1 THE INTERNATIONAL JOURNAL OF ENGINEERING AND INFORMATION TECHNOLOGY (IJEIT), VOL.4, NO.

1,Decmber 2017

Dynamic Monitoring of Tall Buildings

Hattem Abu Sinena Ibrahim M. Abu Sinena
The University of Melbourne, Infrastructure Engineering Misurata University, Department of Civil Engineering,
Department, Parkville, Australia Misurata, Libya
hattema@student.unimelb.edu.au sinenafa@yahoo.com

Abstract - There has been an increasing demand on tall and In response to this need and driven by the advancement
high-rise buildings. In response, structural engineers have in instrumentation and data processing capabilities,
become more interested in improving the design of these dynamic testing of actual structures has evolved rapidly
newly constructed buildings as well as extending the life of in the last four decades [2]. In this regard, Experimental
the existing and aging ones. Field dynamic monitoring is the
Modal Analysis (EMA) provides the most effective way
best method that engineers can rely on to measure the
current performance of tall buildings in order to make to verify and improve the current design practice and
critical decisions regarding the improvement of their theoretical modelling approaches. Indeed, dynamic
designs or regarding the planning of their retrofitting and monitoring has matured to the point where it has often
maintenance. Radar interferometry is a novel remote become an integrated part in long-term Structural Health
monitoring technique that has appeared to be exceptionally Monitoring (SHM) programs such as the one described in
suitable for monitoring of tall buildings. However, the Burj Khalifa Project [3] and Shanghai tower [4]. Such
performance and capabilities of this system relative to other programs not only confirm the structural behaviour of
conventional sensors in not fully understood. This paper buildings, but also provide real-time monitoring of their
reviews the radar system and other commonly used sensors
current status as they become subject to more severe
with a focus on their current status and application.
A model for evaluating the relative performance of the loading events and deterioration over their service life.
different sensors for tall buildings is constructed and it Dynamic testing which is often referred to as
demonstrates that the radar has unmatched capabilities for experimental modal analysis consists of an acquisition
monitoring of high-rise buildings, The comparative case phase and an analysis phase. The whole process aims to
study on the Soul Tower, which is the first of its kind on identify modal characteristics of the structure under test,
such high-rise building, further confirms this conclusion.. namely natural frequencies, modal masses, modal
Consequently, engineers are advised to always consider damping ratios, and mode shapes which can be also
employing the interferometric radar for dynamic estimated from analytical models. In the acquisition
monitoring of tall buildings.
phase a variety of instruments (electro-mechanical,
Index Terms: Interferometric radar, Real Aperture Radar optical, radar, etc.) and techniques (single, multi-point
(RAR), Structural Health Monitoring (SHM), dynamic monitoring) can be used to record the raw physical
monitoring, accelerometers. parameters of a structure over finite time such as
acceleration, velocities, displacements, strains and forces
I. INTRODUCTION [5].
Based on their method of application, sensors can be
T here has been a worldwide rapid growth in the
construction of tall and high-rise buildings thanks to
the recent improvement in design and analysis technique
categorized into traditional contact sensors and remote
(non-contact) sensors. Accelerometers have been by far
the most traditional and popular instruments employed in
and evolution of materials. Understanding the real
the dynamic testing of buildings [6]. The recent
behaviour and performance of such complex structures is
development of wireless communication has eliminated
an imperative part in structural engineering in order to
the effort associated with their wiring when they are used
deliver a cost-effective design solution that satisfies the
in a network to capture the global behaviour of structures.
requirements of safety, serviceability and comfort for
However, their mounting process still involves
their occupants [1].
considerable difficulty that can be a prohibitive factor in
Nevertheless, there is still substantial uncertainty in
some cases.
regards to the actual performance of these structures
For this reason, the innovative remote sensing devices,
relative to the one predicted by analytical models [1]
which do not rely on physical contact with the structure,
or the scaled experimental models such as the ones
have appeared as better options to use [6]. There is a
used in wind tunnel testing.
variety of noncontact devices that employ different
techniques to dynamically measure the response of
Received 28 May 2017; revised 2 June 2017; accepted 6 Jul 2017. structures. Some devices are (a): Laser based such as
Scanning Laser Doppler Vibrometer (SLDV),
Available online 7 Jul 2017. Velocimeters and Light Distance and Ranging device

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 Hattem Abu Sinena and Ibrahim Abu Sinena/ Dynamic Monitoring of Tall Buildings 2

(LiDAR); (b): vision based such as Digital Image

Correlation (DIC) and dynamic photogrammetry; (c):
microwave based as in the interferometer radar.
Application testings have demonstrated that the
aforementioned devices have varied level of applicability
for tall buildings. Limitations include their point wise
approach of measurements, insufficiently short range, (a) 1. System
Figure setup (wired)
Conventional (b) Accelerometer (c) Data
Accelerometer System andacquisition
its
poor measurement resolution and the dependence on Components [8] device
weather conditions. Moreover, most of these devices
require a special surface preparation or installation of
reflectors which subsequently negate the benefit of their
remote use [7]. In contrast, interferometer radar seems
not to suffer from all these limitations and appears to be
an exceptionally suitable measurement system in this
field. This monitoring instrument, which has recently
(a) Individual USB (b) Smart wireless sensor
emerged and become commercially available, has a great interface
potential of being widely adopted as civil engineering Figure 2. MEMS Based Accelerometers [10]
tool in the future. The aim of this research is to evaluate
Buildings and civil structures in general are
the performance and applicability of the interferometer
characterised by limited frequency range (as low as
radar in comparison with other sensors that are
0.1Hz) which translates into low amplitude of
commonly adapted for monitoring of tall buildings.
acceleration specially if the vibration was under low
ambient loads [11]. Consequently, high-sensitivity
II. LITERATURE REVIEW accelerometers with exceptional low frequency
High-quality measurements represent the first characteristics such as piezoelectric and servo transducers
elementary step for a successful dynamic monitoring. are the ideal choice [12]. Low level of vibration (in terms
High precision sensors are preferred as they can of micro-g) can currently be measured by the high-end
effectively monitor the dynamic response of a structure wired accelerometers that are characterised by higher
with less excitation force. Here we review the principals, size, weight and cost. However, one should bear in mind
application and factors affecting the performance of the that the monitoring quality not only depends on the
different monitoring systems; namely accelerometers, resolution of the transducers, but also on the mechanical
inclinometer, GPS and the interferometric radar when and electrical noise from the whole instrumentation chain
used for high buildings. The review does not extend into including cables, amplifiers an data acquisition system,
quantifying financial factors but it is focused on the and undesired ambient interference including thermal,
practical and technical aspects. acoustic, electromagnetic and motion noise [13].
In regards to MEMS and WSN accelerometers, most
A. Accelerometers of their commercial models have serious limitations to be
Accelerometers are the most traditionally used used for buildings as reported by Velez [8], Haritos [14]
vibration sensors in many fields including civil and Nagayama & Jr [11]. Their transducer’s low
engineering due to their relatively low cost and high resolution is the biggest issue that limits their use to
sensitivity [8] . Their conventional modal testing setup vibrations over 20mg which is improbable to occur in
(Figure 1) consists of a number of transducers wired to a buildings. Another factor that contributes to their low
data acquisition device which is in turn connected to a resolution is the embedded Analog Digital Converter
computer that record and process data. The transducers (ADC). Velez [8] developed a prototype of tri-axial
are usually biaxial or tri-axial accelerometers to monitor MEM accelerometer that addresses all these issues. With
vibrations in more than one direction and each axis a minimum resolution between 1 and 0.1mg they
represents a channel. demonstrate successful application in moderate to low
Recent advancement in digital circuitry has led to the vibration scenarios in buildings.
emergence of MEMS (Micro Electro-Mechanical B. Inclinometers and GPS
Systems); a new generation of accelerometers that are Commercially available inclinometers measure tilt
designed to collect, analyse and store or transfer dynamic angle of a mounted sensor relative to the horizon by opt-
data as one unit [9]. The integration of MEMS with electronic means. The inclination measurements are
wireless communication to form a Smart Wireless Sensor simultaneously taken in dual-axes with an accuracy down
(Figure 2) was first realised in 1999. These sensors can to micro-radian precision (0.001mm/m) and sampling
remotely and simultaneously connect to a base station to frequency of 10 Hz while connected to computer [15].
form Wireless Sensor Network (WSN). These measurements can be converted into dynamic
displacements with sub-millimetre levels of accuracy

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3 THE INTERNATIONAL JOURNAL OF ENGINEERING AND INFORMATION TECHNOLOGY (IJEIT), VOL.4, NO.1,Decmber 2017

based on structural models or by relating it with other Monitoring Program were instrumented with GPS and
displacement sensors such as GPS [16]. accelerometers [26], while Shanghai tower incorporated
inclinometer as well for its in-construction and in-service
SHM [4].
C. Real aperture radar
The application of radar in the field of civil
engineering was first demonstrated on a bridge by Farrar,
Darling, Migliori and Baker [27]. The technique was
PC connection based on the interferometry principle, measuring the
dynamic displacement by detecting phase shift of the
backscattered microwaves by a novel coherent radar
Figure 3. Inclinometer - Leica Nivel 210
sensor. In 2004 Pieraccini et al. [28] tested an improved
Transmission system that utilises another principle, namely Stepped
antenna Frequency Continuous Waveform (SF-CW). Henceforth,
PC
connection such system is frequently called coherent Real Aperture
Radar (RAR). The improved system provided the radar
with a range resolution that makes it capable of
measuring the response of several targets simultaneously.
The new technology was developed by the Italian
company IDS in collaboration with the University of
(a) Reference station (b) Roving receiver Florence and was named IBIS-S (Image By
Interferometric Survey of Structures) [29]
Figure 4. GPS Components [17] The most prominent advantage of the interferometer
Global Positioning System (GPS) has long been used radar underlies in its remote monitoring capability. The
for static monitoring of civil engineering structures that device can reliably perform its remote measurements
are subject to settlement, thermal expansion and other without a reflector in almost all cases, thus saving a great
long-term displacement trends. The advent of real-time amount of time and cost associated with the mounting of
kinematic (RTK) surveying technique has made GPS the alternative contact sensors. Furthermore, the
usable for dynamic monitoring. RTK technique utilizes a capability of the device to simultaneously monitor more
reference station (Figure 4) and the phase of signal than one point in its field of view makes it useful in
carrier’s wave to pinpoint, correct and fast track the 3D capturing the overall behaviour of a large structure [30].
coordinates of a roving receiver [18]. Current technology In addition, rather than deriving displacements from
is able to measure the dynamic displacement at sampling acceleration data which often come with considerable
rate of 20Hz or more. In best cases it has ±10mm errors [31], the RAR provides a direct measurement of
accuracy while the best estimate of its resolution is about this interesting engineering parameter. Interestingly, the
3mm in the horizontal plane [19]. measured displacement has an accuracy in orders of
In the last decade, many researchers have investigated sub-millimetre regardless of the monitoring distance
the quality and feasibility of using GPS for continuous and weather conditions while the range can cover up to
dynamic monitoring applications of high-rise buildings several centimetres allowing to monitor structures with
and they had varied outcomes as found in the literature varied degree of flexibility.
[16], [20]–[25]. Major issues includes limited The radar (shown in Figure 5) is commercially
displacement resolution, particularly when good satellite implemented as portable equipment supported by a tripod
geometry is not available, communication issues with and powered by a battery pack. The management of the
base station and most importantly signal noise due to the device is facilitated by system management software
multi-path effect in urban areas.. Nevertheless, all reports preinstalled in an auxiliary portable computer. The
confirm that GPS is accurate enough for monitoring software is also capable of showing real time response
response of high-rise buildings when displacement and performing modal analysis on stored data. Table 1
amplitude is adequately high (as during major earthquake lists the key operational characteristics of the radar.
and windstorm events).
The greatest advantage of the GPS resides in its
capability to measure the static and quasi-static
components of structure’s response to wind which cannot
be otherwise recovered by accelerometers or inclinometer
[22]. This explains why GPS was deployed on the rooftop
of several high-rise buildings in combination with other
precise sensors such as accelerometers and inclinometers.
For example three towers of the Chicago Full-Scale

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 Hattem Abu Sinena and Ibrahim Abu Sinena/ Dynamic Monitoring of Tall Buildings 4

together to evaluate the accuracy of the radar results [33],

[36]. All filed tests confirmed the applicability and
accuracy of the RAR.
Luzi, Monserrat, & Crosetto [37] suggested that SNR
of 70dB or more is required in order to measure vibration
amplitude in the order of 0.01mm. The SNR received
back from an illuminated area of a building is strongly
related to its geometry and the dielectric characteristics of
its surface [38]. As illustrated in Figure 6, the presence of
geometric discontinuities can improve the level of
reflected echo at higher observation angles; however the
SNR is still expected to be lower than the ones obtained
at lower observation angles. It should be highlighted that
the best monitoring scenario for a building usually
Figure 5. IBIS-S Microwave Interferometer [32] involves its upper part as this part exhibits much greater
Table 1. Main Characteristics of IBIS-S displacement response and hence should be the easiest to
measure. However, another complication of the higher
Operating frequency 17.2 GHz (Ku band) observation angles is that the radial component of
displacement (dLOS) can be too small to detect. In this
Max. operating distance (Rmax)
500 m respect, Luzi et al. [6] showed that an observation angle
(@ 40 Hz sampling frequency)
up to 70 degrees was satisfactory in the close radar range
Radiofrequency bandwidth (B) 300 MHz
for certain buildings.
Nominal displacement sensitivity dLOS 0.01 mm
Max. sampling frequency 200 Hz
Sampling interval t 5 ms
Weight of the whole system 12 kg
Max sampling window 5 mins
Max range resolution (R) 0.5m
Antennas half power beam-width 0.18 rad
(Pyramidal horns) (3m2 at 10m)

The elementary sampling volume of a radar

measurement is called a radar bin and it is related to the
field of view (FOV) of the antennas and to the radar Figure 6. SNR Strength and FOV of the Radar
range resolution [33]. Basically any two objects located
in the same bin cannot be individually distinguished. The The SNR measured by the device is called thermal
radar identifies objects on the basis of their measured SNR as it pertains only to the instrumental noise and does
range rather than their angles. Similarly, only not include the clutter generated by other vibrating object
displacements along the line of sight (dLOS ) can be in the same radar bin [29]. Therefore, façade elements
measured. that vibrate autonomously rather than coherently with the
The monitoring procedure of an ordinary building building would have their contribution blended with the
using the RAR involves positioning the radar in the front selected bins causing a dramatic distortion of the
of the investigated structure and orientating it towards the sampling quality. Therefore, high thermal SNR values do
top of the building. The radar then generates a signal-to- not always guarantee high quality of vibration monitoring
noise ratio (SNR) profile for the range bins. From there for the object of interest. The presence of unwanted
the user can select multiple points with the highest SNR spurious vibrating targets can drastically affect the
values to record their displacement-time history. Later monitoring results as was reported by Pieraccini, Dei,
this recorded data undergoes modal analysis so that Mecatti, & Parrini [39] when they failed to monitor the
modal characteristics of the building under testing can be San Gimignano Tower due to vegetation growth on its
estimated. walls.
The literature review has revealed a number of
interesting recent studies to evaluate the radar’s III. METHODOLOGY
performance on buildings, bridges, chimneys, masts and
wind turbines as summarised by Massimiliano Pieraccini The literature review has identified some existing gaps.
[34]. The height of observed buildings in the evaluation Engineers are often faced with the task of selecting an
campaign ranges from 20 meters [6] to 94 meters [35]. appropriate dynamic monitoring instrumentation scheme
In some cases, other conventional sensors were deployed for tall buildings. The selection of sensors is often based

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5 THE INTERNATIONAL JOURNAL OF ENGINEERING AND INFORMATION TECHNOLOGY (IJEIT), VOL.4, NO.1,Decmber 2017

on experience and applicability aspects. The performance

aspect of sensors, however, can be very critical yet not
fully understood due to the different dynamic parameters | A | 12U ---- eq (1)
of the sensors used. Where: 1   f 1 = fundamental angular frequency of
Higher performance sensors are capable of extracting the structure
dynamic properties of a monitored structure under lower
excitation. This is particularly important for EMA of There are several empirical formulas to roughly
constructed buildings, as monitoring is often performed estimate the fundamental natural frequency (f1). The
under AVT and wind speed has to be adequately high for Australian and New Zealand Standard AS/NZS 170.2
sensors to detect buildings response. To put this into formula can be used:
perspective, these measured responses such as 46
accelerations and displacements are approximately f1  --- eq (2)
proportional to the cube of the wind speed [40]. This H
illustrates the great influence wind speed can have on the Where: H = building height in meters
success of EMA. f1 is expressed in (sec-1)
The objective here is to develop full understanding of
the performance of all monitoring systems reviewed
The displacement amplitude along building’s height
earlier with respect to the height of tall building using a
u(y) can be approximately estimated using eq (3):
theoretical approach. In addition and similar to the

approach widely adopted in the literature, an y 
u  U   ---eq (3) [41]
experimental case study of high-rise building monitored H 
by different system will be presented for evaluation.
Where: y= floor height
A. Theoretical model =1.5-2 for cantilever buildings (such as ones
Accelerations measured by accelerometers are not with shear cores)
homogenous with the units measured by displacement- The relationship between displacement amplitude in
based sensors such as RAR and GPS, neither with the tilt the top floor (U) and the corresponding tilt amplitude ()
angles measured by inclinometer. Therefore, we need to can be found by taking derivative of eq (3) using the
find an approximate relationship between all these units lower boundary  =1.5 :
based on theories of structural dynamics. The minimum U
amplitude of acceleration that can be appropriately   u ' (H )  1.5 ----eq (4)
detected by accelerometers needs to be defined based on H
an extensive examination of the available literature and B. Case study
products specifications.
According to Li [22] the main components of a The best case for dynamic monitoring of high-rise
structure’s displacement response to wind are the static buildings was found in the Soul tower described by
component caused by mean wind force and the resonant Barnes, Lee, & Papworth [42]. The tower is located in
component which corresponds to structure’s natural the Gold Coast and comprises of 77 storeys. It was
vibration mode. Figure 7 illustrates this on a building of monitored with multiple dynamic sensors during its
height (H) being subject to dynamic wind loads (F(t)). For construction in late 2010 as it was approaching 200m
the resonant component the structure can be simplified height. Verifying the dynamic properties of the tower was
into a single-degree of freedom model that vibrates in its critical at that stage due to its exposure to coastal winds
first transitional mode. The relationship between and the strict habitability requirements for its residents.
displacement amplitude (U) and acceleration amplitude Figure 8 and Figure 9 illustrate the monitoring scheme on
(A) of the top floor is: the building.
U
D=F/K
F(t) M A
u

= + 

y


quasi-static dynamic
component component
Figure 7. Wind Response Mode

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 Hattem Abu Sinena and Ibrahim Abu Sinena/ Dynamic Monitoring of Tall Buildings 6

IV. RESULTE, ANALYSIS AND

GPS + N FINDINGS
Inclinometer
Y
RAR A. Theoretical model
106 X
A defined precision applicable to common
accelerometers can be established based on the
experimental research carried out by Foss [43] and Velez
[8]. Those experiments were part of two separate
researches to establish the noise floor and relationship
between resolution and detectable acceleration for most
common accelerometers. Here we adopt 0.04mg and 1mg
as the lowest detectable amplitude of acceleration for
conventional accelerometers and MEMS based
accelerometers correspondingly.
It is also useful to put these minimum detectable
Figure 8. Building Monitoring Scheme acceleration amplitudes into perspective with the upper
boundaries expected in tall buildings. Motion perception
at top occupied floors is a design parameter that often
governs the design for high rise buildings [40], [44].
Examining the design practice [44] the lowest perception
threshold for is found to be 5mg of peak acceleration
( with less than 10% probability of being exceeded in any
given year).
All acceleration amplitudes can be approximately
converted into equivalent displacements using eq (1) and
eq (2). The results are function of building height. For tilt
angles, the detectable amplitude for Leica Nivel 220 is
found to be around 0.005mrad with a resolution of
0.001mrad. Using eq (4) one can obtain the equivalent
detectable displacement as a function of height. In
addition 10mm and 0.2mm amplitude of displacements
can be adequately monitored by GPS and RAR
respectively.
Figure 10 shows the developed graph model. For any
Figure 9. View from the Observation Point
given building height, sensors that are lower in the graph
Instead of relying on ambient wind, the test was are expected to perform better under the same conditions.
carried out with forced excitation utilizing the three
GPS (10mm)
erected tower cranes by performing a start-stop loading
sequence with various combinations of weights, positions Motion perception
(5 mg)
and timings to capture all major vibration modes of the Wired accelerometers (0.04 mg)
building. Error! Reference source not found. shows the
adopted excitation and monitoring scheme. Remote Mems accelerometers (1 mg)

monitoring was taken by RAR at106.8m positioned at the Tiltometer

west side of the building. The biaxial inclination sensor (0.005 rad)
1000
Leica Nivel 220 and Leica GPS rover were mounted on
Top floor displacement (mm)

the tip of the shear walls at 182.8m above the ground. 100
All data were supplied in form of graphs as
measurements were processed into the frequency domain. 10
The classical frequency domain peak-picking method is
to be applied to extract modal frequencies from each 1
measurement for comparison. The method is based on the
theory that the amplitude spectra of a structure have 0.1
peaks at its natural frequencies and the assumption is that
the structure is excited with a broadband white noise 0.01 Building height (m)
(random excitation frequencies). 25 50 100 200 400 800

Figure 10. Sensors Performance Model

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7 THE INTERNATIONAL JOURNAL OF ENGINEERING AND INFORMATION TECHNOLOGY (IJEIT), VOL.4, NO.1,Decmber 2017

It can be seen that inclinometer and RAR most often us to create a comprehensive sensor performance model
perform better than MEMS based accelerometers. Low with respect to tall buildings. The model demonstrates
noise wired accelerometers only outperform RAR if that besides its ease of use, the radar is exceptionally
building height is less than 200m. Another remark is that powerful for taller buildings and can easily outmatch the
GPS is only useful for dynamic monitoring of buildings performance of all other commonly used sensors for
higher than 200m and they can outperform MEMS buildings over 200 meters. The comparative case study
accelerometers for buildings higher than 400m. on the Soul Tower has supported the theoretical model
and confirmed the accuracy of this instrument.
B. Case study
The high performance of the real aperture radar is
For the radar observation 6 bins with high SNR and conditional on high echo signal and this requires a careful
interesting range are selected for analysis. By considering setup of the observation geometry with a minimum offset
the observation geometry and their range, each bin can be space. In addition, spurious vibrating elements in the
associated with a building height (y). Bins vibration same view range should be avoided. With respect to dual-
measurements are already transferred into the frequency axis sensors, the only shortcoming identified in the radar
domain and some peaks corresponding to natural is the need to reposition the device to monitor the
frequencies of the building can be clearly identified from building in the other direction and the difficulty in
the peaks. identifying torsional modes. Nevertheless, the
Unlike RAR, which only measures response in its interferometric radar should always be considered as the
direction, these biaxial sensors provide more information first option for dynamic monitoring. Other contact and
about the directional components of vibration modes. invasive sensors might only be more suitable for long
Due to the complex plan shape of the building we can term structural health monitoring.
observe coupled transitional and rotational modes. The
identified modal frequencies obtained from each sensor RESOURCES
are presented in table 2. There are good agreements
between all sensors with discrepancies less than 5%. The IBIS-S interferometric radar and its management
software is supplied by industry partner organisations
Table 2. Modal frequencies Obtained from Inclinometer, GPS and
(IDS Ingegneria Dei Sistemi) in collaboration with the
RAR
Department of Geomatics at the University of Melbourne.
Natural frequency (Hz) All data and observation graphs for Soul Tower were
Mode shape
Inclinometer GPS RAR (Y) obtained from the experimental study of Barnes et al.,
0.26 (X+Y) 1st torsional
[42].
0.29 (X+Y) 0.29 (X+Y) 0.3 1 transitional (X’)
0.32 (X+Y) 2nd torsional ACKNOWLEDGMENT
0.37 (Y+X) 0.38 1 transitional (Y’)
st
The authors wish to thank Misurata University/ Libya
0.61 (Y) 2nd transitional (Y) for giving such an opportunity for publication.
0.67 (X) 0.64 2nd transitional X
0.75 (Y) 2nd transitional Y REFERENCES
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