Since the completion of the I-35W St. Anthony Falls Bridge in September 2008, University of Minnesota researchers have been using “smart bridge” technology to collect and analyze data about the bridge’s structural behavior in a project funded by the Minnesota Department of Transportation (MnDOT).
During its construction, the bridge was instrumented with more than 500 sensors that monitor strain, load distribution, vibrations, temperature, potential for corrosion, and the overall movement of the bridge. Other sensors were installed to monitor the bridge’s security and control automatic anti-icing and lighting systems.
A research team led by civil engineering professors Cathy French and Carol Shield and graduate student Brock Hedegaard has been interpreting data gathered by these sensors during the bridge’s first four years of operation.
The team has used the data to investigate changes in behavior caused by vehicle and environmental loading, evaluate load-rating assumptions, and determine the effectiveness of the “smart bridge” monitoring technology. The information could potentially impact future bridge designs and long-term monitoring plans.
To help them interpret the sensor data, the researchers developed a series of two- and three-dimensional finite element models. To validate the models, team used data collected from static and dynamic truck tests—involving eight loaded sand trucks in various configurations—that were conducted after the construction of the bridge and repeated two years later. These truck tests also provided data that can be used as a benchmark over the bridge’s lifetime to detect changes in behavior.
Overall results of the project indicate that the bridge is performing well and is meeting its design expectations. Other significant findings indicate that environmental factors—such as seasonal and even daily temperature variations—have a more substantial effect on bridge behavior than previously thought.
“The data that’s been collected since the bridge was built is important because it’s helping us understand how this type of structure behaves in this environment and under traffic loads,” says Nancy Daubenberger, state bridge engineer at MnDOT.
In addition, project results have helped MnDOT obtain a more complete, correct, and informative manual for rating the new bridge.
The study also examined the strengths and weaknesses of the instrumentation systems used to monitor the bridge, including various strain gauges, thermistors, accelerometers, linear potentiometers, and sensors monitoring electrochemical activity.
The team found that most of the instrumentation has performed well and would be valuable in future “smart bridge” installations. Recommendations include adding more thermistors in future monitoring efforts to collect more information about temperature variations in the cross section of the bridge.
Currently, the research team is using the initial data to estimate the time-dependent deformations of the structure expected over its design life.
“We look forward to continuing our work with the U of M research team over the next few years on various methods of structural health monitoring,” Daubenberger says.