Structural Analysis of Historical Constructions - Modena, Lourenço & Roca (eds) © 2005 Taylor & Francis Group, London, ISBN 04 1536 379 9

Techniques of structural identification for the monitoring of historical buildings: first experimental results for a masonry tower

P. Carusi, V Sepe & A. Viskovic Università di Chieti-Pescara "G.D 'Annunzio ", Depl. PRlCOS, Pescara (Italy)

ABSTRACT: The techniques of structural identification, and in particular those of modal parameters of linear or linearised models, represent an important tool for analysing existing structures. Like ali civil structures, monumental buildings are constantly subject to environmental actions (wind, traffic, microtremors ofthe ground), that can allow identification avoiding the costs related to artificial excitations, but that are usually unmeasured. In the framework of a research on the application to this kind of structures of the identification techniques with unknown input, already implemented for other kind ofbuildings, this paper report the preliminary results of an experimental investigation on a masonry tower located in the East-Central part of Italy.

INTRODUCTION

The increasing interest in the seismic safeguarding of buildings with strategic or historical interest and the recent seismic new classification of the Italian territory ask to the public administrations to define synthetic cri teria to assign priorities for retrofitting or restoration interventions.

The techniques of structural identification, and in particular those of modal parameters of linear or linearised models, represent an important tool for analysing existing structures. In fact , synthetic infor­mation on damage occurred to buildings can be obtained comparing the value of such modal param­eters (first of ali natural frequencies and damping coefficients) before and after the seismic event.

Like ali civil structures, monumental buildings are constantly subject to environmental actions (wind, traffic, microtremors ofthe ground); the objective di f­ficulty of characterizing and measuring the significant parameters of such types of actions, has motivated the study of techniques of identification under unknown input, which are based on the measurement of the structural response alone in some points; such tech­niques also have the advantages of a limited number of operations to carry out on the building, the small number of sensors for the measurement of struc­tural response and the non-interruption ofthe normal functionality of the building; moreover, they do not require the additional costs related to devices for acting artificial excitations (Brownjohn et aI. , 1989- 1992; Farrar & James, 1997; Katafygiotis et aI. , 1998; Luz & Wallaschek, 1992; Quek et aI. , 1999; Torkamani &

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Ahmadi, 1988; Wang & Haldar, 1994-1997, Wilson & Liu, 1991). Moreover, the high preci sion of the mea­surement instruments allows to record accurately vibrations due to very low actions, for which a linear structural model could be sufficient even in presence of previous damage.

As regards structural systems with linear behaviour, one of the writers and co-authors (Capecchi et aI. , 2002- 2004; De Angelis et aI. , 2003) have recently introduced a procedure in the frequency doma in for the identification of the modal parameters (components of the modal shapes, natural frequencies and moda I damping) in the case ofunknownand possibly non sta­tionary seismic excitation. The procedure, which can be transferred to the time domain (De Angelis et aI. , 2004), is based on the observation that the ratios of the Fourier transforms of the responses in different points of the building, tum out to be independent of the forcing itself, and therefore can give experimental information forthe identificationofmodal parameters.

This procedure, that has successfully been applied to framed plane and three-dimensional structures, can be extended, from a theoretical point of view, to more complex structures, as monumental ones.

However, this task has not yet been dealt with in the present paper, that is part of a research in progress on the techniques of structural identification for historical and monumental building, a theme that is receiving an increasing attention above ali in Italy (Bartoli & Biasi, 1995; Binda et aI. , 1992- 1995; Croci, 2001 ; Pavese, 1992).

As a first step ofthe quoted research, this paperrefer on the preliminary results of an in-situ investigation

recently performed on a masonry tower. In the experi­mentai campaign, that should continue in next months, only low intensity ambient excitation have been used (namely due to the vibration induced by an aerial plat­form (Figure I) and by other building-yard equipments operating near the tower.

Unfortunately, the small amount of experimen­tai data made available so far have not yet allowed to implement refined identification procedures (for theoretical aspects, cf. Capecchi et aI., 2002- 2004; De Angelis et aI., 2003- 2004). However, the com­parison reported in this paper between the modal fre­quencies obtained through a peak-picking (cf. Ewins, 1986) elaboration of experimental data and those given by a finite element model (FEM), confirm that a satisfactory investigation of this kind of structures can be performed under ambient (low intensity and no-cost) excitations, without the need of using arti­ficial excitations (e.g. due to vibrodyne), that are often incompatible with the safeguards ofthe historical heritage.

2 THE "FEBONIO" TOWER IN TRASACCO

The medieval "Febonio" Tower is situated in the small centre of T rasacco (near r.; Aquila, Italy) at about 685 m

Figure I . The "Febonio" Tower in Trasacco (near r; Aquila, ltaly).

above sea leveI on the southern borders ofthe ancient Fucino Lake. The town of Trasacco represents, still today, one ofthe most important centres ofthe Marsica region.

The building, which has a square base section, becomes circular at two-thirds of its height, giving to the tower itself a particular look, unique in the Abruzzo region (Figure I).

The round part, which is a typical consequence deriving from the technical evolution of the fire weapons, is characterized, near the top, by an embat­tled overhang supported by thick shelves in triple order and finely shaped as quarter of circles. In the space inside, completely empty, four pendentives join the square section to the upper circular section (Figure 2); between the square and the circular parts of the tower there is a concrete floor which was built in 1970, together with other interventions required as a con­sequence of structural damages due to the devastating earthquake of 1915.

Few and narrow window openings, with round arches and double vaults, open in the thick masonry, with constructive characteristic similar to the other towers that made up the defence system and control ofthe territories facing the lake.

The dimensions of the bottom square section are 8.20 metres for the externaI sides and 5.40 metres for the internai ones, with a base thickness of about

Figure 2. Inside view (from bottom) of the r.c. floor separating the square and circular parts of the tower.

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1.40 m, and the overall height of the tower is about 29 metres.

3 FINITE ELEMENT MODEL OF THE TOWER

A model of the tower was implemented using the commercial finite elements (FEM) code AIgor®.

Considering the objective of the study, which was to identify the average values of some mechanical parameters, through a comparison between the modal characteristics of the model and those which were obtained during on-site tests , the FEM model was very accurate from the geometrical point of view, while the material was assumed homogeneous and isotropic; quite obviously this dramatic simplification, certainly appropriate for the said objective, excludes the possi­bility of getting reliable information on the very local stress state.

In the first phase of the research, reported in this paper, attention was devoted to the identification of the mass density and the elastic modulus ofthe brick­work through comparison of the natural frequencies ofthe tower and of its FEM model ; in this model, the mass density and the elastic modulus have been varied in a relatively large interval , corresponding to typical values of the characteristics of the masonry typology at issue (cf. 8eolchini, 2000), while a constant value of 0. 18 has been considered for the Poisson coeffi­cient. Some results on the FEM analysis are reported in Table 1, where F l, TI, F2 and V I denote the first flexural mode (Figure 3), the first torsional mode (Fig­ure4) , the second flexural mode (Figure 5) and the first vertical mode, respectively.

Quite obviously, as a consequence of the double symmetry ofthe tower cross sections (square or circu­lar), each one ofthe flexural modes is a multiple mode, corresponding to possible oscillations in any one ofthe vertical central planes of the structure.

Table I. Modal frequencies ofthe FEM model for different values of density and elastic modulus.

Elastic Natural frequency (Hz)

Density modulus Mode Mode Mode Mode (kg/mJ

) (kN/mm2) FI TI F2 VI

1800 3 2,54 7,36 9,84 12,05 1800 5 3,28 9,50 12,71 15,53 2000 2 1,97 5,70 7,62 9,34 2000 2,5 2,20 6,38 8,52 10,44 2000 3 2,41 6,98 9,34 11 ,44 2000 3,5 2,60 7,54 10,08 12,35 2000 4 2,78 8,07 10,78 13 ,20

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Figure 3. First modal shape (1st flexural) of the FEM model: plan (top), lateral view (middle) and axonometric view (bottom).

Figure 4. Second modal shape (1 st torsional) of the FEM model: plan (top), lateral view (middle) and axonometric view (bottom).

Figure 5. Third modal shape (2nd fl exural) of the FEM model: plan (top), lateral view (middle) and axonometric view (bottom).

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4 IN-SITU DYNAMIC MEASUREMENTS

Due to the importance and peculiarity of the mon­ument, it was considered useful to perform in-situ test to get further information on some mechanical characteristics of the tower.

Preliminary dynamic in-situ measurements have been so far performed by the Department ofStructures (Dip. PRICOS) of the University of Chieti- Pescara. In these tests, that should continue in the next months, precision accelerometers are used to record the very low intensity vibrations due to externai "noise" and ambient loads, namely due to the vibration induced by an aerial platform (Figure 1) and by other building­yard equipments operating near the tower during the investigation and restoration works planned by the local authorities.

For the first series of tests three accelerometers of the type PCB 393 A03 were used; they were placed on the structure by way of an articulated aerial plat­form (Figure I). Namely, the three sensors, arranged with a horizontal axis ofmeasurement, were all placed (Figure 6) on the concrete floor which separates the cylindrical part of the tower from the prismatic part underneath; in fact , the FE model ofthe structure (see Section 3) shows how the first modes ofvibration are characterized by oscillations of a global type, both flexural or torsional.

2

1

3

Legend

-+- Accelerometer

Figure 6. Posilion of lhe acceleromelers on lhe r.c . floor between lhe square and lhe circular parIs oflhe lower.

Local oscillations of single parts or the structure, corresponding to modes with higher frequencies, will be the objective of future investigation, that require a larger number of sensors.

In this phase of the analysis accelerometers records have been performed of the low intensity structural vibrations due to the vibration induced by an aerial platform (Figure I) and by other building-yard equip­ments operating near the tower; such tests are denoted in the following with the letter A followed by the num­ber of the sensor and the number of the sequence of the test (e.g. A_s2_05). Other tests, denoted respec­tively with the letters B and C, were carried out with a local type of excitations, consisting of a person jump­ing rhythmically on the concrete floor (mass of about 90 kg, jumps with period of about one second) in a vertical direction (tests denoted with B) or in a lateral direction (tests denoted with C).

The recorded accelerations, some examples of which are shown in Figure 7, have then been filtered and transformed in the frequency doma in, through the c1assical technique of Fast Fourier Transform (FFT).

The FFT of the records in Figure 7 are shown in Figure 8. In spite of some uncertainty, probably due to the low intensity of the vibrations, and in spite of some anomalies that are currently being investigated, most of the tests have shown peaks of frequencies of 2.4 Hz, 7.8 Hz and 13 Hz, approximately.

Considering the location and orientation ofthe sen­sors, the first two frequencies can be associated with the first flexural mode and the first torsional mode, respectively (Section 3).

The comparison between the experimental values of frequency with those given by the FE model for different mechanical characteristics (Table I) , allows to get identified values ofthe mass density and ofthe elastic modulus ofabout 2000 kg/m3 and 3.5 kN/mrn2,

respectively. It must be said, however, that such values represent

only an average estimate for low-intensity vibrations, and therefore the possibility of values locally higher or lower cannot be exc1uded in this phase of the study; in particular, parts ofmasonry with mechanical characteristics lower than the average, due to the pres­ence of hollows or local cracks must be investigated experimentally.

What is more difficult in this phase is the inter­pretation of the third peak of the FFT, that, with this very low levei of vibrations, can be also sometimes difficult to distinguish from peaks due to background noise.

The comparison with the results given by the FE model allows to consider, for this third peak, the hypotheses both of a contribution of the second flex­ural mode, which should however be only slightly relevant at the concrete floor levei where the sensors are located (Figure 5), or of the first vertical mode,

465

8 . ,0"

.. ~~----'0~.5------7------'~5------~----~2 5

0.1

..(),15

1.5

tel'f1lO (S)

2.5 tel'l1»{s)

.s

Figure 7. Example of recorded acceleration time-histories. top: sensor #2, type A excitation, 2.5 seconds middle: sensor #3, type B excitation, 5 s bottom: sensor #2, type C excitation, 5 s.

which is certainly activated by the last two modal­ities of excitation considered (B and C), but whose presence in the FFT should denote an error (imperfect horizontality) in the location of the sensors.

~ A_52_05 0 .00151~ LI. o :; 'C o

:; O rJ1 O 2 4 6 8 10 12 14 16 18

Hz

O 2 4 6 8 10 12 14 16 18 Hz

0.

03

11

I C_s2_03

lo~ O 2 4 6 8 10 12 14 16 18

Hz

Figure 8. Modulus of the Fourier Transforms of the accel­eration time-histories in Figure 7.

Further numerical investigalions, slill in progress, are trying to c1arify these aspects.

5 CONCLUDING REMARKS

This paper refer on the very preliminary results of an in-situ investigation recently performed on a masonry tower. In the experimental campaign, that should con­tinue in next months provided lhe availability of financiai support, only low intensity ambient exci­tation have been used (namely, due to the vibration induced by an aerial platform (Figure I) and by other building-yard equipments operating near the tower).

At the moment, the comparison between the exper­imentai values of frequencies with those given by the FE model for different values ofthe density and elastic modulus, have only allowed to get identif ied average values for the mass density and for the elastic modulus of lhe masonry.

Further investigations, still in progress, are trying to apply to this structure the techniques already imple­mented by one of the writers (see Capecchi et aI., 2002-2004; De Angel is et aI. , 2003- 2004) for other kinds of buildings, and to perform a more accurate

466

updating of the FE model of the tower, taking into account both global and local vibration shapes.

However, the results reported in this paper confirm that a satisfactory investigation of this kind of struc­tures can be performed under ambient (lowintensity and no-cost) excitations, without the need of us ing artificial excitations (e.g. due to vibrodyne), that are often incompatible with the safeguard ofthe historical heritage.

ACKNOWLEDGEMENTS

The mayor and the administration of Trasacco are gratefully acknowledged for making available techni­cal support for the experimental tests reported in this paper.

V Sepe acknowledges also the financiai support of the University ofChieti-Pescara (ex MIUR 60%, 2002 and 2003, Faculty of Architecture).

REFERENCES

Bartoli G. , Biasi C., 11 sistema di monitoraggio della cupola di Santa Maria dei Fiore: problematiche relative ai fun­zionamento degli strumenti ed alia gestione dei dati, in Monitoraggio delle strutture dell ' Ingegneria Civile (Edi­tor P.G. Malerba), International Centre for Mechanical Sciences CISM, 1995, ISBN 88-85137- 09-1 , 183-202 (in Italian).

Beolchini G. Parametri meccanici e legami costitutivi delle murature di pietrame, in Patrimonio storico architet­tonico e terremoto; diagnosi e interventi di recupero (V De Vecchis editor), 2000, 143- 154 (in Italian).

Binda L., Gatti G., Mangano G., Poggi c., Sacchi Landriani G. The collapse of the Civic Tower of Pavia: a survey of the materiais and structure, Masonry Interna­tional , 1992, vol.6, n.l, 11 - 20.

Binda L. , Mirabelli Roberti G., Abbaneo S. The diag­nosis research project, in Monitoraggio delle strutture dell'lngegneria Civile (Editor P.G. Malerba), Interna­tional Centre for Mechanical Sciences CISM, 1995, ISBN 88-85137-09-1,49- 72.

Brownjohn J.M., Oumanoglu A.A., Severn R.T. , Blakebor­ough A. Ambient vibration survey of the Bosporus sus­pension bridge. Earthquake Engilleering and Struc/ural Oynamics 1989; 18, 263- 283.

Brownjohn J.M. , Oumanoglu A.A., Severn R.T. Ambient vibration survey of the Fatih Sultan Mehmet (second Bosporus) suspension bridge. Earthquake Engineering and Structural Dynamics 1992; 21: 907-924.

467

Capecchi O., De Angelis M. , Sepe V Identification of struc­tures subject to unknown seismic excitation. 3rd Joint Conference GIMC and AACME. Giulianova (italy), June 2002.

Capecchi O., De Angelis M., Sepe V Modal model identifica­tion with unknown nonstationary base motion. Meccanica 2004; 39: 31-45.

Croci G. Conservazione e restauro dei Beni Architettonici, UTET, 2001 (in Italian)

De Angelis M. , Sepe V, Capecchi O. Identification of Modal Parameters with Unknown Input, Problems in Struc­tura/ldentification and Diagnostics: general aspects and applications (Ed. Davini and Via/a), CISM Courses and Lectures n.471, 2003, ISBN 3-211- 20492-X.

De Angelis M. , Sepe V, Sibilio E. 2004. Tecniche nel dominio delle frequenze e nel dominio dei tempo per I ' identificazione di strutture sotto forzanti non misurate, Proceed. II-th Italian Conference on Earthquake Engi­neering, Genova, 25- 29 January 2004 (in Italian).

Ewins OJ. Modal testing: theory and practice. Research Study Press- J Wiley & Sons, 1986.

Farrar C.R., James G.H.IlI System identification from ambi­ent vibration measurements on a bridge. Journal ofSound and Vibration 1997; 205(1): 1- 18.

Katafygiotis L.S. , Mickelborough N.C. , Yuen K.V. Statisti­cal identi fication of structural modal parameters using ambient response data. Proceedillgs 3rd Internation Con­ference on Compu/ational S/ochastic Mechanics, Island ofSantorini, Greece, 1998,93- 102.

Luz E. , Wallaschek J. Experimental modal analysis using ambient vibration. The In/erna/ional Journal ofAna/ytical and Experimental Modal Analysis 1992; 7(1): 29- 39.

Pavese A. Studi sul comportamento dinamico di torri in muratura e loro modellazione, PhO Thesis, Oipartimento di Meccanica Strutturale, Università di Pavia, 1992 (in /talian).

Quek S.T. , Wang w., Koh c.G. System identification of lin­ear mdof structures under ambient excitation. Ear/hquake Engineering and Structural Dynamics 1999; 28: 61 - 77.

Torkamani M.A.M., Ahmadi A.K. Stiffness identification of a tall building during construction period using ambient test. Earthquake Engineering and S/ruc/ural Dynamics 1988; 16: 1177- 1188.

Wang O. , Haldar A. Element-Ievel system identification with unknown input. Journal Engineering Mechallics (ASCE) 1994; 120(1): 159- 176.

Wang O. , Haldar A. System identification with limited observations and without input. Joumal Engineering Mechanics (ASCE) 1997; 123(5): 504-511.

Wilson J.C., Liu T. Ambient vibration measurements on a cable-stayed bridge. Earthqllake ElIgineering and Srruc­/lIral Dynalllics 1991 ; 20: 723- 747.