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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

*zhouzhi@hit.edu.cn; phone 86-451-86282664; fax 86-451-86282209

mailto:zhouzhi@hit.edu.cnmailto:zhouzhi@hit.edu.cn

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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

5000

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

1400

1600

1800

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

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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