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IBIS-S
STATIC AND DYNAMIC BRIDGE MONITORING BY RADAR IMAGING OF BRIDGE DISPLACEMENTS AND VIBRATIONS
ImagIng By InterferometrIc Survey
Olson Engineering worked in close cooperation with the University
of Minnesota to demonstrate the IBIS-S technology on the Minnesota
DOT’s St. Anthony Falls Bridge on I-35W in Minneapolis, Minnesota.
Comparisons will be made in the near future of controlled load test
results between the many embedded sensors in this bridge and
the IBIS-S displacement measurements of the southbound bridge.
(in the foreground above).
Olson Engineering provided a demonstration of the IBIS-S on a
girder of a bridge in northern New Jersey that is being studied
as part of the FHWA’s Long Term Bridge Performance Monitoring
project which is being conducted by Rutgers University and
Parsons Brinckerhoff. The results of controlled load tests by Drexel
University with string potentiometer displacement measurements
and the IBIS-S displacement measurements of the bridge girder will
be compared in the near future as part of this research.
FHWA’s Long Term Bridge Performance Monitoring Project
St. Anthony Falls Bridge, Minneapolis, Minnesota
Recent Olson Engineering IBIS-S Projects
3
IBIS-S is an innovative microwave radar sensor,
developed by IDS Georadar of Pisa, Italy in collaboration
with the Department of Electronics and Telecommunications
of Florence University. It is able to simultaneously measure
the displacement response of multiple points belonging to
a structure with an accuracy on the order of a hundredth
of a millimeter (0.0004 inch). Vibration frequencies can be
measured from 0 up to a maximum of 100 Hz.
IBIS-S can be used to remotely measure structural static
deflections as well as vibration displacements to identify
resonant frequencies and mode shapes. In addition to its non-
contact feature, the new displacement vibration measuring
system provides other advantages including quick set-up time
and a wide frequency range of response and portability.
Throughout this brochure, Olson is highlighting various case histories on IBIS-S projects performed
by IDS exclusively, by Olson Engineering exclusively, and those projects performed jointly for various
demonstrations within the USA. Geotechnical engineers may also be interested in the IBIS-L for landslide/
slope stability monitoring. Visit our website at www.OlsonInstruments.com to learn more about our
infrastructure instruments for structural health monitoring (SHM), nondestructive evaluation (NDE)
and seismic geophyical instruments for sale, or visit www.OlsonEngineering.com which specializes in
“Imaging the Infrastructure for Assessment, Monitoring & Repair”. Training is available by one of our
Olson Engineering specialists. Olson is celebrating over 26 years in business and would like to be your
bridge infrastructure specialist for any project, now or in the future. We are located in Wheat Ridge,
Colorado USA — a suburb NW of downtown Denver, CO.
If you are interested in purchasing an IBIS-S System or any of our other instruments, contact:
email: [email protected]
website: www.OlsonInstruments.com
phone: 303.423.1212
If you are interested in Olson’s Professional Consulting Services, contact:
email: [email protected]
website: www.OlsonEngineering.com
phone: 303.423.1212
Table of Contents » IBIS-S Case HistoriesImaging by Interferometric Survey — IBIS-S
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IBIS-S Components
The IBIS-S system is based on interferometric and wide band waveform principles. It is composed of a sensor module, a control PC and a 12 volt power supply unit. The sensor module [A] is a coherent radar unit, generating, transmitting and receiving the electromagnetic signals to be processed in order to compute the displacement time-histories of measurement points. The system has a maximum operational distance (for minimum 40Hz vibration sampling frequency) of 500 m (1640 ft), a maximum sampling frequency of 200 Hz, (permits measuring vibration frequencies up to 100 Hz) and a displacement sensitivity of up to 0.01 mm (0.0004 in). It can measure static displacements up to a distance of 1000m (3280 ft). The IBIS-S system is extremely portable with the entire system weighing less than 100 lbs. The system can easily be rapidly deployed and can be operated in all weather conditions.
The sensor module, including two airhorn antennas [A] for transmission and reception of the 17.1 MHz electromagnetic waves, exhibit a typical super heterodyne architecture. The base-band section consists of a Direct Digital Synthesis (DDS) device to obtain fast frequency hopping. A tuneable sine wave is generated through a high-speed D/A converter, reading a sine lookup table in response to digital tuning and a precision clock source. The radio-
frequency section radiates at a center frequency of 17.2 GHz with a maximum bandwidth of 200 MHz; hence, the radar is classified as Ku-band, according to the standard radar-frequency letter-band nomenclature from IEEE Standard 521-1984. The system was fully approved for use at any site across the USA and its territories in February, 2011 with the operator of the system only requiring a general FCC license for its use. A final calibration section in the module provides the necessary phase stability. Design specifications on phase uncertainty are suitable for measuring short-term displacements with a range uncertainty lower than 0.01 mm (0.0004 in) and an intrinsic accuracy of 0.001 mm (0.00004 inches). The sensor module is installed on a tripod equipped with a rotating head [C], allowing the sensor to be orientated in the desired direction. The module has a USB interface for connection with the control PC [B] and an interface for the power supply module. The control PC (Panasonic Toughbook CF-19) includes the software for system management and is used to configure the acquisition parameters, store the acquired signals, process the data and view the initial results in real time. The system can be powered by any 12 volt battery.
introduction
Imaging by Interferometric Survey [IBIS-S] Basic Description »
[A] Sensor Unit:
■ Signal Transmitter and Receiver
■ Viewfinder
■ Airhorn Antenna
- Additional antenna’s for narrow to wide views and vertical to horizontal
[B] Processing Unit:
■ Control PC with Management Software
■ Parameter Setting:
- Signal Generator
- Signal Acquisition
■ First Result Rendering
[C] Tripod and 3-D Rotating Head
[D] Power Supply Unit (not shown)
[A]
[B] [C]
Airhorn Antenna
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Imaging by Interferometric Survey [IBIS-S] Basic Description »
introduction
IBIS-S Basic Principles
Generally speaking, the IBIS-S sensor module continuously emits from one airhorn and receives in the other airhorn antenna a series of discrete electromagnetic waves for the entire measurement period, and processes phase information at regular time intervals (up to 5 milliseconds, ms) in order to identify any displacement occurring between one emission and the next. The interferometric technique provides a measurement of the line-of-sight displacement of all the reflectors on the structure illuminated by the antenna beam simultaneously that are more than 0.75 m (2.46 ft) apart. Once the line-of-sight or radial displacement d
p has been evaluated,
the vertical displacement d can be easily calculated by employing a geometric projection, as shown in diagram below (Fig.2).
The ability to determine range (i.e. distance) by measuring the time for the radar signal to propagate to the target and back is the distinguishing and most important characteristic of the radar system. Two or more targets, illuminated by the radar, are individually detectable if they produce different echoes. The range resolution is a measure of the minimum distance between two targets at which they can still be detected individually. The bin range resolution refers to the minimum separation that can be detected along the radar’s line of sight and thus the ability to distinguish different targets at different distances away from the unit which is 0.75 m (2.46 ft). This capability is obtained by using the Stepped-Frequency Continuous Wave (SF-CW) technique. Pulse radars use short time duration pulses to obtain high range resolution. For a pulse radar, the range resolution, ∆r, is related to the pulse duration τ by the following (1):
(1)
where c is the speed of light in free space. Since τ = 1/B (the bandwidth in this case is 200 MHz), the range resolution (1) may be expressed as:
(2)
Eq. (2) highlights that range resolution increases (corresponding to a smaller numerical value of ∆r) as the frequency bandwidth of the transmitted electromagnetic wave increases; hence, closely spaced targets can be detected along the radar’s line of sight. The SF-CW technique exploits the above concept to provide the IBIS-S sensor with excellent range resolution capability of 0.75 m (2.46 ft).
The SF-CW technique is based on the transmission of a burst of N monochromatic pulses, equally and incrementally spaced in frequency (with fixed frequency step of ∆f), within a bandwidth B:
fNB ∆−= )1( The N monochromatic pulses sample the scenario in the frequency domain similarly to a short pulse with a large bandwidth B. In a SF-CW radar, the signal source dwells at each frequency fk = fo + k∆f (k=0,1,2, …, N−1) long enough to allows the received echoes to reach the receiver. Hence, the duration of each single pulse (T
pulse) depends on the maximum distance (R
max) to be observed in
the scenario:
cR
T maxpulse
2≥ In the IBIS-S sensor, the SF-CW radar sweeps a large bandwidth B with a burst of N single tones at uniform frequency step, in order to obtain a range resolution of 0.75m (2.46 ft); in other words, two targets can still be detected individually by the sensor if their relative distance is greater than 0.75 m (2.46 ft). The range resolution area is termed range bin. The radar continuously scans the bandwidth at a sampling rate ranging up to 200 Hz, so that the corresponding sweep time ∆t of 5 ms is in principle, well suited to provide excellent waveform definition of the displacement response of typical civil structures.
At each sampled time instant, both in-phase and quadrature components of the received signals are acquired so that the resulting data consists of a vector of N complex samples, representing the frequency response measured at N discrete frequencies. By taking the Inverse Discrete Fourier Transform (IDFT) the response is reconstructed in the time domain of the radar: each complex sample in this domain represents the signal (echo) from a range (distance) interval of length cT
pulse/2.
The amplitude range profile of the radar echoes is then obtained by calculating the magnitude of each bin of the IDFT of acquired vector samples. This range profile gives a one dimensional map of scattering objects in the viewable space in function of their relative distance from the equipment. The concept of range profile is better illustrated in Figure 3, showing an ideal range profile obtained when the radar transmitting beam illuminates a series of targets at different distances and different angles from the system. The peaks in the lower plot of Figure 3 correspond to “good” measurement points and the sensor can be used to simultaneously detect the displacement and the transient response of these points. These reflective points could be either given by the natural reflectivity of features belonging to the structure or by some simple passive metallic reflectors mounted to it. Once the image of the scenario illuminated by the radar beam has been determined at uniform sampling intervals ∆t, the direct line-of-sight displacement response of each target detected in the scenario is evaluated by using the Differential Interferometric technique (see eq. 5, page 6). This technique is based on the comparison of the phase information of the back-scattered electromagnetic waves collected at different times.
2τc
r =∆
Bc
r2
=∆
rh
d
dp
α
rh
d
dp
α
Figure 2. Radial displacement vs. projected vertical displacement.
(3)
(4)
6
Imaging by Interferometric Survey [IBIS-S] Basic Description »3
introduction
Figure 3. Range bin resolution concept diagram, —0.75 m per bin.
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When a target surface moves with respect to the sensor module (emitting and back-receiving the electromagnetic wave), a phase shift arises between the signals reflected by the target surface at different times. Hence, the displacement of the investigated object is determined from the phase shift measured by the radar sensor at the different discrete acquisition times. The radial displacement d
p
(i.e. the displacement along the direction of wave propagation) and the phase shift ∆ϕ are linked by the following:
(5)
where λ is the wavelength of the electromagnetic wave radar signal.
Monitoring Applications of the IBIS-S Radar System
Static Monitoring: Dynamic Monitoring:
Structural Load Testing Structural resonance frequency measurement
Structural displacement and collapse hazards Structural modal shape analysis
Cultural heritage preservation Real time monitoring of deformation
Advantages over Traditional Methods
» Radar for interferometric imaging of bridge displacements in load tests up to an accuracy of 0.0004" with modal vibration measurements and analysis (0 to 100 Hz)
» Real-time simultaneous mapping of deformations
» Fast installation and operation
» Stactic and dynamic monitoring
» Structural vibration sampling up to 100 Hz
» Autonomous operation; 24/7 — in all weather conditions!
» Provides direct line of sight displacements, not derived quantities, in one dimension
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Table of Contents » IBIS-S Case Histories
Type of Structure: Test for: Page
A.Bridges
A1. Olginate Cable-stayed Bridge [Italy] Vibration Monitoring of Stay Cables for Tension Force Balancing 8-9
A2. Bordolano Cable-stayed Bridge [Italy] Dynamic Measurements on the Forestays of a Cable-stayed Bridge 10-11
A3. Highway Flyover Bridge, CO [USA] Static Displacement and Dynamic Vibration Monitoring of a Concrete Bridge 12-13
A4. Manhattan Bridge, NY [USA] Static Displacement and Dynamic Vibration Monitoring of a Steel Bridge 14-15
A5. Capriate Bridge [Italy] Ambient Vibration Testing for Modal Analysis 16-17
A6. Kuranda Scenic Railway Bridge [AU] Dynamic Monitoring of Vertical Displacements Caused by a Train 18-19
Flag Legend: Case Histories performed by IDS Georadar, Pisa, Italy
Case Histories performed by Olson Engineering, Inc. [USA] and IDS Georadar
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Figure 2. IBIS-S position in the survey for the downstream surveys.
Testing Procedure
Figure 1. (a) Elevation and typical cross-sections of the bridge (dimensions in m). (b) First array; testing on the downstream side.
Vibration Monitoring of Stay Cables for Tension Force Balancing »
Project: Cesare Cantú cable-stayed roadway bridge
Project Location: Roadway bridge crossing the Adda River, Italy
Bridge Construction: Pre-stressed concrete deck, formed by a central span of 110 m and two lateral spans of 55 m. The deck is suspended from 24 pairs of cable-stays, arranged in a semi-fan and connected to two H-shaped reinforced concrete towers, reaching the height of approximately 38 m above the foundation. Elevation and plan views are shown in Figure 1.
Project Scope: The measurement of vibrations on one array of stay-cables was performed by simultaneously using conventional piezoelectric accelerometers and the IBIS-S radar sensor, in order to demonstrate the effectiveness of microwave remote sensing and its accuracy in terms of both natural frequencies and cable tension forces.
After the main phase of cable tensioning, vibration measurements were carried out on all cables of the bridge by using conventional accelorometers (WR, model 731A) to check the tension forces. In addition, the global dynamic characteristics of the bridge were determined by ambient vibration testing (AVT), in order to optimize the subsequent phase of adjustment of cable forces. Prior to opening the bridge to traffic, more extensive AVT of the deck and towers were carried out as part of the bridge reception tests. Next, dynamic measurements on two arrays of cables were carried out by simultaneously using piezoelectric accelerometers and the IBIS-S radar system in order to verify the reliability and accuracy of the radar technique.
During the tests, the ambient excitation was mainly provided by two 2-axle trucks with 340 kN gross weight each, crossing the bridge with symmetric and eccentric passages and speeds in the range of 10-40 km/hr. In the test of the first array, cables S'
07 - S'
12 (Fig.1b),
the IBIS-S was placed at the base of the tower on the Calolziocorte side (Fig.2) and inclined at 55° upward; accelerometer and radar data were acquired simultaneously at a rate of 200 Hz over a period of 1700 s. The range profile of the test scenario is presented in Fig. 3; it is observed that after some close and neighboring peaks around the range of 10 m, corresponding to the concrete transverse beam providing the anchorage for cable S'
07, five well-defined peaks clearly
identify the positions of cables S'08
- S' 12
.
Aerial view of the “Cesare Cantù” cable-stayed bridge
Array 1
Downstream —
Array 1
a
b
A1
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A1
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Figure 3. Range profile of the test scenario.
Vibration Monitoring of Stay Cables for Tension Force Balancing »
The analysis of the results obtained by the radar sensor first included the qualitative inspection of the deflection measurements. Figure 4a shows an example of displacement time-history measured on the investigated stay-cable S'
12; it should be noted that, as it has to be
expected for a stay-cable, the fundamental period is clearly detected in the displacement signal. Although radar and conventional measurements refer to different points of the cable (Fig. 1b), the comparison between the time-histories simultaneously recorded by the two measurement systems provides valuable information. An example comparison is given in Figure 4b, where the acceleration computed by twice differentiating the displacement obtained from the radar is compared to the one simultaneously recorded by the accelerometer. It can be observed that the two time series, which refer to two different points almost symmetrically placed with respect to the midpoint of the stay-cable, exhibit the same time evolution and very similar amplitudes. To further enhance this point, Figure 4b is superimposed over Figure 4a, showing the near perfect similarity between the IBIS-S and the accelerometer data.
Although the ASDs of Figure 5 are associated with different mechanical quantities measured (displacement and acceleration) and to different points of the stay cable, the spectral plots clearly highlight an excellent agreement in terms of local natural frequencies of the cable, marked with the vertical dashed lines, and are characterized by equally spaced and well-defined peaks in the investigated frequency range. Global natural frequencies of the bridge-identified in the bridge dynamic survey and corresponding to peaks of the ASDs placed at 0.76, 1.25, 1.66, 1.90 and 2.78 Hz—are also apparent in Figure 5 which also shows that a linear correlation exists between the mode order n and the corresponding natural frequency fn of stay cable S’
12. Hence, the
tension force T can be obtained from natural frequencies and cable properties. The application of the taut string tension force equation, T(f
n) = 4rL2(f
n/n)2 to the cable S’
12 resonant frequency results
(cable length L = 57.24 m, cable weight/m r = 36.51 kg/m and resonant modes 1-5) is summarized in Table 1. The estimates of cable tension obtained from resonant frequencies of the accelerometer and radar sensor are virtually equal. A tension force of 2694 kN was measured by a load cell which compares very well with the average force of 2689 kN for the IBIS-S.
Figure 4. (a) Typical displacement time-history (blue) by the IBIS-S on cable S'12 with (b) typical acceleration data overlaid on top (red).
a
b
Figure 5. Auto-spectrum displacement data comparison of the IBIS-S radar (orange) and accelerometers (blue)on cable S'12
sensort(f
1)
(kn)t(f
1)
(kn)t(f
1)
(kn)t(f
1)
(kn)t(f
1)
(kn)average
accelerometer 2679 2707 2660 2693 2690 2686
iBis-s radar 2679 2707 2679 2693 2690 2689
Table 1. Tensions in cable S'12 obtained from accelerometer and IBIS-S radar measurements.
Test Results
case history
case history
Figure 2. IBIS-S position in the survey for the downstream surveys.
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A2
Testing Procedure
Microwave remote sensing was used to perform dynamic measurements on the arrays of forestays supporting the deck of the cable-stayed bridge crossing the river Oglio between the towns of Bordolano and Quinzano (Fig.1), about 70 km far from Milan, Italy. The dynamic characteristics of the bridge were well-known since ambient vibration tests were carried out in Spring 2004 by the Vibration Laboratory of L’Aquila University using Sprengnether servo-accelerometers. During this test, 10 global modes of the bridge were identified in the frequency range 0−10 Hz and also the dynamic response of one cable (S
2U in Fig.1) has been recorded.
The deflection response of the two arrays of cables to wind and traffic excitation was quickly and safely acquired by positioning the IBIS-S at the base of the upstream-side and downstream-side tower, respectively (Fig.2). Since the test scenario on the two sides was practically the same, the radar image profiles are very similar
and each range profile exhibits three well defined peaks, occurring at the expected distance from the sensor (Fig.3) and clearly identifying the position in range of the cables.
Dynamic Measurements on the Forestays of a Cable-stayed Bridge »
Figure 1. Stay-cable comparison of the auto spectrum displacement data for the IBIS-S radar and accelerometer.
Figure 3. Range profile data of the test scenario of the downstream side.
132
1
3
2
15°15°
Project: Bordolano cable-stayed bridge
Project Location: Bridge over the Oglio River, Italy
Bridge Construction: Steel composite deck, double-plane cables and two inclined concrete towers. Elevation and plan views of the bridge and typical cross section are presented in Figure 1.
Project Scope: The two main arrays of bridge forestays were surveyed, with the main goal of verifying the ease of set-up, and the operational simplicity provided by the IBIS-S microwave remote sensing equipment.
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case history
Dynamic Measurements on the Forestays of a Cable-stayed Bridge » A2
For each array, 3000 s of radar data were acquired at a rate of 200 Hz; the displacement time-history collected on stay-cable S
2U is shown
in Figure 4. Figure 5 presents a direct comparison between: (1) the auto-spectrum of the acceleration measured on stay-cable S
2U by a
conventional accelerometer in the test of Spring 2004 and (2) the auto-spectrum of the acceleration obtained from the radar sensor (and computed by twice differentiating the IBIS-S displacement data). The inspection of the spectral plots in Figure 5 clearly reveals that the values of the first seven natural frequencies of stay-cable S
2U, identified on the basis of the auto-spectra obtained using
conventional and radar measurement systems are virtually coincident (1.84, 3.70, 5.53, 7.37, 9.24. 11.1 and 12.93 Hz in Fig.5). In addition, the peaks of the ASDs placed at 1.06, 2.18, 4.25 and 6.03 Hz correspond to the global natural frequencies of the bridge, identified in the previous dynamic survey of the bridge in 2004.
The inspection of the ASDs in Figure 5 clearly highlights that, as expected, the natural frequencies of the corresponding cables on the two opposite sides; S
1U–S
1D, S
2U–S
2D and S
3U–S
3D (Fig.1) are
almost equal. The response of all cables is characterized by a large number of equally spaced and well-defined peaks so that the tension forces can be computed from cable’s natural frequencies using the taut string model; application of this approach leads to values of cable tensions summarized in Table 2 and very close to the design values. Finally, Figure 6 shows how close the experimental resonant frequencies obtained from microwave remote sensing are to the predictions of the taut string model.
Test Results
Figure 4. Deflection time-history measured by the radar sensor on the stay cable S2U.
Figure 5. Auto-spectrum displacement data comparison of the IBIS-S radar (orange) and accelerometers (blue)on the stay-cable S'2u.
Figure 6. Experimental and taut-string based natural frequencies of: (a) upstream side forestays and (b) downstream forestays.
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Upstream Forestays
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Downstream Forestays
b
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A3
Testing Procedure
The IBIS-S system was deployed under the bridge superstructure (shown top left), illuminating five corner reflectors installed on the west side of the bridge between two bridge piers. The field set-up and all data acquisition required less than 1/2 day. Due to the non-contacting nature of the system and operational range, all testing was performed with no traffic disruption and minimal support requirements.
Each of the corner steel sheet metal reflectors produces a sharp peak on the IBIS-S range profile and therefore a good quality data point whose displacement can be measured by analyzing the phase variations with the differential interferometric technique. Figure 3 presents the IBIS-S Power Profile; a high level of backscattered signal in the range bin in which a crossing beam is located gives a high Signal to Noise Ration and therefore, high accuracy in the measurement of the displacement. The vertical
displacements of the five metallic corner reflectors installed on the west side of the bridge (Fig.3) during the test set-up phase are presented in Figure 4, which shows the resulting displacements from the passage of a testing truck and other vehicles over the bridge deck. The maximum measured peak to peak displacement at mid-span is 2.26 mm. Vertical velocity vibration amplitudes of the measurement points range from +/-4 mm/sec, depending on truck speed. The velocity spectra (Fig. 5) shows six sharp frequency peaks at 1.3, 2.05, 2.45, 2.95, 3.35, and 3.55 Hz, corresponding to the structural resonance modes. The first three should correspond to mainly flexural modes while the second three should be related to the torsional modes. Further analysis could be performed by importing the IBIS-S Displacement Time Series into specific software for ambient modal vibration structural analyses to evaluate, for example, the resonant frequencies, vibration mode shapes and damping.
Static Displacement and Dynamic Vibration Monitoring of a Concrete Bridge »
Figure 1. System configuration and survey geometry of the post-tensioned box girder bridge over Interstate 70 in Colorado, USA.
Figure 2. Installation of the 5 metallic corner reflectors due to the smooth concrete surfaces, each at about 7.5 m spacing (Fig.1).
Project: Flyover Exit Ramp from I-70 EB to CO HWY 58 WB
Project Location: Golden, Colorado USA
Bridge Construction: Post-tensioned concrete box girder bridge
The system configuration and survey geometry is shown in Figure 1.
Project Scope: Primary objective of the demonstration was to measure the deflection time-histories and maximum deflections of the bridge under normal auto and truck traffic loading. Metallic corner reflectors were mounted at 5 locations to monitor vertical dispacements and vibrations.
Reference: Olson, L.D., “Innovations in Bridge Superstructure Condition Assessment with Sonic and Radar Methods”, ASNT, NDE/NDT for Highways and Bridges, Structural Materials Technology Conference, LaGuardia, NY (August 2010).
Corner Reflectors
MetallicCorner
Reflectors
case history
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A3Static Displacement and Dynamic Vibration Monitoring of a Concrete Bridge »
Figure 4. Vertical displacements of the five passive metallic reflectors installed on the bridge caused by random vehicles and a 55,000 lb. test truck (shown) passing over the bridge deck.
IBIS-S Power ProfIle
A sharp peak corresponding
to each corner reflector on the
bridge is clearly identified in
the profile.
Figure 3. Portion of the IBIS-S Power Profile from radar reflections from the five metallic corner reflectors (CR1-CR5).
Metal Corner Reflector
Figure 5. .Velocity Frequency Spectrum for the 5 corner reflectors from displacement time domain data showing 6 resonances.
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Testing Procedure
A4
A demonstration test was performed using the high precision IBIS-S radar system, which was deployed on the Brooklyn side bank below the bridge superstructure (Fig.2). The field set-up and all data acquisition required less than 1/2 day for the IBIS-S radar system. Due to the non-contacting nature of the system and operational range, all testing was performed with no traffic disruption and minimal support requirements.
Two tests were performed pointing the sensor first towards the center and then to the edge of the main span with an attempt to estimate the torsion given by the asymmetric configuration of the traffic. In both situations,
the IBIS-S radar system was placed under the bridge (Fig.2) on the Brooklyn side illuminating most of the main span with the radar beam, therefore allowing the accurate displacement measure ment of around 80 visible points along the bridge span at the same time (steel cross beams at 5.5 m spacings). The excellent natural reflectivity of the micro-wave from the steel floor beams provided
equally distributed measurement points along the tested span without the need for artificial reflectors, oftentimes required for concrete or other non-metallic structures. Each of the floor beams generated a sharp peak on the IBIS-S range profile and therefore a good quality point whose displacement can be measured by analyzing the phase variations through the differential interferometric technique.
Figure 2. View of the IBIS-S system setup on the Brooklyn side bank below the bridge superstructure.
Static Displacement and Dynamic Vibration Monitoring of a Steel Bridge »
Project: Manhattan Bridge
Project Location: Brooklyn Borough, New York City, NY USA
Bridge Construction: Steel truss and suspension cable bridge weighing over 14,680 tons. The bridge traffic layout is shown in Figure 1.
Project Scope: The primary objective of the demonstration was to measure the deflection time histories and maximum deflections of the centerline and north edge of the Manhattan Bridge under normal automobile/truck and subway train traffic loading. A comparison test was made with previously collected displacement data from Global Position System (GPS) measurements versus IBIS-S radar system displacements.
Reference: L. Mayer, B. Yanev, L.D. Olson, A. Smyth, “Monitoring of the Manhattan Bridge and Interferometric Radar Systems”, IABMAS 2010 Proceeding, Philadelphia, PA USA (June 2010)
Figure 1. Current Manhattan Bridge traffic layout: cross-section shown facing north.
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A4
case history
Static Displacement and Dynamic Vibration Monitoring of a Steel Bridge »
Figure 3 presents a portion of the IBIS-S Power Profile: a high level of backscattered signal in the range bin in which a cross floor beam is located gives a high Signal to Noise Ratio and therefore high accuracy in the measurement of the displacement.
Figure 4 shows the vertical displacement of the central section of the floor beams in the main span resulting form the passage of a single train over the deck. The measured peak to peak displacement at mid-span is 33.82 cm but it decreases moving towards the bridge piers. The slow train entrance can be clearly identified by the high delay between the maximum vertical deflections of points at the different cross-sections of the span. Further, a positive vertical deformation of 5.31 cm can be observed when the train passes to the side spans. Measured peak to peak vertical deformations were about 1 cm under normal vehicular traffic when no trains were on the bridge.
Maximum peak to peak deformations measured during the passage of more than one train both for the central and side section of the main span are shown in Figure 5. Both deformed static curves are close to symmetrical with respect to the center of the span. However, the one related to the edge measurement shows a maximum deflection value at mid-span which is 6.67 cm higher than the one measured for the central section. Because the scans were taken at different times, the difference between the maximum deflections mid-span must be considered as a lower bound of the torsional movement. The center of the cross section would not be at its lowest during the greatest torsional deformation. Further, Figure 5 shows significant torsional behavior at quarter-span related to the second torsional mode, as should be expected.
Jointly analyzing the results in the frequency domain through the computation of the Displacement Cross-Spectrums of the whole set of range bin combinations and averaging the results allows the identification of the frequencies which are common to all measurement points, excluding frequencies potentially given by scattered noise effects. This kind of spectral analysis on the displacement time series, measured both for the central and for the side section, leads to the clear identification of three main resonant structural frequencies. As shown in Figure 6, the first resonant frequency is at 0.23 Hz, the second one is at 0.30 Hz while the third is at 0.49 Hz.
The frequencies obtained by the GPS were 0.23 Hz, 0.31 Hz and 0.50 Hz respectively. Further, a resonant frequency peak at 0.016 Hz can be identified and was shown to be related to the slow static deformation of a passing train. Also, similar magnitude of displacements were measured by the earlier GPS measurements by Columbia University to the IBIS-S displacements.
Test Results
Figure 3. Portion of the IBIS-S Power Profile.
Figure 4. Vertical displacement time series of the crossing beams given by the passage of a single train over the deck.
Figure 5. Peak to peak vertical deflection of the central and side sections of the main span from trains from maximum IBIS-S measured displacements of 80 floor beams.
Figure 6. Average displacement frequency spectra showing 3 resonances.
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Figure 2. (a) Corner reflectors on left adjacent to accelerometer sensors and (b) IBIS-S located under Capriate bridge near pier in circle above
Ambient Vibration Testing for Modal Analysis »
Two series of ambient vibration tests were conducted and the response of the bridge was measured at selected points using WR-731A piezoelectric sensors for comparison with IBIS-S measurements of 6 corner reflectors. Figure 1 above shows a schematic diagram of the vertical sensor layout. A series of three set-ups were required to cover the measurement points identified in Figure 1, with the accelerometers placed at points 5-6, 21-22 being used as reference measurements; the same figure identifies the locations of the corner reflectors installed on the downstream side of the deck (Fig.2a). As shown, both accelerometer and corner reflectors were positioned as closely as possible to each other.
Figure 1. Accelerometer Sensor and Corner Reflector layout for ambient vibration testing of the Capriate bridge (dimensions are in m).
Project: Capriate Bridge, Italy
Project Location: Traffic bridge crossing the Adda River between the towns of Capriate and Trezzo
Bridge Construction: Multi-span concrete arch bridge
Project Scope: A comparison was made using accelerometers and the IBIS-S radar system for ambient vibration testing to determine modal vibration frequencies and shapes.
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A5Ambient Vibration Testing for Modal Analysis »
The range profile of the stronger reflectors, including the six corner reflectors, is shown in Figure 3.
Typical velocity signals recorded for the accelerometer at TP22 on the downstream side and the adjacent corner reflector of the IBIS-S system are superimposed on each other in Figure 4. Review of this figure indicates an excellent agreement between the accelerometer (integrated to velocity units) and the IBIS-S (differentiated to velocity units).
Frequency domain comparisons of the IBIS-S and accelerometer are presented in Figure 5. Again, there is an excellent agreement between the two systems with the IBIS-S having the advantage of faster setup time and remote monitoring (no cables) of the displacements and vibrations.
Two of the vibration modes apparent in Figure 5 are plotted for all 6 corner reflectors versus the accelerometer responses in Figure 6. Review of this figure shows that the IBIS-S data agrees very well with the accelerometer derived mode shapes and frequencies. The evaluation of the modal parameters from the radar signals was first carried out by applying the Frequency Domain Decomposition (FDD) technique to all available velocity records obtained by the IBIS-S. The use of a greater number of corner reflectors would have thus fully defined the mode shapes for the bridge just as was done with the accelerometers which are more fully plotted in Figure 6.
Test Results
Figure 3. Range profile data results from the downstream side.
Figure 4. Comparison of velocity (mm/s) vibration responses versus time (14 s) comparison for the accelerometer and IBIS-S corner reflector measurements at TP22.
Figure 5. Comparison of frequency domain responses from 3000 seconds of data between the IBIS-S and Accelerometer.
Figure 6. Modal shape comparison with IBIS-S and Accelerometer. Modal shape comparison with IBIS-S and Accelerometer.
case history
Project: Bordolano cable-stayed bridge #2
Project Location: Bridge over the Oglio River, Italy
Bridge Construction: Steel composite deck, double-plane cables and two inclined concrete towers. Elevation and plan views of the bridge and typical cross section are presented in Figure 1.
Project Scope: The two main arrays of bridge forestays were surveyed, with the main goal of verifying the ease of set-up, and the operational simplicity provided by the IBIS-S microwave remote sensing equipment.
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Figure 2. The existing transversal beams acted as “natural reflectors”, and were exploited as measurement points.
Dynamic Monitoring of Vertical Displacements Caused by a Train »
A measurement was performed on a bridge of the Kuranda Scenic Railway using the IBIS-S radar system. The IBIS-S was installed 2.7 m below the bridge deck (Fig.1) in order to measure its deflection during the passing of the train. The existing transversal beams of the bridge could be exploited as measurement points (Fig.2).
The IBIS-S radar system illuminated the entire metallic portion of the bridge from below. In Figure 3, it is possible to see the radar image of the bridge. Every peak in the image is an exploitable measurement point. A total of 18 peaks occured between 16 and 41 m which correspond to the beams on the central span.
The whole measurement consisted of 2 sessions of 10 minutes each across two train passages. The results from the first measurement session in terms of displacement vs time and frequency analysis of the measured displacements are shown on the next page.
Figure 1. The IBIS-S system was installed under the bridge (circled area) in order to measure its deflection during the passage of a train.
Project: Scenic railway bridge, completed in 1891. It is said to be the most photographed bridge in all of Australia.
Project Location: Kuranda Scenic Railway, Cairns, AU
Bridge Construction: Winding its way through dense rainforest, steep ravines and picturesque waterfalls, this steel trestle railway incudes over 49 bridges and 19 hand-made tunnels.
Project Scope: The IBIS-S radar system was utilized to determine the maximum displacement of the now century old bridge for accessing the current stress and fatigue of the bridge deck. The experimental results consist of the visualisation of the bridge deck displacement; and identification of the resonance frequencies of the structure.
Operational Parameters
Maximum Range Distance 120 m
Sampling Frequency 70 Hz
Spacial Resolution (in range) 0, 5
Measurement Parameters
Test Width (across train passage) 10 ft
Bridge Length ~ 50 m
Bridge Deck Height (above IBIS-S) 2.7 m
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A6Dynamic Monitoring of Vertical Displacements Caused by a Train »
Figure 4. A frequency was obtained by applying the frequency analysis to the measured displacements linked to a train passage.
Figure 5. Line of Sight displacement projected along the vertical direction, according to the radar to bridge measured distance (2.7 m).
Figure 3. IBIS-S radar range profile of the illuminated rail (Fig.2).
Steel pillar
1/4 Bridge Length 1/2 Bridge Length
Steel pillar End of Bridge
IBIS-S Radar Range Profile
Steel pillarSteel pillarSteel pillarSteel pillar
1/4 Bridge Length1/4 Bridge Length
BRIDGE
Test Results
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