Historical centers in seismic area: methodology for conservation, prevention and reconstruction. The case study of Vi lla Sant’Angelo

Davide INDELICATO 1 (1) Faculty of Engineering and Architecture and Motor Sciences, University of Enna “Kore”, Enna, Italy davide.indelicato@unikore.it Abstract In recent times, the recurrence of seismic events has suggested again to pay careful attention to the ways to intervene on the existing buildings whit the aim of increase their security requirements together with maintaining high levels in urban quality. In particular, it is a shared opinion that the conservation of historical city, especially in earthquake affected areas, should take into proper consideration the theme of seismic security. Nevertheless, this particular topic is confronted to operating methods which are still confused and often ineffective. This paper describes a methodology for the analysis of building aggregates of historical city, aimed at reducing their susceptibility to earthquake induced damage. This methodology has been applied to a block in the historical center of Villa Sant'Angelo, one of the towns near L'Aquila affected by the seismic event occurred in April 2009. The aim is to show that, even in situations of so high seismic risk, the deep knowledge of building constructions and of their environment is a basic condition in order to find operating solutions based on sustainability and cost saving principles. In this way, the restoration projects which spring from these principles, can find a good alternative to the easier but less sustainable solution of city delocalization. The methodology here proposed can be applied both in post-earthquake rebuild conditions and in the most advantageous prevention activities. In a period of so hard economic crisis, the lesson learned from last earthquakes imposes us to move our resources from emergency to prevention activities.

Keywords: conservation, prevention, reconstruction, historical centers, Villa Sant’Angelo

1. The research methodology This study, based on part of a doctoral thesis, is structured on the basic concept of building aggregates. The complexity of urban historical texture is due to all its transformations, alterations, permanence, that makes hard the interpretation of constitutive nature of the buildings. The complexity of historical city is reflected on its seismic response. Buildings in historical centers, in fact, since they belong to a whole, are tightly reciprocally connected. So, the research proposes the building aggregates as a minimal unit to be analyzed and as an average component between the urban and the architectural scale. The building aggregates is defined as “a buildings group structured in time and connected by a historical self-regulation system”[1]. The components of building aggregates, by this aggregation procedure, acquire different features characterized by typical individual elements. The building aggregates is endowed with something essentially different from its individual parts which have an inner relation level permitting to consider it as a unique object. The research methodology here adopted is structured in three steps: the knowledge, the interpretation and the project. It is also developed on three scales: the urban scale (the city), the building aggregates scale (the city fragment) and the architectural scale (the building). 2. The knowledge phase 2.1. The earthquake of April 6th 2009 and the seism ic history of L’Aquila On April 6th 2009, at 03:32 AM, the district of L’Aquila was shocked by a strong earthquake with the main-shock magnitude 5.8. The distribution in plan of the shakes repetition shows that the area

involved in the seismic sequence, called “seismic crater”, is 30 Km parallel to Appennini mountain chain, in direction NW-SE. This first main-shock, on April 6th, was situated at the south-west of L’Aquila, near Tornimparte, Lucoli and Scoppito. The stronger aftershock, registered on April 7th at 19:47 (Ml=5.3) has involved the southerner area of seismic crater, near Villa Sant’Angelo, San Martino d’Ocre, Fossa, San Felice d’Ocre. On April 9th a third aftershock, with magnitude 5.1, shacked the northern area, near Barete and Pizzoli [2]. The study of the main seismic events which involved the area damaged by the 2009 earthquake, highlighted a constant and particular intense seismic activity. The strongest historical earthquakes which involved L’Aquila district (registered in Catalogo Parametrico dei Terremoti Italiani, 2004) happened on September 9th, 1349 (Me 6.4, MCS 9) and on February 2th, 1703 (Me 6.7, MCS 9). These two earthquakes were more powerful than 2009 events, however we can compare it whit other seismic events happened in this area in 1461 (Me 6.4, MCS 9), in 1762 (Me 5.9), in 1916 (Me 5.2) and in 1958 (Me 5.2), which damaged L’Aquila and its surroundings [3]. The goal of studying the seismic history of a specific geographic area is to recognize the hardest earthquake in relation with adjusting the consolidation project. At the same time, this study will allow to evaluate the efficacy reinforcement operations made after every post-seismic rebuilding, as a real time test.

Fig. 1: Location and classification of L’Aquila seismic sequence from April to June, 2009. Fig. 2: Plan of “seismic crater”. 2.2. Critical survey and damage survey Villa Sant’Angelo is a typical hilly village of 5,26 square kilometres, at 570 meters above sea level. Only 436 people lived there before 2009 earthquake (ISTAT, January 1st, 2007). The village is composed of two built up areas: Villa Sant’Angelo, that is the administrative centre and Tussillo, a small hamlet. As a consequence of latest urbanization process they tend to connect themselves, but the two historical centres remain detached. The “villa” (rural area) is situated at the base of the hill; Tussillo is endorsed on the “Sant’ Petr’ “ mountain [4]. In this urban contest, the building aggregates considered as study case is representative of a typical damage condition of this area. This block is situated in the southern area of the historical centre of Villa Sant’Angelo, in closeness of the city entrance. The geometric survey has highlighted, amongst other things, a hard altitude gap between upstream and downstream level that influenced the architectural, morphological and typology features of the buildings. The collection of information on the structural relationship among buildings as well as on constructive techniques of each building is the so called “critical survey”, in which the buildings are examined in such a way to point out all the significant structural details, both at the scale of single buildings and in their mutual connections. The critical survey has carried out also observing the ruins with the aim to acquire information about constructive features of collapsed building to link, in the second step, at its damaging scenario. The evident collapse condition make easier the identification of constructive elements, of the walls juxtaposition, of the trace of historical passed away configuration, of historical anti-seismic aid. All this features were precisely annotated and drawn. In this step it was carried out a survey of damage and collapse status at two scales: at urban scale it was surveyed in swift way the damage and the ruins of all the building aggregates of the historical centres of Villa Sant’Angelo; at individual building aggregates scale it was analyzed the specific damage status.

Fig. 3: Damage survey of Villa Sant’Angelo. Urban and building aggregates scale analyses. 2.3. Supposed pre-seismic configuration of the buil ding aggregates The critical examination of damage and collapse condition of building aggregates as well as of the ruins allowed supposing the pre-seismic configuration of the building aggregates. This hypothetic reconstruction is finalized to understand the reason of the buildings collapse or damages which can be strictly connected with their conservation status or constructive typology. Starting from the upstream area, it was supposed that the upper exstremity of the architectural unit could have three elevations: two elevations were in masonry, one elevation was in concrete brick. The roof was made of steel beams, hollow clay blocks and reinforced concrete slabs. The inner buttress walls of first level didn't continue in the other two levels, so two couple of steell beams were placed in support of the roof. 2.4. Origins and development of building aggregates Supposed reconstruction of pre-seismic configuration of building aggregates is essential, amongst other things, to retrace backwards its origins and typological development process [5]. The sequence of building process is discovered by recognition of walls juxtaposition, by assign a related date to masonry and, in consequence, to corresponding buildings. The site occupation started contemporarily from upperstream buildings and downstream buildings. The upper extremity architectural unit is a one-celled building duplicated in the back. They originally had only one elevation and the cells ware separated. The following elevation and unification occurred in two distinct periods: when a staircase was inserted at the upper levels and when the floor was changed. The development of the block proceeded to the south direction, by occupation of east front towards main street, then by the occupation of the back space toward the country areas. The development of downstream area started with a barn and followed by other rural underground buildings, till rejoining with residential building of northern area.

Fig. 4: Supposed pre-seismic configuration of building aggregates.

Fig. 5: Origins and development of building aggregates.

2.5. Constructive survey and local seismic culture The Abruzzo district has been victim of many catastrophic events in all its history. So, local builder were induced to use particular devises in order to make buildings resistant to earthquake. This condition allowed to develop an historical anti-seismic constructive culture is still in progress and that we can understand by studying and careful observing historical buildings [6]. The building aggregates chosen as study case in the historical centre of Villa Sant’Angelo has many features of local constructive tradition and also the same constructive “alteration” that 2009 earthquake put to the test (for example reinforced concrete edging or roof). The evaluation of the various modern and historical constructive devises, in consequence of seismic effects, leads us to reflect both about the technical rebuilding manner and the way to reduce seismic vulnerability of the other buildings.

Fig. 7: Historical constructive culture 3. The interpretation: from analyses to project 3.1. Vulnerability and resistance seismic factors In the interpretation step it was compared the reconstruction of supposed conformation of building aggregates before the earthquake with the post-seismic survey. This analysis allowed deducing the vulnerability and resistance seismic factors really existing in the building aggregates. Different kinds of vulnerability factors were identified: innate vulnerability factors (such as extreme horizontal and vertical thinness of masonry walls, pushing roofs, low quality of masonry walls…); alteration and interaction vulnerability (for instance deficiency of adjacent walls anchoring, reinforced concrete edging); position vulnerability (such as slender facade). Some of resistance factors detected in the studied block are, for example, the well anchored masonry corner, the wooden chains inside the masonry walls, the metal chains, the wooden trusses whit external anchoring. 3.2. Seismic damage scenario Founding on previous analyses it was possible to suppose the seismic damage scenario happened due to 2009 earthquake and the individual unstable mechanisms of collapse [7]. The façade out of plane overturning collapse mechanism didn’t affect the underground or ground planes but just the second levels. In lower levels there were often masonry vaults with metal chain and they resisted both when they were parallel to the façade and when they were orthogonal to the façade. Moreover the historical anti-seismic devises were particular efficient. For instance, the wooden trusses whit external anchoring in the barn prevented the collapse of external façades. Also the wooden chains inside the masonry walls and the metal chains were efficient, as we can know by observing the collapse edges. The same level of seismic protection was not guaranteed by modern anti-seismic devises in reinforced concrete. The detailed knowledge of the block and the accurate description of damages and collapses it has suffered are essential for identifying probable causes that led to the nature and extent of the surveyed scenario and, at the same time, recognizing structural solutions that could prevent even greater damages.

Fig. 8: Vulnerability (in red) and resistance (in green) seismic factors of building aggregates

Fig. 9: Seismic damage scenario. In red the collapse mechanisms.

4. The project: safety and conservation design at u rban, building aggregates and building scale The third step of methodology is the project. It consists of a rubble removal plan and safety and a conservation project at urban scale, building aggregates scale and building scale [8]. The purpose of rubble removal plan is the conservation of what remains of Villa Sant'Angelo, avoiding widespread culling of the surviving portions, in order to not remove the traces of the ancient centre. So, the project considers some local interventions with minimal impact on surviving buildings, aiming at select what can be preserved and what we must renounce to guarantee safety and start rebuilding. The rubble removal plan combines together conservative need and safety need. With regard to the securing operations, these include various types, such as removing of unstable component of roof or balcony and the shoring walls. The need for the removal of only unrecoverable portions is motivated by the desire to minimize the loss of important cognitive elements. Some interventions are specifically aimed at the conservation of the surviving portions, for example the closing of "the edge of collapses" in order to limit the decay due to atmospheric agents, or the selection, cataloguing and storage of stones having an architectural value. This project is inspired by extremely conservative and environmental economic sustainable criterions, because it proposes the re-use of material rubble, trying to preserve what earthquake didn’t destroy. 5. Conclusive considerations: from emergency to pre vention To engage a so extended damage level is a hard endeavour that could induce to considerate cursory and no social and economic sustainable solutions. The city relocation, for example, could create a great urban ruin and the establishment of new towns with no social and historical references. Also the extensive demolition is likewise inadmissible because it involves great economic resources and the removal of ancient city with no signs of the earthquake tragedy. This study proposes to intervene in gradually differentiate way to rebuilding the city, both whit creation of new architecture and conserving as much as possible. This analyses method can also be applied in prevention way. Much consideration about the block damaged by the earthquake can be carry out in pre-seismic phase, whit prevention purpose [9].

Fig. 10: Rubble removal plan and safety and conservation project at building aggregates and building scale.

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[3] ROVIDA, A. CASTELLI, V. CAMASSI, R. STUCCHI, M. Terremoti storici nell’area colpita dagli eventi sismici dell’aprile 2009, Istituto Nazionale di Geofisica e Vulcanologia, http://www.ingv.it., 2009

[4] ANTONINI, O. Villa Sant’Angelo e dintorni. Tussillo, Casentino, Fontavignone, Stiffe, Campana, S. Eusanio, Fossa, Ocre. L’architettura religiosa. L’Aquila: One Group edizioni, 2006

[5] CANIGGIA, G. MAFFEI, G. Ragionamenti di tipologia: operatività della tipologia processuale in architettura. FIRENZA: Alinea, 1997

[6] RANELLUCCI, S. Manuale del recupero della Regione Abruzzo. ROMA: DEI, 2004 [7] GIUFFRÈ, A. Sicurezza e conservazione dei centri storici. Il caso Ortigia. BARI: Laterza, 1993 [8] CAROCCI, C.F. CATTARI, S. CIRCO, C. INDELICATO, D. TOCCI C. A methodology for approaching the reconstruction of historical centres heavily damaged by 2009 L’Aquila earthquake. In Structural Analysis of Historic Constructions, Periodical of Advanced Materials Research. Switzerland: Trans Tech Publications, 2010, Vols. 133-134, pp 1113-1118

[9] CAROCCI,C. INDELICATO, D. Seismic safety and preservation of historical urban texture: a study case in Catania. In AA.VV. Proceedings of Seismic risk and rehabilitation of stone masonry housing. Horta: SerSilito-Empresa Gràfica, 2008, p. 231-235