Results of InService Monitoring of a Sample of Typical Highway Bridges

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Results of In-Service Monitoring of a Sample of
Typical Highway Bridges

• Michael Rakowski, Brianna Brookes,
• Michael J. Chajes, and Harry Tripp Shenton
• Center for Innovative Bridge Engineering
• Dept. of Civil and Environmental Engineering
• University of Delaware
• Newark, Delaware
• April 24, 2008
• ASCE Structures Congress
• Vancouver, BC

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Overview of the Presentation

• Motivation and background for the work
• Inventory of the monitored bridges
• Monitoring procedure
• Sample results
• Load rating based on in-service data
• Summary and conclusions

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Motivation

• Deteriorated state of our nations bridge
  inventory
• Recognized need to gather more quantitative data
  as part of the normal biennial inspection process
• At the core of the new FHWA LTBPP
• Greater knowledge of the condition of our bridges
• Improved management of the inventory
• Extend the life of our bridges
• Better use of limited financial resources
• Need for quick and inexpensive tools for
  measuring bridge response due to site-specific
  traffic

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Background

• In-Service Bridge Monitoring System (ISBMS)
  developed at UD in 2000
• Field and laboratory tested
• 2006 Second generation ISBMS developed

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In-Service Bridge Monitoring System

• Small, battery operated
• Rapidly deployable
• Single strain transducer
• Data acquisition
• High speed (1200 Hz)
• User defined trigger
• Records only peaks
• Date and time stamp
• Signal conditioning balance and shunt
  calibration
• Runs, unattended for up to two weeks

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ISBMS Installed
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Current Project

• 2006 DelDOT funds project to use ISBMS in
  regularly scheduled biennial inspections of 12
  bridges in the state
• Close coordination between UD and DelDOT
  technology transfer
• 2006 6 bridges are monitored
• 2007 6 bridges are monitored
• Date collected and In-Service Load Ratings are
  calculated for all 12 bridges

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Criteria for Selecting Bridges

• Due for biennial inspection
• Easy access (usually with at most a ladder)
• Slab on steel girder
• High ADTT

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Inventory of Monitored Bridges
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2007 Bridges
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Monitoring Procedure

• UD team meets DelDOT inspection crew at bridge
  site during scheduled biennial inspection
• UD/DelDOT crew configure ISBMS
• Transducer and system are mounted to girder by
  inspection crew and armed
• System allowed to collect data for two week
  period
• UD retrieves ISBMS and downloads data

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Summary of Results
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Bridge 1-826Timeline Plot
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Bridge 1-826Stress Histogram
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Bridge 1-149Timeline Plot
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Bridge 1-149Stress Histogram
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Bridge 2-918Timeline Plot
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Bridge 2-918Stress Histogram
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Application of the Measured Data

• Record of peak stress experienced due to
  site-specific traffic.
• Biennial monitoring provides historical record
• Changes over time may be indications of damage,
  deterioration, or changes in traffic
• Monitor response to overloads and permit vehicles
• In-Service Load Rating

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In-Service Load Rating

• Theoretical ratings tend to be very conservative
  lead to lower ratings and load postings
• Best model of the bridge is the bridge itself
• Measured data records the maximum live load
  affect experienced during the monitoring period
  can calculate the true rating for the (two week)
  monitoring period
• Need to use the two weeks of measured data to
  predict the maximum live load affect for the
  desired period
• 2-years, 50-years, 75-years

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Procedure for Predicting Maximum Live Load Strain

• Measured data used to calculate inverse standard
  normal (reliability index)
• Linear regression to tail of the curve
• Determine maximum strain for desired interval
  (2-, 50-, 75 years).
• Use predicted maximum strain to calculate live
  load moment
• Adjust rating factor based on predicted LL

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Strain Prediction Bridge 1-826
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Strain Prediction Bridge 2-918
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Strain Prediction Bridge 1-149
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In-Service Rating Factors
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Summary and Conclusions

• Results presented of an ongoing project to use
  in-service strain data to better manage bridges
  in Delaware.
• Twelve bridges monitored to date
• From the data the timeline plot and histogram
  have been created. These data and plots will
  become part of the permanent inspection record
  for the bridge.
• A procedure for developing a load rating based on
  the recorded data has also been discussed.
• The data can be used to predict a 1-, 2- or 75-
  year load rating. These ratings can be used to
  better manage our bridge inventory.

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Acknowledgements

• Authors would like to thank the Delaware
  Department of Transportation for their support
  for this project

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Thank you!