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International Workshop on Structural Health Monitoring and Damage Assessment, National Chung

Hsing University, Taichung, Taiwan, ROC, December 14-15, 2006.

Structural Health Monitoring Activities of ApplyingOptical Fiber Sensors in Taiwan

Chung-Yue Wang Hao-Lin Wang Ming-Hung Chen

Center for Bridge Engineering Research, National Central University, Chungli, Taiwan 32054, ROC

[email protected]

Abstract: Applications of optical fiber sensors on the structural health monitoring of the

deformations of bridges, viaduct of high speed rail systems, railway tracks, airport pavements, and

geological faults are introduced.

2006 Optical Society of America

OCIS codes (999.9999) Structural Health Monitoring

1. IntroductionDeveloping deteriorated infrastructures is a serious problem for most countries in the world. This makes inspection,

monitoring, damage assessment, retrofitting and management of civil infrastructures become the major work of most

developed countries to prevent severe deterioration and hazards. Implementation of successful structural health

monitoring programs requires selection and placement of sensors for the measurement of key parameters that

influence the performance and health of the structural system. In additional to the structural considerations, sensors

need to be chosen based on compatibility with the materials and the scope of the measurements. Conventional

sensors and strain gauges perform well in many applications. Fiber optical sensors provide good signal quality,

durability, flexibility and practicality in some other situations, where the conventional sensors are either not capable

of or feasible for making the appropriate measurements [1]. Many fiber optical sensor types and configurations

have become available over the past two decades. This article is intended to provide a description of the recent

activities of applying optical fiber sensors on structural health monitoring in Taiwan.

2. Development of the structural health monitoring systemThe most critical technique in structural health monitoring is the backward analysis of applying measured data to

identify the current condition of the structure to be monitored. Engineers should design the monitoring system

based on a physical model that can reasonably and efficiently reflect essential information about the health condition

of the structure being studied. To the authors, it is the most important index to evaluate the intelligent capability of

engineers and to show the value of investment by the owner on structural health monitoring system. Engineers

have to use these measured data to identify the dynamic characteristics, deflection, settlement, inclination, cracking,

and creeping conditions of the structures. Various kinds of optical fiber sensors of high and durable signal quality

have been developed for those physical quantities to be measured. However, the success of the applications of

these state-of-the-art optical fiber sensors on real structures depends on the teamwork among diagnosticians,

installation engineers, sensor manufacturers and structure owners.

3. Applications of optical fiber sensors on structural health monitoring in TaiwanFollowing are the application examples of applying optical fiber sensors on structural health monitoring in Taiwan.

(1) Performance of airport pavementChou and Cheng [1] embedded interferometric optic fiber sensors produced by the Smartec Group [2] in the

airport concrete slab as shown in Fig. 1 for the measurement of pavement joint movements due to seasonal

environment changes. The measured data were further used to study the averaged slab expansion/contraction rate

and joint maximum opening. In addition, some applications, such as the maximum compressive and tensile

stresses that slabs may sustain, were calculated.

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Fig. 1 Airport concrete pavement slab performance monitoring [1].

(2) Axial force and deformation monitoring of railway trackThe axial stress distribution and variation of continuous welded rail tracks under the action of environmental

temperature will be related to the deformation of the track and the safety of the rolling stock. A track deformation

monitoring system using optical fiber sensors has been developed by the authors and Fibera Inc. [3]. As shown in

Fig. 2, there are 3 Athermal type fiber-Bragg grating (FNG) sensors are installed at each cross section of a track.

These three axial strains can be used to back calculate the axial force, bending moments and deformation curvatures

at their section. Using the calculated curvatures distributed along the track, one can calculate the deflection curves

of the monitored track based on the method proposed by Inaudi et al. [4]. Recently, a monitoring project of

installing 123 FBG athermal sensors in a region of 200 meters on the continuous welded track of a bridge was

conducted to study the axial stress distribution and train-track-bridge interaction behavior.

Fig. 2. Installation of the FBG sensors and monitoring of the performance of railway tracks.

Fig. 3. Fastening of the FBG sensors on concrete structure.

(3) Deterioration monitoring of bridge girderAs all structures are aging, it is very important to know about their status, especially if they are structures used

by the public, such as bridges or other infrastructures. A bridge restricted to a gross vehicle weight up to 25 tons

due to deterioration and exceeding vibration under moving loads was installed with 18 Athermal FBG sensors [3]

distributed at 9 locations of the span and associated real time monitoring and safety warning system to understand its

long term performance. In each location there are two sensors separated by 0.5 m along the depth of a girder (see

Fig. 3) to measure the axial strains and to calculate the curvature of the corresponding beam section. Curvatures

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measured along the edge girder of the bridge are used to calculate its deflection curve [4].

(4) Loading test of bridgeA Deflection curve is the most useful information for characterizing the health conditions of bridge structure.

Besides the curvature integration method [4], the integration of rotating angle is another method used to obtain the

deflection curve along the bridge axis. The authors developed an algorithm [5] to calculate the deflection curve of a

structure by the measuring relative rotating angles at some locations along the beam axis. As shown in Fig. 4, the

relative rotating angle is measured by the optical tilting meter developed by POFC [6]. Through the temperature

compensated for strains measured by FBG sensors, the resolution of the tilting angle is 4/10,000 degree. The

performance of this calculation algorithm was verified by the conventional surveying method in a bridge loading test

as shown in Fig. 5.

Fig. 4. Slope-deflection calculation method and the optical fiber tilting meter..

Fig. 5. Loading test and deflection measurement by optical slope-deflection meter.

Fig. 6. Performance monitoring of viaduct of high speed rail system due to passage of rolling stock.

(5) Performance monitoring of viaduct of high speed rail system due to passage of rolling stockSince the optical tilting meter possesses the advantages of high resolution and high sensitivity, it is suitable for

measuring small deformations of high frequency in very stiff superstructures. Two 33-meter optical tilting meters

as shown in Fig. 6 were used to measure the dynamic deflection curves of the viaduct of high speed rail systems due

to passage of rolling stock at speeds ranged from 180 km/hr to 305 Km/hr. Two test lines were measured

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simultaneously, which makes the measurement of the cant induced by transverse rotation be available.

(6) Creeping of faultIt is well recognized that there is some relationship between fault movement and the occurrence of earthquakes.Techniques for monitoring surface movements of faults have been well established. However, techniques for

subsurface monitoring that can clearly indicate the creeping movements of fault are still in demand. A 30-meter

long optical-fiber-type tilting meter (OFTM) for geotechnical engineering developed by POFC [6] was installed into

a borehole located in the southeastern area of Taiwan by the authors of the paper (see Fig. 7). This measurement

system can detect the deformation curve of a borehole due to slight movements of the ground. The research group

can do the real-time monitoring from the National Central University located in the northwestern part of the island

through an internet system.

Fig. 7. Monitoring of the creeping of fault.

(7) Scouring monitoringLocal scour is one of the major factors causing bridge failure. A real-time monitoring system for bridge

scouring, using FBG sensors, has been developed and installed in field [7]. This FBG scour-monitoring system can

measure both the processes of scouring/deposition and variations in the water level.

4. ConclusionsA few application cases of applying optical fiber sensors on civil infrastructural health monitoring and diagnosis are

presented. Test results demonstrate that the optical fiber sensors are considerably good candidates for smart

structure applications. Due to the small size and weight, electrical isolation, environmental ruggedness, and ability

to be multiplexed, the optical fiber sensors enable the use of health monitoring structure systems that would be

difficult or impossible to implement using conventional electronic technology. It is obvious that the optical fiber

sensors have great potential to combine with a wide variety of instrumentation techniques for engineering

applications to form an integrated whole field infrastructural health monitoring system.

5. References[1] C. P. Chou and H. J. Cheng, Analysis of concrete joint movement and seasonal thermal stress at the Chiang-Kai-Shek

International Airport, Journal of the Eastern Asia Society for Transportation Studies, Vol. 6, 1217-1230 (2005).

[2] Smartec S.A., Via Pobbiette, 11, CH-6928 Manno, Switzerland. http://www.smartec.ch.[3] Fibera, Inc., 3350 Scott Boulevard, Bldg 56, Santa Clara, CA 95054, USA. http://www.FiberaInc.com[4] D. Inaudi,, S. Vurpillot, N Casanova, and P. Kronenberg, "Structural monitoring by curvature analysis using interferometric optic fiber

sensors." Smart Materials and Structures, Vol. 7, 199-208 (1998).

[5] C. Y. Wang, H.-L. Wang and M.-H. Chen, Applications of FBG Sensors on Bridge Health Monitoring & Diagnosis," in Proceedings ofthe 5th Int. Conf. on Structural Health Monitoring 2005, 1717-1726 (2005).

[6] POFC, Prime Optical Fiber Corporation, No. 11, Ke Jung Rd., Science-Based Industrial Park, Chu-Nan 350, Miao-Li County, Taiwan,ROC. http://www.pofc.com.

[7] Y. B Lin, J. C. Chen, K. C. Chang, J. C. Chern and J. S. Lai, Real-time monitoring of local scour by using fiber Bragg grating sensors,Smart Materials and Structures, 14, 664-670 (2005).

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