FBG Application
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Zhou Zhi, OU Jinping. A new kind of smart bridge cable based on FBG sensors SPIE v.5502 2004: 196-199
A new kind of smart bridge cable based on FBG sensors
ZHOU Zhi*, ZHAO Xuefeng, WANG Chuan, ZHANG Zhichun, HU Qingli, OU Jinping
School of Civil Engineering, Harbin Institute of Tech., Harbin, Haihe Road 202, China,150090
ABSTRACT
The feasibility and advantages of smart bridge cable based on FBGs are discussed. And the sensing properties of FBGs
installed in the cables under dead load are tested. The experimental results and practical applications show that the smartbridge cable is proper to be used in bridge cables transfer, construction control and long-term health monitoring.
Keywords: FBG sensor, smart bridge cable, structural health monitoring, strain
1. INTRODUCTION
Cables are the key components of cable-stayed bridges, suspension bridges, hanging bridges and so on. The bridge
cables in service are easily damaged due to the factors of environment corrosion, fatigue, materials aging, stressredistribution, etc, which result in that the cables can not stand as long as designed. Nowadays, engineers depend onmanometers and accelerometers, even electro-magnetic sensors to measure the cable stress. However, it is still difficultfor engineers to evaluate the cables state of stress or damage and give decision-making of maintaining or rejection.Optical Fiber Bragg grating (FBG) is now developing very fast since 1989
[1~4], which show great advantages such as
small, high accuracy, electro-magnetic resistance, quasi-distribution sensing, absolute measurement and so on. FBGs arenow can be available to measure structural strain and temperature[5~9].
In this paper, we have developed a new kind of cables installed with FBGs during manufacture of the cables for LuoguoBridge in Panzhihua, and Binzhou Yellow River Bridge in China, and their strain sensing properties are also studied.
2. FABRICATION OF SMART CABLES BASED ON FBGS
The main idea of smart cables based on FBGs is that FBGs sensors are installed in the cables during their fabrication
course. The FBGs in the cable can be used to measure the stain state of steel wire, which can show the stress state of thecable. According the fact that whatever damage occurring on the cable will be revealled by the inner stress state, we can
easily evaluate the safety of cable and take right action on it. Due to that FBGs diameter is very small, only 125micrometer or so, it doesnt affect the cables mechanical properties at all. Theoretically speaking, the feasibility of
smart cables based on FBGs are assured by the facts that the strain resolution of FBG can reach 1~2 , and the stresserror of each steel wire of the cable is blow 2%. The transaction of smart cables based on FBGs is depicted as Figure 1.
FBG sensors
Steel wire
PE layer
Figure 1: Transection sketch of smart cable based on FBG sensors
*[email protected]; phone 86-451-86282664; fax 86-451-86282209
mailto:[email protected]:[email protected] -
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Taken casting anchor cable as example, the manufacture procedure of smart cable based on FBGs sensors is depicted as
figure 2, which doesnt change the basic fabrication procedure of common cables. The key procedure of smart cablesfabrication is the installation technique of FBGs on the steel wire, which is depicted as figure 3~5.
Figure 2 Manufacture techniques of Smart cable with FBGs sensors
Figure 3 Techniques of FBGs installation on steel wire of cables
Figure 4 Smart hanger cable based on FBGs for Luoguo Bridge
Figure 5 Smart cables based on FBGs for Binzhou Yellow River Bridge
Adhesive tape open Sand paper polish Acetone Clean Glue sticking J39 cover
Silica gel Cushion Transmission line PE protectionAdhesive recover Jumper protection
Steel wire tension Mould shape Adhesive tape wrap Adhesive tape unfold FBG Installation
PE protection Transmission line protection Anchor annealAnchor cast Jumper protection
FBGFBG
FBG
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The FBGs in the smart cable are well protected, and the smart cable can show FBGs sensing properties. The fabrication
technology depicted above can be easily realized, and the cost is very low. The authors have made 24 smart cables forBinzhou Yellow River Bridge and 4 for Luoguo Bridge, which are now in service.
3. SENSING PROPERTIES OF SMART CABLES
In order to test the strain sensing properties of smart cables, we make full use of the last procedure of the cablesfabrication, excess extension of the cables, to assure the superiority of the FBGs sensors. Taken the cable, N26-3
embedded with 3 FBGs for Binzhou Yellow River Bridge as example, it consists of 287 7, and elastic module andoriginal length are 1.95105MPa and 219.591m respectively. The final extension length of the cable is 86cm. Themeasurement results from the 3 FBGs are given as figure 6~9. Here due to that the time for experiment is very short,
temperature effect is not considered.
0 500 1000 1500 2000 2500 3000 3500 4000
0
2000
4000
6000
8000
10000
Load(kN)
strain (
loading course of FBG1
Unloading course of FBG1
0 500 1000 1500 2000 2500
0
1000
2000
3000
4000
5000
Load(kN)
strain ( )
loading course of FBG2
unloading course of FBG2
Figure 6 Strain sensing of FBG1 in N26-3 Figure 7 Strain sensing of FBG2 in N26-3
0 500 1000 1500 2000 2500
0
1000
2000
3000
4000
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load(kN)
strain ( )
Loading course of FBG3
unLoading course of FBG3
0 500 1000 1500 2000 2500 3000 3500 4000
0
200
400
600
800
1000
1200
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1600
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2000
strain(
)
load (kN)
FBG1 under loading
FBG2 under loading
FBG3 under loading
FBG1 under unloading
FBG2 under unloading
FBG3 under unloading
Figure 8 Strain sensing of FBG3 in N26-3 Figure 9 Comparison of results from 3 FBGs
We can find that the strain sensing properties of the 3 FBGs are very inspiring, which show good linearity and repetition.
When the load reaches the extra extension value of 8502kN, the strain gotten from FBG1 is 3800.8, which agrees wellwith that from calculation, and the value is 3916.4. The error is only 2.9%. So it is a good way for FBGs installed incables to monitor the inner strain state, and the smart cables based on FBGs can work well.
4. TEMPERATURE COMPENSATION OF SMART CABLES BASED ON FBGS
As the temperature for a bridge in service changes a lot, even to 60~80 during the 4 seasons in one year, and the
theoretical and experimental results shows that it is essential to consider the temperature compensation when FBGs inservice for strain sensing. Considering the operational convenience, we install another suspension FBG in a steelcapillary along the FBG strain sensor to sense the temperature, which can not be affected by load, The basic sensingprinciple can be interpreted as follows. Not considering the coupling effect of FBG under strain and temperature simultaneously, we
can give the expression of FBG affected by strain and temperature at the same time and that of FBG affected only by temperature,which are given as (1) and (2)
1111/ TKK
Tbb+=
(1)
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2222/ TK
Tbb= (2)
where , , T and K mean central wavelength of FBGs, strain, temperature and coefficient respectively.
Because the 2 FBGs are close enough,1
T equals 2T . Thus, we can get the strain with the consideration of temperature
compensation:
KKK
bTbTbb/))/(/(
222111= (3)
5. CONCLUSIONS
Aiming at structural health monitoring of bridge cables, a new kind of smart cable based on FBGs is developed, and itsfabrication techniques and temperature compensation in service are also brought forward. 24 smart cables for BinzhouYellow River Bridge and 4 for Luoguo Bridge have been developed, which are now in service. From the results of cablesunder loading and unloading, the FBGs installed in the cables show inspiring strain sensing properties. With the FBGs in
the cable, the stress state can be obtained during transfer and construction, especially for the cables long termmonitoring in service. The cables with installed FBGs show the advantages of FBG sensors such as high accuracy,electro-magnetic resistance, quasi-distribution sensing, absolute measurement and so on.
ACKNOWLEDGEMENT
Project Supported by China Postdoctoral Science Foundation, National Science Foundation of China (50308008)and 863High Tech. Program.
REFERENCES
1. G. Meltz, Overview of Fiber Grating Based Sensors. SPIE, 2838, 20002. Y.J. Rao,Recent progress in applications of in fiber Bragg grating sensors. Optics and lasers in Eng. Vol.31, 19993. P.M. Nellen, R. Bronnimann, et al. Structurally Embedded fiber Bragg gratings: Civil engineering applications. SPIE,
Vol.3860, 1999: 44-544. P. L. Fuhr, S. Spammer.Fiber optic sensors in the Waterbury Bridge. SPIE, 1998, 3489:124-1295. J. Sein, E. Udd, W. Schulz.Health monitoring of an Oregon historical bridge with fiber grating strain sensors. SPIE,
3671,1999: 128-1346. R. C. Tennyson, T. Coroy and G. Ducket al.Fiber optic sensors in civil engineering structures. J. Civ. Eng., 20007. ZHOU Zhi, Optical Fiber Smart BRAGG Grating Sensors and Intelligent Monitoring System in Civil Infrastructures , Doctor
Dissertation, Harbin Institute of Tech., 2003
8. ZHOU Zhi, WU Zhanjun, Ou Jinping. Technique and Application of In-situ Monitoring for Concrete Structures withFBG Sensors, Pacific Science Review, Far East Technical University of Russian,Vlasivostok,Vol.4, 2002
9. OU Jin-ping, ZHOU Zhi,WU Zhanjun. The sensing properties and practical application in civil infrastructures ofoptical FBGs. SPIE., 5129, 2003