I-35W bridge one of several projects on CSE researcher's plate
Lauren Linderman had just graduated from college in St. Louis with her bachelor’s in civil engineering when the I-35W bridge collapsed in 2007, making international headlines. Little did she know that a decade later, she’d be conducting cutting-edge research in Minnesota on how to increase infrastructure safety— including monitoring the replacement for that very bridge.
Linderman studies smart technology applications in two broad categories: monitoring the stability and performance of structures and buildings, and limiting their response—in hopes of minimizing injury and damage—in catastrophic events like earthquakes.
The assistant professor in the Department of Civil, Environmental, and Geo-Engineering scrutinizes measurements from hundreds of sensors on the 1-35W bridge to glean clues about what kind of changes it’s undergoing, potential implications of those changes, and how future bridge design could improve even more.
She’s collaborating with Professor Carol Shield on the bridge-monitoring project.
They look at sensors—placed by MNDOT on the bridge since its 2008 inception—that measure forces like acceleration (or vibration), strain, temperature, and displacement.
Keeping an eye on bridge creep
“There are other structures that have used this technology, but I think this is the only structure where we’ve had sensors on there since inception that are still operating,” Linderman said. “That’s been really interesting, because it’s allowed us to look at confirming some design considerations.”
One such consideration is called “creep.”
As Linderman explained, “Concrete creeps. In lay terms, what that means is that under constant load, it will continue to get shorter with time. That’s really hard to model accurately.”
“Here we were able to capture the long-term creep of the structure over the first 10 years, which is when the majority of the creep happens,” she said.
Linderman and Shield believe their analysis suggests that after a decade, the I-35W replacement bridge isn’t quite done creeping—a subtle revelation that wouldn’t have been accessible without current technology and a growing knowledge of how to deploy it.
The stress on bridges in places with wildly variable temperature and humidity is greater, Linderman said, and the task of monitoring them more complex.
“The top of the deck might have a different temperature than the bottom of the bridge. So it’s not uniformly expanding and contracting. There’s a lot of loading on the bridge due to temperature changes,” she explained. “It’s important. It seems like the temperatures that we see in the structure often exceed the design gradients that the code would specify.”
The holy grail for bridges
In a related project, Linderman is examining sensor selection and placement—trying to discern what’s the most efficient and cost-effective way to use available smart technology.
“Longer term, our goal is to incorporate the reliability of the sensors into where you put them,” she said. “By that I mean, what if a sensor fails? Do I also want something [another sensor] that’s redundant?”
Through her work, Linderman often imagines some of the smart tools that don’t yet exist.
“There are still sensors that I think would be exciting to develop,” she said.
“Corrosion sensing [for bridges] is probably the holy grail. One of the big concerns with concrete structures is if the rebar is corroding, or the prestressing strand is corroding, you can’t see it.”
Instead, engineers rely on a complicated process of deduction.
“If you could detect that,” she added, “that would be pretty cool.”
Story by Susan Maas
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