Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, JapanSTRUCTURAL ASSESSMENT OF A REPAIRED CABLE
BRIDGE DAMAGED IN 1999 CHI-CHI EARTHQUAKE
Kuo-Chun CHANG1, Zheng-Kuan LEE 2 and Chien-Chou CHEN3
1

Director, National Center for Research on Earthquake Engineering
Taipei, Taiwan, kcchang@ncree.narl.org.tw
2
Associate Research Fellow, National Center for Research on Earthquake Engineering
Taipei, Taiwan, zklee@ncree.narl.org.tw
3
Professor, Yunlin University of Science and Technology
Yulin, Taiwan,ccchen@ce.yuntech.edu.twABSTRACT: The Chi-Lu cable-stayed bridge was damaged during Chi-Chi Earthquake
just before completion of its construction. Urgent repairs were made following theearthquake without having made a detailed assessment of the damaged structure. To
assess the structural safety, a series of field tests were implemented, including load tests,
ambient vibration test and forced vibration test. This paper presents the assessments on
the structural safety of the bridge by field tests before the official re-opening, and thehealth monitoring of this bridge since its reopening to general traffic conditions.
Key Words: Chi-Chi Earthquake, cable-stayed bridge, cable force, wind/rain cablevibration, bridge health monitoring, optic-fiber sensor

INTRODUCTION
Chi-Lu Cable-Stayed Bridge, located at central Taiwan, crossing Juosheui River and linking
north Chi-Chi Township and south Lu-Ku Village, is a modern design pre-stressed concrete
cable-stayed bridge. As shown in Fig.1, the bridge is composed of a single pylon (58 meters
height above the deck), two-row fanned cables (68 cables), and a streamline-shape main
girder (2.75 meters in depth and 24 meters in width). The girder rigidly connects the pylonwith spans 120 meters to each side bent. On September 21, 1999, during the final
construction stage of the bridge, Chi-Chi Earthquake with 7.3 ML magnitude seriously struck
central Taiwan. Fig.2 shows the fault line and the bridge’s location. Fourteen kilometers awayfrom the epicenter, the nearby Sun Moon Lake Strong Motion Station reveals 989 gal in
East-West direction and 423 gal in North-South direction. Chi-Lu Bridge experienced thenear-fault strong ground motion, much greater than its seismic design 0.28 G (Chang et al.
2004). After two-stage repair, following the re-opening of the bridge, a FBG based structuralmonitoring system has been implemented to the structure to monitor the structural behavior of thebridge during routine operation. Based on the field observation and experiments from the last
few years, this paper briefly describes the damage, the repair process, the loading tests, andthe long-term structural monitoring system on Chi-Lu Cable-Stayed Bridge.

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Fig. 1 Profile of Chi-Lu Cable-Stayed Bridge

Fig. 2 Map of Chelungpu fault, Chi-Lu Bridge, and Sun-Moon Lake1823

DAMAGE AND EMERGENT TREATMENT
Shaken by Chi-Chi Earthquake, the Chi-Lu Cable-Stayed Bridge was known as the firstdamaged modern prestressed-concrete cable-stayed bridge. The observed damage is as follows.
The protective concrete cover of the pylon spalled (Fig.3a); one cable fell onto the deck (Fig.3b);
the soffit of the girder at the juncture with the pylon cracked (Fig.3c); the rebar in thetransverse direction of the girder buckled in compression (Fig.3d); the cap beams of the twobents were damaged by the vertical and lateral pounding forces (Fig.3e); the eccentric deviceof the guide pipes come off (Fig.3f); and the elevation profile of the deck was lower than that ofthe design profile (Fig.4). For preventing further damage from more earthquakes, 18
supporting frames were set up (Fig.5) before completion of the repair.
abcdefFig. 3 Chi-Lu Cable-Stayed Bridge damage after Chi-Chi Earthquake

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Chi-Chi

Fig. 4 Elevation profile of the deck after the earthquake

Fig. 5 One of the 18 supporting frames

REPAIR OF THE DAMAGE
Chi-Lu Bridge was unexpectedly damaged by the quake during its final construction stage.
The related engineering lawsuit delayed the repair schedule, and the repair process wasimplemented in two stages. Stage 1: All reinforced concrete components were repaired by July
2001. Stage 2: The cable system was repaired by August 2004. Fig.6 shows the repair of thegirder near the pylon. Fig.7 shows the repair of the bent. Fig.8 shows a cable with a newanchor was hanged from the deck to the pylon, and the cable force was being tensioned.
baFig.6 Repair of the girder near the pylon: (a) new rebar mesh, (b) hydraulic jacks inside thegirder to balance the compressive force

1825Fig. 7 New cap beam reconstruction

abFig.8 Repair of the cable system：(a) a cable with new anchor was hanged to the pylon, (b) thecable force was measured and tensioned inside the girder

SAFETY LOADING TESTS ON THE DECK
After Repair Stage 1, to partially relieve the traffic demand, the bridge owner entrusted
NCREE (National Center for Research on Earthquake Engineering) with the 1st truck loading
test, as shown in Fig.9. After the 1st loading test, Chi-Lu Bridge was opened to passersby andpassenger cars only.
After Repair Stage 2, the bridge owner entrusted NCREE for the 2nd truck loading test.
After the 2nd loading test, as shown in Fig.10, Chi-Lu Bridge was officially opened to thegeneral traffic. During the 2nd loading test, to measure the response of the girder and pylon,
both LVDT and tilt meter were deployed as shown in Fig.11. The LVDT were installed on thetop of the scaffoldings, setup above the riverbed. Two critical loading patterns were applied onthe deck. One is the two-span loading, and the other is single-span loading, as shown in
Fig.10a and Fig.10b, respectively. Table 1 lists both the experimental results and thenumerical results by the FEM model used the original design. In the two-span loading pattern,
the displacement response is close to that of the design value; in the single-span loading
pattern, the displacement response is more conservative than the design value. After theloading test that ensured the safety of the bridge, Chi-Lu Bridge then was officially opened tothe general traffic.

1826Fig.9 The 1st loading test after Repair Stage 1baFig.10 The 2nd loading test after Repair Stage 2

Chi-ChiFig.11 Sensor deployment for the 2nd loading test

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Table 1 Comparison between the experiment result and the design analysis
ChannelTwo-span loading
Experiment /Design

Single-span loading
Experiment /Design1

4.0 cm / 3.7 cm (↓)

7.4 cm / 9.3 cm (↓)

2

2.0 cm / 2.0 cm (↓)

5.2 cm / 6.3 cm (↓)

31.8 cm /2.0 cm (↓)

3.6 cm / 4.3 cm (↑)

43.9 cm / 3.7 cm (↓)

5.3 cm / 5.7 cm (↑)

50.0013° / 0 (+)

0.0405° / 0.0436° (+)

60.0009° / 0 (+)

0.0161° / 0.0113° (+)

Displacement

Slope

LOADING TEST ON CABLES
During the Repair Stage 2, the cables were replaced. The cable forces and cable vibrations
were measured simultaneously by hydraulic jacks (Fig.8b) and velocity-meters (Fig.12a)
respectively. The identified cable forces by FEM analysis together with the velocity spectrum
were compared with those measured by the hydraulic jacks.
Consider the example of Cable R33, one of the four longest cables of this bridge. Fig.12bshows the measured velocity spectrum with 23 characteristic frequencies. On the other handin the FEM analysis, a beam model subjected to gravity and geometry-nonlinearity wasadopted. The parameters to be determined are the cable force and the moment inertia. Afterthe optimization process, the identified cable force is 1.66E6 N, and the moment inertia is
5.0E-6 m4. Table 2 compares the characteristic frequencies from the measured and those fromthe analysis. The analyzed cable sag, shown in Fig.13, is 49.0 cm.
To verify the analyzed cable sag value, two surveyors measured the sag. From the survey,
the sag is 50.0 cm, which is close to 49.0 cm. To verify the identified cable force 1.66E6 N, thehydraulic jack in Fig.8b read the cable force 1.69E6 N, which is also closed to the identified
cable force1.66E6 N. Without applying formulas, and not knowing the sag and the moment
inertia in advance, the identified results yield good agreements with the surveyed sags as wellas those measured cable forces. With the identified sag and the identified moment inertia, andthe measured frequencies, by practical formulas (Zui H. et al. 1996), one can check on thecable force.
abFig.12 Cable vibration measurement and its signal spectrum

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Table 2 Characteristic frequency comparison between the measured and the identified
Mode
Number
1
2
3
4
5
6
7
8
9
10
11
12

Measured
(Hz)
0.769
1.499
2.248
2.997
3.751
4.511
5.279
6.044
6.833
7.591
8.386
9.176

Identified
(Hz)
0.769
1.496
2.247
2.999
3.755
4.514
5.278
6.047
6.822
7.604
8.393
9.190

Mode
Number13
14
15
161718192021
2223

Measured(Hz)
9.978
10.802
11.63112.45513.30314.16715.02115.907
16.82117.73318.635

Identified
(Hz)
9.996
10.81
11.63
12.47
13.32
14.18
15.05
15.93
16.83
17.74
18.67

Fig.13 The identified cable configuration with sag 49 cm

FBG-BASED LEVEL SENSORS
After opening to the general traffic, the bridge is concerned with its routine performance.
Therefore, to survey the level of the deck is necessary for the bridge management. Instead ofmanually operating a theodolite, FBG-based level sensors were applied to monitor the level ofthe girder. The level sensor is a device used to measure the change in elevation between
points by using the connected pipe principle, the buoyancy principle, and Hooke's Law of theFiber Bragg Grating. Fig.14 illustrates how it works (Carlos et al. 2011, and Halliday et al.
1997).
The level sensors were placed inside the girder, shown in Fig.15, where P3 and P4 wereimmediately beside the pylon and serve as reference points. Beginning at noon one day, andending at noon the next day, the girder elevation was continually monitored for 24 hours inFig.16. At 5:00 pm, the bridge temperature peaked, the steel cables lengthened, and P2 andP5 elevations decreased, whereas between 3:00 am and 8:00 am, the bridge temperature was
the lowest, the steel cables contracted, and P2 and P5 elevation increased.

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Fig.14 FBG-based level sensor

Fiber opticlevel meter

ChiChi -ChiPylon

Side-BentP3

P5 P6LuLu - ku
P1 P2P4

Fig.15 Deployment of FBG-based level sensors

Fig.16 Monitoring the level change for 24 hours
Furthermore, to observe the elevation change induced by traffic, the quasi-static signal inFig.16 could be filtered out. As shown in Fig. 17, two trucks drove onto the deck at different
time. The truck that first arrived had driven from Chi-Chi; thus, P1 and P2 simultaneously
dropped and P5 and P6 simultaneously rose. When the truck passed the pylon and crossed
over onto the Lu-Ku end, P1 and P2 increased to their original positions and P5 and P6
dropped simultaneously. When the truck left the bridge, the girder returned to its original
stable level. About 200 s later, the second truck drove over the bridge from Lu-Ku. The change
in the girder’s elevation was similar to that under the first truck, only the order and timing ofthe dropping and rising was reversed. It is demonstrated that the FBG-based level sensors
can trace the girder level influenced by temperature change and traffic loading. From theregular observation, the girder level is stable without obvious settlement since the bridge’s
opening to the general traffic.

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Fig.17 Vertical response to two trucks

FBG-BASED VIBRATION SENSORS FOR CABLES
Cable forces compose an important health index of a cable-stayed bridge. For evaluating thecable forces, conventionally, electric sensors are attached on cables to measure the vibration.
As shown in Fig.12a, through electric wires, sensors are parallel connected to the datalog.
With limited expensive sensors and channels, to permanently measure all 68 cables
simultaneously becomes very difficult if not impossible. Besides the economic factor,
lightening and rain are hazardous to electric instruments.
To overcome the difficulties mentioned above, self-assembled FBG vibration sensors areinstalled on the cables as illustrated in Fig.18. One optic channel measures 17 cables, andonly four channels are required for the 68 cables. Through the Internet, the cable vibration
spectrum are automatically analyzed and reported back. By this way, all cable forces can beevaluated simultaneously without routine manual labor works. From the measured data, allcable forces are considered to be normal since the bridge’s re-opening to the general traffic.

Fig.18 One optic channel measuring 17 cables’ vibration

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WIND/RAIN CABLE VIBRATION
Geographically, Taiwan is constantly suffering the threat of both earthquakes and typhoons.
Typhoons bring torrential and strong wind, cause floods to damage bridges, and probably
induce the wind/rain cable vibration on cable-stayed bridges. With the FBG vibration sensors
for cables, the wind/rain cable vibration of Chi-Lu Bridge had been measured and video-taped.
On July 28, 2008, Fung-Wong Typhoon went across Taiwan. Fig.19 shows the vibration
spectrum of Cable R33 before and during the wind/rain vibration. From Fig.19, the vibration
spectrum before the wind/rain vibration was distributed along each mode, but was moreconcentrated and stronger on the second mode during the wind/rain vibration. Besides, thestrong cable vibration was also video-taped. The field observation demonstrates that some
cables of Chi-Lu Cable-Stayed Bridge may behave in wind/rain instability.
1 × 104

Before wind/rain vibration
風雨馳振前

4 × 105

During wind/rain vibration

HzFig.19 Spectrum comparison before and during wind/rain vibration

To further study the inherent cable damping (Pacheco, B. et al. 1993), a pull-and-release
experiment, as shown in Fig. 20, was carried out to determine the damping ratio and theScruton Number:

Sc =mξ
ρD 2(1)

Where m is the mass per unit length, ξ is the damping ratio, ρ is the air density, and Ｄ is thecable diameter. Table 3 lists the Scruton Number for three cables. According to PTI (1993), if
the Scruton Number is less than 1.7, wind/rain induced vibration will readily occur. The
measured wind/rain vibration signal, the taped video, and the pull-and-release experimentagree with the PTI recommendations.
R33 First Mode0.879 Hz

ξ=0.2 %
ξ=0.36 %ξ=0.58 %Fig.20 The pull-and-release forced vibration and the damping analysis

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Table 3 Scruton Number for the first three modes of three cables
Mode
1st2nd3rdR33
1.86
1.12
1.21

R31
2.72
2.96
2.96

R29
3.31
2.60
1.89

CONCLUSIONS

Because of Chi-Chi Earthquake, Chi-Lu Cable-Stayed Bridge performs as a rare but valuable example
for research on cable-stayed bridges about the near-fault strike, damage patterns, emergent treatments,
repair processes, safety loading tests before re-opening, and the long-term structural monitoring. The
following summarizes this study.
(1) About the girder, loading test on the deck was performed and analyzed. In addition, FBG-based
level sensors are installed and demonstrated effective to temperature change and traffic loading.
(2) About the cables, loading test 、 force evaluation 、 wind/rain instability observation during
typhoon、pull-and-release test, and FBG-based sensors for all 68 cables, etc., were carried outand analyzed.
(3) From the regular measurements, the cable system and the deck elevation are normal since opening.
However, some long cables may be insufficient to wind/rain stability.
(4) To reduce routine labor work in survey, FBG-based sensors are self-assembled and installed on
the bridge. It’s demonstrated that FBG sensors have substantial potential for monitoring
cable-stayed bridges.

ACKNOWLEDGMENTS
The authors wish to express their thanks to Directorate General of Highways, MOTC for theresearch funding and the field experiment assistance on Chi-Lu Cable-Stayed Bridge.

REFERENCES

Carlos, R., Carlos, F., and Joaquim, F. (2011). “Fiber-optic-based displacement transducer to measurebridge deflections,” Structural Health Monitoring, Vol. 10, No. 2, 147-156.
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Halliday, D., Resnick, R. and Walker, J. (1997). “Fundamentals of Physics,” Wiley Interscience
Publications, New York, USA.
Pacheco, B., Fujino, Y. and Suelk, A. (1993). “Estimation curve for modal damping in staycables with viscous damper,” Journal of Structural Engineering, ASCE, Vol.119, No. 6,1961-1979.
Post-Tensioning Institute Committee on Cable-stayed Bridges (1993) "Recommendations forstay cable design, testing, and installation."
Zui H., Shinke T., and Namita Y. (1996), “Practical formulas for estimation of cable tension byvibration method,” Journal of Structural Engineering, ASCE, Vol.122, No. 6, 651-656.

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