Seismic response of masonry buildings in historical centres struck by the 2016 Central Italy earthquake. Calibration of a vulnerability model for strengthened conditions

Brought to light by the 1997 Umbria-Marche (Italy) earthquake for the first time, the theme of damage patterns of masonry buildings with structural interventions became urgent again in the 2016–17 Central Italy seismic swarm. On a sample of 2306 buildings within the seismic field, the 85% bears the signs of structural interventions. With the aim of inferring the vulnerability factors from damage of strengthened buildings, a close-up analysis revealed that interventions can cause damage to the structures on which they are applied because of i) poor detailing and ii) incompatibility among parts, old and new. These conditions limited the attainment of a building's box-like behaviour and, therefore, an unfavourable contribution of interventions was identified. Conversely, a favourable contribution was determined. The analytic comparison between the behaviour of building types with interventions and that of the vulnerability classes of the European Macroseismic Scale (EMS-98) obtained a vulnerability model for empirical data. Buildings whose interventions had an unfavourable contribution are comparable with ‘original’, unreinforced ones; conversely, a favourable contribution permitted them to approach the behaviour of modern masonry structures. This work represents a first step towards the inclusion of strengthened buildings in the EMS-98 vulnerability system.