A multidisciplinary cognitive approach aimed at the safeguard of the ruins of Mongialino’s tower in Mineo (Sicily) Mariateresa GALIZIA,1 Alessandro LO FARO1, Mariagrazia SALERNO1, Cettina SANTAGATI1 (1)
Department of Civil Engineering and Architecture, University of Catania, Catania, Italy
[email protected];
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[email protected] Abstract
This paper presents the outcomes of the multidisciplinary study carried out on the tower of Mongialino in Mineo (Sicily), a complex architectural heritage today reduced into ruins. The site has a strong landscape and territorial value, the vestiges of the donjon rise on a rock promontory that dominates the valley of Margi, already inhabited in ancient greek age. The unstable conditions of the building, affected by important collapses and lesions, require a safeguard intervention and that implies a deep knowledge and understanding of the construction. The first step of this process is a suitable documentation and representation of the artifact through the integrated use of current technologies of surveying able to provide a metrically accurate three-dimensional model of the architectural object. The circular planimetric shape of the donjon and its towering position require to solve both the problems related to the 3D data acquisition and those related to the representation of a cylindrical shape object. In this paper we propose a multidisciplinary approach concerning both the reading of the technological, typological and material elements that constitutes the constructive apparatus of the building (masonry, ribs, ring-like vault), and the geometric analysis addressed at the virtual reconstruction of the original shape. The peculiarity of the object, perhaps an unique case in Sicily of a cylindrical tower with inner cylindrical nucleus, its status of ruin immersed into a context of remarkable environmental value, encourage this cross-disciplinary approach. This way it is possible to catch the essence of the object to protect and therefore to foreseen the necessary interventions that will be able to pass it down to future generations. Keywords: Digital Cultural Heritage, 3D modeling, geometrical analysis, building technology
1.
Introduction
Nowadays data acquisition research on cultural heritage concentrates increasingly on the identification of rigorous methods of investigation, which must be able to define a suitable methodological approach for the creation of more detailed guidelines. For this reason the study of the castle of Mongialino in Mineo, Sicily, has been carried out using a method which involves the integration of specific investigations aiming towards the documentation of the architectural element in its complexity and its totality. This research has analysed the historical-documental, geometric-spatial and typological-formal aspects of the defensive structure, interesting scholars from different interdisciplinary fields of study. Its typological features as well as its severe state of decay and danger have made it necessary to carry out a survey using a 3D Laser Scanner, which was the most appropriate method for its conditions and provided a synthetic drawing which includes the various investigations of the scholars involved. The study, in fact, uses a three-dimensional model obtained from laser scans as a model of objectively accurate research, through which it is possible to identify the geometric rules as well as the classification of the materials and of the building techniques – both traditional and imported – in order to validate the historic-documental theory.
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Moreover, the 3D model obtained gives us an appropriate analysis of the wall structures and of their textures in order to define their state of conservation as well as being able to achieve reliable results about the level of their stability. This provides an accurate evaluation of the state of conservation of the architecture and, the fundamental components being known, guidelines for an intervention plan for its renovation, which will be more coherent since there is the opportunity to analyse carefully the result, understanding its principles and the criteria which document the data acquisition process.
Fig. 1: The tower of Mongialino rises on a rock promontory that dominates the valley of Margi
2.
Reading the signs: historical and territorial overview
Mariagrazia Salerno Sicily is a territory of castles. The island, which is a flourishing land in the heart of the Mediterranean, has been, in its history, the crucial set of fighting between kingdoms which competed for the control of the Mare Nostrum. The fortified architecture in Sicily is an important testimony of the history and of military and constructive techniques: in the island, that was in a permanent state of siege, the defensive constructions changed appearance or maintained partly functional elements, reflecting the modus operandi of new lords; all this leads to a diversification of the fortress theme. Numerous castles arose: next to and in the towns, often equipped with sighting towers with protective walls; others are located along the coast or protecting ways that connected the urban sites [1]. The south-eastern Sicily, like the rest of the island, has been cradle of civilization since prehistoric period. In the centuries various dominations, from the Greeks to Northern Europe people, have followed: not only the lands were fruitful, therefore suitable for agriculture and human settlement, but the particular physiognomy of the (Catania) Plain, surrounded by the southern foothills of Erei and, in the north-west, by Mount Hyblaean, makes these locations strategic for the control of major waterways and terrestrial ones. In this study, Mineo’s territory is analyzed, in which, throughout the medieval period, there are several defenses guarding a portion of the Plain called “valle dei Margi”. Here rises the Tower of Mongialino, placed to protect the territory and the internal road that connected the stronghold of Mineo with the Agrigento one. The fortalice, otherwise referred to Montalfone or Griffon or Falcone or Gaffone or Montecaffuri, because of the different naming it took over the centuries, is located on a rocky and steep spur (315 m above sea level) overlooking the valley of Casalgismondo, ploughed by the Pietrarossa river, a feeder of the Monaciand Gornalunga rivers, which, introducing in the Iron River, flows, after a long arcuate path, into the river of Margi. At the foot of the rocky outcrop rose a small village and a religious complex, which is ruined too. The date of construction of the manor is uncertain: Gambuzza [2] and Tomarchio [3] reported that the building was constructed in the period of Arab incursions: in 829 the Arabs conquered Mineo and its lands, in which they erected the "Malgia Khalil". The first mention of Malgia Khalil is that of the Arab chroniclers Ibn 'atir, An Nuwayrî and Ibn Haldun. They narrated the Muslim conquest of the Byzantine fortress, which was stripped of the peripheral elements of defense (we do not have any information of the consistence of this previous fortress) and reduced to a garrison of secondary importance. The signs lead approximately to the location of the castle. Near the castle, there was a large farm, preceding the birth of the castle as reported by the Arab chroniclers and subsequent notary deed until the frederician age; De Spuches asserts that the castle was built by Manfred, lord of Butera, who was proclaimed lord of Mongialino in 1143; it has recently been suggested that the construction of the fort
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is due to the presence of the Cistercians in the territory of Mineo; it is assumed that the two churches of the feud of Mongialino were dedicated to S. Cataldo and some architectural elements, placed in the way of other more recent ones, show a typical Cistercian craftsmanship [4]; finally, a last hypothesis is that the manor is dating back to the Angevin age [5]. Indeed in 1273 Charles of Anjou established a commission to assess the condition of defensive garrisons in the area; the commission began its work in Mineo in April 2 and ended in June 18 of the same year in Enna. Their objective was to assess existing safeguards to reorganize, renovate and build new ones where there was a need or a garrison destroyed. The historical reconstruction has proven that the life of the castle was closely linked to that of Vetustissima et Jucundissima Mineo. The town of Mineo took part at the Catania Emirate and followed its destiny until the Normans occupied the island. It is supposed that Count Roger, between 1062 and 1063, has occupied the fortified places in the interior of the island, and among these Mineo and surroundings. The feudal catalog from 1320 reveals that since the foundation of the Norman kingdom were instituted the feuds of Mongialino, Serravalle, Balchino, Camemi, Lamia, Bucalca or Bucalta (today Buscialca), Anicara (today Nicchiara) [6]. These populated feuds did not became part of the state property of Mineo, but they were linked to it because the officers of the city extended their authority or jurisdiction over there, and finally because the feuds were subjected to the payment in cash to take advantage of civic uses. Vast estates of the territory of the state property of Mineo, however, were granted to knights, who became direct vassals of the crown. The feud Mongialino was never divided in other lordships, and was composed of the following subfeud: Casalvecchio, Frasca, Serri, in the territory of Mineo, Pietrarossa Soprana, Pietrarossa Sottana, San Cataldo, and Montecaffuri, where today stands the ruins of the castle. Several feudal lords succeeded in Mongialino but the long-lasting dynasty was that of Enrico Statella, who in 1408 came into possession until the end of 1800. This feud, under Charles V, was still subjected to the city of Mineo and so forced to comply with the dictates of civic uses; this is confirmed by the acts referring to the Civic Council of 1530, year in which were established the "feghi dell’Università”. The 1693 earthquake, which destroyed many towns of the Val di Noto, caused extensive damages to the dungeon. The time and the carelessness of the owners have accentuated the decline, but nevertheless it is possible to understand its features. From 1800 until 1988 the castle and part of the feud became property of privates. Only in that year the town of Mineo bought the castle of Mongialino to build there a Museum of defensive garrisons and of the sacredness of the landscape.
Fig. 2: The valley of the Margi river system, elaboration on the map of Von Schmettau 1720-1721
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Fig. 3-4: Views of the ruins of Mongialino Tower
3.
Reading the signs: digital survey and the project for knowledge
Mariateresa Galizia The medieval castle of Mongialino lies on Mount Alfone, an inaccessible promontory in the territory of Mineo, Sicily, which overlooks the whole valley of the Margi and is so perfectly integrated with the surrounding area that a survey of the whole territory is seen as necessary for the graphical representation of a part of the rock on which it rises. Access to the castle is possible by means of a steep ramp following the lines of slope of the terrain, which leads to the top of the limestone elevation. In the lower part of the fortress there is a religious building with a church with two small rooms on the ground floor and one room on the upper floor. A barn and a henhouse formed part of it as well. The church, which dates back to 1600, was in use until 1800. Services were held for the people coming from the surrounding countryside. The planimetric map of the architectural unit shows a ‘whistle-shaped’ cylindrical tower overhanging both the north-west side and the south-west side where it is possible to observe what remains of the polygonal surrounding walls, which are instead clearly visible on the north-east and south-east sides. In the upper part the polygonal surrounding walls, of which we can see the remains of the first ring along the hilly path, enclose a courtyard with warehouses and pens. On the walls it is possible to see the merlons, the putlock holes placed at equal height, which make us imagine the existence of a walkway to guard the territory, and the arquebus embrasure to protect the castle. From the digital survey the circular dungeon shows a huge wall thickness of about 2 metres with a widening at the base: a section with an embankment which ensures the stability of the building. A central column unit, which is hollow inside and concentric with the tower, was used as a tank for collecting rain water. The inner room is a ring-shaped corridor about 4 metres wide. On the wall surface it is possible to see some holes which leads us to assume the existence of a wooden stairway accessing the upper level. It seems that its floor lay on a stone stringcourse, placed at a height of 3.90 metres, overhanging the inner walls onto which it is fixed. The ring-shaped barrel vault is arranged in pointed arches with two centres placed radially. Through a narrow square passage which opens onto the vault it is possible to access – once probably by using a ladder – the upper floor (third floor). This covers only half of the circumference of the base of the tower, and leaves the other half uncovered forming a terrace, thus giving the defensive structure what is defined as a ‘whistle shape’. Moreover, evidence of this shape is to be found on the third floor in the toothing between the wall surfaces which are placed at right angles to one another, and in the upper part of the drainpipe, which is built into the brickwork and placed at the base of the top floor parapet (which has a regular height). From a vertical viewpoint the cylindrical front of the tower shows, on the second and third floors, two different types of openings: splayed and straight loopholes (some of them having a seat). In all likelihood at the time of the construction there were only the splayed loopholes and, over time, some of them were rearranged and turned into windows fitted with seats. This hypothesis is supported by the interruption of the stringcourse where the windows with seats are, as well as the lack of plastering by these openings. Two of the three splayed loopholes have been damaged by the collapse of the structure, and now it is possible to see only partial traces (one of the two has been walled, so it is possible to see it only from the inside). The third loophole is in fairly good condition and is placed at about the middle of the arches supporting the vault. The instrumental survey of the geometric-spatial, static-conservational and environmental characteristics of the place was conveniently carried out using the 3D terrestrial laser scanner Riegl VZ-400 [7,8,9]. The phase of the in situ survey took into account a territorial and architectural scale approach. The data acquisition project involved a total of twelve stations, five of which were placed along the driveway lying under the fortress and at a distance of about 550m between each of them for
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the territorial scale representation, and 7 additional scans were placed near the object for the architectural scale representation. During the phase of acquisition homologous points were used for the following union of clouds instead of targets. In addition, during the scanning phase high resolution (Mpixel) photographic images were obtained from a Nikon D 700 Fx format -14 mm release, integrated with the instrumentation. The obtained data were processed using Riscan Pro software for the alignments of the clouds and, then, they were exported onto Cyclone, where layers were created for each scan, by choosing different colours for the single clouds. On these clouds slices were carried out on the model using vertical, radial and horizontal planes. The model thus obtained was cleared of intrusive elements so as to make the graphical representation on Cloudworks more accurate. This digital model came to represent a collection of information which was decoded through single fields of study related to each other. Indeed, it represents a simulation model still open to further research in order to carry out more detailed structural exams, which will be able to clarify some doubts about the interpretation of certain formal and constructive elements, thus providing the basic principles for a rigorous construction theory of the architecture. The outcome of the digital survey is a 3D representation model forming the basis for a computer reconstruction of the castle according to the principles of the London Charter [10], which established that digital heritage visualization has to be both intellectually and technically rigorous (reliability, documentation, intellectual integrity, sustainability and access). The obtained model is the result of a logical process in which the various sources for the reconstruction theories to be put forward (material remains, current and previous graphical documentation, current and previous photographic documentation, written sources, iconographic apparatus, typological comparisons, metrical observations, stylistic observations) were used and made public. The result obtained to date represents a product to be included in a possible Cultural and Environmental Heritage databank: a digital archive with 3D information which can be shared by scholars in this field, is open, can be constantly updated and integrated with other types of data, or can simply be used for study and for the dissemination of knowledge. Furthermore, as required by the London Charter, the model of 3D visualization makes it possible to devise a strategy which may ensure the long-term sustainability of the documentation and of the results of digital visualization concerning cultural heritage in order to prevent loss in mankind’s cultural, economic, social and intellectual heritage.
Fig. 5-6: In situ survey: territorial scan positions (on the left) and architectural scan positions (on the right)
Fig. 7-8: Views of the point clouds in confidence mode
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Fig. 9: North-West elevation
4.
Reading the signs: the geometric interpretation of the architectural shape
Cettina Santagati The interpretative process aimed at understanding the architectural shape of Mongialino’s Tower has taken into account the few written sources that describe the consistence of the castle [12,3] and, above all, the metric and spatial data by 3D laser scanning survey. The several collapses have largely affected the structure so now it is difficult to get a unitary spatial reading of the architectural text, indeed essential for its proper understanding and, therefore, conservation. The digital model of the tower – its virtual replica – has been an effective and irreplaceable research tool in this phase, as it allows in real time: -the close exploration of inaccessible or dangerous areas easily subjected to collapses; -the control of the geometrical/spatial interpretative hypothesis that have been formulated; -the volumetric quantification of the masonry portions that have been spoiled; the virtual anastylosis of the collapsed pieces still present on site. The first step of our approach forecasted the geometrical study of the circular plan of the tower [12]. At a first metrological examination, we confirmed that the planimetric dimensions are compatible with the ancient local measurement unit – canna (about 2,062 m) – according to multiples and sub-multiples. As previously described (Section 3), the external diameter is 20,64 m (almost 10 canne), the thickness of the mansonry is 2,06 m (almost 1 canna), then follows a ring-like vaulted room and the central cylindrical core whose diameter is 8,18 m (about 4 canne) and that holds inside a cavity used as holding tank. Furthermore, we also have verified that the radial arches that support the ring-like vault divide the plan in uniform circular sectors. The theoretical geometric reconstruction of the collapsed area brings to a plan divided into twelve parts, according to a regular dodecagon structure. The dodecagon is one of the principal regular polygon and it is done by dividing the circle by number 3. This is a geometric shape whose tracing is very simple, because it is based on the construction of equilateral triangles with side equal to the radius of the circumscribed circle. Often, in castle’s architectures we can recognize symbolisms linked to numbers mystique. In this case the number 12 could be linked to the time and to humans (like 12 months, 12 zodiacal signs, 12 hours, 12 doors of heaven), to the divine trinity, to the multitude, to the 12 apostles.
Fig. 10: Studies on the plan: a) metrological analysis; b) geometrical analysis and plan reconstruction of the first two elevations; c) geometrical analysis and plan reconstruction of the third elevation.
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In the second step we investigated the shape of the inner room to have proofs on the Vito Amico description. The presence of a cantilever stone stringcourse of about 3.90 m of height on both sides of the walls brings to the supposition of a wood planking level and so to the division into two levels. The stringcourse is made by supporting and decorative ashlars. The firsts act like cantilevers, pierce the solid wall and support the floor frame, the others give continuity to the floor. Furthermore, the presence of a planking level is beyond confirmed by the sings of some holes at different height that leads to imagine a stair that connected the two mentioned levels. Among these, 4 holes placed from one side to the other of the room, at the same height and under the only intact splayed arrow loop, lead to hypothesize a halfway landing. The openings of the tower belong to two different typologies: splayed arrow loops and right-angle arrow loops (some of them have a seat). It seems that the planimetric distribution of the openings does not comply with a strict symmetry: some of the openings are in axis with the dodecagonon sides, others are not. We could assume that in a defensive architecture the opening of an arrow loop should respond more to some functional needs than to aesthetic ones. Nevertheless, if we consider the intrinsic symmetry of the used circular shape and the division in twelve parts, the choice to not have a balanced succession of plenums and voids, it disadvantages the stability of the building and, maybe, it has started weakness mechanisms. The next step was the geometrical study of the ring-like vault [13]. This is a toric surface supported by radial pointed arches whose ratio between span and rise is 4/7, with a division in modules M of about 60 cm (that corresponds to the double roman feet unit). The springline is at about 5.40 m from the ground (9M) and the distance between the stringcourse and the springline is equal to 1.50 m (2,5 M). The vault directrix profile is a segmental arch whose springline is at about 7.00 m from the ground and whose centers are at the same height of radial pointed arches springline. As regards the upper level, it is likely the access had place through small passages into the vault. The reduced dimensions of the only surviving passage leads to imagine the use of a retractable stair. The current impossibility to access this level and the few surviving ruins do not allow to recognize the two other elevation described by Vito Amico, but only one of them. We suppose that the plan of the upper level was a semi-cylinder, that corresponds to the collapsed zone, with a terrace. Some of the still present signs confirm that hypothesis: the traces of a radial wall that closed the upper level, the continuous parapet of the terrace, the logline for the water disposal that is placed at the end of the parapet and is build in the masonry. Some further checks on the digital models highlighted that the radial wall of the upper level lies on the vault rather than on the supporting pointed arches and the entrance to the tower is perfectly in axis with the pointed arches. These two circumstances are not so optimal from a static point of view. Finally, we studied the geometry of the upper level vault by using the few surviving parts. The first thing that we noticed is that the upper radial supporting arches are not in axis with the lower ones. The absence of other elements of reference does not allow to carry out a deeper check that could help to understand if this is a building tracing mistake. Furthermore, starting from the existing traces of the vault we can suppose another toric surface supported by pointed arches and whose directrix is a buttress arch with springplane staggered of about 1.50 m (2,5M).
Fig. 11: Cross section and geometrical analysis of the supporting radial pointed arch.
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The geometrical studies and the proofs highlighted up to the present allow to propose a first hypothesis on the original shape of the tower: a cylindrical building whistle-like, with terrace. On the base of these evidences and interpretations we are working on the virtual reconstruction of the Mongialino Tower. Our reconstruction hypothesis and their representations will be guided by London [10] and Seville Charter [14] on Cultural Heritage Computer – Based visualization principles. In particular, we are applying and verifying the principles that deal with Authenticity (principle 4), Historical Rigor (principle 5) and Scientific Transparency (principle 7). Due to the weakness of the sources, the methodological approach will attempt to follow a rigorous path highlighting all the estimated choices sharing and making them visible also to the community. The goal is to make the methodological approach visually transparent the both to the scientific community and also to non experts [15,16].
5.
Reading the signs: materials and constructive techniques
Alessandro Lo Faro The massive bulk of Mongialino’s tower binds strongly on the rocks on wich it stands through the use of construction materials also. All the building elements that constitute the dungeon are in fact made using the limestone carried out Mineo’s territory. Probably the same rock on which the tower is has contributed to its construction. The sands of the nearby rivers were mixed with lime for mortar that bind the masonries and plasters that cover them. We interpreted the masonry of the dungeon through the detailed examination of the portions of walls now ruined and the laser scanner acquisition: the arrangement of the ashlars is largely readable as it is detached from plaster. The eminently defensive function justifies the massive thickness of solid masonry with an obvious variation between the tower and the outer fence, perhaps built later and reshaped several times. Analyzing the masonries according a constructive point of view, we are recognized 3 different typologies of walls, that we call M1, M2 and M3. The tower has a cylindrical outer thickness of about 206 cm; the columnar inner core is equal to 308 cm. In both cases it is possible to distinguish a cobblestone wall, called M1, consisting of 2 outer irregular coursed rubble wall, with visible face of approximately 25x30 cm. These sets have containment function and thickness varying from 30 to 50 cm. The core of both walls is constituted by a conglomerate in mortar and rubble limestone of small size (average size 3-30 cm). The settlement is every 50 cm, generally. They seem completely absent squarely dressed stone having function of ligament: the external sets are without necessary bond. In the upper part of the dungeon, in correspondence of the third elevation, is possible to recognize another typology of wall, which we define M2. Its thickness decreases by 2 rows of ashlars, reducing about 140 cm. It is always a cobblestone wall, with outer surface thick from 30 to 40 cm and concretional core of rubble and mortar. Compared to the type M1, the beddings are highlighted by narrow joints with fragments of tiles and bricks placed at each row of blocks. The surrounding walls and the buildings near the tower have been realized using an additional type of walling, here called M3. Its constructive technique is particularly poor and close to described masonry previously: a double surfaces in unsquared dressed stone, with a core of limestone mortar and rubbles. The thicknesses ranging from 70 to 100 cm, with particularly thick mortar joints. Greater care is found only at the corners, where are readable the dressed stone having ligament function. The different types of wall texture find here are all cobblestone wall: the impressive thicknesses (up to 3 meters in the columnar core) are a consequence of the defensive function of the tower but above all by the walling technology. It requires considerable thickness, with a massive behavior of masonry, against the ineffective bond between the blocks and the weak cross connection between the outer sets. The horizontal closures of the intermediate elevations, as recalled by Vito Amico, were wooden floors, of which today remains the stone shelves and the traces of the scaffolding holes: there the timber joists supporting the continuous timber flooring, with a wheelbase of about 90 cm. The second and the final elevation of the dungeon are defined by an annular vault realized in dressed ashlars supported by polycentric arches, set at a distance of about 320 cm [17]. In the first elevation the arches are composed of a succession of blocks, according to the sequence (a / 2 + a / 2) - a - (a / 2 + a / 2), i.e. a parallelepiped of dimensions 45x50x (7 ÷ 20) cm, then with variable height (Fig. 12). The arches, whose visible edges are slightly grooved, resting on stone cantilevers, just deeper (60 cm) than other ashlar. The laying of the arcs occurred initially placing the cantilevers inside the wall, and the first of these blocks over them, while also the masonry rises to a 30° inclined plane. The bar rel vault rises from this height. Thanks to the instrumental survey, it was possible to trace both the shape of the support arch than that of the vault. It has the geometry of a segmental arch because its centre is about 4 meters below the impost. The barrel vault in the first elevation is realized using dressed blocks of tender limestone, organized according the rowlock course. The height of ashlars is generally 30 cm, variable width (11 ÷ 18 cm)
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and length (30 ÷ 55 cm). The courses have staggered joints, about ¼ of total length, i.e. 13 cm. They are also occasionally used blocks having lateral squared surface to compensate for any unevenness. The support arches and barrel vaults do not have any bond from each other. At the surfaces of contact between the arch and the vault, you can see traces of the ribs, i.e. the housing for the wooden joists on top of the arches. Set up the centre, that is tracked stone arches, these are placed on the shutterings for the assembly of the blocks that form the barrel vault [18]. The abutment, as stated clearly due to the collapse, is made up of "heavy" materials: rubble stone tied with plenty of mortar, following the principles inherited from the Roman caementum. We have no trace of the vault that closed the last elevation, with the exception of the supporting arches, of which some remains ashlar abutment.
1 2
5
4 3
Fig. 12-13: Detail of the vault system: 1) blocks of the barrel vault; 2) ashlar of support arch, so called a; 3) cantilever stone block; 4) M1 masonry; 5) outer set of M1 type masonry. On the right a view of the ruined vault.
The modus construendi of this vault is similar to that of the lower level, although its geometry is different: it rested about 150 cm higher than the impost of the supporting arches. The vault's ashlars have more regular size, equal to 24x (30 ÷ 40) x 30 cm. The support arches are resting on brackets slightly protruding, of which only traces remain evident in the columnar core. Here also decorative pieces seem to have assumed, by their strength, functional value. The poor and unadorned simplicity of the adopted solutions, both spatial and constructive models, seem to recall the first French Romanesque, specifically the large common rooms and the cloister's aisles of the Cistercian abbeys. In those spaces the use of the barrel vault, (round and / or lancet arch) interspersed with reinforcement arcs of stonework, was the technological solution most frequently adopted. Here in Mongialino the working technique of stone blocks appears cruder and have a variability in size cannot be found in the examples of France. The function to compensate for the irregularities of the stone shall be entrusted to the mortar joints. In medieval fortified architecture the Cistercians workers were well estimated by contemporaries, so that the Emperor Frederick II came to entrust them with the construction of its castles [19]. It well designed, that is, the power of their logic and essential art. The lesson widespread in Sicily by Frederick architectures (Castle Maniace in Syracuse, Tower of Frederick in Enna) about the stoneworking according to the canons of stereotomy, however, appears in our tower still far away.
6.
Conclusion
This study is important not only for a proper historical/typological placement of the building, which is unique in the Sicilian landscape, but also for its preservation and transmission to future generations. The 3D model becomes an essential tool for the understanding of the static behavior of the structure, of the causes that led to the collapse and the proper conservation project. The reading of the cracks and understanding of damage mechanisms give some necessary indications to the choice of rehabilitative therapy [20]. The apparent symmetry of the plan tower’s collides with its inherent weaknesses, which partially justifies its current state of ruin. Among these we highlight: the presence of openings (the entrance, some loopholes) close to the impost of the supporting arches, i.e. where the thrusts are concentrated; the third elevation masonry stay directly on the vault; the weak crossconnection between the sets of masonry; the heavy abutment; the lack of connection between vaults and arches of reinforcement, are just a few examples. The present study would have these future outlooks: analysis of typological and formal models of the tower; the virtual reconstruction of the object; verification and measurement of the cracks.
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Acknowledgment Authors would like to thank Microgeo srl (Simone Orlandini, Enrico Cabrucci, Vincenzo Ippolito) for their support in the surveying phases.
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