Unreinforced masonry buildings in aggregate of urban settlements: Current approaches and critical issues for the seismic vulnerability assessment

The paper explores the architectural complexity of unreinforced masonry buildings in aggregate typical of urban historic centers, focusing on their seismic vulnerability. These buildings often stem from the gradual transformation of urban areas over centuries, also leading to significant variability in materials, geometries, construction techniques, maintenance status, and connections between adjacent Structural Units (SUs). The study outlines the key features and vulnerability factors of this structural typology, with a particular focus on the methodologies proposed in the literature for the seismic safety verification of a single SU, as well as for assessing the seismic vulnerability of buildings in aggregate at the urban scale. The paper highlights the peculiarities and shortcomings of each currently available methodology, emphasizing the unresolved issues and drawing possible solutions, with particular reference to the “aggregate” effect, i.e. the interaction between the SUs. This study underscores the complexity of assessing the seismic vulnerability of buildings in aggregate, emphasizing the importance of considering both in-plane (IP) and out-of-plane (OOP) mechanisms, as well as the pounding effect.