Eur. Phys. J. Plus (2013) 128: 33 DOI 10.1140/epjp/i2013-13033-1

THE EUROPEAN PHYSICAL JOURNAL PLUS

Regular Article

A study of the Civic Tower in Ravenna as an example of medieval towers’ preservation problems Stefania Bruni1,2,a , Giuseppe Maino1,3,b , Elena Marrocchino4,c , Carmela Vaccaro4,d , and Lisa Volpe5,e 1

2

3 4

5

ENEA, Agenzia nazionale per le nuove tecnologie, l’energia e lo sviluppo economico sostenibile - via Martiri di Monte Sole, 4, 40129 Bologna, Italy NEREA NEtwork per il REstauro Avanzato, laboratorio delle Reta Alta Tecnologia Regione Emilia Romagna, presso Dip. DISMEC, Ravenna - 48100 Ravenna, Italy Facolt` a di Conservazione dei Beni Culturali, Universit` a di Bologna - sede Ravenna, via Mariani, 5, 48100 Ravenna, Italy University of Ferrara, Facolt` a di Scienze Matematiche, Fisiche e Naturali, Earth Sciences Department, Block B - via Saragat, 1 44122 Ferrara, Italy TekneHub, Laboratory of Ferrara Technopole, Department of Earth Science, Ferrara University - Ferrara, Italy Received: 10 December 2011 / Revised: 7 February 2013 c Societ` Published online: 14 March 2013 –  a Italiana di Fisica / Springer-Verlag 2013 Abstract. Structural stability is a major item when considering very high masonry buildings made of stones, bricks, etc., that can start sudden structural failures and collapses, often without any obvious signs of warning. A famous example is the collapse of the belfry of the Basilica of San Marco in Venice —the implementation of it began in the ninth century— which took place in July 1902 a few days after the appearance of a fissure. This paper discusses the scientific investigation performed on the Torre Civica (Civic Tower) in Ravenna (North-East Italy), in order to characterize its constituent materials, namely bricks and mortar. All this information and relevant data merge in a multimedia database which will help to design appropriate conservation and restoration works, mainly concerning the reconstruction of the apical part of the tower, that was foreshortened ten years ago for safety reasons, starting from the original materials catalogued and preserved up to the present day.

1 Introduction In Emilia Romagna as well as in the whole Italian territory, the medieval architecture, in most cities, has witnessed the organization of the city walls and towers, whose presence is a tangible sign of the historical events that have occurred. The analysis of the organization planning cannot ignore the evolution of the role that the towers have had both as a system of control and protection and as historical buildings and public administrative sites. Ravenna, in particular, is an example of the continuous adaptation of architectural buildings in past times. The tower, for example, with a succession of structural changes and architectural history has linked its temporal evolution to the concept of city spaces and street furniture, becoming a symbol and a reference for the citizens of Ravenna. As a matter of fact, cities are the concrete manifestation of a social, economic, cultural and architectural heritage that has modified and adapted over the centuries with changes due to both natural events and, above all, the work of men who lived and live in them. Ravenna is therefore the manifestation of a succession of historical events that have characterized the urban structure and its buildings, starting from the well-known Byzantine period up to the present day. Among the many historical buildings in this town, one of the most representative and symbolic, although less known than the Roman and Byzantine ones for the historical evolution of building techniques over the centuries, is the Civic Tower (Via Paolo Costa, Ravenna) —property of the city of Ravenna— called “Tower of the Butchers” (figs. 1a and b). It is also interesting for its present status of preservation, and is representative of a class of buildings that deserve adequate safety measures. a b c d e

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Fig. 1. (a) The Civic Tower in Ravenna, Italy. It is 38 m high. (b) The Civic Tower from a different viewpoint.

By means of the samplings —representative of the whole structure— performed during the consolidation of the tower, which is subject to failures and structural problems, namely mortar and bricks belonging to different structural layers, we proceeded to an analysis of the composition of materials and their conservation status, defining the structuraltextural character and the composition of the brick mortar. The study was carried out through optical microscopy with reflected and transmitted light, SEM (scanning electron microscopy) and chemical microanalyses. Finally, in order to estimate the porosity and the behaviour with respect to the humidity of the tower bricks, an NMR investigation was performed.

2 Historical notes After the death of Justinian, in a political climate changed following the decadence of the Exarchate of Ravenna (VI and VIII centuries AD) and owing to the first invasions of the Lombards, in Ravenna there was a change also in the architectural forms and styles, even though the construction techniques were not modified and the main materials were often re-used. It is customary to think that the tower is a distinctive feature of the urban landscape; towers actually appeared in various historical periods with different functions, and aims associated with the new uses: – in Roman times, for example, the tower was considered a structural element of defense with military functions such as sight control and protection of city walls; – in the Dark Ages, a type of towers, known as “cautious tower”, was located detached from the city walls and had the purpose of watching the city markets or the roads; – starting from the XII and XIII centuries AD, towers could provide private residences representing the status of the family that had commissioned their construction.

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Fig. 2. Lack of bricks and mortar.

Historical documents, considering the wall thickness (1.20 meters at the base) and the lack of windows in the old stretch of the Ravenna tower, have suggested scholars that the Civic Tower had no residential function, but was used to control and/or to defend a strategic hub of the city. Very often, in the Late Antiquity and in medieval times, these structures were built in the vicinity of the main bridges, bordering ancient rivers. The tower, object of this study, in fact, stood near the bridge on the ancient river Padenna, the most important waterway mentioned in various documents relevant to the medieval city of Ravenna. Previous studies have shown that the base of the tower is at the same level in which streets paved with trachyte (the plan of use in the Late Antiquity) were found. Over the centuries, the tower has undergone several changes: The curtain walls are, in fact, heavily reworked in several places, such as the edges and the lower stretch of the wall overlooking XIII Giugno street, in addition to the lower segment of the bell cell. At about one third of its height, the tower narrows. The stretch of the lower wall was built with sesquipedalian bricks, most likely made of reclaimed material, according to an ancient building technique typical of the civil and religious traditions of the high-medieval Ravenna [1]. On the contrary, the upper part presents bricks of primary use, as they are found in buildings of the thirteenth century as, for example, the church of Santa Chiara and the chapel built against the north side of the Basilica di San Giovanni Evangelista (the first half of the fourteenth century AD) [2]. Therefore, in our investigation we considered several samples, which are representative of both situations.

3 Structural characteristics and conservation status The Civic Tower is about 38 meters high with a square base. The walls have different thicknesses depending on the height; in fact, the wall section tapers from bottom to top, and one can see various structures [3]. The tower shows major structural problems and a steep slope due to differential settlement, subsidence and problems related to the fact that Ravenna is a seismic zone. From the geological point of view, the tower is located in an area prone to seismic risk, since several earthquakes occurred from 1483 to 1900 [4]. Moreover, further problems arise because of the occurrence of major fractures in the curtain walls and the bad state of conservation of the materials, especially those exposed outdoors. Given these causes of degradation, even small vibrations due to the vehicular traffic on the adjacent streets facing the tower, might facilitate the detachment of material and worsen the state of conservation. For this reason, the municipality has taken steps by establishing traffic-limited areas in the vicinity of the tower in order to avoid, for instance, the passage of trucks whose vibrations could not be borne by the structure. During our visits —winter 2010/spring 2011— the decline was clearly visible even at microscopic scale, both at the mesoscopic and microscopic levels. The surface of the exterior and interior walls shows, in fact, at different heights, chemical and physical degradation products such as salt efflorescence, leaching of mortar which are deeply eroded, and presents furrows of several inches, often reconstructed with unsuitable binders (cement), as shown in figs. 2–4.

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Fig. 3. Cracks and joints between the apparent replacement of missing bricks and mortar with a cement mix.

Fig. 4. Deposition of pollutants and black crusts.

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Fig. 5. Anthropogenic degradation: The base of the tower.

Peeling and alveous processes are found, involving an increase in secondary porosity, disintegration, pulverization, wear, loss and separation of materials of brick and mortar bed (figs. 2 and 3), especially in the upper zone (due to atmospheric factors including wind and rain) and at bottom of the tower (due to the infiltration of rising damp). Although the tower is in a restricted traffic zone, numerous forms of degradation were found on the surface of the building, mainly due to the deposition of atmospheric particles, solubilization of the binding and hydrolysis of the silicate matrix. In parts of the masonry not subject to a special flow of water, black crusts, as well as salt efflorescence and lack of material can be observed. The latter emphasizes the presence in the curtain walls at the bottom of the tower, subject to the rising damp, of poor-quality bricks, which are characterized by heterogeneous mixtures, of partly undercooked (albasi) and partly overcooked (ferioli) bricks. Probably, these bricks are not original and have been used to replace bricks deteriorated from the capillary rise (fig. 4). Both the black crust and the presence of bricks with heterogeneous physical-mechanical and thermal properties promote the rapid deterioration of the curtain wall of the base, so that even the bricks that have been introduced recently have already degraded. In addition to microfractures, furrows with large size are also found. The biological degradation is also important, linked to the presence of weeds (lichens, mosses, etc.), organic patinas (algae, etc.), guano and fauna, sometimes devoid of life (birds, rats, cockroaches, etc.), which caused a visible decay of the wooden beams used to support the structure of the tower. Considering, finally, the current state of conservation of the tower and the context in which it is inserted, one can verify the presence of a pronounced degradation of anthropic type (fig. 5). It is mainly due to the improper use of restoration materials, incompatible with the original materials and construction techniques; to the placement of technological (cables, etc.) and structural (scaffolding security, etc.) elements; finally, to acts of vandalism and lack of maintenance. Figures 6–8 show the reinforcing structures outside and inside the tower, while figs. 9 and 10 show details of the interior of the tower. It was considered appropriate, therefore, to evaluate the state of conservation of the tower, through the study and knowledge of the constituent materials.

3.1 Sampling For the study of the building materials, many samples —20 for each considered case— were taken from the internal structure at the height of 2 meters (see, for instance, sample 40/9, consisting in mortar and brick) and 8 meters (see, for instance, sample 40/73, brick). Ten other samples were taken from the top of the tower at 38 m, in order to perform NMR measurements for water content and diffusion. The number of samples allowed us to obtain statistical results

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Fig. 6. Anchors of consolidation at the base of the tower.

Fig. 7. Reinforced concrete at the base of the tower.

for the analysis of the building structure. However, in the following sections, we present analyses referring only to the first two described samples, since they are representative of all the others and of the whole structure. More detailed results will be presented in a forthcoming report.

3.2 Results First of all, preliminary analyses were performed at the stereomicroscope using an OPTIKA SZ674STR (total magnification 90×), equipped with a webcam MOTICAM 2005 5.0 Mp. The investigation on the bricks, conducted at the optical microscope both in reflected and transmitted light at the Department of Earth Sciences, University of Ferrara, already showed differences in the studied materials.

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Fig. 8. Reinforcing structures inside the tower.

The Brick 40/9 (fig. 11) shows a red colour, lighter (it nearly corresponds to pink) than that found in the 40/73 sample area, and a medium/fine grain size which is relatively homogeneous with little porosity and medium pore size. The sample No. 40/73 (fig. 12), however, shows a medium/fine grain size and homogeneous porosity, with frequent but small pores. At the stereomicroscope, it was possible to confirm the widespread presence of efflorescence and salt subflorescence, which often fill both the primary and secondary porosity (figs. 12 and 13). The traces of whitewash, characterized by red pigmentation faux brick, are often covered by sulphation crusts, so they are recognizable only by microscopic observation. The observation has also allowed us to recognize the use of earthenware as temper. As regards the installation mortar sampled in the area 40/9, a first observation conducted under the stereomicroscope, shows that it consists of sand (inert) and calcium carbonate, due to the burning of lime used as binder (fig. 14). In mortar the earthenware was probably used to enhance the hydraulic properties. The characteristics of 40/9 and 40/73 samples are summarized in table 1. The sand has a fairly fine and uniform grain size; grains are small with blunt edges, characteristic of river transport. The sand, used as feedstock for mortar, appears to be formed, in addition to silicate sands, by carbonate lithic fragments [5] that enhance the light color (fig. 15). The silicate part is mainly composed of quartz, micas, amphiboles, feldspars. From a direct observation, finally, the ligand-lime relationship appears to be 1:1, with a good distribution of aggregates in the binder.

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Fig. 9. Details of the interior of the tower.

Fig. 10. Other details of the interior of the tower at a different level.

3.3 Analysis of the Scanning Electron Microscope (SEM) and microanalysis (EDXRS)

The scientific investigations by means of SEM/EDXRS, were performed at the Electron Microscopy Laboratory of ENEA-Bologna-UTSISM, with an equipment (Inspect S model) that has a particularly innovative technology compared to normal standards. This technology, in fact, allows one to carry out direct observation and microanalysis of samples of 5–10 cm in diameter up to magnifications of 3 nm without previous treatment of the sample (namely, preparation and metallization); therefore, samples can have subsequent reuse for further scientific investigations and/or in the process of restoration.

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Fig. 11. Sample 40/9. Stereomicroscope image (1.35×).

Fig. 12. Sample 40/73. Stereomicroscope image (1.4×).

3.4 Brick The material used for the building is mainly composed of clay bricks. The term “clay” refers to a very fine lithified sediment with a granular size of less than 2 μm in diameter, consisting mainly of alumino-silicate hydrates belonging to the class of phyllosilicates. From the morphological study carried out by SEM, the presence can in fact be seen, in both samples of brick, 40/9 and 40/73, of fine particles with relatively homogeneous sizes, rare crystals of medium size and lamellar minerals (clay minerals).

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Fig. 13. Sample 40/73. Stereomicroscope image (9×): Efflorescence.

Fig. 14. Mortar. Sample 40/9. Stereomicroscope image (1×). Same length scale as in fig. 12.

The sample “brick 40/73”, in addition to having a greater number of crystals with respect to the sample “brick 40/9”, shows the presence of salt diffused into the surface, as further confirmed by the electronic microanalysis (figs. 16 and 17). The ceramic material used to make bricks, is generally prepared from a mixture of clays (15–45%) together with quartz (15–30%) and calcium carbonate (15–25%); in addition, there is a small percentage, approximately 2% on the whole, of iron oxide, water (4–6%) and alkaline oxides (K2 O, etc.). The microanalysis by EDAX reveals, in fact, the presence of chemicals related to this composition with some differences in the concentrations in the analyzed brick samples. In the “brick 40/9” sample, aluminum, silicon, oxygen,

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Page 11 of 20 Table 1. Properties of the samples. Sample 40/9

Sample 40/73

Colour

Light red

Dark red

Granulometry

Medium/fine size and quite homogeneous

Medium/fine size and homogeneous

Porosity

Few pores of medium size

Many pores of small size

Salt effluorescences

Widespread

Present in large number and subfluorescences

Pollutants

Paints and particulate

Particulate

Other

–

Widespread sulfation traces of whitewash Red inclusions black crusts

Fig. 15. Mortar. Sample 40/9. Stereomicroscope image (9×).

potassium, calcium and iron are present in a greater concentration than in the “brick 40/73” sample, which however contains traces of magnesium and a high concentration of carbon (figs. 18 and 19 and tables 2 and 3). The different concentrations of iron in the two samples also explain the different colors of the bricks themselves. Moreover, analyses were performed on the surface deposits of sample “brick 40/73”. The microanalysis, in fact, showed a major component of calcium, sulfur and carbon and alkaline elements confirming the hypothesis that, on the surface of the samples, there are black crusts, possibly due to the surface sulfation, efflorescence and deposition of oil related to vehicular traffic. It is worth mentioning that the SEM semi-quantitative analysis aims to quantify the amount of an element in a sample and must compare the signal from the sample to that of a known standard. Software packages included with all modern probes and equipment already contain standard data and perform numerical corrections to the measured SEM data. To first order, counts from the sample and counts from the standard are directly related to the concentration, kratio = Isample /Istd = Csample , where kratio must be corrected for sample effects by means of a multiplicative factor, commonly referred to as “ZAF ” corrections. ZAF means we have to make three types of corrections to our sample data: Z is the so-called atomicnumber correction made up of stopping power and backscatter terms; A is the absorption correction which takes into account that some of the X-rays produced in the sample volume do not make it out of the sample. Finally, F is the fluorescence correction for X-ray–induced excitation in the sample [6].

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Fig. 16. Brick. Sample 40/9: SEM back-scattered electron (BSE) imagery.

Fig. 17. Brick. Sample 40/73: SEM BSE image.

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Fig. 18. Chemical microanalysis by EDXRS of the “brick 40/9” sample (recto). Table 2. Semiquantitative analysis of the “brick 40/9” sample (recto). Element

Wt%

C

20.53

O

53.91

Mg

1.50

Al

4.59

Si

15.60

K

1.00

Ca

1.58

Fe

1.28

Total

100.00

There is an alternative correction process utilized by many of the modern probes; it is the “Phi–rho-z” method that basically combines the Z and A effects into one method. Our results do not differ very much by assuming both types of correction.

3.5 Mortar The SEM analysis of the samples showed a mixture of fairly homogeneous mortar with grains of variable size and the presence of some crystallized phases (salts). (See figs. 20–22 and table 4.) The electronic microanalysis confirmed the inorganic nature of the binder (limestone, CaCO3 ). Moreover, the sulfation of mortar itself resulted, because of the presence of CaSO4 , as well as the presence of elements such as potassium, K, related to alkaline salts. Analyses have also detected concentrations of elements such as silicon, aluminum, magnesium, iron, etc. due to the different grains of sand in the inert constituents of mortar. Preliminary results both on bricks and mortar have been presented at conferences [7] and [8].

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Fig. 19. Chemical microanalysis by EDXRS of the “brick 40/9” sample (verso). Table 3. Semiquantitative analysis of the “brick 40/9” sample (verso). Element

Wt%

C

38.23

O

45.72

Mg

1.17

Al

3.34

Si

8.89

K

0.72

Ca

1.18

Fe

0.76

Total

100.00

4 H-NMR analysis of samples taken from the Civic Tower of Ravenna In order to study the effects of the climate on the three samples taken from the Civic Tower of Ravenna and belonging to the same set of samples used in the analysis of sect. 3 but of greater size, an investigation was made by means of nuclear magnetic resonance (NMR), up to the depth of 2.85 mm in each sample, probing the porous matrix with a resolution of less than 200 μm. Finally, we compared the matrix of porous “aged” samples with that of the previously obtained “non-aged” ones. NMR relaxation studies of fluids in porous media have been stimulated by the oil industry for the study of reservoir rocks. Depending on the translational diffusion of the fluid molecules, the apparent relaxation time distribution is influenced, in different ways, by the wall relaxation and internal gradient fields. The determination of penetration depth and distribution of water at the surfaces is essential to the knowledge of the state of conservation of cultural heritage items and materials, such as frescoes, stone, brick, wood and paper. Water can penetrate the surface of an object, coming from either an external or an internal source, and in general the moisture content of the surface region is the cause of various decay phenomena such as microfractures and disintegration.

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Fig. 20. Mortar in sample 40/9: SEM BSE image.

Fig. 21. Mortar in sample 40/9: SEM BSE image.

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Fig. 22. Chemical microanalysis by EDXRS of the “brick 40/9” sample corresponding to the mortar zone. Table 4. Semiquantitative analysis of the “brick 40/9” sample corresponding to the mortar zone. Element

Wt%

C

29.28

O

53.65

Al

0.97

Si

4.35

S

1.31

K

0.40

Ca

10.03

Total

100.00

The NMR approach can be very powerful for the evaluation of the state of fine-arts materials, thus complementing the previous microscope analyses. Not only can the water saturation and/or the porosity of the material be evaluated but also information on material pore size distributions can be obtained by monitoring the distributions of the relaxation times of the transverse T 2 and longitudinal T 1 components of the H magnetization of the trapped water. For small pores and fluids of low viscosity the relaxation times, T 1 and T 2, are approximately proportional to the pore diameter, and the pore size distribution is obtained by the inverse Laplace transform of the relaxation curve.

4.1 Samples The brick is a ceramic material consisting mainly of clay, formed by silica in varying percentages, around 50%, plus a 20–30% calcium carbonate, as well as a small percentage, around 2%, of iron oxide and a percentage around 4–6% of water.

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Fig. 23. Scheme of T 2 measurement. Table 5. Values of sample 1. T 2 average (ms)±σ 25 ± 2

XSig(u.a.) 8

Table 6. Values of sample 2. T 2 average (ms) ±σ 25 ± 2

XSig(u.a.) 11

Table 7. Values of sample 3. T 2 average (ms) ±σ 16 ± 2

XSig(u.a.) 6

In this NMR investigation, we studied three surface samples of brick taken from the Tower at different heights (2 m, 8 m and 38 m), having the following dimensions: height 3 cm; width 3 cm; depth 3 cm. The instrumentation used was a Profiler PM10. In many cases, not only high spatial resolution but also a large penetration depth is desired. It can be achieved with the PM10, which provides a penetration depth of 10 mm. This version of the profile NMR-MOUSE generates a sensitive volume, where B0 = 0.25 T, and G0 = 11.1 T/m. Although most NMR analyses are performed with the object inside the magnet, stray-field techniques use NMR magnets placed next to the object [9]. The first small device of this kind was the NMR Mobile Universal Surface R [10], which was recently optimized to measure depth profiles through coplanar layers Explorer, or NMR-MOUSE of a diverse range of objects with a depth resolution higher than 10 μm. There is great interest in using this Profile R for the noninvasive analysis of objects of cultural heritage such as the stratigraphy of paintings, the NMR-MOUSE degradation of paper, wood, bones and mummies, and other materials containing hydrogen, including consolidation treatments of porous building materials of interest in our case. 4.2 Methods of measurement The samples were put in capillary absorption condition for 120 minutes in order to verify the hydrophobic porous matrix of the samples themselves. The pulse sequence used is the Carr-Purcell-Meiboom-Gill (CPMG)one [11], adopted for obtaining a measure of the intrinsic T 2 of the system, as shown in fig. 23. From the measurement of the various intrinsic T 2 values of the system, it was possible to get a reliable estimate of the size of the porous matrix. In fact the following relation holds: S 1 =ρ , T V where ρ is a constant having the dimension of a velocity (μm/s), V is the volume of the pore and S is its surface. 4.3 Analyses and comparisons of the samples The CPMG performed on samples from the “aged” conditions resulted in the values of T 2 shown in tables 5–7.

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Fig. 24. Sample 1 (height 2 m): Graph sample 1 (4B 150 cycles inv).

Fig. 25. Sample 2 (height 9 m): Graph sample 2 - inv 150 cycles 10E.

The charts in figs. 24–26 show the trend of the density signal as a function of the spin-spin relaxation time T 2 in order to estimate the distribution of pores in the porous matrix. From the graph for the sample number 1 (height 2 m), one can observe a predominance of small-to-medium–sized pores because the values of T 2 are thickened between 0.2 ms and 50 ms. Sample 2 shows a distribution of relaxation times similar to sample 1 and compatible with the type of natural ageing to which both these samples have been submitted. In fact, being in the lower parts of the tower both samples manifest the effect of ageing caused by climatic phenomena to a lesser extent.

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Fig. 26. Sample 3 (height 38 m): Graphics 150cycles in inv.

Sample 3 shows a class with smaller pores than the previous ones; this may be attributable to the higher location of the sample. In fact, being at the top of the tower that sample was more prone to windy weather and more intense phenomena that resulted in a greater sensitivity to the phenomena of infiltration of water, which resulted in a reduction of the pore space, due to the recrystallization inside the pore space as a result of the accelerated ageing by climatic phenomena. It is just the formation of these salts that leads to reducing the interconnections between the pores resulting in a lower porosity accessible to water, or small pores.

5 Conclusions This study allowed us to verify that the Civic Tower of Ravenna has a widespread environmental degradation mainly due to two factors, namely to its outdoor location and to limited maintenance work. The static unstable conditions of the tower can also determine structural damage to adjacent buildings. The building, therefore, under these conditions, definitely needs an effective and complex restoration. The analyses performed in laboratory on the samples of bricks and mortar taken at different heights permitted to check the status of deterioration in building materials at the microscopic level. The brick sampling has a different nature and composition with respect to the constituent material which affects the characteristics of the brick itself (colour, porosity, etc.). The bricks with large porosity, such as sample no. 40/73, are more prone to the phenomena of efflorescence and salt subflorescence, as well as to sulfation and possible formation of black crusts than other samples. Moreover, some areas of these samples show traces of whitewash, suggesting a different construction technique that is likely related to another historical period than the medieval one. The mortar is made of equal parts of binder and inert: Lime is the used binder, while the inert is supposed to be a mixture of sand, mostly quartz. The sand used for the mortar has minerals of small size, with fairly uniform grain size and mostly rounded grains, indicative of river transport. Most of the grains of sand has a light colour, which would explain the origin of sialic rocks. The combination of different but complementary techniques —optical microscopy, SEM, microanalysis and NMR— has allowed to obtain meaningful and important data necessary for a possible restoration and consolidation of the Civic Tower of Ravenna and may be a first step towards the realization of a shared protocol of procedures for the analysis of similar structures.

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