Seismic behaviour of ordinary masonry buildings during the 2016 central Italy earthquakes

Between August 2016 and January 2017 nine shallow earthquakes ranging from 5.0 and 6.5 of moment magnitude affected Central Italy, involving several municipalities wherein unreinforced masonry buildings are more than three quarters of all constructions. Damage state has been very severe, with sixteen settlements belonging to the municipalities of Amatrice, Arquata del Tronto, Accumoli, Castelsantangelo sul Nera and Norcia experiencing a cumulative European macroseismic scale intensity larger than IX. Ground motion demand in terms of peak ground velocity was approximately two or three times what expected for a 475 years return period while the pseudoacceleration response spectra showed values between once and twice gravity acceleration for the period range typical of two and three storeys unreinforced masonry buildings. Moreover, since October 2016, such large seismic demand acted on structures damaged from previous shocks testifying the effects of damage accumulation, too. The significant shaking alone cannot explain the extremely severe damage of some settlements, with large portions of whole blocks completely collapsed, highlighting the need for investigating the specific vulnerability factors and construction features of unreinforced masonry buildings in the affected area. In fact, although some deficiencies already highlighted in previous Italian earthquakes (e.g. inadequate structural connections) have been surveyed also during this sequence, a marked vulnerability of masonry and its mortar has been noticed, in particular in the area between Amatrice and Arquata del Tronto. On the contrary, the historical constructions in Norcia performed much better, as a result of the 1860 seismic code and of the retrofitting interventions implemented after the different earthquakes occurred in the last two centuries. Finally, a number of demolished and rebuilt constructions performed very well, and this was also the case also of modern hollow clay blockwork buildings that protected not only human life, but also cost of construction and continuity of use.