桥梁监测

CR5000s from CSI helpresearchersplan for better infrastructure health and design
Evaluating bridge deck performance in Montana

 Amidst a vast expanse of farming and range lands,bridgedesigners from the Montana Department ofTransportation(MDT)recognized a unique opportunity to evaluatehighway bridgedeck design. Near the small town of Saco, Montana,three bridgeswere constructed less than one mile apart along thesame highwaycorridor, Montana State Route 243.  Consequently,each bridgewill experience identical demands from weather, traffic,and wintermaintenance.  Each bridge has three 15 m spans,incorporatinga rein-forced concrete deck-on-pre stressed-stringerdesign withidentical global dimensions and substructurecomponents.  Theonly difference among them is the compositionof the rein-forcedconcrete deck.  The "Conventional" deckrepresents thestandard deck used in Montana – a conventionalconcrete with astandard rebar pattern.  The "Empirical" deckalso usesconventional concrete, but employs approximately one-thirdtheamount of reinforcing steel.  The "HPC" deck uses astandardrebar pat-tern with a high-strength concrete. MDTcontracted withThe Western Transportation Institute (WTI) atMontana StateUniversity to design and install an instrumentationsystem, acquiredata from this system and perform subsequentanalysis to evaluatethe performance of the three bridge decks.

Spread Spectrum radios and an Internetlinktransmit data from bridges to MSU.

Instrumentation

During the planning phases of the project, finiteelementanalysis was done on the three deck configurations toidentifycritical locations in the bridge decks for placingstraingages.  Based on this analysis, the instrumentationisprimarily concentrated in a single driving lane of one 15 m spanofeach three-span bridge (approx.1/6th of the bridge). Thestrain gages were placed in each bridge deck prior to castingtheconcrete.  The gages were positioned to monitorbothlongitudinal and transverse strains at three differentdepthsthrough the thickness of the deck. Three different straingagetechnologies were used in each bridge: 35 Vishay foil straingagesbonded to reinforcing steel, 7Vishay embedment-type straingagessuspended in the concrete and 16 Geokon vibrating wire straingagessuspended in the concrete.  Additionally, 16 temperaturesarerecorded via thermistors internal to the vibrating wirestraingages.  To monitor ambient conditions, a weather stationwaserected at the Saco Public School, located approximately onemilefrom the bridge sites.  Temperature, barometric pressure,windspeed/direction, and relative humidity values are measuredat15minute intervals and posted to the Internet for publicviewingathttp://wtigis.coe.montana.edu/saco/Saco_Current.htm

Typical strain output for a series of gagesoverthe bent during a single-truck, live load test.

Data Acquisition

All strain data is collected and stored using a singleCR5000Measurement and Control System mounted under eachbridge.  Thebonded and embedded gages require WheatstoneBridge arrangementsdesigned and fabricated by WTI. Correspondingvoltages are routedthrough a single AM16/32 multiplexer. Vibratingwire strains andtemperatures are read using a single AVW1 VibratingWire Interfacecoupled with an AM16/32multiplexer.  All gagesare read onceevery hour.  Data from each bridge isperiodically transmittedto WTI through the Internet via a networkof RF400Spread Spectrumradios based at the Saco School. Weather data is monitoredusing a CR10X Measurement and ControlSystem and transmitteddirectly to WTI via the Internet.

Long-Term Evaluation

One of the primary objectives of the Saco Bridge DeckEvaluationis the comparative analysis of long-term bridgedeckperformance.  This objective will be accomplishedthroughqualitative and quantitative analyses of bridge deckconditionsover a two-year period.  Ideally, the project willbe extendedfor several years to more fully evaluate relative deckperformancethroughout their service lives.  Qualitativeanalyses includecrack mapping and surveying operations.Quantitative analyses aremostly focused on how strains in thebridge decks change whenexposed to changing weather conditions andvehicle loadevents.  In addition to hourly monitoring of deckconditions,a subset of the strain gages remains continually activeto capture"large vehicle events."  When calibrated againstportableweigh-in-motion (WIM) traffic records, large event datawill offeran estimate of the cumulative demands placed on thebridges fromtraffic loading.

Live Load Testing

A second objective of this project is to compare theloadcarrying mechanisms of the three bridge decks.  Thisobjectiveis being realized by a series of live load tests. Beforebeing opened to traffic, heavily loaded vehicles were drivenslowlyalong the full length of each bridge along severaldifferentlongitudinal paths. During each test event, 41 channelsofrebar-bonded and concrete-embedment strain gage datawererecorded.  To accommodate the number of sensors in eachbridgeand a rapid data acquisition rate, all three CR5000sweresimultaneously used during live load testing.Longitudinalpositioning of the truck during the test was recordedvia ahand-held electronic button, engaged as the test trucktraveledevery two meters along the deck.  In addition to thelocaldeck strains recorded by the internal strain gages,globalbehaviors were monitored using gages temporarily affixed tothebottom of the stringers, supplied by Bridge Diagnostics,Inc.(BDI).  A majority of the tests were conducted usingonetandem-axle dump truck.  To simulate "worst case"behavior,two vehicles were driven side-by-side along the length ofthebridge.  High-speed tests were also con-ducted using asingletruck traversing the bridge at 60 mph.  A second liveloadtestis scheduled to take place after the bridge shave beeninservice for two years.

Outcome

At present, comparative investigation of bridge deckbehaviorwith this level of detail and control of variables hasbeenlimited.  This study presents a unique opportunity todevelopa better under-standing of local reinforced concrete bridgedeckbehaviors and load paths through the deck andsubstructure.Researchers and departments of transportation alikewill benefitfrom a heightened understanding of bridge deckperformance undervehicle loading and varying, long-term weatherconditions.Application of this knowledge will lead to betterplanning forinfrastructure health as well as improved bridgedeckdesign.