Failures and damages of historical masonry structures induced by 2012 northern and 2016-17 central Italy seismic sequences: Critical issues and new perspectives towards seismic prevention

The work analyses some observed structural criticalities in historical masonry structures emphasized by the seismic events of May 2012, in Emilia, and of August-October 2016, in Central Italy. The causes that brought to different damage scenarios in the historic centres and to the different degree of damage evolution of the buildings, have been investigated, comparing their construction and dynamic features, such as the natural vibration periods of the different construction typologies with the spectral accelerations and the amplification periods distinctive of the seismic events. In particular, starting from the evidence of the damage observed on the built heritage of some of the centres most affected by the two seismic sequences, and from the characteristics of the seismic signals recorded at the considered sites, it is proposed a study which analyses some criticalities, in relation to preventive purposes of the technical standards, specifically with reference to historical constructions. Among the main criticalities highlighted in the present work, it should be noted the remarkable difference between the design spectra and the spectra of the recorded seismic events (real earthquakes), and the circumstance that in reality the constructions generally are interested by more seismic shocks (multi-events) characterized by higher intensities with respect to the ones expected by the building code. Such feature gives rise to a criticality for the operators acting in the emergency phases of rescue and temporary securing, who find themselves operating in high hazardous conditions, due to the high vulnerability of already damaged built heritage induced by the main-shock, and by the high probability (near certainty) that, in the short period, events having intensities and characteristics similar to the main-shock may occur. Moreover, the evolution of the damages, with progressive crumbling associated to global or partial collapses, due to the sequence of shocks (multi-events), which had interested thousands of buildings in the historical centres (and not only) subjected to the seismic sequence, bare witness of criticalities in the preventive approach proposed at national level. The analysis of the criticalities highlighted in the present work allows to envisage an approach which considers for the structures, historical or not, adequate performance requirements with respect to the emergency, temporary and permanent securing phases, accounting for the multi-event seismic context typical of the Italian territory.