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St. VITALE IN RAVENNA: A SURVEY ON MATERIALS AND

STRUCTURES.

L. BINDA1, G. MIRABELLA ROBERTI2, F. GUZZETTI3.

1 Professor, Faculty of Architecture, Dept. Structural Eng., Piazza L. da Vinci32, Politecnico di Milano,20133 Milano, Italy

2 Assoc. Professor, PAU, Faculty of Archtecture, Univ. Reggio Calabria, Salita Melissari Feo del Vito,89124 Reggio Calabria, Italy

3Research Associate, Faculty of Architecture, Dept. Environmental Eng. & Survey, Piazza L. daVinci32, Politecnico di Milano, 20133 Milano, Italy

ABSTRACT

St.Vitale, a very well preserved central plan byzantine basilica, is the object of the study.The special features of the building: complex volumes including columns, piers, arches andvaults, a light dome made with clay tubes, large bricks and thick mortar joints, stimulate a carefulapproach for the preservation and conservation of such an important monument.

The methodology adopted for the survey is described and the results obtained in the firstyear of structural monitoring are shortly presented with the aim of contributing to a betterunderstanding of the structural behavior.

1. INTRODUCTION.

Byzantine buildings play a very important role in the Mediterranean architecture. Ravenna,

being the capital of the Western Roman Empire from 402 A. C., had three of the most important

monuments of that period: St.Vitale, St.Apollinare in Classe and St. Michele in Africisco.

St. Vitale is certainly the most important, still

preserved, brick masonry church of theJustinian age in Italy. The possibility of

studying its materials and structure, on behalf

of the historical monuments office of Ravenna

(Soprintendenza ai Beni Architettonici e

Ambientali, responsible: Arch. A.M. Iannucci)

Fig. 1 San Vitale, Ravenna: a view of the north side was welcome by the authors.

The Basilica of St. Vitale in Ravenna,

built between 526 and 547 A.C. [1] under the age of Justinian (Fig.1), has been since a long time

the object of archeological and historical studies, as well as stylistic criticism. Very little interest

has been shown for its structure and materials up to few years ago.

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The authors have dealt with the geometrical survey of the complicated three dimensional

structural elements and their connections, the study of the material brick and mortar composition

and of the role of thick joints in the masonry. One year ago, a static monitoring network was set

up, with the main goal of registering all the differential displacements of the bearing structure,

and also of measuring the settlements of the whole structure compared to the surroundings.

About 80 measuring points are monitored: the horizontal displacements inner pillars and of three

outer pillars. The crack movements are surveyed in about 40 positions all around the gallery at

the first level.

An overall view of the problems is presented in the followings, far from giving final results

of a research which will proceed for some years.

2. DESCRIPTION OF ST. VITALE AND OF ITS CONSTRUCTION PHASES

The Basilica of S. Vitale is composed by two concentric octagonal prisms. The outer and

lowest one contains two levels of galleries, covered by a systems of vaults built in the XII cent.

The, inner and highest one climaxes with the dome over the octagonal drum connected to the

pillars of the antes octagon by a series of semi-circular arches. They are connected together by

the arches delimiting the exedrae on the sides of the inner octagon. The external pillars are

connected by thick walls stiffened by a couple of built-in columns at regular intervals. Two long

buttresses were built after the construction of the church against the two northern external pillars.

The dome has a spherical shape with a diameter of approximately 16 m. It is constructed of

concentric rings made of clay tubes that narrow towards the top.

The octagons are both covered by timber roofs. The east side opens to the presbyterion and

the apse. Nearly opposite, the main entrance is preceded by a nartex shaped as a forceps and built

tangent to one of the sides of the external octagon. Two towers are situated at the two sides of the

narthex, one of which became a bell tower in the X-XII Centuries.

St. Vitale was built between 540 and 547-48 BC by the Emperor Justinian, according to F. W.

Deichmann [1]. From the X century up to 1797 St. Vitale became a property of the Benedictines

and part of a monastery. The most important transformation was the substitution of the timber

floors at the two levels with barrel vaults with lunettes and the construction of the two external

buttresses [2] (XII or XIII cent.). The random distribution of the vault springs is a sign that the

vaults were not part of the original construction.

Between 1562 and 1781, many changes took place: (i) a cloister was built adjacent to the

narthex [1], (ii) the SS. Sacramento Chapel and the Sacristy were erected; (iii) the bell tower

collapsed in 1688 under an earthquake, (iv) the church floor was elevated (about 70cm) in 1702

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and new chapels were then built, (v) in 1781 the vault of the presbiterium and some original

mosaics were damaged by an earthquake (Fig.2).

Fig. 2 Ground level plan, XV century

In 1797 the Benedictines had to leave the

Monastery annexed to St. Vitale, which was

given to the army. Between the end of the XIX

Cent. and 1934 a continuous restoration work

was carried out on St. Vitale under the direction

of Corrado Ricci [3], [4]. These interventions

have to be considered of great importance, since

the main objective was to reconstitute the

supposed original architecture, by means of

demolitions and reconstruction of the

correspondent parts of the boundary walls.

Starting from 1904, G. Majoli working under C.

Ricci had detected an out of plumb of pier EP7 and a correspondent crack pattern in the vaults

and transversal arch P7-EP7 (Fig.3). A system of ties connecting the inner and outer piers at the

second level of the church was carried out (1908-1916) (P2-EP2, P3-EP3, P5-EP5, P7-EP7)

(Fig.3).

Finally from 1911 and 1930 the floor of

the church was again lowered to the original

level and a system of pumps to drain the water

from the floor was realized in the same time [5];

in fact the level of the original floor was at that

time only 20 cm above the mean sea level.

Fig. 3 Present situation of the ground level

3. THE STRUCTURAL PROBLEMS

The structure, as well as many non

structural parts of St. Vitale have been subjected

to so many changes along the centuries that it is

difficult to understand how the Basilica can

show only few structural problems. Furthermore

soil settlements of 120 cm from the original construction time to nowadays were found by a

recent goetechnical survey due to subsidence; nevertheless the diagnosis was not pessimistic for

the Basilica structures.

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The highest number of cracks is concentrated at

the second level, interesting particularly the vaults

connected to the sides 1, 2, 5, 6, 7 of the outer octagon.

In the vertical walls the cracks appear at the top of the

exedrae connecting the internal piers P1-P2, P3-P4, P4-

P5, P5-P6. Vertical and inclined major cracks are also

present in some of the columns (Fig.4). This crack

pattern can be the result of synergic causes, of new

formation or already stabilized so that the interpretation

is not easy.

4. STRUCTURE AND MATERIALS SURVEY

In order to give an answer to the above question isFig. 4 Crack on one of the top columns ofthe exedra

being carried out in-situ and in laboratory. After the results of the investigation, a modelization of

the structure can be attempted and the diagnosis can be effectuated.

4.1 Geotechnical investigation

It was carried on starting from 1982 to 1986 on St.Vitale and on other surrounding areas by

G. Ricceri from Padova University [6],[7]. Subsidence in Ravenna is due to natural and anthropic

origin; the ground level has sunk approximately 1500mm for St. Vitale and of the nearby Galla

Placidia Mausoleum with a contemporary rise of the same order of the sea level.

Four layers of different types of soil were found under St. Vitale: (i) alluvial soil mixed

with anthropic materials and with pieces of bricks and cotto, 3.80 to 6.30 m thick, (ii) fine sand,

(iii) silty clay, (iv) medium-fine gray sand. The Dutch cone strength of the three deeper layers

varies from 2MPa to 1323 MPa. The wall foundations are 1.50m thick, 3.00m deep, consist of

limestone elements and trachite blocks supported by oak piles reaching 3.00m below the

foundation level. The conclusion from the survey was quite optimistic. In 3 years crackmovement was found to follow seasonal temperature trends with practically no relative residual

movements. Nevertheless a continuous monitoring system was required.

4.2 Topographic and photogrammetric survey of the geometry

Due to the impossibility of finding a large budget, the survey was limited to the collection

of the most important information on geometry and dimensions. A first set of data was collected

about the global geometry and about some specific parts of the church. More detailed

topographic survey was carried out for: (i) the determination of the thickness of the dome; (ii) the

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detailed definition of the tridimensional geometry of one of the eight sectors of the building,

(between piers P2 and P3 denominated S2, in Fig.3).

At first, a topographic network was set up all around the church and in all the sectors of the

structure; by a traditional polygonal scheme, the three coordinates of 16 points were defined in a

local reference system.

Starting from the geodetic network vertices, the dome survey was planned as difference

between intrados and extrados determination. The intrados survey was made with the

determination of about 335 points. 269 points were surveyed in the lower part of the extrados,

with difficult accessibility. Using a Digital Object Model (DOM), the extrados profile was

obtained with a grid perfectly coincident with the intrados one. The result is very important: the

thickness of the vault is evaluated between 16 cm and 24 cm, with an accuracy of 1 cm (Fig.5)

[8].

Fig. 5 Representation of the dome sectionmeasured in various meridian directions

Using the same methodology, also theexternal and internal lateral surface of the

construction was surveyed and also the out of

plumb of the eight inner pillars at four different

levels. There is a clear distortion of the plan of the

octagon, but the interpretation and explanation of

the causes are far from being easily understood. Also the outer pillars are out of plumb, and in

particular EP7.

An approach based on topographic survey integrated with simplified photogrammetric

software was adopted for the survey of sector S2,

previously selected for the structural modeling. These

tools are qualified to obtain sufficiently precise

tridimensional determination of complicated

structural geometry in the cases when the traditional

photogrammetric approach is too expensive or, for

other reasons, impossible [9].

Fig.6 gives a three-dimensional restitution of

the sector S2 survey. A mesh discretization has

already been traced on the surveyed geometry; with

some few adjustments the geometry can be easily used

for numerical modeling of the structural behavior ofFig. 6 3D-resitution of sector S2

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this sector.

4.3 Masonry construction technology and materials

The masonry of St. Vitale are built with large Julianean bricks (310 510 40mm) and very

thick mortar joints: the ratio mortar/brick thickness is 1/1, very frequently found in Byzantine

buildings in Italy, Turkey and Greece [10], [11]. This aspect gives the Byzantine walls a peculiar

appearance with evident horizontal strips (Fig.7). Visual inspections of the wall section confirm

that the masonry is solid, two to three brick thick, with an average thickness of 900mm, wich

increases in the pillars and built-in columns. The mortar joints of the masonry were certainly the

most interesting material from St. Vitale. The

material can be considered a conglomerate rather

than a mortar due to the max dimensions of the brick

pebbles and of the other aggregates which can reach

a value >16mm.

The results of the chemical analyses show the

presence of aggregates which are partially siliceousFig. 7 Detail of the masonry of San Vitale

and partially calcareous and of a binder based on

hydrated lime. The soluble silica 0.33% is low in the large fractions of the aggregates and

significant (13.15%) in the small fractions with dimensions below 0.075mm. The petrographic-

mineralogical analyses confirm the presence of new formation products between binder and brick

pebbles; thus allowing for the hypothesis that a pozzolanic reaction between lime and bricks

occurred after the construction of the masonry.

After a laboratory research the characteristic of these masonries seem to be a good strength

and a very high deformability at early age, allowing to follow soil and masonry settlements

without important crack formation [12].

5. THE MONITORING NETWORK

The monitoring system installed in St. Vitale is made of three separated sub-systems, each

of them recording the following set of displacements as follows: (i) differential settlements of the

structural elements; (ii) horizontal movements of some vertical elements; (iii) crack opening

variations during the time.

5.1 Survey of the differential settlements

Three different types of altimetrical points have been installed: to the ground, on the wall

and suspended on wire.

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Fig. 8 Position of altimetrical points: internal network at thethe Gallery (right)

The points on the ground allow the high precision ( 0.02mm) replacement of invar staffs

2m high. They are placed near the bearing elements of the building structure and unprotected, but

far from the tourist way. The measuring points on the wall are made by little cantilever brackets

fixed to the wall and protected by semi-spherical bowls. They allow high precision replacement

of staffs 25 cm high.

Finally the measuring points suspended on wire are precision brackets employed to replace

the staffs onto invar wires connecting the Gallery with the ground floor.

A total number of 86 points have been placed in strategical position for a precise and

exhaustive measurement of the displacements. In Figs. 8 and 9 the position of these points and

the scheme of the leveling connections employed are shown. Beside the new altimetrical points

installed, 5 others were connected to the network: 2 of them are IGM points, the other 3 are

altimetrical points placed by the Geological Service for the Municipality of Ravenna to measure

the local subsidence. So a total number of 91 points is involved in the network. The highest

number of altimetrical points was adopted for the gallery level, due to the crack pattern; at the

ground level only the measuring points only strictly necessary for the connection to the network

were localized.

ground level (left) and in

Fig. 9 Position of altimetrical points external network

Measurements are made by means of

the high precision automatic level with

parallel-plate micrometer. The highprecision leveling network is made of closed

polylines. The complete network iscomposed of 117 sides connecting the 91

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altimetrical points, so that the redundancy of the network is very high (117 91=26 redundancies)

assuring a high reliability of the results. The measured values have been then processed by a

mean square error (MSE) computation software. This error is a very good index of significance

of the possible movements recorded: until today, the measurements gave always a maximum

MSE around 0.15mm, allowing to consider displacements over 0.3 mm. The point 22, on

external wall, was considered fixed in the calculations

of the differential settlements of the structural

elements.

Fig. 10 Placement of the translation stage withthe Zenith level

Fig. 11 Measurement with removabledeformometer

5.2 Out-of-plumb surveying subsystem

In order to monitor the out of plumb of the pillars

sustaining the dome, 8 measuring positions wereinstalled on the 8 inner pillars, and 3 outer pillars

corners not restrained by other elements. Each position

has a reference bracket on the top of the element under

control, and a device, useful to reposition the

measuring instrument, made by a precise translation

stage and a very accurate zenith level (see Fig.10).

For the sake of precision, each value is calculatedas the mean value of 4 measurements taken in different

positions of the zenith level (in X and Y direction and

backward); each is in turn the mean value of 3 different

readings.

5.3. Crack movements monitoring subsystem

The aim of this subsystem is the control on the

variations of the macroscopic crack pattern existing onthe Gallery level. Nearly 40 relevant cracks were

chosen to indicate the actual movements of the structure. The measuring instrument is a

removable deformometer (Fig.11); each value recorded is the mean value of multiple readings (3

at least).

6. FIRST RESULTS OF THE STRUCTURAL MONITORING

The results obtained in the first year of survey are very interesting; 5 different measurementcampaigns have been realized every three months, from November, 1998. The fifth one, in

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November 1999, concluded the first year and ended an yearly cycle. So it is already possible to

distinguish between the effect of the environment interaction and the possible to residual

displacements, since the initial environment conditions are restored. In fact while the partial

variations, are quite large, the total variations are almost all smaller than the significance limit.

That is, appreciable movements are recorded, but after one year of monitoring the structure is

again back in conditions similar to those of November 1998.

The higher recorded differences in the Z coordinate are those between August (temperature

of 30C) and November 1999 (around 7C). This temperature difference caused important

differential movements (of the order of 0.5 1 mm) of all the points at the Gallery level, but

produced very small variations (less than the significance limit) for the points on the stone

columns. So differential displacements of the order of 1 mm occur between the pillars and the

columns a little more than two meters distant, in only three months.

Therefore the structure is subjected to rather large differential displacements which

probably have effect on the crack pattern. Nevertheless, correlations between the crack pattern

and the vertical displacements are not yet possible due to delays in the monitoring of cracks.

The measured variations for the cracks are really small and only in few cases over the level

of significance. It will be interesting to see in the future whether residual displacements will be

confirmed or not.

In figure 12 the variations of the out of plumb, after the first year, are indicated. Also in this

case the resulting displacements are very small, usually less than the sensitivity level, even if the

partial variations are clearly greater, indicating a relevant movement of the structure under the

environmental effects (Fig. 13); at the moment there are no clear sign of danger.

It is important to underline that monitoring

systems like the one installed in St. Vitale become

increasingly meaningful in the long run: in fact a

large number of measurements is needed.

Fig. 13 Partial variation of the out of out ofplumb (November 1998-August 1999)

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Setting up a classical (non automatic) instrumentation system allows to reduce the costs

and to extend the duration of investigation. When the critical points will be known, it will be

convenient installing on these points an automatic monitoring system.

Finally, if the next measuring campaigns will confirm the results obtained till to day,

differential settlements will be simulated by a numerical model of the structure in order to

evaluate the structural response to the cycling seasonal variations.

7. CONCLUSIONS AND FURTHER DEVELOPMENTS

At the present stage of the research, some remarks can be made on the contributions given

by each phase of the investigation: (i) the history of the building construction and restoration

allows to understand the evolution of the structure and hence the history of loading and to

explain their effects on the soil settlements; (ii) the geotechnical investigation will in the future

be used to understand the soil-structure interaction; (iii) the investigation on the technique of

construction are giving informations on the mechanical and physical masonry parameters which

have influenced the short and long term behaviour of the structure; (iv) the crack pattern and

geometry monitoring helps in the interpretation and detection of the long term behaviour of the

structure and on the causes of damage.

Further research is needed in-situ and in laboratory on the mechanical behaviour of the

materials and of the structure. After the structural analysis based on elastic FEM will be

completed, nonlinear models will be also used to study the behaviour of the damaged structural

elements and calculate their carrying capacity.

8. ACKNOWLEDGMENTS

The authors wish to thank arch. A.M.

Iannucci, Soprintendente dei Beni Ambientali e

Architettonici di Ravenna, and her collaboratorarch. E. Agostinelli, for the financial and

technical support; architects N. Lombardini, C.

Tedeschi, P. Brugnera, and the technicians P.

Perolari, M. Antico and G. Ghilardi for their

contribution to the research.

Fig. 12 Total variation of the out of plumb(November 1998-November 1999)

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9. REFERENCES

1. Deichmann F.W., "Ravenna Hauptstadt des sptantiken Abendlandes", Wiesbaden-Stuttgart,1969-1989.

2. Gerola G., "Le volte delle loggie e la decorazione delle pareti in San Vitale" in "Atti delReale Istituto Veneto di Scienze, Lettere ed Arti", A.A. 1915-16. Venezia, 1916.

3. Ricci C., "Ravenna e i lavori fatti dalla Sovrintendenza nel 1898", in Emporium vol. III, n.48, Bergamo, 1899.

4. Ricci C., "Monumenti di Ravenna, Tavole storiche dei mosaici - S.Vitale", Roma, 1935

5. Iannucci A.M., "I mosaici pavimentali di S. Vitale: cronotassi e criteri di restauro", inCCARB, XXX/1983,

6. Ricceri G., "Studi e ricerche nell'area di San Vitale, Galla Placidia e Santa Croce inRavenna", S.G.E. Padova, 1992.

7. Mazzucato A., Ricceri G., Selleri F., Soranzo M., "Remarkable case histories regarding St.Vitale church, Ravenna, Italy", Dynamics, Repairs & Restoration, Structural Studies of

Historical Buildings IV, Vol. 2, Computational Mechanics Publications, Southampton,Boston, 1995.

8. Astori B., Bezoari G., Guzzetti F., "Analogue and digital methods in architecturalphotogrammetry", XVII International Congress of Photogrammetry and Remote Sensing -Commission V, Washington, August 1992.

9. Brugnera P., Guzzetti F., Lombardini N., Trebeschi A., "Moderni rilievi topografici efotogrammetrici della Basilica di San Vitale in Ravenna", Recupero e Conservazione n.24,1998.

10. Baronio G., Binda L., Lombardini L., "The role of brick pebbles and dust in conglomeratesbased on hydrated lime and crushed bricks", Conv. 7NAMC, Notre Dame, 1996.

11. zsen G.A., Akz F., Yzer N., zkaraman, "The structural evaluation of Kk AyasofyaMosque - Sts. Sergius and Bacchus in "Istanbul, Spatial Structures: Heritage, Present andFuture", IASS, International Symposium, 1995, Milan, 1995, pp. 1261-1270.

12. Binda L., Tedeschi C., Baronio G., "Mechanical behaviour at different ages, of masonryprisms with thick mortar joints reproducing a byzantine, 8NAMC (North American MasonryConf.), Austin, USA, pp. 382-392, 1999.