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COMPARISON OF NATURAL FREQUENCIES OF VIBRATION FOR A BRIDGE OBTAINED FROM MEASUREMENTS WITH
ACCELEROMETERS AND PIEZOELECTRIC CABLE SENSOR
Nobuhiro SHIMOI1, Masahiro SAIJO2, Carlos CUADRA3, and Hirokazu MADOKORO4
ABSTRACT
Deterioration or ageing of bridges structures and damages due to strong earthquakes may conduce to collapse of the bridge, sometimes with catastrophic consequences. Therefore investigation of structural condition of bridges is required for secure safe road operations. This paper presents a prototype of piezoelectric-cable sensor for vibration monitoring system that permits easy evaluation of integrity of a bridge structure. For this study a bridge located at Yurihonjo city, Japan was chosen as target structure. The structure is a continuous beam type bridge with steel beams of variable sections and reinforced concrete slabs to support the asphalt carpet. The bridge consists of seven spans with a total length of 256 m. The experimental measurements were performed on the first span near the left abutment.
Quantitatively, natural period of vibration can be obtained from signal recorded by proposed data acquisition system. Then, results from proposed system are compared with those obtained from common accelerometers. Since piezoelectric sensor emits a signal when a change in the stress condition occurs, the sensor was setup at the support of the bridge where large changes in the stress level is expected to occur. The target bridge was also subjected to moving loads, and its vibration response was also obtained. The experiment was performed using a track of 20 t of equivalent weight. Responses were captured appropriately using the proposed system and they are comparable with responses obtained from accelerometers.
Although the general response pattern is obtained appropriately it is necessary to improve the accuracy of the proposed system to have more reliable data. In this paper, the general outline of the proposed system is described mentioning main specifications and discussing possible ways to improve the data acquisition system to permit a stable and accurate monitoring of bridge structures.
INTRODUCTION
In Japan many bridges were constructed as a part of the reconstruction program after the Pacific War. Therefore old constructions of that date are now in danger due to the deterioration of their structures. On the other hand in the decade of 1980 of the last century constructions of large projects of building and bridges were initiated. Then high demand of materials, in special for reinforced concrete structures, induced the used of sea sand which contains salts that in reaction with water and cement originates the corrosion of steel reinforcement. Outside Japan an example of the necessity of health monitoring was evident in the collapse of the bridge I-35W on Mississippi river in USA. During the evening rush hour 1 Professor, Akita Prefectural University, [email protected] 2 Eng., OYO corporation, [email protected] 3 Associate Professor, Akita Prefectural University, [email protected] 4 Associate Professor, Akita Prefectural University, [email protected]
on August 1, 2007, the bridge suddenly collapsed, killing 13 people and injuring 145. In the case of Japan the Ministry of Land and Transpor121 cases of bridges in danger havconcrete bridges (weathering) and corrosion of steel bridges.
A continuous or real timeMonitoring systems already exist however they are in general structures and are expensive. For existing old structures, simple and cheap sensor systems are required.
In this research a new sensor that is based on piezoelectric cable inserted into a boltdeveloped and its applicability to perform health monitoring of structures is verified by means of a series of measuments on vibration of a target bridge. The selected bridge is located Akita prefecture, Japan. The results obtained with this new simple smart sensor are comparable with those obtained with more sophisticated and expensive commercial sensor
TARGET BRIDGE
A general view of selected structure for this studyAsuka Oohashi constructed in the year 1979 and spans256 m.
The target is a girder type bridge with 7 spans as is shown in Figure 2(a). For measurements only the first span (encircled left span in Figure 2(a)) was selected. The bridge structure is formed by steel beams which support a reinforced concrete slab as is shown in Figure 2(b). The bridge has 2 abutments at both ends and 6 intermediate piers and therean abutment at one end and a pier at other end, with a span length of 31 m.
2(a) Elevation and plan views of bridge
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August 1, 2007, the bridge suddenly collapsed, killing 13 people and injuring 145. In the case of Japan the Ministry of Land and Transportation has performed a study of vulnerability of bridges and 121 cases of bridges in danger have been reported. The risk of bridges is due to deterioration of aged concrete bridges (weathering) and corrosion of steel bridges.
uous or real time structural health monitoring could help to prevent damages. Monitoring systems already exist however they are in general designed to be installed in new structures and are expensive. For existing old structures, simple and cheap sensor systems are required.
In this research a new sensor that is based on piezoelectric cable inserted into a bolticability to perform health monitoring of structures is verified by means of a
series of measuments on vibration of a target bridge. The selected bridge is located Akita prefecture, Japan. The results obtained with this new simple smart sensor are comparable with those obtained with more sophisticated and expensive commercial sensors.
view of selected structure for this study can be observed in Figure 1. This bridge is called ted in the year 1979 and spans over the Koyoshi river with a total length of
is a girder type bridge with 7 spans as is shown in Figure 2(a). For measurements the first span (encircled left span in Figure 2(a)) was selected. The bridge structure is formed by
steel beams which support a reinforced concrete slab as is shown in Figure 2(b). The bridge has 2 abutments at both ends and 6 intermediate piers and therefore the selected span for measurements has an abutment at one end and a pier at other end, with a span length of 31 m.
Figure 1. General view of target structure
Elevation and plan views of bridge 2(b) The bridge in three dimensional an illustration
Figure 2. General scheme of target span
August 1, 2007, the bridge suddenly collapsed, killing 13 people and injuring 145. In the case of vulnerability of bridges and
bridges is due to deterioration of aged
structural health monitoring could help to prevent damages. designed to be installed in new
structures and are expensive. For existing old structures, simple and cheap sensor systems are required. In this research a new sensor that is based on piezoelectric cable inserted into a bolt shape is
icability to perform health monitoring of structures is verified by means of a series of measuments on vibration of a target bridge. The selected bridge is located at Yurihonjo city, Akita prefecture, Japan. The results obtained with this new simple smart sensor are comparable with
can be observed in Figure 1. This bridge is called over the Koyoshi river with a total length of
is a girder type bridge with 7 spans as is shown in Figure 2(a). For measurements the first span (encircled left span in Figure 2(a)) was selected. The bridge structure is formed by
steel beams which support a reinforced concrete slab as is shown in Figure 2(b). The bridge has 2 fore the selected span for measurements has
The bridge in three dimensional an illustration
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VIBRATION MEASUREMENTS PLAN
Points of vibration measurements on selected span are shown in Figure 3. Bolt type sensors were located at seven points of measurements together with accelerometers to compare both results. In addition at middle of span a laser displacement transducer was setup near point No. 7.
Elevation
Figure 3. General setup of points of measurements
4(a) Sensor No1,3 and No2,4 4(b) Sensor No5,No6
4(c) Sensor No7 and Accelerometer 4(d) Radio unit
4(e) Laser sensor 4(f) The 19t truck for moving load
Figure 4. Details of sensor setup and measurements
Plan View
Figure 4 shows some details of sensors setup. Basically, relative displacements between girders and support structure were measured at each point. For this purpose, sensors were fixed firmly to girder flange with its end in contact with support surface. Seway as can be observed in Figures 4(a) and 4(b). Figure 4(c) shows the sensor setup at point 7 which is the centre of span. Figure 4(d) shows the (radio unit) and corresponding battery. This unit computer for data acquisition. Figure 4(e) Shown the setup of laser displacement transducer. Also vibration due to moving loads was measured and in Figure 4(f) the truck test is shown.
CHARACTERISTICS OF PROPOSED PIEZOELECTRIC SENSOR
Basic scheme of measurement systems are shown in Figure 5. Figure 5(a) shows proposed system were signals from piezoelectric bolt sensors are transmitted to computer by means of a module which contains a microcomputer board. As can be observed, thand it is easy to install in-situ. Former measurements systems require more equipment like energy generator, signal amplifiers, etcetera, as can be observed in Figure 5(b) for the case of measurements using accelerometers.
5(a) Proposed system 5(b) Former systems
The piezoelectric bolt sensor is shown in Figure 6(a). Basically consists of a
located in its inner core and external cover of urethane resin resulting in a nominal diameter of 15 mm. The structure of piezoelectric cable is shown in Figure 6(b). electric condensor emits voltage when the cable is subjected to an extedeformation on the cable, especially in case of dynamic action or vibration.
6(a) Bolt sensor 6(b) Detail of piezoelectric cable
Figure 6. Proposed
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Figure 4 shows some details of sensors setup. Basically, relative displacements between girders and support structure were measured at each point. For this purpose, sensors were fixed firmly to girder flange with its end in contact with support surface. Sensor 1, 2, 3, 4, 5 and 6 were setup in this way as can be observed in Figures 4(a) and 4(b). Figure 4(c) shows the detail of sensor setup at point 7 which is the centre of span. Figure 4(d) shows the wireless
) and corresponding battery. This unit receives signal from sensor and sent it to the for data acquisition. Figure 4(e) Shown the setup of laser displacement transducer. Also
vibration due to moving loads was measured and in Figure 4(f) the truck of 19 t that was used for this
OF PROPOSED PIEZOELECTRIC SENSOR
Basic scheme of measurement systems are shown in Figure 5. Figure 5(a) shows proposed system were signals from piezoelectric bolt sensors are transmitted to computer by means of a module which contains a microcomputer board. As can be observed, the proposed system is simple
situ. Former measurements systems require more equipment like energy generator, signal amplifiers, etcetera, as can be observed in Figure 5(b) for the case of measurements
5(a) Proposed system 5(b) Former systems
Figure 5. Scheme of measurement systems
bolt sensor is shown in Figure 6(a). Basically consists of a r core and external cover of urethane resin resulting in a nominal diameter of 15 mm.
of piezoelectric cable is shown in Figure 6(b). The piezoelectric film which acts as electric condensor emits voltage when the cable is subjected to an external action that produces deformation on the cable, especially in case of dynamic action or vibration.
6(a) Bolt sensor 6(b) Detail of piezoelectric cable
Figure 6. Proposed piezoelectric bolt sensor
Figure 4 shows some details of sensors setup. Basically, relative displacements between girders and support structure were measured at each point. For this purpose, sensors were fixed firmly to
nsor 1, 2, 3, 4, 5 and 6 were setup in this of accelerometer and
wireless transmission unit s signal from sensor and sent it to the
for data acquisition. Figure 4(e) Shown the setup of laser displacement transducer. Also of 19 t that was used for this
Basic scheme of measurement systems are shown in Figure 5. Figure 5(a) shows proposed system were signals from piezoelectric bolt sensors are transmitted to computer by means of a wireless
e proposed system is simple situ. Former measurements systems require more equipment like energy
generator, signal amplifiers, etcetera, as can be observed in Figure 5(b) for the case of measurements
5(a) Proposed system 5(b) Former systems
bolt sensor is shown in Figure 6(a). Basically consists of a piezoelectric cable r core and external cover of urethane resin resulting in a nominal diameter of 15 mm.
The piezoelectric film which acts as rnal action that produces
6(a) Bolt sensor 6(b) Detail of piezoelectric cable
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As is shown in Figure 7(a), the sensor is connected to a radio unit which sends a wireless signal to a controller connected to a computer. The sensor itself does not need input energy to emit its response however radio unit requires a source of energy. Signals from sensor can be monitored from a control room as is shown in Figure 7(b). In this research a prefabricated portable room was installed near target bridge to perform measurements.
7(a) Sensor unit 7(b) Control room
Figure 7. Sensor signal transmission and general layout of control room Figure 8(a) shows general layout of a vibration test of bolt sensor using a small vibration
machine that were carried out to verify sensor response to dynamic actions. These tests were performed at fixed amplitudes and therefore it is supposed that the sensors behave under a fixed harmonic force. For comparison displacement of the vibration machine was measured by mean of a laser displacement transducer. Vibration test were performed for 0.05 mm, 0.1 mm and 0.2 mm of amplitude respectively. For each amplitude value the frequency was set up from 2 Hz to 20 Hz. The curves that relate the frequency, amplitude and the maximum output voltage of the sensor are shown in Figure 8(b).
8(a) Vibration test setup 8(b) Frequency and output voltage relationship
Figure 8. Vibration test using bolt sensors
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ESTIMATION OF PREDOMINANT FREQUENCY OF TARGET BRIDGE
As illustrative example of measurement results, responses at measurement point No. 1 and No. 2 are shown in Figure 9. Upper part of the Figure 9 shows acceleration responses obtained by means of accelerometers and bottom part of Figure shows the voltage output from the bolt sensors.
From recorded signals, predominant frequencies were obtained by Fourier analysis. In Figure 10 Fourier spectrum for signals from point No. 1 to No. 6 are shown. Upper part of each figure corresponds to accelerometers results and bottom part corresponds to proposed sensor results. Results for point No. 1, No. 2 and No. 4 show good agreement between accelerometer results and proposed sensor results. The difference of results at point No. 5 and at point No. 6 it is believed that is due that proposed sensors and accelerometers are not set up at same location. However in the Fourier spectrum for proposed sensor a second peak near 3.6 Hz is observed which is comparable with the predominant frequencies obtained from accelerometers.
Figure 11 shows a comparison of proposed sensor response and laser displacement transducer response when the truck of 19 t runs on target span at a velocity of 20 km/h. Although this measurement corresponds to relative displacement between girder and slab and therefore only small displacement can be detected, external action (in this case load from truck) was clearly captured by both sensors as can be observed in the disturbance at central part of the figure.
Figure 9. Measurements signals from accelerometers and from bolt sensors
(a) Predominant frequency of sensorNo1 (b)Vibration of sensor No2
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(c)Vibration of sensor No3 (d)Vibration of sensor No4
(e)Vibration of sensor No5 (f) Vibration of sensor No6
Figure 10. Predominant frequencies from Fourier analysis
Figure 11. Comparison of sensor response and laser transducer response
CONCLUSIONS
In this research a new smart simple piezoelectric sensor and its corresponding data acquisition system were developed to be used for real time structural health monitoring of structures. Installation of the system in selected portion of selected bridge structure has permit to perform the monitoring of the structural response for free vibration and vibration in case of moving loads (vehicle loads). The applicability of this new bolt- type sensor for structural health monitoring and estimation of dynamic characteristics of a bridge structure was verified. Simultaneously commercial accelerometers were used for vibration measurements to compare with those measurements using proposed system. In general predominant frequencies obtained from in-situ measurements using accelerometers and proposed bolt sensors show good agreement.
It was also verified that proposed sensor could be used also to detected impact loads acting on bridge structures. In this case verification was done by performing measurements using moving loads
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from a truck of 19 t. Measurements results from laser displacement tranducer and proposed sensor are comparable.
Acknowledgement
The authors acknowledge Japan Society for the Promotion of Science for the Grants-in-Aid for Scientific Research on Smart Structure Sensing System Using Piezoelectric Cable, under which this research was conducted.
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