A Comparison Between Empirical Procedures for the Definition of Vulnerability Classes of Masonry Buildings: Application to Five Historical Centres Struck by 2016 Central Italy Earthquake

The definition of seismic risk scenarios necessarily depends on the attribution of a vulnerability class to each building of a stock. In the macroseismic scale (EMS-98) the vulnerability class – from A to F – results from the combination between horizontal and vertical structures. On the basis of post-event surveys carried out in Italy after the earthquakes occurred in the last 50 years, many rules for converting the masonry quality and the stiffness of horizontal diaphragms into a vulnerability class have been proposed. However, despite the now high number of retrofitted or strengthened buildings in Italy, structural interventions are not mentioned in these procedures, except for metal tie rods and r.c. tie beams. The paper proposes a critical approach to the definition of vulnerability classes, by the means of applying the conversion rules to the same sample of 525 masonry buildings located in five historical centres struck by 2016 Central Italy earthquake: Acquasanta Terme, Campi Alto di Norcia, Castelsantangelo sul Nera, Muccia and Vezzano. They have been chosen due to the extensive strengthening campaigns that had been carried out after earlier seismic events. The preliminary recognition of the structural features of each building happens at the terms of the MUSE-DV Masonry, a rapid visual screening procedure recently proposed by the authors. The damage probability matrices (DPMs), obtained from each conversion, are compared to those from a theoretical model proposed for the EMS-98. Given the same poor masonry quality, the existing rules classify buildings in class A or B depending only on floors’ stiffness and horizontal connections. As a result, both low and high damage may appear in the same vulnerability class causing a bimodal trend in the damage distributions. Conversely, the MUSE-DV procedure allows to reduce these two frequency peaks by considering interventions. In fact, overall interventions, even on very poor masonry structures, may lead to a very low damage and, consequently, to low vulnerability classes (even C or D), while uncontrolled interventions could obtain a high damage and a high vulnerability. The twofold consequence is that a) structural interventions have a ‘relative’, i.e. positive or negative, contribution; b) the usual limitation to A and B vulnerability classes for random masonry buildings needs to be widened to better explain the damage observed in the 2016 Central Italy earthquake.