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Construction and Building Materials 16(2002) 385–395

0950-0618/02/$ - see front matter� 2002 Elsevier Science Ltd. All rights reserved.PII: S0950-0618Ž02.00041-7

Design choices and intervention techniques for repairing andstrengthening of the Monza cathedral bell-tower

C. Modena *, M.R. Valluzzi , R. Tongini Folli , L. Bindaa, a b b

Universita di Padova, Department of Construction and Transportation Engineering, Via Marzolo 9-35131 Padova, Italya `Politecnico di Milano, Department of Structural Engineering, Piazza L. da Vinci 32-20133 Milano, Italyb

Received 6 July 2001; accepted 31 May 2002

Abstract

A presentation is given of the fundamental design choices and of the selection of the most appropriate materials and techniqueswhich have been made for strengthening the Monza cathedral bell-tower, based on investigation and structural assessment carriedout prior to and during the design process. The results of the experimental and numerical investigation will first be given in orderto explain the reasons for the design choice.� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Bell-tower; Brick masonry; Strengthening; Creep; Confinement repair

1. Introduction

In 1996 the Department of Structural Engineering(DIS) of the Politecnico of Milan received the firstresearch contract supported by the parish of the cathedralof Monza concerning the investigation and diagnosis forthe bell-tower of the cathedral. The 16th century toweradjacent to the cathedral of Monza was showing anevolution of some damages for which it was alreadybeing monitored since 1978. The western and easternload-bearing walls of the tower, which are 74 m high,bear two major passing-through cracks with more than70 years.From 1996 to 1998 an experimental investigation was

carried out both in the laboratory and on site wherestatic and dynamic important parameters were measured.Elastic FE static and dynamic modelling was performedtaking into account the effect of dead loads, wind,temperature and bell ringing. All the models werecalibrated with the on site and laboratory measurements.The geometrical survey together with the survey of

the crack pattern allowed to realise that the tower wallshave a dangerous distribution of a net of thin vertical

*Corresponding author. Tel.:q39-049-8275613; fax:q39-049-8275604.

E-mail address: [email protected](C. Modena).

cracks from a level of 10 m up to 30 m. Flat-jack testswere performed in strategic points of the structure inorder to measure the state of stress caused by the deadload and the stress–strain behaviour of the material andthe results compared with the values given by the FEmodel. Sonic pulse velocity tests were also carried outon some parts of the walls in order to define the damageextent and distribution.The dynamic response of the tower to the vibrations

caused by the bells and by the wind was also experi-mentally and analytically investigated.Long-term observations of movements occurring

along cracks, laboratory and in situ investigations onthe existing material properties and on the state of stressof the monuments provided evidence of the real natureof its structural weakness.It is first of all the poor quality of the units and

mortar, combined with mechanical damage, whichmakes some portion of the tall structure very prone tosuffer progressively increasing damage propagationwhich in the long-term could possibly lead to the failureof the masonry.An already existing strengthening procedure, based

on the introduction of steel bars into horizontal mortarjoints (reinforced repointing), has been specificallyupgraded and investigated, both experimentally and

386 C. Modena et al. / Construction and Building Materials 16 (2002) 385–395

Fig. 1. Monza bell-tower.

numerically, in order to counteract the formation andpropagation of cracks on masonry subjected to highcompressive stressesw1x. Brick-masonry prisms, builtwith weak bricks and mortars in order to simulate themasonry of the tower and with recessed mortar joints,have been subsequently reinforced and re-pointed withthe same materials which will be used for the towerrepair. They have been subjected to compression withincremental loading kept constant for a chosen time ateach increment, in order to check the long-term effect-iveness of repair.Details will also be given on how the technique will

be actually applied, taking into consideration the resultsof an accurate survey of the zones where cracks appear,thus evidencing dangerous structural deterioration, thecomplexity of the geometry and of the constructionproperties of the tower.As a way of confining the whole structure, the

positioning of horizontal ties at the level of the internal,now partially missing, floors of the tower, will be carriedout as described below.The overall structural behaviour of the tower is in

this way improved, as the formation andyor propagationof large (single) cracks which tend to separate thestructure into different portions, is limited.

2. Description of damage symptoms

The study of historical documents concerning theconstruction and following events of the tower(Fig. 1),gave useful information for the diagnosis of thestructure.The tower construction started in 1592 perhaps fol-

lowing the design of Pellegrini and ended in 1605w2x.

The fact that the construction of the tower only required13 years suggests that uniformity of construction tech-niques and materials were used, as in fact it wasdetected.The only serious damage to the tower reported by the

documents occurred in 1740 and was due to the firewhich developed in the bell-tower and caused thecollapse of the bells and of the supporting frame downto the vault of the first floor at 11 mw3–5x. In 1755 theheavy clock was installed and some restoration workswere carried out. Nevertheless, already in 1927 someglass devices were applied across the cracks to detecttheir movements. In fact the main, now passing throughcracks, are visible on pictures taken in the 1920s on thewest facade of the tower. Deep large cracks also appearat the four corners of the tower at a height of approxi-mately 10–30 m. Other cracks can be seen from theinternal walls of the tower; they are very thin, verticaland diffused along the four sides of the tower. From1978 some major cracks have been surveyed withremovable estensometers showing a slowly increasingof their opening along the time. From 1988 the rate ofopening seems to be increasing; this fact is causing agreat concern among the people responsible for thesafety of the tower itself.

3. On site and laboratory investigation

The first step of the investigation was dedicated tothe geometrical and crack pattern data which were notyet available. Unfortunately no scaffolding existed, sothe external parts of the structure could not be reached.A geodetic net set up in the square for the cathedral in1993 was used as support; based on some points of thenet, some significant points of the west facade weresurveyed and used to straighten and return a series ofphotographic images. The belfry structure was not sur-veyed; its precise dimensions were not needed for thestructural analysis for which this part was assumed onlyas a dead load. The restitution of the images was carriedout with a computer program for simplified photogram-metry (Elco Vision 10) w6x. The internal prospects wereobtained mainly by direct measurement and matched tothe external ones. No relevant leaning was measureddue to the small subsidence which was taking place inthe square. The cracks were surveyed visually andphotographically and reported on plans, prospects andsections(Fig. 2a,b). Special attention was also paid tothe vertical cracks at the edges of the northern andsouthern facades of the tower; they unfortunately couldnot be monitored or investigated before the scaffoldingwas not set up, but now it is clear that they are deepinside the masonry with a tendency to detach the builtin corners(Fig. 3).The survey campaign also reveals a distribution of

thin vertical cracks crossing the bricks inside and outside

387C. Modena et al. / Construction and Building Materials 16 (2002) 385–395

Fig. 2. External and internal crack pattern of the west facade of the¸tower.

Fig. 4. Thin cracks at the entrance wall.

Fig. 3. Detachment of the corner of the tower. Fig. 5. Crack increasing in time of the base 2a.

starting from 12 m up to two-thirds of the tower. Thecracks go 400 mm deep inside the masonry walls aswas shown by removing some bricks and mortars joints.

The distribution of the thin cracks is shown in Fig. 4,representing the wall corresponding to the entrance ofthe tower.The removal of the bricks together with some coring

also showed that the walls, approximately 1800 mmthick, were made only with solid bricks and did notcontain any rubble.In the meantime themeasurement of the main cracks

continued while an automatic monitoring system wasbeing set up. Fig. 5 shows the monitored opening of themain crack from 1978 to 1997 with a clear tendencytoward a faster increase from 1988. The tendency ofopening of the three surveyed cracks was calculated as30.6, 31.3 and 39.7mmyyear from 1978 to 1995.Actually if this tendency is considered from 1988 to1997 the values change, respectively, to 41.2, 35.2 and56.2.


Fig. 6. Pulse velocity frequency histogram on zone 1 and zone 2.

Fig. 7. Single flat-jack tests performed, respectively, at 5.4, 5.6, 13,14, 31.5 and 38 m.

Fig. 8. Double flat-jack test on the west side of the tower(5 m).

The materials sampled from the walls of the towerweretested in the laboratory to be characterised. Mortarsand bricks were subjected tochemical, mineralogical–petrographical analyses, physical and mechanical testsand ND evaluation.Mortars are mainly based on putty lime and siliceous

aggregates, coming from near the Ticino river. Thesemortars are very weak and rather de-cohesioned andthey could not be tested mechanically.The bricks belong to two types different in colour:

brown and light red. These two types also show greatdifferences in their physical and mechanical character-istics. The brown brick is less porous and absorbent(absorption by total immersion 13%), and the strongest(compressive strength between 28 and 33 Nymm and2

elastic modulus between 2050 and 5300 Nymm . The2)

red brick has a higher water absorption(18%) and lowerstrength(between 4 and 12 Nymm ) and modulus of2

elasticity between 500 and 1330 Nymm . Petrographic–2

mineralogical observation confirmed that the red brickwas produced at a temperature lower than 8008C andhence with low characteristicsw7x. Unfortunately the redbrick is the most diffused in the construction; this meansthat the masonry components in the tower are very weakand, therefore, the masonry is also weak.As a confirmation of the state of damage of the

structure,sonic tests have been carried out on the mostdamaged parts of the walls. The equipment used for thesurvey is composed of an instrumented hammer astransmitter, an accelerometer as receiver and an oscil-loscopew8,9x. Two areas were detected at approximately12 m height and with a depth of the masonry wall of1800 mm.Zone 1—at 2.28 m from the floor and 1.55 m from

the entrance to the tower, where the masonry wasconsidered in good condition. A surface of 800=800

mm was controlled with 16 measurements bytransparency.Zone 2—at 0.50 m from the floor and 0.22 m from

the entrance where the wall presents a series of thinlarge cracks(Fig. 2b). The velocity values were gener-ally very low compared to other masonries, due to thefact that the materials are rather weak and also to thepresence of a large number of mortar joints in the wallwhich are attenuating the sonic waves. A comparisonbetween the two zones was possible and zone 1 gavesystematically much lower results than zone 2. Thedifference can be clearly seen in the frequency histo-grams of Fig. 6.

Flat-jack tests were carried out to measure the valueof the vertical compressive stress and the stiffness ofthe materialw10,11x. Seven single flat-jack tests wereperformed, respectively, at 5.4, 5.6 13.0, 14.0, 31.5 and38.0 m height of the tower and the detected stress valuesare reported in Fig. 7. The highest values seemed to beparticularly dangerous, taking into account the strengthvalues usually detected on this type of masonry.


Fig. 9. Vertical stresses due to the dead load.

Table 1Flat jack test results

Test number Single test Double test

State of On set of Elastic Modulus Poisson Coeff.ns

stress cracking E(20-50%)s (20–50%)(Nymm )2 (Nymm )2 (Nymm )2

TMJ1,2 0.98 1.87 985 0.13TMJ6,7 1.67 2.62 1380 0.19TMJ8,9 0.69 2.24 1372 0.07TMJ10,11 0.48 2.24 465 0.2

Therefore, somedouble flat-jack tests were carriedout in order to check the mechanical behaviour of themasonry under compression. Fig. 8 shows, as an exam-ple, the results obtained in the case of the weakest pointamong the ones which were tested; the stress valuemeasured at that point is clearly above the elastic limit.Table 1 reports the values at which the masonry

started cracking in the four cases, showing that the stateof stress seems to be not so far from unsafe, if thesafety factors adopted by the codes for the new masonryare taken into account. The results also explain thediffused cracks in the walls of the tower.In order to verify the response of the structure to

dynamic loading and their effect in terms of stressvariation, two dynamic tests were planned using theenvironmental excitation: the first one measured theeffects of the bells ringing, the second one measuredthe effects of a strong wind.The response of six horizontal servo-accelerometers

mounted in pairs at different heights from the groundlevel, was monitored and elaborated, together with thereadings of some transducers mounted across the majorcracks near the front window at the base of the tower.The sensitivity of the accelerometers was in the range

of 0–700 Hz, that is from the low(0.5–5 Hz) frequen-cies expected for the tower movements to the frequen-cies associated with the sound propagation in themasonry. Comments on these tests are given in theSection 4.

4. Modelling

Numerical analyses were carried out to evaluate thestate of stress and strain in the masonry. The tower wasmodelled by F.E. method under three load conditions:(1) dead loads;(2) wind; and (3) temperature varia-tions. The compressive stresses due to the dead loadincrease from the top to the bottom with a concentrationat the edges near the large window where they reach2.2 Nymm . The wind action increases the values on2

the west side by 0.25 Nymm . Also the maximum value2

calculated for the temperature variation is"0.25 Nymm . Figs. 9–11 show the results on the most stressed2

wall of the towerw12x.

By means of an appropriate analysis technique, accel-eration histories were transformed in displacement his-tories at the different levels, giving a peak-to-peakmaximum difference of nearly 4 mm in the W–Edirection and of 1.9 mm in the N–S direction when allthe bells ring together, and somewhat less(2.5 and 1.5,respectively) when only the major bell is ringing. InFig. 12 the modal form associated to the third frequencyof the tower is shown.Also the transducers mounted across the major cracks

were sensitive to the dynamic excitation induced by thebell ringing, giving a maximum peak-to-peak(openingto closing) of 28 mm, that should be compared with adaily variation of approximately 100mm due to temper-ature effects.The analysis of the collected data allowed also to

detect an important structural property, that is the fre-quency of the first mode of vibration of the tower.


Fig. 10. Vertical stresses due to the dead load and to the wind. Fig. 12. Modal form to the third frequency.

Fig. 11. Vertical stresses due to the dead load and to temperaturevariations.

Table 2Frequencies of the first six modes of the structure

Mode Frequency(Hz)

1st flexural E–W 0.6541st flexural N–S 0.663Torsional 3.1782nd flexural E–W 3.2322nd flexural N–S 3.311Axial 5.715

This value can be directly compared to the frequencyobtained from a F.E. model of the structure, allowingthe identification of a dynamic elastic modulus that canbe subsequently employed to calculate the effects(interms of stresses) of the applied force history. For a

more realistic approach, the modulus obtained in theunloading–reloading branches of some cyclic tests car-ried out on prisms recovered from the walls of the Cryptof the cathedral built with the same materials used forthe tower was calculatedw13,14x. The average valueobtained was 3742"314 Nymm . The value adopted in2

the analysis was 3400 Nymm , corresponding to a lower2

limit (but the specimens were cut from the Crypt) andgiving a very close match to the first frequency(0.654Hz). Table 2 shows the frequencies calculated for thefirst six modes of the structure.A first dynamic analysis was made applying a har-

monic force calculated from the movements of the massof the main bell, in the formFsF sin(2pft), withh

F s24 700 N, andf s0.34 Hz is the frequency of theh

bell oscillation. These calculations gave displacementsvery close to those measured experimentally, encourag-ing the use of the model for other verifications.

5. Remarks and suggestions for repair

Towers are generally subjected to heavy dead loadsdue to their weight and their massive construction


Fig. 13. South side section of the tower with the location of the different types of the reinforcing rings:(a) A1 type; (b) B type; (c) A2 typewith details of the strengthening of the corner;(d) C type; (e) reinforcement ring at the foundation level(F type) and particular the verticalsection.

technique. As a consequence, the state of stress at groundlevel cannot be far from the compressive limit strengthof the material with the development of increasing creepdeformation under constant loads. In the long-term this

behaviour induces continuous damage of the materialand can lead to its collapse.In the case of the Monza tower since the crack pattern

has developed slowly along the years a possible time-


Fig. 14. Section(a) and view(b) of the west side of the tower with the location of the repair techniques and details of the reinforced repointing(e) to the strengthening of the pilaster strips(c) and the walls(d).

dependent behaviour of the material can be supposeddue to the heavy dead load. This phenomenon, togetherwith the effects of cyclic loads as wind and temperaturevariations can eventually cause the collapse of thestructure. The situation can be considered serious andsimilar to other casesw15–17x.

All the tests and modelling carried out show adangerous situation which could bring about the suddencollapse of the tower. Therefore, the Report prepared byDIS invited the responsible of the tower safety to preparea design for intervention as soon as possible.

6. Repair and strengthening interventions

Following the diagnostic investigation and the finalsuggestion the parish decided to repair the tower and C.Modena was chosen as designer for the intervention.In order to improve the structural behaviour of the

tower, a series of repair and strengthening techniqueswas planned to be executed both at a local and globallevel. This consisted of the application of:(i) metallichorizontal reinforcing rings on several sections alongthe height of the tower to confine the masonry and to


Fig. 15. Reinforcement position and bonding in the reinforced prisms.

Fig. 16. Reinforced repointing technique: insertion of reinforced barsinto the joints on site.

improve the connection between the contiguous walls;(ii) the application of the reinforced repointing techniquediffused on various portions of the walls to counteractthe creep damage, and concentrated on some pilasterstrips to strengthen the corners;(iii ) local interventionsof injection, rebuilding and pointing of the mortar joints,to restore the zones having high material deterioration.Complementary intervention will be aimed to retrofitthe stairs structure, the support of the bells, and theopenings distributed along the tower.A series of stainless steel reinforcing rings have been

planned to be applied in different horizontal sections ofthe tower. The general scheme is given in Fig. 13. Theyare properly designed depending on the particular localconditions detected on the structure. The ‘A’ types arecomposed by two internal rods(30 mm in diameter)anchored to stainless steel plates which will be possiblyincluded in proper recesses successively closed by themasonry. The general solution for the anchoring of theties is the ‘A2’ type, characterised by the local strength-ening of the masonry by means of a series of steel barsdiffused along the connection zone. The vicinity of thechurch at the first level led to the ‘A1’ type solutions,performed by the transversal connection of the anchoringties (see also the ‘B’ type).The presence of a vault at the height of 11 m, led to

the choice of a diaphragm solution for the reinforcingring at that level: it is composed by a bolted net structureof C-shape metallic components(properly protectedfrom corrosion) and tie rods(Fig. 13b) anchored byplates not visible from the outside. If the in situconditions are favourable, such structure will be incor-porated within the floor at the extrados of the vault. Forboth the described types of rings, the presence of screwcouplings allows the easy tensioning of the rods fromthe inside of the tower.The only reinforcing ring external to the structure is

placed at the cornice level composed by two tie rods(36 mm in diameter) (Fig. 13d); due to the difficult

accessibility of the site, it is completely made of stainlesssteel.Finally, the results of soil investigations will allow to

evaluate the feasibility of applying a further reinforcingring at the foundation level(Fig. 13e), executed by fourreinforced concrete beams connected by a series of post-tensed bars inserted in cored holes.Pilaster strips highly deteriorated and walls exhibiting

the typical creep damage(i.e. thin and diffused cracks)will be repaired and strengthened by the reinforcedrepointing technique.It is based on the insertion of small diameter rein-

forced bars(stainless steel or FRP bars or plates can bealso considered) into the joint previously excavated andthen refilled by mortars having generally better charac-teristics than the original ones. However, the use ofpointing materials at most as possible compatible withthe existent mortars is a fundamental condition to assurethe proper effectiveness of the intervention and thedurability of the structure. In order to improve theconfining action of the bars, the connection between thereinforced layer and the internal core of the masonry isgiven by a series of transversal stainless steel U shaped


Fig. 17. Experimental results of panels strengthened by reinforced repointing techniques subjected to creep loads: vertical(a) and horizontal(b)strains vs. stress diagrams.

pins, inserted into sloped cored holes successively sealedwith mortar(Fig. 14).The structural effectiveness of such intervention was

supported, in recent years, by a series of experimentaltests performed at the Politecnico of Milan on clay brickmasonry prisms reinforced with two 6 mm diameterreinforced bars. The influence of several parameterswere considered, namely the type of loading(monotonicor creep simulating), the position of the reinforcement(at one or both sides of the wallets), the anchoringpattern of the bars, and the repointing andyor sealingmaterial (different hydraulic mortars andyor resins). Afirst campaign was aimed to detect the effectiveness ofthe technique in repairing the masonry after damagingdue to previous monotonic compressive tests, whereasthe second one was devoted to detect the behaviourunder creep loads in the strengthened condition of thepanels. The long-term actions can be performed byprogressive increments of compressive loads, kept con-stant for a fixed time(e.g. 3 h) up to the appearance ofexcessive deformations and to the collapse which is dueto the tertiary creep phase. The results of the twodifferent campaigns have been given separately in earlierwork w1,18x. The general features of the panels arepresented in Fig. 15. Fig. 16 shows how the techniqueis applied on site.The results of the tests carried out on prisms subjected

to monotonic and pseudo creep loads(both from thefirst and the second series) showed how the maincontribution of the technique is given to counteract thedevelopment of the cracking pattern by limiting thetensile stresses in the bricks(approx. 40%, for thespecific reinforcement condition) and the horizontaldilation of the masonry. As for the strength the resultsshowed that there is no influence of the technique inthe peak strengthw18x.The creep tests contributed to clarify the mechanical

behaviour of the strengthened and non-strengthenedpanels(plain), as shown in Fig. 17, where the trend of

the vertical and horizontal strains detected on the widersides(the most representative of the whole walls) aregiven. It is possible to notice that, also in this case, thetechnique has no significant influence on the strength(20% higher) and on the vertical deformation of thepanels, whereas the horizontal dilation is clearly reducedby almost 70% in comparison with the non-strengthened(plain) case.

7. Conclusions

● The investigation carried out on site and in thelaboratory on the materials and structure of Monzabell-tower allows the detection of the details ofdamage, which is evolving toward the failure.

● The static and dynamic tests were useful to calibratethe FE mathematical model.

● The design following the guidelines given by thediagnosis and the experimental research, is now basedon a technique which, respecting as much as possiblethe original structure, provides an improvement whichsaves the preservation concepts.

Acknowledgments

The research was supported by CNR-GNDT, COFIN2000 and by the parish of Monza. Authors wish to thankC. Poggi, A. Zasso, A. Saisi, C. Tiraboschi, C. Arcadi,M. Antico, M. Cucchi, M. Iscandri and P. Perolari andfor their help in experimental work on site and inlaboratory, and the students E. Bardelli, L. Cantini, S.Taiocchi and A. Testa.

References

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