GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 1.1 71 of EPA and the isoseismals calculated as interpolation of the MCS intensities evaluated in 305 localities in the macroseismic surveys performed after August 24 (Galli et al., 2016). The figure shows also the strong motion stations (blue triangles) together with the value of EPA. The largest values of I MCS are concentrated in a rather restricted area around the epicenter. The significant difference between the damage in Amatrice (IX-X MCS) and Norcia (VII MCS, NOR and NRC station) could be attributed to the different building characteristics (many of the buildings in the area of Norcia were strengthened after the Umbria-Marche 1997 earthquake). The differences in building stock vulnerability also explain the discrepancy between the isoseismal pattern, mainly oriented in N-S direction, and the ground motion pattern oriented in the NW-SE direction, according to the directivity effect of the causative fault. The survey, done in Amatrice by the authors accompanied by a crew of the Italian Fire Brigades on September 12 2016, allowed to assess the level of damage, vulnerability factor and main damage mechanisms of 240 buildings out of about 300 in the historical center of the town (Tab. 2), thus obtaining a picture of the destructive effects of the earthquake and the explanation of the factors that lead to such a devastating result. The masonry buildings are 89% of the total, the RC buildings 9%, while the remaining buildings are mixed RC/masonry and steel structures. Tab. 2 - Damage level (EMS: Grünthal, 1998) assigned for each building type in Amatrice. Damage level D0 D1 D2 D3 D4 D5 Total Masonry buildings 5 13 25 26 41 104 214 RC buildings 4 6 2 7 1 1 21 Other 0 1 0 1 3 0 5 Total 9 20 27 34 45 105 240 Building types The parameters assigned to the inspected buildings have been inserted into a QGIS map shown in Fig. 2. The map shows that many of the buildings which suffered major damage or collapse lied along Corso Umberto, where the majority of the historical buildings are located (32 collapses out of a total of 105); it appears also that the area located eastward of Via Roma has a larger percentage of collapses (D5). By analyzing the damage on the map and the results shown in Tab. 2 it is possible to count a 49% of D5 damage level and 19% of D4, so almost 70% of the damage is heavy. Fig. 3 reports the vulnerability factors identified for 102 masonry buildings with damage level ranging from D1 to D4. The most relevant factors (more than 10 buildings affected) are: - ev4: poor quality masonry (28%); - ev12: local discontinuities (21%); - ev1: lack of appropriate connection between walls (13%); - ev3: poor connection between floors and vertical structures (11%). Focusing on masonry structures, the majority of the inspected buildings had multileaf masonry walls, formed by two panels of irregular stones connected by poor quality mortar joints, filled with poorly cemented rubble stones and without bondstones connecting the inner and outer panels. The presence of specific vulnerability factors such as the lack of strong connections between walls, the poor connection between walls and floors and especially the inadequate quality of masonry, were the main causes of the activation of mainly out-of- plane damage mechanisms. The result has been a weakening of the buildings bearing capacity bringing to partial or total collapse, despite the fact that about 40% of masonry buildings had steel tie rods or other retrofitting systems. Many of the strongly damaged buildings had been modified using reinforced concrete and many as well presented timber horizontal elements, showing both a detrimental effect in some cases and a useful contribution in others. Once more