Case Study - Bitan Suspension Bridge Monitoring

Hsin Dien, New Taipei City, Taiwan 2013

Bitan Suspension Bridge Monitoring System 2

Aim Monitor the Bitan Suspension Bridge to enhance pedestrian safety during crowded periods

Location Bitan, Hsin-Dien, New Taipei City, Taiwan

System Integrators

China Engineering Consults, Inc. (CECI), Taipei, Taiwan 3L Technologies, Inc., Jhu-Nan, MiaoLi, Taiwan

Date On going since June 2011

Instrument Micron Optics sm130-500 Optical Sensing Interrogator

Sensor 2 X FS530, 3-D FBG accelerometer, 3L Technologies Inc. 10 X FS520, 2-D FBG accelerometer, 3L Technologies Inc.

Software CECI Custom Sensing Software, Micron Optics ENLIGHT Software

FBG Benefit

•  Multiplexed sensors minimize cable runs and simplify installation

•  Rugged sensors perform well in long term outdoor installations •  Long range capability allows the interrogator to be located far

from the bridge relative to electrical sensor.

Bitan Suspension Bridge Monitoring System 3

Bitan Suspension Bridge Monitoring System

•  The 200-meter long suspension bridge was first constructed in 1937, renovated 2001

•  A total of 12 2D/3D FBG accelerometers are installed on the lateral sides of the bridge

•  2 sets of 3D FBG accelerometers are installed near the center of the bridge

•  As the bridge has become a historical landmark and a popular tourist attraction, loading on the bridge has increased substantially

•  Municipal government monitors the behavior of bridge continuously as a safety measure

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Sensor and instrumentation placements 5

•  Each interrogator channel is allocated 3 sets of FBG Accelerometers

•  The sm130-500 interrogator is located inside a monitoring station 1km away from the bridge

•  Spacing between adjacent sensors is 40 meters

•  Spacing between opposing sensors is 4 meters

•  Sensors are installed at the far ends of steel support beams

Installation of FBG Accelerometers 6

FBG Accelerometer signals in all 4 channels 7

Channel 1, southeast end, 3 sensors, 13 FBG’s, 2-way insertion loss ~ 7dB

Channel 3, northeast end, 3 sensors, 13 FBG’s, 2-way insertion loss ~ 9dB

Channel 4, northwest end, 3 sensors, 12 FBG’s, 2-way insertion loss ~ 9dB

Channel 2, southwest end, 3 sensors, 12 FBG’s, 2-way insertion loss ~ 8dB

Bridge structural analysis of sensor data - 1 8

•  A total of 26 axial acceleration parameters are monitored at 100Hz

•  During high-traffic periods, lateral movements of +/-15cm are observed

•  Data is viewed in both time- and frequency-domains to gain a better understanding of the structural dynamics of the bridge

•  Results will be used to determine if structural modifications can be made to diminish lateral movement and enhance pedestrian safety

Example Time and Frequency Data from One Sensor 9

Channel S3 FBG Accelerometer data, June 6 2011

Same Sensor Two Days Later 10

Channel S3 FBG Accelerometer data, June 7 2011

Bridge structural analysis of sensor data - 2

•  The previous two pages show data acquired over a ten minute period on two different days, June 5 and 7, 2011

•  Data clearly shows there was significantly more foot traffic on June 5

•  All 3 axes (X,Y,Z) have strong frequency content at 4.5Hz – this is the vibration frequency of the lateral support beam, not the bridge main structure

•  Main frequency of Y-axis falls at 0.5Hz, Z-axis at 1.5Hz, and the X-axis also at 0.5Hz

•  Typically, pedestrian foot-step frequency is 1.65~2.35Hz, and flexing vibration frequency is around 0.44Hz. This is in good correspondence with the measured 0.5Hz on X- and Y-axial

•  With more crowd and more vibration amplitude on June 5, there is coupling of acceleration from Y-axis to Z-axis, as 0.5Hz was also observed on Z-axis

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Conclusive Remarks 12

•  An FBG-based accelerometer vibration monitoring system has been successfully installed on the Bitan Suspension Bridge

•  Data retrieved from the system provide valuable dynamic information on bridge, during peak and off-peak hours

•  The bridge showed significant twisting and lateral movement when more crowd is on the bridge, and virtually no twisting behavior and little lateral movement when some or little crowd is on the bridge

•  The bridge owner wants to minimize bridge twisting and lateral movements to provide a safe and leisurely environment for pedestrians visiting the bridge

•  The monitoring system has been in full operation since June 2011, with zero down time

Acknowledgements 13

•  Micron Optics International, AG I-Wen Wu, Tel: +886-37668229, Mobile: +886-93982658, moi.agsea@.msa.hinet.net web: www.micronoptics.com

•  3L Technologies Inc. Ms. Melanie Tsai, Tel: +886-37637448, Mobile: +886-989382786, melanie@mail.llltec.com web: www.llltec.com

•  China Engineering Consultants, Inc. Dr. Ming-Hung Chen, Tel: +886-287325567, (M):+886-935122510, mhchen@ceci.org.tw web: www.ceci.org.tw