Observed and simulated urban-scale seismic damage of masonry buildings in aggregate on soft soil: The case of Visso hit by the 2016/2017 central Italy earthquake

The paper focuses on the seismic response of masonry buildings in the historical centre of Visso, severely hit by the 2016–2017 Central Italy earthquake. This represents an emblematic case of site effects and, also, of soil-foundation-structure (SFS) interaction. Maps of the observed damage at urban scale are compared with those estimated from fragility curves derived for the building stock that characterizes the Visso municipality. Similarly to what is quite recurring in typical small historical centres in Italy and Europe, the key feature of Visso is to be mostly composed by unreinforced masonry (URM) buildings (85%), the majority of which is in aggregate (93%). Fragility curves were developed through a numerical procedure based on nonlinear dynamic analyses on 3D equivalent frame models, both fixed and compliant base, accounting for site effects and SFS interaction. The procedure aims at balancing the computational effort with the potential of defining fragility curves customized for the building under investigation and, thus, hopefully capable to improve the reliability of risk assessment. The fragility curves for URM buildings in aggregate explicitly account for the recurring failure mechanisms that post-earthquake evidence has testified, i.e. in-plane (IP) and out-of-plane (OOP) responses, as well as the interaction effects among adjacent units and pounding effects. The numerical procedure has been applied and validated in previous works of Authors to various archetypes representative either of an isolated school and one aggregate. In this work, additional archetypes have been analyzed and, then, the whole set of achieved fragility curves has been generalized to be applicable for a risk assessment at urban scale and develop damage scenarios of Visso's urban settlement. To this aim, simplified modification factors have been proposed to be applied to fragility curves of rock soil condition to account for the possible effect of soft soil as well as the activation of OOP mechanisms. Results have shown that the match between the observed and simulated response improves when the interaction between the structure and the soil is considered.