The main shocks of the 2016-2017 seismic sequence of Central Italy are reported in Fig. 4 while Fig. 5 shows the two horizontal components of acceleration and pseudo-velocity response spectra obtained from the recordings of three accelerometric stations of the National Accelerometric Network (RAN) close to Campi Alto and Norcia. The processed strong-motion data are obtained from (Luzi et al 2016).
The CMI station was 1 km far from Campi Alto and thus its data can be considered representative of the seismic input of Campi Alto. This is a temporary station installed after the first earthquake of 24th August 2016 and, unfortunately, there are no records for the seismic event of the 30th October. The NOR e NRC stations are located in proximity of the historic centre of Norcia and they are permanent accelerometers. The spectra are cut short at 2.5 seconds because the masonry structures analysed in this paper certainly have a lower principal period.
Table 1 summarizes the main parameters recorded by these three stations during the seismic sequence: the Peak Ground Acceleration (PGA), the acceleration corresponding to a period of 0.3 seconds (Sa_0.3), the maximum value of spectrum (Sa_max) and the corresponding period, the Peak Ground Velocity (PGV), the pseudo-velocity (Sv_0.3s) corresponding to a period of 0.3 seconds, and the maximum value of the pseudo-velocity spectrum (Sv_max). Table 1 shows that all seismic parameters resulted generally more severe in the East-West direction with respect to North-South one. With reference to East-West seismic components, it is possible to note that for earthquakes actions occurred on 26th October, the CMI station recorded values significantly higher with respect to those of NOR and NRC stations, Fig. 5 and Table 1. With reference to the earthquake occurred on 30th October, for which only NOR and NRC station records are available, the most severe actions were recorded by NRC station. Both PGA (707 cm/s2 versus 476 cm/s2) and Sa_max (1991 cm/s2 versus 1931 cm/s2) were lower for NRC station with respect to CMI; whereas the parameters corresponding to a period equal to 0.3s were higher for NRC station. This clearly indicates that the seismic actions recorded in Campi Alto were higher or comparable to those recorded in Norcia.
Table 2 summarizes two other seismic intensity measures: the Housner Intensity, IH (Housner 1952), and the modified Housner intensity, mIH (Mouyiannou et al. 2004). They are calculated as integral of the pseudo-velocity spectrum between 0-2.5 s and 0-0.5 s, respectively. These parameters are two energy-based intensity measures that may providing a better correlation with the structural damage with respect to parameters reported in Table 1.
The trend of the parameters reported in Table 2 is similar to that of parameters in Table 1: during the earthquakes of 26th October, CMI station recorded higher values than those of the two stations close to Norcia; however, the values are fully comparable with those recorded by the NRC and NOR stations during the subsequent earthquake of 30th October.
Several studies demonstrated that local seismic effects, due to soil properties and topography configuration, can significantly affect the structural response of structures (Jahromi and Karkhaneh 2019; Panzera et al. 2018). Thus, to understand if the different response of the buildings in the historic centre of Norcia and Campi Alto were affected by different amplification factors, the results of a seismic microzonation study, carried out after the 2016 earthquake by SGA (Studio Geologi Associati -Perugia - Italy), is depicted in Fig. 6.
In this study (available on website www.regione.umbria.it) a monodimensional model was adopted for Norcia, characterised by a flatten topographic surface, whereas in Campi Alto the presence of a complex morphology made it necessary to model the topographic amplification through a bidimensional analysis. Fig. 6 reports three detailed maps describing the areal distribution of the Amplification Factor (AF) for three different period ranges: 0.1-0.5s, 0.4-0.8s, 0.7-1.1s. The centre of Norcia is totally characterised by an amplification factor in the range of 1.1-1.4, regardless of the period of the structures, whereas in Campi Alto two subzones with an amplification factor of 1 or in the range of 1.1-1.4 can be identified. Thus, the amplification factor computed on Campi Alto is in every case lower (or equal) than that related to the centre of Norcia.
Comparative analysis on structural characteristics and damage: buildings of Campi Alto and Norcia
The comparative analysis of buildings characteristics in Campi Alto and Norcia is reported in the following based on the data collected by in situ surveys and relevant AeDES forms (Baggio et al. 2007). The forms were filled by technicians of the Umbria Seismic Risk Office following the 2016 seismic sequence. In Campi Alto, 49 AeDES forms were analysed covering the total number of ordinary buildings inside the walls of the village: 44 masonry structural units (MSUs) of residential buildings and 5 warehouses. By contrast, the data related to buildings in the centre of Norcia involve 670 MSUs but they are not the total number of buildings (i.e. a significant percentage of about 80%).
Fig. 7 and 8 show the percentage of MSUs as a function of age of construction (C) and renovation (R), masonry quality, type of horizontal structures, and roof type. Note that for renovation, the year 1982 has been considered as a reference year to distinguish MSUs with renovation works realized before and after the 1979 seismic event; note that UR (unreinforced) means that no renovation works were recorded.
The majority of MSUs (65% and 60% for Campi Alto and Norcia, respectively) has been built before 1982 and strengthened after this date. However, there are no MSUs dated after 1982 in the sample related to Campi Alto. Thus, only few MSUs in Campi Alto (6%) have regular layout/good quality masonry, while 55% show irregular layout/bad quality masonry and 29% is characterised by both types of masonry (namely “mixed”). By contrast, the quality of masonry of MSUs in Norcia is evenly spread in the sample: regular layout/good quality masonry (27%), mixed (34%) and irregular layout/bad quality masonry (38%). Fig. 9 reports a comparison between Campi Alto and Norcia typical masonry types. The masonry of buildings in Campi Alto generally consists of rough stone elements (generally limestone, conglomerates, or travertine). It is commonly strongly irregular, and it is rarely possible to recognise the presence of horizontal bed joints (Fig. 9a,b). Furthermore, in several cases the masonry is made by disconnected layers, small elements and powdery mortar with lack of any cohesiveness. By contrast, the masonry of buildings in Norcia is mainly characterized by worked stone elements placed in a regular way (Fig. 9c,d). However, in some cases also in Norcia irregular masonry can be found as depicted in Fig. 9e. Furthermore, buildings of Norcia are commonly plastered.
The difference of MSUs in Campi Alto and Norcia is related not only to the masonry quality but also to the type of horizontal structures. Buildings in Norcia mostly presents beams with Rigid or Semi-Rigid (R/SR) slabs on each floor (i.e. 56% of the dataset) while in Campi Alto this kind of horizontal structures are often limited to the upper floors and on the ground floor there are Vaults (R/SR+V in Fig. 8). Finally, in terms of roof types, the most significant difference between Campi Alto and Norcia is related to the presence of Non-Thrusting Heavy structures (NT-H), 63% and 81% of the MSUs dataset for Campi Alto and Norcia, respectively. Non-Thrusting Light (NT-L) roofs represent 10% and 9% of the dataset for Campi Alto and Norcia while Thrusting Light (T-L) and Thrusting Heavy (T-H) roofs are uncommon.
In the AeDES form (Baggio et al. 2007) different usability ratings are reported: A. Usable buildings; B. Building usable only after short term countermeasures; C. Partially usable building; E. Unusable building. Fig. 10a shows that in Campi Alto 92% of the residential MSUs (45 MSUs) resulted unusable (E rating), and the remaining 8% (4 MSUs) usable only after short term countermeasures or partially usable (B/C rating). In Norcia, the percentage of usable MSUs (A rating) was strongly greater than in Campi Alto and equal to 26% of MSUs; the remaining part was almost equally distributed in B/C or E rating (42% and 32%, respectively). This clearly indicate a better response of MSUs in Norcia with respect to Campi Alto. To better understand the differences in the behaviours of the MSUs, the Damage Index (DI) obtained from the damage levels and extent occurred on vertical structures, according to (Dolce et al. 2017), is investigated. It varies from DI 0 to DI 5 and the percentages of DI levels related to Norcia and Campi Alto are depicted in Fig. 10b. The figure clearly confirms that a more severe level of damage was detected on MSUs of Campi Alto with respect to Norcia: a DI lower or equal to 2 was detected on 78% of MSUs in Norcia against 26% in Campi Alto. Furthermore, in Campi Alto 29% of MSUs resulted affected by local or global collapse, DI 5. The plan distribution of DI in Campi Alto and Norcia is reported in Fig. 11 while Fig. 12 reports the cumulative percentages of MSUs with a DI lower than a fixed value. In particular, the trends are plotted with reference to MSUs built after 1982 (C>1982) or before such year (C<1982) but renovated before or after 1982 (R<1982; R>1982) or unreinforced (UR). It is noticeable that in Campi Alto 80% of the MSUs renovated after 1982 has a damage index lower than or equal to 3 (usually only local damage), while those renovated before 1982 reach the 50% mark. All the unreinforced MSUs had a damage index equal to 5.
For the dataset of MSUs in the historical center of Norcia, it resulted that 97% of MSUs renovated after 1982 and about 95% of those renovated before 1982 show a damage index equal to or lower than 3.
Furthermore, according to AeDES forms and in situ inspections specifically carried out by the authors resulted that in Campi Alto 13 MSUs collapsed (i.e. 9 full collapse and 4 partial collapse, see Fig. 13), corresponding to 26% of the entire built-up area. In particular, the full collapses affected 4 MSUs never consolidated, 4 renovated before 1982, and 1 renovated after 1982 while 4 partial collapses affected MSUs renovated after 1982. Fig. 13 also shows the three collapses that affected cultural heritage buildings inside the walls of Campi Alto: Santa Maria della Neve’s church, Sant’Andrea’s church and Madonna di Piazza’s church; the latter two were examined in Penna et al. 2019. In Norcia, only 4 MSUs collapsed and 7 partially collapsed corresponding to about 2% of the dataset. However, the damage on cultural heritage buildings was diffused and severe also in Norcia as demonstrated by the collapse of several churches and heritage buildings inside the city walls.
The diffuse and severe level of damage on cultural heritage buildings both in Norcia and Campi Alto is a clear consequence not only of the high vulnerability of such constructions but also of the severe intervention restrictions imposed on these buildings in order to preserve their original characteristics.
Critical analysis of strengthening intervention effectiveness
The data discussed in the previous sections show that, even if most of MSUs in Campi Alto and Norcia were renovated following the 1979 seismic events, the damage provided by the 2016 seismic sequence was more severe in Campi Alto with respect to Norcia. To focus on such a different behaviour, a detailed analysis of 20 strengthening projects (related to 26 MSUs) realized in Campi Alto between 1984 and 2012 is herein reported, see Fig. 14. The original documentation has been examined thanks to the collaboration with Umbria Region offices, and it involves 26 MSUs corresponding to 53% of the entire built-up area of Campi Alto. Thus, the sample of data may be reasonably representative of the strengthening works carried out in Campi Alto. A summary of building characteristics (number of stories and average floor surface) and of relevant strengthening intervention data (year of project of the intervention, type of the intervention on vertical and horizontal structures) as well as details on empirical damage detected after the last seismic sequence (damage index and type of external damage) on such MSUs is reported in the Annex section.
Fig. 15 shows the distribution of the damage index evaluated according to data collected in the post 2016 earthquake sequence by AeDES forms and in-situ inspections carried out by authors. Note that in the latter case, it was possible only to evaluate the external condition of the buildings, whereas the AeDES data considers also the internal damage (unless the building was inaccessible). The 26 MSUs have been grouped in three categories based on the amount and type of damage detected by surveyors: MSUs with no external damage (N.E.D.); MSUs with external damage (E.D.), and partially collapsed MSUs (P.C.).
In general, the analysis of the documentation related to the 26 MSUs (see Annex) showed that the intervention on vertical structures commonly involved the replacement of existing vertical walls of the floors above ground floor, combined with strengthening of undemolished walls portions by using several solutions: grout injections (G.I.), reinforced plaster jacketing (R.P.), steel connectors (S.C.) and tie roads (T.R.). In particular, G.I. was often combined with a wide use of S.C. at the orthogonal wall intersections. The R.P. was used to strength the bearing walls in only 6 projects, however this intervention rarely involved all the walls. Moreover, the architectural restriction of maintaining the fair-faced aspect of the masonry forced to apply R.P. only on the internal side of the external walls.
The intervention on horizontal structures involved, in all cases, the strengthening of existing vaults through the removal of the filling material and the construction of a RC slab at the extrados. Furthermore, existing wooden slabs were always replaced by new ones made of: RC beams and hollow bricks (9 projects); new wood beams (4 projects); steel beams (1 projects); or a combination of the previous ones (5 projects). The existing roofs were in most cases replaced by RC structures or wooden beams (10 and 7 projects, respectively), by steel beams in 2 projects. The new horizontal structures were connected to the surrounding walls by a RC ring beam partially built in the masonry wall thickness (or with the same wall dimension in case of demolition and reconstruction).
In the N.E.D. group, 3 MSUs had a damage index equal to 0 or 1, see Fig. 16. Although different in size and number of floors, a common factor on such MSUs was the extensive use of strengthening intervention on vertical walls. In particular, the walls that suffered severe damage after the 1979 and 1997 earthquakes were restored by means of several local dismantling and subsequent rebuilding (the so-called “scuci-cuci” intervention), and strengthened through G.I. or combining G.I. and R.P. (e.g. project 10, see Annex). Connections between orthogonal walls were improved by inserting S.C. or T.R. (e.g. project 17 and 18, see Annex). Furthermore, a strong replacement of existing walls was carried out by using block masonry (project 10a) or stone masonry (project 17). Thus, extensive intervention on both vertical and horizontal structures clearly led to satisfactory seismic performances of the MSUs. In the remaining 7 MSUs, the damage index resulted in the range of 2-3, however the lack of visible damage prevents understanding the problems that affected these buildings.
In the E.D. group (i.e 10 MSUs) the damage index resulted always equal or greater than 2; in 5 MSUs, horizontal cracks, mainly localized at the level of intermediate floors, were detected (see Fig. 17). The width of the cracks was millimetric and revealed the activation of mutual translation between the two portions of the building, above and below the crack. This type of damage was detected on buildings where the upper floors were completely rebuilt, and the cracks were localized on the contact surface between the new floors and the existing ones. The behaviour of such buildings clearly confirms that, in case of reconstruction of floors, it is crucial to properly connect the new floors to the existing ones and vertical structures to horizontal ones. Indeed, in such 5 MSUs the poor attention to details in the adopted strengthening solution was the main reason of the damage detected after the earthquake.
In the remaining 5 MSUs with external damage, the damage was caused by deficiencies correlated to the original vulnerability of the building which were unsolved by renovation works: lack of connections on orthogonal walls and insufficient use of steel connectors as a strengthening technique, (see Fig. 18 related to MSU of project 13); reduced distance between openings and/or presence of openings close to the corners of the building and insufficient use of strengthening aimed at increasing the vertical walls bearing capacity, (see Fig. 19 related to MSUs of projects 6 and 7). In this latter case, MSUs belonging to a row building (adjacent masonry structural units related to each other) reported damages localized at its far end. By analysing the ground floor plan (Fig. 19a), it can be noted that the openings are very close to the corners and the distance between openings is very small; these two features cause a lack of masonry area that led to the formation of shear cracks in two bearing walls (Fig. 19b and Fig. 19c).
The P.C. group involved 6 MSUs with a damage index ranging between 3-5. In all cases the bad quality of masonry led to partial collapses. In particular, the uncomplete replacement of original existing wall in the strengthening interventions designed after 1979 seismic events led to partial collapse of ground floor with limited damage on the upper ones. In particular, the load-bearing walls of the first, second and third floors were demolished and rebuilt with block masonry while ground floor vertical walls were strengthened by G.I. in project 4, see Annex A and Fig. 20. After the 2016 seismic sequence the building experienced a rigid rotation of the three upper floors (with no damage, Fig. 20a) with respect to the ground floor where a large portion of the façade collapsed (Fig. 20b). Thus, the G.I. on ground floor resulted ineffective or insufficient to improve the quality of the masonry at the ground floor and this, along with the presence of two too wide openings in relation to the wall dimension, led to the crumble of the wall; tie rods were clearly ineffective because of poor quality of masonry wall. Similarly, the masonry at left corner of ground and first floor crumbled due to masonry bad quality in the MSUs of project 8 (see Fig. 21a); also the upper floors suffered damages limited to the walls that were not rebuilt after 1979 seismic events. In the MSU of project 2 an extensive collapse was detected: the original stone masonry of the ground floor and first floor crumbled (Fig. 21b); in addiction, the external layer of stone masonry, applied only for aesthetic reasons, collapsed due to the lack of connections with the load-bearing block masonry behind it.
Severe damage after the 2016 sequence was also observed in case of MSUs strengthened by using a very heavy and rigid roof on a bad quality walls which were not adequately reinforced by extensive use of G.I. and R.P. as originally designed, (e.g. MSU of project 11, see Annex A and Fig. 22).