SSHM and DSHM for a better knowledge and risk

SSHM and DSHM for a better knowledge and risk prevention of historical buildings: the cases of the Two Towers in Bologna and the Cathedral in Modena Baraccani Simonetta, Palermo Michele, Stefano Silvestri, Giada Gasparini, Tomaso Trombetti University of Bologna DICAM Bologna, Italy [email protected]

Abstract— In the last recent years, structural monitoring has acquired an increasing importance in the diagnosis and control of buildings, especially for historical buildings whose preservation is essential to the safeguard of cultural heritage. The aim of this study is to introduce a standardized approach for the interpretation of the large amount of data acquired from a monitoring system of historic buildings. This approach is based on the definition of specific reference quantities (extrapolated from the recorded time series) able to characterize the main features of the structural response and the preliminary identification of the order of magnitudes of these quantities. These reference quantities may be collected in a database and may become fundamental for comparing the response of similar buildings. This type of analysis has been applied to the data obtained from the Static Structural Health Monitoring (SSHM) system of two significant Italian monuments: the Cathedral of Modena and the Two Towers of Bologna (Asinelli and the Garisenda Towers), North Italy. This paper provides, also, the preliminary results of the experimental data as obtained from the Dynamic Structural Health Monitoring (DSHM) of the Asinelli Tower conducted by the Italian National Institute of Geophysics and Volcanology (INGV). Keywords—Static SHM; Dynamic SHM , reference quantities, dynamic identificationl, traffic-induced vibrations

I. INTRODUCTION Cultural heritage sites represent inestimable values and not removable resources of most of European countries, which have to be preserved for future generations in order to transmit their history, culture and art. Cultural heritage sites and, more specifically, historical buildings are built during decades

(sometimes centuries) by using various construction techniques, workmanships of different expertise, with the results of a complex structure, characterized by a high degree of uncertainties. Moreover due to the fact that they stand from centuries, their actual state of conservation is the result of not only the natural effects of aging, but also the consequence of climate changing, and extreme natural hazards (such as earthquakes) which are happening worldwide with increasing frequency. The inherent complexity of most of historical sites, together with the natural aging due to material degradations and the effects of natural hazards, render the assessment of the “state of the health” of the construction extremely difficult and associated with high uncertainties, at the point that each site can be referred to as an “unicum”. Moreover, all these effects tend to inevitably reduce the level of safety and thus to increase the risks to future extreme events. Therefore, it clearly appears the need of a robust estimation of the actual level of safety in order to plan effective interventions, which requires an in-depth knowledge of the historical building, including material properties. In principle, a better knowledge of the material properties could be gathered by mean of extensive insitu tests. Nonetheless, when dealing with monumental buildings it is often prohibited to carry out destructive tests and therefore non-destructive methods are preferred. In this respect, Structural Health Monitoring (SHM) techniques are more and more widespread for the diagnosis and control of historic buildings. The SHM involves the installation of several devices that permit to monitor the time evolution of parameters concerning the structural behavior of buildings - like the strain and stress state of structural elements, the development of specific existing cracks, the inclination at specific critical

points of the structure …. The obtained data need to be analyzed in order to gather some useful information on the condition of the building under observation. Analyses carried out on these data can have multiple purposes. The present paper is aimed to introduce a standardized approach for the SHM data analysis with a specific reference to historical buildings. The approach is applied to two important Italian monuments: the Cathedral of Modena and the Two Towers of Bologna (Asinelli and Garisenda Towers), North Italy. The approach is aimed at: (i) introducing reference quantities based on the identification of the main physical components characterizing the recorded data, (ii) providing order of magnitudes of these physical quantities and (iii) identifying the presence of potential structural criticalities. The data obtained from the dynamic monitoring system installed for experimental tests on the Asinelli Tower have been also presented. II. STRUCTURAL HEALTH MONITORING (SHM) SYSTEM

The main target of a monitoring system is the evaluation of the building “structural health”. A reliable SHM strategy requires that the building be monitored for a sufficient long time (at least some years) in order to understand the seasonal effects and be able to eventually detect the presence of potential structural criticalities and/or incoming damages. The word “damage”, when used with reference to structures, describes situations that can influence their present or future behavior in an adverse way. Damages may evolve slowly when related to temperature, fatigue, corrosion, subsidence phenomena, or also sudden increase as a consequence of extreme events such as earthquakes, hurricanes and explosions [1-4]. SHM systems can be distinguished into: (i) Static SHM (SSHM) systems that typically acquired 2 to 4 data per hour and (ii) Dynamic SHM (DSHM) systems which acquire data in continuous. The SSHM cannot provide information regarding system dynamic identification and is generally used to monitor the evolution of the “state or condition” of the monument. A stationary behavior reasonably suggests that the structure is in a safe condition, whilst a non-stationary response (especially in the cases where a clear trend is observed in the data) may indicates a significant evolution of the state of damage, which may preclude the structural safety of the monument. The data gathered from a SSHM system may be used in conjunction with structural analysis to identify the

main vulnerabilities associated with the relevant hazards and the main structural criticalities in order to conceive targeted solutions and avoid heavy interventions. A SSHM is composed of several devices which typically monitor the strains and stress state evolution at specific locations, the opening of existing cracks, the inclination of walls, climate conditions, …. The recorded raw data need to be processed in order to detect eventual trends from the seasonal responses. On this regard, various signal processing techniques have been proposed during the years [5-6]. Nonetheless, to the knowledge of the authors, a simple method for the structural interpretation of the recorded signal is still not available in the scientific literature. The first purpose of the present work is to introduce a simple approach for a structural interpretation of data and a risk assessment with specific reference to historical buildings. It is assumed that the recorded time series may be decomposed into two fundamental components: the first one related with the natural actions (the actions that depend on the weather effects) and characterized, in absence of extreme events (such as explosion earthquake, hurricanes,…), by a substantial periodic behavior, the second one related to the other factors such as the evolution of the state of the structure due to material degradation, soil settlements and others. For historic buildings (composed of massive masonry walls), the temperature is typically the natural action which mainly affects the structural response (in usual operational condition). Based on the above assumptions, reference quantities are first introduced to describe the main features of the structural response (Table 1). Then, for instance, order of magnitudes of these reference quantities are provided for two different monumental buildings: Cathedral and tall tower. These reference quantities may be collected in a database and used to compare the response of similar buildings and to distinguish between stable conditions (if the values of the quantities falls within specific ranges) from potential dangerous situations (if the values of the quantities go outside the ranges) and establish an alarming system. The specific monuments, which are here analyzed, are the Asinelli and the Garisenda Towers of Bologna and the Cathedral of Modena together with the adjacent Ghirlandina Tower.

TABLE I.

REFERENCE QUANTITIES FO OR THE ANALYSIS OF THE RECORDED DATA. TABLE STYLES

Reference quantity Daily Amplitude Mean Daily Value

nition Defin

δ day, j = ⎡⎣max x ( ti ) − min m x ( ti ) ⎤⎦ ∀ti ∈ dayj

∀ 1 nj

mday , j =

nj

∑x(t )∀t ∈ dayj i

i

i =1

Annual Amplitude

Σ year , j = ⎡⎣ max x ( ti ) − min x ( ti ) ⎤⎦ ∀ti ∈ yearj

Mean Value

Myear , j =

Annual

1 nj

nj

∑x(t )∀t ∈ yearj i

The monitoring system installled in the Cathedral is composed of biaxial and triaxiial joint meters (MGBMGT), inclinometers (FP), deformometers (D) and thermometers (T) to monitor the t main cracks across the walls and vaults, the incllination of the external longitudinal walls, the relative displacements between the Cathedral and the Tow wer and the internal temperature, respectively. A plan view with the indication of all installed instrruments is provided in Figure 2.

i

i =1

Daily residual on δ

rδ day , j ( k −l ) = δday , j | year k −δday , j | year l

Daily residual on m

rmday , j( k −l) = μ day , j | year k −μ day , j | year l

Annual residual on Σ Annual residual on M Drop

RΣyear, j( k −l) = δ year, j | year k −δ year, j | year l RMyear, j( k −l ) = μyear, j | yearr k −μyear, j | year l

Δ

RAL OF MODENA III. SSHM SYSTEM OF THE CATHEDR

The Cathedral of Modena annd the adjacent Ghirlandina Tower are part of the UNESCO site of Piazza Grande, since 1997. Thee Cathedral is a masterpiece of Romanesque architecture and sculpture of northern Italy (Figure 1). 1 Its construction was realized between 1099 and 1319, when the construction of the Ghirlandina waas completed The Cathedral has a basilica plan with w three naves culminating in three apses. Thhe Cathedral is connected to the contiguous Ghirllandina Tower (a tower of about 86 meters heigght) through two masonry arches. During the centuriies, the monument experienced various interventions annd transformations [7, 8]. Recently, a conservation project, which is currently under development, has been b planned with the purpose of strengthening the main walls, the vaults and of providing the struccture with a box behavior. In the context of this retrofit r project, a SSHM system was also installed in the t 2003.

Fig. 2. Location of the device installed on the Cathedral

As illustrative example, Fiigure 3 displays the complete time series over years y 2004-2015 with reference to the biaxial joint meter m MGB1 placed on a crack on the south longitudinal wall of the Cathedral. With the exception of few spiikes, this series clearly shows the daily and annual perriodicity with an almost constant average value, thus noot showing a significant evolution of the state. For instance, with reference too crack displacement of the Cathedral, some averagge values of selected reference quantities are reporteed in Table 2.

Fig. 3. The complete time series oveer years 2004-2015 recorded by the Biaxial joint meter installed on south longitudinal wall of the Cathedral of Modena

Fig. 1. The Cathedral of Modena

TABLE II.

REFERENCE VALUES FOR R THE BIAXIAL AND TRIAXIAL JOINT METERS INSTALLED IN TH HE CATHEDRAL OF MODENA

CATHEDRAL OF MODENA mean values over the entire observation period Δt (2004--2015) Device

Deformomete r [Biaxial]

Deformomete r [Triaxial]

Comp. X Comp. Y Comp. X Comp. Y Comp. Z

δ day

Σ yearr

rm ,day

RM , year

0.02

0.35

-0.012

-0.03

0.00 6

0.10

-0.005

-0.01

0.03

0.65

0.027

0.75

0.01

0.30

-0.008

-0.02

0.00 8

0.20

0.037

0.05

The monitoring system instaalled is composed of deformometer (D), long basse deformometer (F), extensimeter (E), inclinometer (I), laser displacements sensor (L) and thermometers (T) to monitor the opening of the main cracks, thee masonry compression at critical locations, the variatioon of the stress level of the steel ties , the variation inn the inclination of the towers and temperature, respecctively (Figure 5).

WERS OF BOLOGNA IV. SSHM SYSTEM OF THE TWO TOW

During the XII and XIII centuriees, a large number of towers were erected by some of the t richest families of Bologna [9]. Some of them stilll stand nowadays and the two most majestic, the Garisenda and Asinelli are the symbol of the city (Figure 3).The Garisenda, the older of the two, can be dated around a the last two decades of the eleventh centuury. During the construction phases, the foundatioon soil underwent important subsidence phenomena, which caused a visible tilt of the Tower [10]. The Tower is 48 m high and has a slope of 3.22 m towards South-East. The Asinelli tower was most probably completed c in 1119 reaching a height of almost 100 m. It I tilts toward West of 2.23 m. Several strengthening interventions i were performed in the last decade (19988 - 2008) on both Towers. After the end of the works, at a the beginning of year 2011, a SSHM system was insstalled in the Two Towers.

Fig. 4. The Two Towers of Bologna: the Garisenda G Tower (left) and the Asinelli tower (right).

Fig. 5. Monitoring system of Garisennda Tower (left) Monitoring system of Asinelli Tower (right).

Figure 4 display the completee time series over year 2011-2015 with reference to thhe sensor installed at the south-west corner base of the Asinelli Tower measuring the horizontal masonry deformation (displacement between two points). No data are recorded from July to Auugust 2011due to an interruption of the system. Daata oscillates around an almost constant average valuee during the first three years and an increasing trend is observed during the last year of monitoring. For the sake of conciseness, thhe average values of the reference quantities are reporteed only with refer to the Garisenda Tower (Table 3).

Fig. 6. The complete time series over yearrs 2011-2015 recorded by the Deformometer installed at the south-w west corner base of the Asinelli tower TABLE III.

REFERENCE VALUES FOR FO OR EACH TYPOLOGY OF INSTRUMENTAL INSTALLED IN THE GAR RISENDA TOWER

GARISENDA TOWER -mean values over the entire observation period Δt (2011-2015)) Device

δ day

Σ year

rm ,day

RM , year

Long base Deformometer [mm] Deformometer [mm] Extensimeter [με] Laser displacement sensor [m] Comp.X Inclinometer[°] Comp.Y

0.06

0.45

-0.02

-0.03

0.01 22

0.08 90

0.02 -11

0.02 -11

0.010

0.044

-0.008

-0.005

0.006 0.007

0.06 0.06

0.003 0.003

0.003 0.003

slope of the Tower [11]. Thee seismic stations were located at 35 m, 70 m and onn at the top. The fourth station was installed at the basement. Although the seismic sequence in Emilia--Romagna was active during the monitoring interval but no earthquake was effectively recorded due to thhe strong seismic noise background of the city centeer. Therefore, the used dataset consists essentially of seismic ambient noise: the ground vibration induced by natural or artificial sources that propagate along thhe tower. Spectral analysis on the reccorded data allows to identify the fundamental freqquencies of the tower (Figure 3 and Table 2). The measured m ranges of the first three lateral periods are inn good agreements with the results of the numerical moodels [12]. The first three flexural frequuencies fall within the range of 0.32-0.33 Hz, 1.3-1..5 Hz and 3.0-3.3 Hz, respectively.

V. DSHM SYSTEM OF THE ASIN NELLI TOWER Beside the SSHM also data regaarding DSHM are available for the Asinelli Tower. Thhe data have been used for the dynamic identification of the Tower and for the assessment of the traffic-innduced-effects on the tower responses. It clearly appears that, from a structuural point of view, it is of fundamental importance to evaluate the mechanical and dynamic propertiess of the historical monuments, especially in the case of historical towers, which, due to their geometrical configuration, are particularly prone to seismic dam mages. In 2012, following the seismic seequence of Emilia Romagna, started on 20th May with an M 6.2 earthquake, the Istituto Nazionalee di Geofisica e Vulcanologia (INGV) was calleed by the local authorities to design an experiment for the dynamical monitoring of the Tower. Four seismic stations were insttalled inside the Asinelli Tower. All the stations weere equipped with triaxial seismometers (Lennartz Le3d5s, 0.2 Hz eigenfrequency) coupled to AD 24-bit converters (Reftek 72A/07). Data, sampled at a 200 sps, were continuously recorded from 22th June J 2012 to 17h September 2012. The location of thhe three sites was driven by considerations about the observed o structural changes along the Tower and on thee results of studies conducted by the University of Modena M about the

Fig. 7. FFT of the recorded signal TABLE IV.

EXPERIMENTALLY MEEASURED PERIODS (RANGES).

Periods

T1,f(s)

T2,f(s)

T3,f(s)

T1,t(s)

MSS

3.0-3.3

0.72-0.76

0.30-0.32

0.42-0.45

h also allowed to The dynamic monitoring has recognize the occurrence of o frequent transients propagating along the Tower that can be probably ascribed to the passage of heavy vehicles along the streets surrounding the Toower. The oscillation produces a clear effect of dam mped beating (Figure 5) whose duration and amplitudee can be related to the amplitude of the triggering siggnal. The frequency of the beat and the damping ratioo can be related to the physical and geometric propeerties of the structure, understanding the mechanism m and modelling the phenomenon can help to better b investigate the dynamical behavior of the Tow wer [13-15]. Other experimental tests are planning for the next months in order to better investigate i the trafficinduced-effects on the Towers responses.

REFERENCES [1]

[2]

[3]

Fig. 8. The free vibration response after the excitation due a signal that can presumably be associated to the transit of a bus

[4]

VI. CONCLUSIONS An approach for a standardized analysis of the data recorded by a Static Structural Health Monitoring system installed in an historical building has been presented. It has been assumed that the recorded time series may be decomposed into two fundamental components. The first one related with the natural actions and characterized, in absence of extreme events (such as explosion earthquake,…), by a substantial periodic behavior. The second one related to the evolution of the state indicating possible evolution of structural damage. Once the recorded signal is depurated from the first components (the periodic one), the eventual evolution of the state of the structure can be more easily detected. Aside the identification of the eventual evolution of the structural damage, the other main objective of the work was the introduction of “reference” quantities as descriptors of the recorded data. The reference quantities could be collected in a database and used as to evaluate the records of similar buildings to have a more reliable interpretation of the SMH data. The approach has been applied to the Cathedral of Modena and the Two Towers of Bologna for which the orders of magnitudes of the “reference quantities” have been identified. For instance, with reference to masonry and crack displacement, some average values of selected reference quantities are as follows: • daily amplitude δ of about 0.2/100 mm for the “Two Towers” of Bologna; • annual amplitude Σ of about 2/10 mm for the “Two Towers” of Bologna; • daily amplitude δ of about 1/100 mm for the Cathedral of Modena; • annual amplitude Σ of about 2/10 mm for the Cathedral of Modena. Finally, the dynamic properties and the assessment of the traffic-induced-effects on the responses of the Asinelli Tower have been evaluate trough a dynamic monitoring

[5]

[6]

[7] [8] [9] [10]

[11]

[12]

[13]

[14]

[15]

Doebling, S. W., et al., (1998) "A Review of Damage Identification Methods that Examine Changes in Dynamic Properties," Shock and Vibration Digest 30 (2). Farrar, C. R., and Doebling, S. W. (1999). "Vibration-Based Structural Damage Identification," accepted for publication of Philosophical Transactions: Mathematical, Physical and Engineering Sciences, Royal Society, London, UK Cawley, P. (1997) "Long Range Inspection of Structures Using Low Frequency Ultrasound," in Structural Damage Assessment Using Advanced Signal Processing Procedures Proceedings of DAMAS '97, Univ. of Sheffield, UK, 1-17. Doebling, S. W. and C. R. Farrar, (1997) "Using Statistical Analysis to Enhance Modal-Based Damage Identification," in Structural Damage Assessment Using Advanced Signal Processing Procedures Proceedings of DAMAS '97, Univ. of Sheffield, UK, pp. 199-210 Huang, N. E., Shen, Z., Long, S. R., Wu, M. C., Shih, H. H., Zheng, Q., ... & Liu, H. H. (1998). The empirical mode decomposition and the Hilbert spectrum for nonlinear and non-stationary time series analysis. Proceedings of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences, 454(1971), 903-995. Claerbout, J. F. (1976). "Fundamentals of Geophysical Data Processing: With Application to Petroleum Prospecting", McGraw-Hill, New York. M. Armandi). Lanfranco e Wiligelmo. II Duomo di Modena. 1984 T. Sandonni. Cronaca dei restauri del Duomo di Modena (1897-1925). Baracchi Giovanardi, O., Modena, 1983 T. Costa, "Il grande libro delle Torri Bolognesi", Studio Costa, 1984. C. Ceccoli, P. Diotallevi, P. Pozzati, L. Sanpaolesi, G. Dallavalle, "Indagini inerenti le strutture murarie e fondali e consolidamento delle parti in elevazione della Torre Garisenda". Gruppo di studio della Torre Garisenda Bologna, 2001. Bertacchini, E., Boni E., Capra A., Castagnetti C., Dubbini M. (2010). Terrestrial Laser Scanner for Surveying and Monitoring Middle Age T owers. In: XXIV FIG Congress. Sydney, Australia, 2010, vol. TS 4D - number 4445, p. 1-13, Copenaghen:FIG Federation INternational des Geometres, ISBN: 9788790907877 Ceccoli, C., Benedetti A., Trombetti T., Silvestri S., Gasparini G., Lometti A. (2011). Valutazione delle caratteristiche dinamiche e sismiche della torre degli asinelli sita in Bologna, unpublished manuscript. Trombetti, T., Silvestri, S., Gasparini, G., Pintucchi, B., & De Stefano, M. (2008). Numerical verification of the effectiveness of the "Alpha" method for the estimation of the maximum rotational elastic response of eccentric systems. Journal of Earthquake Engineering, 12(2), 249-280. Trombetti, T. L., & Conte, J. P. (2005). New insight into and simplified approach to seismic analysis of torsionally coupled one-story, elastic systems. Journal of sound and vibration, 286(1), 265-312. Palermo, M., Silvestri, S., Gasparini, G., & Trombetti, T. (2013). Physically-based prediction of the maximum corner displacement magnification of one-storey eccentric systems. Bulletin of Earthquake Engineering, 11(5), 1467