On the vulnerability features of historical masonry buildings in aggregate

Damage and losses caused by several seismic events revealed the significant seismic risk of existing unreinforced masonry (URM) structures in aggregate, especially when belonging to historical centers of small municipalities. Small historical centers are frequently the consequence of a centuries-long process of building expansion that results in interacting units with varying materials, construction techniques, heights, states of preservation, and, in some cases, spontaneous repairs. All these complicated factors contribute to the fact that seismic assessment of URM buildings in aggregate still constitutes a challenging topic and an open issue both at research level and in engineering practice. Structural irregularities, connection quality among structural parts, diaphragm flexibility, and the lack of aseismic devices are all well-known factors impacting the seismic response of historic masonry structures. Despite that, no univocal answers are available in the literature on the beneficial or detrimental effect for structural units to be in aggregate or not (apart evident cases) neither standardized procedure to quantity such effects as well as the possible interaction between in-plane (IP) damage and local out-of-plane (OOP) mechanisms. After describing the various possible methods for assessing the seismic vulnerability of masonry aggregate, the paper summarizes the key results of an analytical-numerical approach applied to various URM aggregates located in different locations with moderate to high seismic risk. The paper examines the “aggregate-effect” as well as the effects of combining IP and OOP mechanisms on the fragility curves of the case studies. For the first aim, results of structural units analyzed as isolated or inserted in the aggregate are compared by using both nonlinear static analyses (i.e. pushover) and nonlinear dynamic analyses. For the second aim, the findings concern a newly integrated approach for assessing local behaviors and combining them with the global response. In particular, the IP response of URM buildings was simulated through a 3D Equivalent Frame model of the structures, whereas out-of-plane OOP mechanisms were evaluated separately but using floor accelerations derived from post-processing of data from the global 3D model as seismic input, aiming to explicitly consider the filtering effect provided by the non-linear dynamic response of the structure at the different building levels. The outcomes highlight the difficulties of adequately capturing the seismic response of mutually interacting structural units through pushover analyses as well as the impossibility to combine IP and OOP without using sophisticated numerical models.