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SURVEY AND INVESTIGATION FOR THE DIAGNOSIS OF DAMAGED MASONRY STUCTURES:

THE TORRAZZO OF CREMONA

L. Binda1, R. Tongini Folli

1, G. Mirabella Roberti

1

Abstract

Some recent cases of failure of ancient massive structures lead to the hypothesis of possible

continuous damages occurring to these structures due to dead and cyclic loads (wind, temperaturevariations, etc.). The long term behaviour of historic masonry has also been detected in laboratory bycreep and cyclic tests. An investigation procedure was applied by the authors first to the Bell-Tower ofthe Cathedral in Monza and then to the Bell-Tower of the Cathedral of Cremona (Torrazzo). The resultsare here reported and discussed. These experiences indicate that laboratory and on-site investigationtogether with structural analyses are particularly important for tall bell-towers.

1 Introduction

The sudden collapse of the Civic Tower of Pavia (on March 17, 1989) (Fig.1), which was not previouslyconsidered at risk, encouraged studies about the safety and stability of similar structures (Binda et al.1992). Tall and heavy buildings, like towers, or heavily loaded structural elements, like piers, aregreatly influenced by the persistent high compressive stresses due to dead loads. Furthermore, the

stress distribution in the bearing elements is generally non-uniform. The main causes are: 1) thepresence of openings (doors, windows) in most cases wider in the lowest part of the towers, 2) the highinhomogeneity of the load-bearing walls, often made by multiple-leaf, and in many case low-strengthmasonry, 3) the presence of staircases built inside the load bearing walls. Also the fatigue effects dueto cyclic actions, induced by temperature variation and wind, can as well cause synergetic damageeffects. The damage manifestation are thin or large vertical cracks which tend to propagate theirdimension with time; this is due to creep deformations which can increase with variable rate until they

bring the structure to failure.An experimental research lasting nine months wascarried out on the masonry specimens recovered fromthe ruins of the Civic Tower. This material is uniqueand the possibility of performing tests on a real ancientmasonry was of great help to understand its long term

behaviour. In fact no physical model even built withvery carefully chosen materials could have given somany reliable data as it was the case of the PaviaTower masonry. The results of the tests confirmed thehypothesis that, when the masonry is heavily loadedfor a long time, a creep behaviour can developedwhich can bring the material to collapse (Binda et al.

1991 and 1993). Some cases of historic buildingswhich collapsed in the past decade can be presentedas a validation of the hypothesis.

1 Dept. of Structural Engineering, Politecnico di Milano, Piazza L. da Vinci, 32, 20133 MILANO, Italy

Fig. 1 Failure of the Civic Tower of Pavia, Italy,1989

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The Cathedral of Noto (Sicily) was damaged in 1990 byan earthquake which caused cracks to the piers andvaults. The piers of the left hand side of the centralnave together with part of the dome collapsed only 6years later on March 13, 1996; the earthquake was alsoresponsible but not the main cause. In fact the removal

of a rendering applied in the sixties during a repairintervention, showed that the piers were alreadyvertically cracked at that time and had been roughlyrepaired by filling the cracks with the gypsum mortarused for rendering (Fig. 2). Very likely, the structurehad already suffered long term effects and crackingwhen the rendering was applied in the sixties.The Cathedral of Pavia was initially monitored to studythe effects of the damage occurred during the collapseof the adjacent Civic Tower in 1989. The investigationshowed that the massive multiple leaf stone masonrypiers bearing the main dome of the Church arevertically cracked and that spalling and deep

delamination of their external leaf is taking place andworsening with time (Macchi 1996). The very heavycompressive state of stress measured locally by flat-

jack tests in the external leaf (up to 30 N/mm2) is

unfavourable when combined with the effect of ahorizontal thrust estimated by FE calculation around 9N/mm

2.

A simple survey carried out on 60 Italian ancient towersconfirmed that a few of them are to be considered at

risk; their crack pattern and their state of stress, roughly calculated assuming the dead load action andcross section area, compared to the one calculated for the Civic tower indicates a serious state ofdamage (Binda et al. 1997). The authors have then developed their research in two main directions: 1)study of procedures for on site investigation of massive buildings(Binda et al. 1998), 2) implementation

into numerical models of constitutive laws taking into account the creep behaviour of old masonry, inorder to predict the time when collapse may occur(Papa et al. 1994).This paper will deal only with the first one. Furthermore two similar cases of investigation will bedescribed and compared: the bell-tower of the Cathedral of Monza (in 1998) and the bell-tower of theCathedral of Cremona (Torrazzo), still under investigation nowadays and the main subject of thispaper.

2 Investigation procedure for massive structures

It was mentioned above that simple calculations or crack pattern survey can be useful to becomeaware of structures at risk. Nevertheless, only a deep knowledge of the structure damages and load-carrying capacity can help in preventing failures and in choosing the right strengthening techniques. Onthe basis of their knowledge the authors also have developed investigation procedures for the safety of

these structures; the idea came at first when studying the collapse of the Civic Tower in Pavia.The procedure is based on the following steps: (i) historic research to know the evolution of thestructure over time, (ii) geometrical and crack pattern survey, which allow to understand the evolutionof the structure, to calculate weights and give a first interpretation of the crack pattern, (iii) geognosticinvestigation and monitoring, to understand the soil-structure interaction, (iv) on-site mechanical andnon-destructive testing (radar, sonic, etc.) to define local states of stress and stress-strain behaviour ofthe material, (v) chemical, physical and mechanical tests on mortars, brick and stones to find theircomposition and their characteristics, (vi) if necessary, passive and active dynamic tests on site tosurvey the overall structural behaviour, (vii) monitoring system applied to the structure whennecessary.

2.1 The Bell-Tower of the Cathedral of MonzaThe Bell-Tower of the Cathedral of Monza, a town close to Milan (Italy), is a masonry structure 70 m

high, with a square plan (a side is 9.7 m large) with solid brick walls 140 cm thick (Fig. 3). The towerconstruction started in 1592, probably following the design of Pellegrino Tibaldi, the architet of thePavia Tower belfry, and ended in 1605 (Scotti 1989). The only damage to the Tower reported by thedocuments occurred in 1740 and was due to a fire which developed in the Bell-Tower and caused the

Fig. 2 Pillar of the left hand side of the centralnave of the collapsed Cathedral of Noto; crackpattern.

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collapse of the belfry dome and roof and the fall of the bellswith their supporting frame down to the vault of the first floorat 11 m. No damages were reported in other known events,as lightening or thunderstorms occurred along the centuries.Nevertheless cracks are present since 1927 or even beforeas it can be seen from old pictures. From 1978 the cracks

have been surveyed with removable extensometers: theyshow a slow increase of their opening along the time. From1988 the rate of opening seems to be increasing faster. Thetrend of opening of the three main cracks was calculated as30.6, 31.3 and 39.7 micron/year from 1978 to 1995. Actuallyif this trend is considered from 1988 to 1997 the valueschange respectively into 41.2, 35.2 and 56.2.The first step of the investigation procedure (Binda et al.1998)was the geometrical survey. A geodetic network setup in the square of the Cathedral in 1993, was used assupport; based on some points of the net some significantpoints of the west facade were surveyed and used toproduce a series of orto-photografic images (Astori 1992).

No relevant leaning was measured due to the smallsubsidence which is taking place in the square. Two distinctproducts were obtained: 1) a detailed three-dimensional

model from which the external and internal prospects and the vertical sections were obtained, 2) asimplified model for which only the essential aspects of the geometry were preserved for the structuralanalysis.The survey of the crack pattern allowed to realise that the tower walls have a dangerous distribution ofpassing-through cracks on the western and eastern load-bearing walls since more than 50 years, and

of a net of thin vertical cracks from a level of 11 m up to 30 m (Fig. 4).Other cracks can be seen from the internal walls of the tower; they are very thin, vertical and diffusedalong the four sides of the tower and deeper at the sides of the entrance were the stresses are moreconcentrated. The thin diffused cracks run 450 mm deep inside the section, reducing its total workingthickness from 1400 mm to no more than 900 mm.

From laboratory tests it was found that the mortar is very weak and made with putty lime and siliceousaggregates (Fig. 5); also the bricks were of poor strength (between 4 and 12 N/mm2).

Fig. 3 Monza Bell-Tower

Fig. 4 Survey of the crackpattern- west and east sides

[%]

SiO2

Al2O3

F2O3

CaO

MgO

Na2O K2O SO3

LoI

Ins.Res.

Sol.Sil

CO2

Components of mortar

0

5

10

15

20

25

30

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40

45

50

55

60

65

70

75

Fig. 5 Chemical Analysis of mortars from Monza

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On site single flat-jack tests were carried out atdifferent heights of the tower (5.4, 5.6, 13.0, 14.0,31.5 and 38.0m) and the stress values against theheight are plotted in Fig. 6. The maximumcompressive stress acting in the tower, measuredon site by flat jack test, is about 2.2 N/mm

2. The

most interesting information came from the doubleflat-jack tests results compared with the single,where it was possible to see the real riskysituation.Passive dynamic tests using the bell ringing werealso carried out monitoring the dynamic excitationof the extensometers applied across the maincracks, giving under these cycling stresses amaximum peak to peak (opening to closing) of 28

m that has to be compared with a daily variation of 100 m due to the temperature effects. Other

information on the results of the investigation are given in (Macchi et al. 1996).The diagnosis based on the experimental survey and on the FE modelling lead to the conclusion thatthe Bell-tower is a high risky building and needs a quick intervention.

3 The Torrazzo of Cremona

The bell-tower of the Cathedral of Cremona, an interesting historic town not far from Milan (Italy), wascalled since long time ago with the nickname "il Torrazzo". The Tower is situated at the northern side ofthe Cathedral and it is connected to it by a Loggia called "Bertazzola". The geometry of the tower israther complex, being composed by: a lower part (romanesque Tower) with a square plan of 13 m sideand 70 m high, an upper part, the Ghirlandina, with an octagonal plan (2.5 m side), more than 40 mhigh. The Torrazzo is known as the tallest Medieval Bell-tower in Europe being 112 m high (Fig. 7).The lower part of the tower, with a square plan is a massive construction with few openings localisedon the western and on the eastern sides. The upper Ghirlandina appears as a light structure witharches and large openings on all the four sides.The staircase from the lowest level up to the Girlandina level was built within the thickness of the walls

(approximately 3.3 m thick). Along the staircase, covered with a barrel vault, the thickness of theexternal wall is circa 1m, while the thickness of the internal wall is 0.7 to 1m being the span of thestaircase 1.3 to 1.6m. The staircase allows to reach some internal vaulted rooms.

3.1.Historic notesArchive research did not clarify the date of construction; nevertheless the highest number of referencedata collected locates the date of construction between the 8

thand the 13

thcentury. In 1491 the porch

of the Bertazzola was added connecting the Torrazzo with the Cathedral and in 1519 the Loggia was

built resting on the arches of the porch. Maintenance works werecarried out starting from the 15th century. These works mainlyconcerned the highest part of the tower damaged by storms andlightening, especially the stone and brick columns which weresometimes also substituted. The last intervention at the Girlandina

was carried out in 1977 by M.T. Saracino, architect of the Culturalproperties Office in Milan. The works performed were the following:connection of structural and decorative elements, construction of aconcrete frame sustaining the twin columns of the "Stanza delle Ore"(at 85m height) and surface treatments of stone and brick elementswith an epoxy resin.

3.2 On site investigationThe investigation procedure proposed above was adopted. The aimsof the investigation were the following: (i) to survey and interpret thecrack pattern and the eventual other mechanical damages, (ii) tomeasure on site local states of stress (in compression) to be used asa check for the structural analysis based on numerical models, (iii) to

characterise the masonry and its components (chemically, physicallyand mechanically) and to define some characteristic parameters(strength, deformability, etc.) as input data for the mathematical

Fig. 6 Single flat-jack tests of Monza

Fig. 7 Torrazzo of Cremona

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models, (iv) to study the dynamic behaviour of the structure under cycling loads, (v) to state whether amonitoring system should be applied to the tower in order to store environmental parameters and staticdisplacement values, (vi) to choose appropriate techniques for conservation and repair.

Geometrical surveyThe first step was the geometrical survey

of the tower carried by the DIIARDepartment of the Politecnico of Milan andsubsequently integrated by the DIS,Politecnico of Milan. A principal networkdefining fixed points in the horizontal andvertical plan was set up having 21 nodesinside and around the Tower made withfixed nails. The co-ordinates of the nodeswere determined with a T2000 WILDequipment (Fig. 8). The vertical andhorizontal profiles were determined by rays

starting from the network nodes,using a TC1600 DIOR system and

an auto scanning Laser SystemMDL. A photogrammetric survey ofthe external prospects was alsocarried out using TC1600-DIORand T460* DISTO equipment. Theprospects were obtained by aRollei special software, MSR (Fig.9). The Ghirlandina and theBertazzola were not surveyed. Theprospects were successivelycompleted with topographicalexternal profiles and the missedparts (Ghirlandina and Bertazzola)

by scanning existing surveys. Thesurvey allowed to find someirregularities of the structure: (i) a21 cm horizontal displacement ofthe center of the tower in directionnorth-east, calculated from theground level to the top at 112 m, (ii)a non symmetrical reduction of theplan dimensions from the groundlevel to the top: 31 cm for thenorth-east corner and 66 cm for thesouth-west corner, (iii) theGhirlandina is not perfectly

centered on the square part of thetower, but with a slight counter-clockwise rotation toward west.

Crack pattern surveyThe presence of a diffused crackpattern particularly on the west, onthe east side of the tower and on

the Ghirlandina can indicate high states of stress due to the dead loads, the temperature variationsand/or to a slight leaning. The survey was carried out on the outer surfaces by reaching the height of60m thanks to a special crane. The crack pattern is certainly also influenced by differential movementsdue to temperature variation between one side and the other of the tower. The highest variationscertainly occur between the north and the south side. The west side has a diffused fissuration with

passing through cracks; the cracks are mostly vertical and start from approximately 20 m on. Importantcracks appear also between 48 and 60 m from the ground level (Fig10a). The north side is cracked inthe center between 27 and 40 m and at the north-east corner. The east side is cracked between 6 and20 m from the ground level and between 35 and 60 m (Fig.10b). The south side has few cracks

Fig. 8 Topographic Network

Fig. 9 Rectification of

the West side

Fig. 10a,b - survey of the crack pattern-west and east sides

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located between 14 and 27 m. The Ghirlandina shows the most important cracks, on the buttress andon the brick columns particularly on the south-west corner. Also the internal part of the tower, along thestaircase and inside the rooms shows a diffused crack pattern with some passing through cracks.Three thresholds were established concerning the measure of the opening of the cracks: 10 mm. The crack pattern survey allowed to understand and interpret roughlythe mechanical damage and to locate the position of the monitoring system.

Material characteristics and decay surveyThe inspection of the masonry surface and on the inside of the walls allows to make the followingdescription: (i) the walls are made with solid bricks and no rubble was used for the inner part of thesection; (ii) the bricks are regular with the varying dimensions: 24-28 x 10-12. x 5.5-7 cm, (iii) theexternal walls of the Ghirlandina are irregularly scaled and tooled, (iv) the colour of the bricks isvariable read- dark red, yellow, orange etc., (iv) the mortar joints are regular with thickness variablefrom 1 to 3 cm, (v) different techniques of jointing and pointing can be found and often the vertical

joints seem to be void or recessed, (vi) the masonry texture is also regular with header and stretcheralternatively positioned, (vii) an external leaf one brick thick with a weak collar joint is certainly presentalong the staircases, in the internal rooms and at the level of the Bertazzola and more research isneeded to test the real extension of this leaf along the tower, (viii) in the staircase walls a rowcomposed by 4 to 22 header is repeated at rather regular intervals as if it should represent a

connection of the external leaf to the internal one., (ix) several scaffolding holes externally closed canbe seen along the masonry walls, (x) numerous restorations by brick substitution can also be seenexternally and internally.Some structural elements (columns in the Stanza delle Ore) and decorative elements (stonescovering the base of the tower, the columns of the Bertazzola, some columns of the Ghirlandina, thepinnacles) are made with stone.Together with the geometric survey an accurate survey of the material decay was carried out. A map ofthe decay has been represented on plans and prospects by AUTOCAD 14. Spalling, scaling andpowdering are the most diffused signs of deterioration for the bricks. These phenomena are localisedat the external surfaces at a height of 57 m, on the walls of the staircases near the openings at 47.70m. Diffused spalling can be found on the walls of the Ghirlandina which had been treated apparentlywith epoxy in 1967. The decay of the mortar joints is mostly sanding on the external surface of thewalls, especially on the west facade at 64 m, some on the east side at 24 m height. Furthermore at the

spring of the vault of the staircase and under the openings. The same deterioration appears at thelevel of the bell fry. Many vertical joints are missed or recessed inside Stanza dell'Orologio, Stanza delLaboratorio and also at the Ghirlandina).The stone elements are also interested by soiling and erosion, particularly the base of the tower, thecolumns of the belfry, the columns and pinnacles of the Ghirlandina.Concerning the reinforced concrete frame (85 m level) the columns between the north and north-eastand the east and south-east side show washout of the binder, formation of carbonates near the stirrups

with partial detachment of thereinforcement cover (no more than 1cmthick) and reinforcement corrosion.Sixteen samples of bricks and mortarswere collected from the masonry: 5from the facades, 4 inside from the

walls of the internal rooms, 4 along thestaircase, 3 from the external andinternal walls of the Ghirlandina. Themaximum depth of sampling was300mm. All sampling operations weredocumented graphically or photo-graphically.Laboratory tests were carried out onmortars and bricks. Chemical analyses(Fig. 11) show that the mortar binderwas hydrated lime (probably lime putty)and the aggregates were mainlysiliceous. Two types of bricks were

used which differ in colour (red andbrown), but also in properties. The redbricks have high absorption (21-28.8%),low strength 8-12.4 N/mm

2 in

[%]

SiO2

Al2O3

F2O3

CaO

MgO Na2O K2O SO3

LoI

Ins.Res.

Sol.Sil

CO2

Components of mortar

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

Fig. 11 Chemical analysis of mortars from Cremona

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compression and in tension (0.1-1.6 N/mm2) and low modulus of elasticity (1000-2175 N/mm

2); the

brown brick have lower absorption (18.5-21.7%), higher compressive strength (9.4-25.43 N/mm2) and

tensile strength (2.2-2.6 N/mm2) and modulus of elasticity (1725-4417 N/mm

2). The two types of bricks

are present everywhere in the tower, so an average between the two bricks can be considered asreference.The samples taken from the Ghirlandina (2 bricks from the north side at 70 m, 1 brick from the south

side at 75 m) were chosen in order to detect the existence and type of the 1977 surface treatment.In order to detect the suspected existence of an external cladding in use during the Middle Age as afalse curtain to hide the roughness of the real wall, bricks were sampled from the external wall of theBertazzola at 6m level and from the walls of the "Stanza dei Contrappesi" at 13.6 m level. This externalleaf was confirmed and its thickness is around 12 cm. The sampling allowed to find large areas wherethe leaf seems to be detached from the rest of the wall; following these results the application of NDTtechnique was required in order to map the detached areas which represent structurally a reduction ofthe wall section to be taken into account when modelling.

All the areas were samples were taken were then repaired with similar bricks and mortars.Boroscopic observation were carried out inside the existing coring performed by MT. Saracino in 1977.The aim was to confirm the existence of the external leaf and whether it is diffused everywhere. Theoperation was done using a COMEG stiff modular boroscope with frontal and lateral view and a lengthof up to 83 cm. A Panasonic CP410 Camera with COMPUTAR optic (6 mm) was used to collect data.

The results confirmed that the wall is regularly constructed in bricks with local detachments betweenthe external 12 mm leaf and the rest of the wall.Flat-jack tests- Single and double flat-jack tests were carried out on the Torrazzo. The single flat-jacktest was also used to study the behaviour of the external leaf of the wall. Different types anddimensions of flat-jacks were used: (i) 24 x 12 cm rectangular jacks where the detachment of theexternal leaf was suspected, (ii) 40 x 20 cm rectangular jacks and (iii) 35 x 24 cm semicircular jackwere no detachment was supposed and for the double jack test.21 tests were carried out of which 19 with single flat-jack and 2 with double flat-jack: 3 tests between 1and 5 m from the ground, 7 at 7 m, 10 between 15 and 18 m, 1 at 22 m. The double flat-hjack testswere carried out at 7.2 and 19 m from the ground.The results of the single jack tests are reported in Fig. 12 and show clearly two situations: a state ofstress varying between 0.4 and 0.9 N/mm

2 where the test found a detached leaf, a state of stress

varying from 1.01 and 1.81 N/mm2 where no detatchment was found.

Also double flat-jack tests were performed and Fig 13a,b show the stress-strain plots. It wasimpossible to carry out tests at higher levels due to the lack of scaffolding and of appropriate means forcarrying the jack equipment. In the future other tests will be carried out.

4 Comparison between the two Towers

Since the bell-tower of Monza is considered a building with high risks of collapse, a comparisonbetween the data collected on both towers seems to be useful to understand better the real situation ofthe Torrazzo.

As said above the mortar compositions of the two bell-towers do not differ too much one from the other(Figs 5 and 11) even if theTorrazzo mortar seems to bemore consistent. The bricks of

the Monza Tower aregenerally weaker than those ofthe Torrazzo (Figs 14a and b)except for the brown typewhich is mainly used on theoutside surface of the bearingwalls and very seldom used inthe interior. On the contrarythe brown and the red bricksare evenly distributed in theTorrazzo walls.It is also interesting tocompare the results of single

and double flat-jack testscarried out on the two towers.

1.7 m

7.2-7.4 m

16.6-17.8 m

15.2-16.5 m

7.2-7.7 m

16-17.8 m

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

Stress[N/mm2]

masonry section 3.3 m

masonry section 1 m

presence of veneer

inner walls (rooms)

Fig. 12 Single flat-jack tests of Cremona

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h

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m2]

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Fig. 14a Stress-strain plot for Monza Towerbricks Fig. 14b Stress-strain plot for Torrazzo bricks

Local stateof stress

-3.00 -2.00 -1.00 0.00 1.00 2.00 3.00

Strain [m/mm]

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1.00

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2]

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-3.00 -2.00 -1.00 0.00 1.00 2.00 3.00

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0.00

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1.00

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2]

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Fig. 13a Stress-strain plot at 7m height Fig. 13b - Stress-strain plot at 19m height

Local stateof stress

-2.00 -1.00 0.00 1.00 2.00 3.00 4.00 5.00

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Fig. 15a Monza Tower stress-strain plot at 5m higth Fig. 15b Monza Tower stress-strain plot at 13m higth

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In the following the results of four tests, two for each tower are commented. In Fig 13a and b the

maximum stress reached with the double flat-jack test and the single one respectively at 7 and 19m

height are considered, showing an elastic linear behaviour up to respectively 2.45 and 2.7 N/mm2. The

maximum stress reached when cracks clearly appear is respectively 3.77 and 3.77 N/mm2 and the

state of stress measured is 1.5 and 1.5 N/mm2. So in these two cases the safety coefficient at collapse

is more than 3. In Fig. 15a and b two cases of tests carried out on the walls of the Monza tower are

considered. If four double flat-jack tests are taken into account carried out approximately at the sameheight 5 and 13m on the Monza tower. Here the linear elastic behaviour stops at respectively 1.65 and

1.1 N/mm2and the maximum stress reached before cracks propagated was 2.62 and 1.87 N/mm

2.

The measured local state of stress was respectively 1.67 and 0.98 N/mm2. In these two cases the

safety coefficient at failure is much lower than in the first one and less than 2. Furthermore in the case

of the Torrazzo the modulus of elasticity is much higher and the Poisson ratio much lower than in the

case of the Monza tower.

5. Conclusions

The two experiences of investigation on tall towers allow some concluding remarks:- the on site and laboratory tests carried out following the methodology described in the first section

allowed to detect situations of danger and to characterise the materials and calculate inputparameters for the structural analysis;

- the laboratory tests were able to show the difference of properties of the masonry in the twobuildings and to see that where the weaker materials are used, there is the most difficult situation;

- the flat-jack test is a powerful tool to calculate the actual state of stress in compression and todetect the mechanical behaviour of the masonry, so that two different situations (Torrazzo andMonza tower) can be compared;

- the investigation allowed to state that the situation of the Monza tower is very difficult and that aquick intervention has to be done;

- for the Torrazzo, even if the state of damage is not considered dangerous, a monitoring system hasbeen set up, and the tower will be under control for 4 to 5 years at least in order to study its furtherevolution; in the meantime some repairs are being done for the external surfaces.

FE Numerical models were used for the static and dynamic analysis of the two towers (Binda et al.1998; Binda et al. 2000). The results of the experimental research were used to calibrate the FEmodels.

6 Aknowledgements

The authors wish to thank the students: M. Dellavedova, P. DellOrao, A. Rinoldi, M.S. Rizzotto, P.Condoleo, and the laboratory technicians: M.Antico, M. Cucchi, M. Iscandri and P. Perolari for theirhelp in laboratory and on-site. The investigation on the Monza tower was supported by Parish of St.Giovanni Battista - Duomo di Monza; the one on the Torrazzo was supported by Church Cattedrale diCremona. Also partial support came from MURST-Cofin 98.

6. References

Astori B., Bezoari G., Guzzetti F., 1992. Analogue and digital methods in architectural photogrammetry,Proc. XVII Int. Congress of Photogrammetry and Remote Sensing, Commission V, Washing.

Binda L., Anzani A., 1993. The time-dependent behaviour of masonry prisms: an interpretation, TheMasonry Society Journal, vol.11, n.2, pp.17-34.

Binda L., Anzani A., 1997. The safety of ancient masonry towers: A survey on the effects of heavydead loads, Int. Conf. on Studies in Ancient Structures, Istanbul, Turkye, pp. 207-216.

Binda L., Anzani A., Gioda G., 1991. An analysis of the time-dependent behaviour of masonry walls,Proc. 9

thInt. Brick/Block Masonry Conf., 2 pp.1058-1067.

Binda L., Gatti G., Mangano G., Poggi C., Sacchi Landriani G., 1992. The collapse of the Civic Towerof Pavia: a survey of the materials and structure, Masonry International, vol. 6, n.1, pp.11-20.

Binda

L., Falco

M.,

Poggi

C., Zasso

A., Mirabella Roberti

G., Corradi

R., Tongini Folli R.,2000. Staticand Dynamic Studies on the Torrazzo in Cremona (Italy): the Highest Masonry Bell Tower in Europe,

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