Influence of the Aggregate Effect on the Seismic Vulnerability of Italian Historical Centers

The recent seismic events in Italy have once again highlighted the high seismic vulnerability of its historical centers, generally composed of building aggregates. In particular, the most significant damage and collapses occurred in ordinary unreinforced masonry buildings, highly vulnerable to out-of-plane mechanisms. Therefore, the assessment of the seismic vulnerability of unreinforced masonry buildings in aggregate in regard to the out-of-plane mechanisms is a fundamental and very current issue for the seismic protection of historical centers. In this context a central aspect to investigate is the influence on the out-of-plane behavior of buildings of the interaction between adjacent units, named aggregate effect. The main aim of the present work is to analyze the contribution of this effect on the out-of-plane response of buildings arranged in aggregate. To this purpose, the friction force that develops between the façade wall of a unit subject to an overturning out-of-the-plane mechanism and transversal walls shared with the adjacent units, is accounted for. Then, the influence of this effect has been analyzed through a nonlinear kinematic approach with reference to a real case study of an Italian historical center.