit. The code provides indications on how to identify each structural unit and, once it has been singled out, modeling and analysis can benefit of some simplifications: each floor can be analyzed and verified separately, the axial force variation in the walls can be disregarded, and the global torsional behavior can be neglected. These common-sense simplifications aim at avoiding complex FEM models, which would overexert the simplifications introduced, and encourage the development of more efficient assessment procedures, especially for large scale territorial risk studies, where the required level of detail (LOD) is generally low. Along these lines, this study proposes a procedure formulated in analytical form, which allows developing fragility curves of structural units belonging to masonry building clusters based, both, on gross geometrical data available from typological surveys, and on typical mechanical properties taken from the Italian code. Recognizing that the analysis tool should be commensurate to the knowledge level, the pro-cedure has been implemented in a spreadsheet. This allows producing, with limited computational effort, fragility curves to be used in comprehensive risk assessment studies of historical centers. An application on a masonry building cluster in Umbria, Italy, is presented, which shows that the procedure yields accurate results with concise input data and that the corresponding fragility curves can be obtained with minimal computational effort, which is an extremely useful feature when performing risk analyses of historical city centers.