Hong Kong’s Tsing Ma Bridge is the world’s longestspan suspension bridge carrying both road and

rail traffic. As with other long-span cable-supportedbridges, it can move from several centimeters to sever-al meters under different types of loading conditions.

Although these displacements or deformations maynot create hazardous conditions for traffic actually onthe bridge, as they increase in size, they significantlyaffect the bridge’s structural integrity and maintenanceneeds. Real-time measurement accuracy of GPS hasimproved to centimeter-level precision recently, makingit well-suited to monitor variations in three-dimension-al bridge motion in response to wind, temperature, andtraffic loads. This article describes the layout and tech-nical performance requirements of such a system anddiscusses the results in bridge structural health moni-toring.

The Highways Department of Hong Kong SpecialAdministrative Region designed the Wind andStructural Health Monitoring System (WASHMS) forthree large cable-supported bridges, Tsing Ma Bridge,Kap Shui Mun Bridge, and Ting Kau Bridge, in the TsingMa Control Area (TMCA) of western Hong Kong. TsingMa Bridge measures 1,377 meters across the Ma Wanshipping channel. Built at a cost of approximately

Kai-yuen Wong (Eur Ing, chartered engineer) is senior engineer in the Bridge Health Section of Tsing

Ma Control Area Division, Highways Department,Hong Kong Special Administrative Region.

King-leung Man and Wai-yee Chan (Ir, chartered engineers) are engineers in the

same Bridge Health Section.

Monitoring Hong Kong’s Bridges

Real-Time Kinematic Spans the Gap

Modern cable-supported bridges carryenormous loads across great distances, in

part due to their designed capability tomove, ever so slightly, under varied

conditions. In Hong Kong, a real-timekinematic (RTK) GPS monitoring system

provides the centimeter-level accuracy, inall weather conditions, to detect bridge

movements beyond normal ranges.Engineers can then conduct inspections

or maintenance needed to maintain longterm structural health.

by Kai-yuen Wong,King-leung Man,

and Wai-yee Chan

HK$7.2 billion (US$925 million), the suspension bridgeprovides high-speed rail and road connections to HongKong’s busy international airport. Ting Kau and KapShui Mun bridges, 1177 and 820 meters long, respec-tively, use cable-stayed construction.

We saw an opportunity to improve the efficiency andaccuracy of the existing monitoring system with GPStechnology, to monitor the displacements of the mainsuspension cables, decks, and bridge towers. We usethese measured displacement values to derive the rele-vant stress status acting on major bridge components.

Displacement of bridge structure serves as an effec-tive indicator of its structural performance condition.We are establishing Finite Element Models (FEM) withreference to GPS measurement for the three bridges;these models will enable us to identify critical structur-al components, for the purpose of long-term inspectionand maintenance.

The real-time GPS On-Structure InstrumentationSystem (GPS-OSIS) constitutes five sub-systems: theGPS receivers themselves, local and global data acqui-sition systems, a computer system, and a fiberoptic net-work. GPS-OSIS improves the efficiency and accuracyof WASHMS monitoring and evaluation activities by:

� reporting displacements, reflecting loading andstress conditions

� providing more information to estimate distribu-tion of stresses/strains in major bridge components

� documenting abnormal loading incidents such astyphoons, earthquakes, traffic overloads, and ship colli-sions with bridge piers

� detecting damage or accumulated damage inmajor bridge components

� estimating bridge load-carrying capacities and val-idating design assumptions and parameters

� providing information for planning and schedul-ing bridge inspection and maintenance activities.

OverviewStructural design of cable-supported bridges is basedon displacements. Displacements or movements ofcables, decks, and towers that deviate from the design’s

geometrical configurations will redistribute stressesand strains among bridge components, affecting theload-carrying capacity of the whole bridge.

Until recently, the WASHMS for Tsing Ma Bridge(TMB), Kap Shui Mun Bridge (KSMB) and Ting KauBridge (TKB) consisted of 774 sensors in seven majortypes: anemometers, temperature sensors, dynamicweigh-in-motion sensors, accelerometers, displacementtransducers, level sensing stations, and strain gauges.The commissioning of GPS-OSIS in January, 2001,brought an additional 29 sensors into the overall sys-tem. Now fully operational, GPS-OSIS functions as anadditional system integral to the existing WASHMS forimproving the efficiency and accuracy of bridge healthmonitoring works.

Although the annual TMCA geodetic survey mea-sures the geometrical configurations of the threebridges, this serves for monitoring geometrical varia-tions due to permanent and long-term structuralactions such as dead loads and super-imposed deadloads. Geodetic surveying cannot detect the bridges’instant responses to transient and variable structuralactions such as primary live loads. Before GPS deploy-ment, accelerometers monitored geometrical varia-tions, but this method, due to uncertainties or unknownintegration constants, only provides relative local dis-placements at measurement locations, and does notgive overall absolute displacements.

This difference becomes very important in the case,

The three bridgesand major high-ways of westernHong Kong’s TsingMa Control Area.

Bridge AcronymsGPS-OSIS GPS On-Structure

Instrumentation SystemHKSAR Hong Kong Special

Administrative RegionHyD Highways Department

KSMB Kap Shui Mun BridgeLFC-OSIS Lantau Fixed Crossing —

On-Structure Instrumentation System

TKB Ting Kau BridgeTMB Tsing Ma Bridge

TMCA Tsing Ma Control AreaWASHMS Wind and Structural Health

Monitoring System

Receiver atop TsingMa Bridge tower,with Kap Shui MunBridge in the distance

for example, of a strong wind swaying the bridge deckalignment steadily to one side, or daily temperaturevariation raising or lowering bridge deck level, and sus-taining these changes over a period of time beforereturning to normal position. Accelerometers cannotdetect such continuous or steady displacement of themassive deck, only instant local vibration, and they donot give an overall true or absolute displacement. Theirlimitation in detecting slow global displacement wouldintroduce error in mathematical derivations.

History. During the initial WASHMS design stage in1992, we considered other measuring technologiessuch as infrared and laser for displacement monitoring.As these required visibility over a definite distance andcould not operate fully under severe weather condi-tions, we dropped them from consideration. We alsoconsidered and then dropped GPS, due to its less thanrequisite accuracy at that time.

As real-time accuracy of post-Selective AvailabilityGPS for survey application has improved to approxi-mately 20 millimeters vertically and 10 millimeters hori-

zontally, the Highways Department (HyD) of HongKong Special Administrative Region (HKSAR) re-inves-tigated GPS use for monitoring the bridges’ geometricalconfigurations. The reported suitability of GPS as anall-weather system also became an important factor inits adoption. This has yet to be fully demonstrated,although the impending typhoon seasons should put itto the test.

The Case for GPS-OSISIn the past, level sensing stations and servo-typeaccelerometers monitored bridge responses in TMCA.The level sensing stations, at a sampling rate of 2.56 Hz,provide real-time monitoring of displacements, with ameasurement accuracy of approximately 2 millimetersat typical deck sections at vertical planes only. Theselevel sensing stations are used only on the TMB andKSMB, but not on TKB because the installation cost ofpipeline systems along the bridge would be high (alevel-sensing system measures pressure changes in sen-sors installed along a network of flexible hoses).However, they cannot measure the lateral and longitu-dinal displacements of the deck sections in the horizon-tal plane.

High precision servo-type accelerometers monitorthe local vertical, lateral and rotational accelerations ofthe decks, cables and bridge-towers of TMB, KSMB andTKB. Displacement monitoring requires double-integra-tion of the measured acceleration data to determinethe three-dimensional motions of the three cable-sup-ported bridges. Because the natural frequencies of thedecks are of very low values, double-integration oftheir acceleration data cannot reflect actual displace-ment values.

After trial tests on TMB in 1999 and 2000 demonstrat-ed GPS applicability and the required accuracy level,HyD decided to employ GPS with Real Time Kinematic(RTK) function to monitor displacements of the threecable-supported bridges. GPS-OSIS monitors real-timemotions of the main suspension cables, decks, andbridge towers of TMB, KSMB, and TKB, derives the rele-vant stress status acting on the major bridge compo-nents, and works with other WASHMS instrumentationsystems to monitor overall bridge health and scheduleinspections and maintenance activities.

System DescriptionFigure 1 shows the five major system components. TheGPS sensory system consists of two base reference sta-

FIGURE 1 GPS-OSISstructure

FIGURE 2 Location ofGPS rover receiverson Tsing Ma Bridge

tions and 27 dual-frequency, 24-channel, carrier-phaserover stations, collecting data at a 10 Hz sampling rate,with position latency less than 0.03 seconds.

The local data acquisition system consists of threejunction cabinets with fiberoptic multiplexers, one oneach bridge. One cabinet with fiberoptic multiplexersin the bridge monitoring room constitutes the globaldata acquisition system. The GPS computer systemcomprises two workstations and a custom software system. The fiberoptic network includes 6.5 kilometersof single mode fiber and 18 kilometers of multimodefiber.

Real-time bridge monitoring with up to centimeter-level precision has a time delay of approximately twoseconds for display of bridge motions in the monitor-ing room due to data transmission, processing, andgraphical conversion times. As an integrity monitoringsystem, two base reference stations monitor the qualityof the RTK correction broadcast and RTK performance,including GPS availability and geometry. GPS-OSISoperates 24 hours a day with automatic control for dataacquisition, processing, archiving, display, and storage.Post-processing the GPS data with relevant data fromother WASHMS sensors provides the structural evalua-tion needed by HyD engineers.

Receiver LocationWe installed the rover receivers on bridge locationswhere we expect to see maximum displacements: at theedges of deck sections at midspan and quarterspans,where applicable, and at towertops. Figure 2 showsTMB rover locations and measured displacements.Table 1 shows normal bridge displacements.

Most GPS receivers are adjacent to level sensing sta-tions or to accelerometers, so that we can compare theGPS results with data from other sensors.

The reference stations carried choke-ring antennaswith radomes to reduce interference, RF jamming, andvibration. Because of size and weight considerations,rover positions used micro-band antennas. To reducesignal interference from physical obstruction, wemounted all rovers with view angles greater than 15

degrees, taking care to avoid obstructions caused bydoubledecker buses and high-body vehicles running inthe slow vehicle lane closest to bridge edges.

TMB receivers. Four pairs on deck, one pair on maincables, one pair each at Tsing Yi and Ma Wan towertops;

KSMB receivers. One pair on bridge deck, one paireach at Ma Wan and Lantau towertops;

TKB receivers. Two pairs on bridge deck, one each atTsing Yi, Central, and Ting Kau towertops

Reference stations. Two base reference stations sitatop the storage building adjacent to the bridge moni-toring room.

GPS-OSIS provides real-time motions of bridge decksat 10Hz, derives the variation in geometrical configura-tion at centerlines of deck sections and records thetime-history motions of the bridges at measuring loca-tions, and performs similar monitoring procedures forthe towers.

Position latency in the GPS sensory system is lessthan 0.03 seconds, significantly better than when wefirst evaluated GPS systems in 1992. The RTK techniquemeasures bridge displacement even when bridge com-ponents move at irregular speed, for instance, the aero-dynamic effects under varying wind conditions. To fullymonitor bridge structural health, we must be able tosee structural response to such conditions (whosetime of occurrence is captured by other sensors) withinthis 30 millisecond timeframe. The “raw” GPS data cap-tured by the RTK GPS meets our basic requirements for

TABLE 1 Normal bridge movements measuredby GPS

Normal Range Normal RangeTMB Deck Vertical 650 mm Tower Longitudinal 100 mm

Lateral 250 mm Lateral 50 mm

KSMB Deck Vertical 80 mm Tower Longitudinal 40 mmLateral 50 mm Lateral 50 mm

TKB Deck Vertical 150 mm Tower Longitudinal 60 mmLateral 50 mm Lateral 80 mm

FIGURE 3 Location of GPS rover receivers on Kap Shui MunBridge

FIGURE 4 Location of GPS rover receivers on Ting KauBridge

real-time displacement monitoring.Processing and analysis by the computer system

takes two seconds, then the system graphically dis-plays the patterns of real-time information as shown inFigures 5 to 8.

GPS-OSIS Information CommunicationThe monitoring system must continue to function atfull capability under the adverse weather or severeenvironmental conditions that would validate thebridge structural design. Although it is unlikely thatthe extreme design conditions would occur frequentlyduring the 120-year design life for the three bridges, asevere storm would induce higher stress/strain in thestructural components, bringing them closer toextreme design conditions. The correspondingstress/strain measurement of structural responses incritical structural components can validate the designvalues of environmental loading and design parame-ters, confirming the confidence level of safe operation.

For this reason, the stability and reliability of datatransfer must meet higher than normal specifications.GPS-OSIS therefore uses a stable, effective fiberopticnetwork for data transmission. Optical fiber, insensitiveto electromagnetic waves and lightning effects, pro-vides high quality and speed of data transmissionunder bad weather and severe environments such asthe strong electromagnetic field generated by high volt-age circuits. (High voltage power cables run along thebridge service corridors, in the same cable ladder sys-tems as the fiberoptic cable, on separated, higher levelracks.) In addition, advanced fiber transceivers candetect any interruption of data transmission and sendan alarm to alert maintenance personnel and provideproblem locations.

Each set of receivers uses three asynchronous serialports for data collection, differential signals transfer,and remote monitoring. Each port has a data transmis-sion rate of 19,200 Baud. Multimode fibers deliver thereceiver signals to local data acquisition stations wherethey are converted to high frequency signals and deliv-ered to the global data acquisition station by high qual-ity single-mode fiber.

The longest backbone fiber network, connectinglocal and global data acquisition centers, extendsabout 3.5 kilometers. The longest branch fiber network,connecting receivers to local data centers, measuresabout 1.5 kilometers. Replacing several hundred cop-per data transmission wires with one single-mode fibersignificantly improves data transmission efficiency andsimplifies network maintenance.

Processing and AnalysisThe fiberoptic network delivers geodetical coordinatesfrom the 27 rover stations synchronously to the GPScomputer system. The computer system uses one oper-ational workstation, named GPS-OW, for processing,archiving, storage, and graphical treatment to displaythe real-time bridge motions. The GPS-OW also handlessystem operation and controls the GPS receivers, localdata acquisition stations, the global data acquisitionstation, and the fiberoptic network. When a problem

FIGURES 5 and 6 Screen displays of instantaneous displacements of deck,cables and towertops (above) and time series deck displacements(below) of Tsing Ma Bridge. Photo at top shows receiver on TMB suspen-sion cable at mid span and adjacent post-mounted receiver.

occurs or the measured data deviation exceeds pre-setparameters, the system alerts the operator.

A second workstation for analysis, GPS-AW, post-processes the GPS data and works with the otherWASHMS sensor data to evaluate current bridge healthconditions. These two workstations are both connectedwith other WASHMS workstations and servers. ShouldGPS-OW break down, GPS-AW can assume its role tokeep the GPS-OSIS operating normally.

Comparison with Level Sensing. To verify GPS-OSISaccuracy and compatibility, the system compares itsresults with those from the level sensing stations of theLantau Fixed Crossing — On-Structure InstrumentationSystem (LFC-OSIS) for TMB and KSMB. Figure 9 showsthe vertical displacements from the two systems. Thelevel sensing stations’ sampling rate and accuracy are2.56 Hz and �2 millimeters respectively. The matchingresults indicate accuracy and compatibility of GPS-OSISdata to LFC-OSIS.

Applying the ResultsGPS-OSIS integrates with other structural analysis soft-ware to assess stress status in major bridge compo-nents. The bridge health monitoring system currentlyevaluates the bridges in three aspects: load-carrying

capacity, serviceability and durability.� Load-carrying capacity relates to structural stabili-

ty and ultimate strength of materials. Estimating load-carrying capacity targets quantification of the bridge’sactual safety capacity, to avoid catastrophic damage.

� Serviceability relates to the deformation, crackand vibration of the bridge under normal loading con-ditions.

� Durability assessment focuses on damage to thebridge, causes of damage, and its influence on physicalproperties of materials.

Together, these three aspects will provide valuableinformation facilitating planning and scheduling ofinspection and maintenance activities.

The WASHMS, including GPS-OSIS, is not required toassist decisionmaking regarding bridge operationsbecause a separate monitoring system produces refer-ence information for operational purposes, for examplediverting wind-susceptible vehicles during severeweather conditions.

Bridge Health MonitoringWind Load and Response Monitoring. In cable-supportedbridge design, wind structure/parameters for stabilityanalysis and windtunnel testing are based on winddata collected by weather observation stations far fromthe bridge site. Therefore we must observe wind condi-tions at the site itself to validate design assumptionsand parameters. WASHMS wind load and responsemonitoring has five main steps:

� record wind speeds in three orthogonal directionsand responses of accelerations, displacements, andstrains;

� extract the parameters of hourly mean winds,three-second gust winds, roughness of terrain, winddirections, wind incidences, wind turbulence intensi-ties, and wind power spectra from the wind data underboth typhoon and non-typhoon conditions. These para-meters will be compared with the design extreme val-ues for monitoring of wind loading on the bridges;

� plot the instant wind speeds against instant bridgemotions at tower tops and mid mainspan deck sectionsunder typhoon conditions. We will compare these plotswith similar plots from aeroelastic windtunnel testingfor design validation and wind response monitoring;

� plot bridge response spectra under typhoons todetermine the mechanical magnification factor oradmittance function of the bridge structures;

� plot the stress demand ratio diagrams in key windresisting components for wind loading monitoring.

GPS-OSIS plays an important role in plotting windspeeds against bridge motion (third item, above)because the GPS data can display the global motionover the full length of the bridge, which in the pastcould not be accurately obtained by accelerometers.

Temperature Load and Response Monitoring. Ambienttemperature, wind speed fluctuations, material proper-ties, surface characteristics, and section geometry allaffect bridge thermal response. The range of verticaldeflection at mid-span of TMB is around 400 millimetersdaily and can increase to more than one meter in sea-sonal variation over the year.

FIGURE 7 Screen dis-play of instanta-neous displacementof Kap Shui Munbridge deck

FIGURE 8 Time seriesdisplacement of KapShui Mun bridgedeck and towertops

WASHMS temperature load and response monitor-ing includes temperature measurements (includingeffective bridge temperatures and differential tempera-tures in sections of decks, towers, and cables) in air,structural steel sections, stainless steel sheets, con-crete sections, and asphalt pavement, and plottingmeasured temperatures against time and/or bridge dis-placements at instrumented locations. Then computa-tions predict critical temperature, estimate or calcu-late thermal-induced stresses in the bridgesuperstructure, and assess the significance of thermal-induced stresses to serviceability and ultimate loadcharacteristics. GPS-OSIS measures thermal move-ments in suspension cables, decks, and towers. Theseresults will not only be used in computation of ther-mal-induced stresses in decks, but also in monitoringthe performance of the polytetrafluoroethylene (amaterial with very low friction coefficient, used tofacilitate sliding movement) bearings supporting theTMB and KSMB decks.

Highway Load and Response Monitoring. Traffic jamsconstitute the major design assumption in deriving therequired intensity of vehicular traffic loading for long-span bridges. This in turn relies on the traffic loadedlength, which is based on the number of traffic jamsformed daily, the position, duration, and vehicle distri-bution of traffic jams, and some assumptions about traf-fic flow at traffic jams.

Traffic load and response monitoring in WASHMSfirst measures traffic flow and traffic loads usingdynamic weigh-in-motion sensors installed on the car-riageways. Then the system estimates fatigue damagein key structural components induced by vehicles,based on measured load results and measuredresponse results from strain gauges, level sensing sta-tions, and GPS-OSIS.

Railway Load and Response Monitoring. Two trains trav-elling in opposite directions can induce a 650 millime-ter vertical deflection at mid-span of TMB. Normal high-way loadings are fairly nominal in comparison.

The railway trackforms of TMB and KSMB are sup-ported by longitudinal I-girders installed under thedeck section cross-frames, distributing railway load tothe deck-sections. As there is no bridge sensory systemto measure the dynamic train loading, railway load andresponse monitoring is carried out indirectly withstrain gauges installed in the longitudinal I -girders. Thestrain results then help plot the influence lines inducedby each set of multiple-axle loads, and the total train-loading is obtained by superposition of individual influ-ence lines. GPS-OSIS measurements at towertops,cables, and deck sections will significantly improve theaccuracy in estimating the structural effects, particular-ly for fatigue damage, under dynamic train loading.

Cable Force Monitoring. The bridges’ cable systems arethe key components for load resistance, particularly forvertical loads. For the TMB suspension bridge, GPS-OSIS measures the displacements of main suspensioncables and deck-sections. For the cable-stayed bridges,GPS-OSIS measures the displacements of deck-sectionsand towers. In both cases, measured net displacementsin cables are converted to cable forces and then com-

pared with their design values at both serviceabilityand ultimate limit states.

Stress Monitoring. Structural design is usually basedon structural idealization models, where componentsare idealized as linear members at section centerlines.If a bridge’s axes of towers and deck deviate fromthose of idealized models, its load-carrying capacityand internal force distribution may change as well.

Structural health evaluation work by GPS-OSIS startswith the input of the derived displacement values tothe computerized global structural models, which sim-ulate the global stiffness of the whole bridge. Matrixcomputations obtain the distribution of internal forces

FIGURE 9 Comparisonof vertical displace-ment measurementby GPS and levelsensory system

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-400300 600 900 1200 1500 1800 2100 2400 2700 3000

Time (seconds)Tsing Ma Bridge - at mid-main span of bridge deck

GPS measurement (North)Level sensor measurement (North)

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-600 100 200 300 400 500 600 700 800 900 1000

Time (seconds)Kap Shui Mun Bridge - at mid-main span of bridge deck

Ver

tical

dis

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illim

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s)

Towertop receiveron Tsing Ma Bridge

and apply the results to the respective local structuralmodels of key bridge components such as cross-frameand longitudinal truss and connections at deck-towerand at cable-deck, to obtain more detailed informationon distribution of stress/strain. Stress/strain distribu-tions in different structural components located at dif-ferent locations have different distribution patternsunder the actions of dead and live loads.

Employing GPS-OSIS for displacement monitoringand other instrumentation systems (LFC-OSIS and TKB-OSIS) for stress/strain and load monitoring can verifyload-deformation relationships in key components, andestimate or determine load-carrying capacities and po-tential damage locations in key structural components.

ConclusionGPS monitors the motions of key bridge locations inreal-time. By measuring these variations in geometricalconfigurations and incorporating results from other

sensors, we can more accurately and reliably monitorand evaluate structural performance and health condi-tions of major cable-supported bridge components. Wecan then apply the results to planning and implement-ing inspection and maintenance activities.

AcknowledgementsThe authors wish to express thanks to Mr. Y.C. Lo,Director of Highways, for his permission to publish thispaper. We also extend thanks to colleagues in TMCADivision of Highways Department for their cooperationin GPS installation, to the Electrical & MechanicalServices Department for GPS supervision work, to LeicaGeosystems Limited for GPS installation work, and last-ly to Dr. C.K. Lau, former Deputy Director of Highways,for his support of GPS work in TMCA. Any opinionsexpressed or conclusions reached in the text are entire-ly those of the authors. �

ManufacturersLeica Geosystems AG (Heerbrugg, Switzerland) MC500receivers serve as both rovers and reference stations inthe GPS-OSIS, with Leica AT504 choke-ring antennas onthe reference stations. The system also utilizes customGPS software from Leica Geosystems, TC1885 andTC8108 fiberoptic multiplexers from Nova Electronics,and two Silicon Graphics 540 workstations.

Tsing Ma Bridgespans the Ma Wanshipping channel.

MC500 FEATURES• 24 channels (12 of L1 & 12 of L2)• Built-in RTK positioning at up to 10 times per

second• 50 msec position latency• Automatic OTF phase ambiguity resolution• ClearTrak™ technology for robust data

acquisition even in difficult trackingenvironments

• Output of RTK, CMR and differential RTCMcorrections

• Designed for unattended or remotely controlled operation

For information on Leica products, contact:USA Tel: +1 770 447 6361 Fax: +1 770 447 0710Switzerland Tel: +41 71 727 31 31 Fax: +41 71 727 46 74Singapore Tel: +65 776 9318 Fax: +65 774 7145

www.leica-geosystems.com

MC500 installed on the TsingMa bridges in Hong Kong

©Reprinted from GPS WORLD, July 2001 AN ADVANSTAR ★ PUBLICATION Printed in U.S.A.