The I-35W Saint Anthony Falls Minneapolis collapsed in August 2007, killing 13 people and injuring 145. Over 100 vehicles had been on the bridge when it fell.

Its replacement, which opened to traffic just over a year later, was instrumented with more than 500 sensors to record its behaviour and evaluate the effectiveness of different monitoring strategies. A 10-year review of the bridge’s monitoring system has now been produced by University of Minnesota researchers.

“The measurements collected over the 10 years of monitoring, with the aid of material tests, truck tests, and finite element modelling, provide insight regarding the relative strengths of the instrumentation systems and potential considerations for future system deployments,” said Lauren Linderman, assistant professor in the Department of Civil, Environmental, and Geo-Engineering (CEGE) and the project's principal investigator.

Researchers leveraged the monitoring system in different ways throughout the project. A key finding from the project is that seasonal and daily temperature variations have a big impact on the bridge’s behaviour. As a result, the instrumentation needed to measure temperature and its impacts is the most important for long-term monitoring.

“Currently, one cross-section is heavily instrumented to capture temperature data, but behaviour at this cross-section varied significantly from behaviour near the piers,” said Linderman. “For future bridge deployments, we recommend instrumenting more cross-sections, with fewer sensors, to get a more accurate representation of the temperature distribution throughout the structure. This could be essential as the temperature-dependent behaviour starts to dominate the bridge’s response, and the time-dependent effects slow down.”

These large thermal stresses - in particular, those caused by low temperatures during morning rush hours - could lead to concerns about deck cracking. "This bridge used a conservative design approach, so we believe its service life won’t be affected,” she said. “Comparable bridges, however, could encounter issues. The insight gained from this study could be key for their design and life span.”

The continuous operation of the I-35W bridge monitoring system makes it one of the longest-lived in the country, said Linderman. The system incorporates many sensor types, including strain, temperature, and acceleration and displacement sensors, and has generated a large data set. “This data set offers an unmatched opportunity to evaluate long-term monitoring approaches,” she said.

The project was sponsored by the Minnesota Department of Transportation (MnDOT). “This project has given us more knowledge in how we may be able to better instrument bridges in the future if we find it necessary to monitor or investigate their behaviour,” said Ben Jilk, principal engineer with MnDOT's Bridge Office. “It has also given us confidence that the design criteria we have been utilising on bridges in Minnesota like the I-35W bridge is conservative with respect to the behaviour observed.”