Researchers at OSU help make sure Oklahoma's bridges are safe
Friday, September 17, 2021
Media Contact: Harrison Hill | Research Communications Specialist | 405-744-5827 | harrison.c.hill@Okstate.edu
Partnership with ODOT is win-win
During rush hour on a Wednesday in 2007 in Minneapolis, hundreds of drivers were headed home across the I-35W Mississippi River bridge when the pavement below them suddenly gave way.
The aftermath of this collapse was the catalyst for stricter regulations on bridge maintenance and a federal mandate — all major bridges needed to be inspected. National Bridge Inspection Standards requires all bridges to be inspected once every two years.
One such bridge is the I-235 bridge in Oklahoma City, where issues recently arose because of a construction fault in the 1980s.
After repairing the bridge, the Oklahoma Department of Transportation reached out to researchers at Oklahoma State University to inspect and monitor the bridge repair.
“ODOT engineers contacted us directly to help with this particular project,” said Dr. Robert Emerson, associate professor of civil and environmental engineering. “They knew we had the expertise for this project because of the experiences it had working with OSU over several years.”
In the early 2000s, Emerson and his team worked with ODOT to develop a repair method for timber pile bridges in Oklahoma and to monitor other bridges.
“We’ve load-tested bridges before by equipping them with strain transducers and monitoring the response under load to use it as a predictor of condition,” Emerson said.
Although the I-235 bridge is safe to drive on, the issues arose from its original construction, with it having a flaw from the very beginning.
“It is a post-tensioned concrete structure,” Emerson said. “This means there are ducts that go through the concrete. And prestressing strands are passed through those ducts, and then stretched — pulled under tension — then a grout is injected to flow through those ducts to fill all the void spaces.”
Once this is done, the strands are cut at the ends, anchored to the rest of the bridge because they are bonded with the grout, which is bonded to the duct, which is bonded to the concrete.
“It’s all supposed to work together,” Emerson said. “But if it’s not, issues can arise.”
According to Emerson, an inspection by ODOT and its partners of the I-235 bridge found a point of concern. The grout used in the 1980s was not filling all the void spaces.
In the bridge’s construction, the ducts follow the location of tensile stresses in the bridge as vehicles move across it, Emerson said. At the midspan, that duct is near the bottom to carry tension. But whenever you have a continuous beam that goes over a support, it flips, creating a high spot where tension is at the top and compression is at the bottom for bending moments, he said.
“Through the inspection, ODOT found that in those high spots over the supports, the grout didn’t actually flow through,” Emerson said. “There was an air pocket, which over time would create a much bigger issue.”
Concrete is a permeable material, so water would gradually seep through it and fill up that space, causing corrosion of those prestressing strands, Emerson said.
“The fear was that this would be true for all of [the high spots] across the width
of that bridge at that location,” Emerson said.
ODOT and another partner identified and repaired the bridge, then contacted OSU.
“ODOT wanted our team to come up with a project to monitor the bridge and its repairs over a period of time to see if it was stable,”
Emerson said. “We installed gauges and have been monitoring the bridge ever since.”
The team is currently in its fifth year of testing and plans to add another year onto the project.
The load tests — the third one being done in August — provide an immense amount of data for the team.
“The load tests were conducted the first year, the third year, and the third one will
be the fifth year of this project,” Emerson said.
Usually working late at night or early in the morning, the team has ODOT drive large, heavy trucks over the bridge.
“We break it up into five lanes and pass the trucks in increasing weight — a single truck then two trucks together and then four trucks, over the bridge,” Emerson said. “This is the best way for us to get an increase in loading and see the bridge’s response to the weight increase.”
The trucks are driven slowly across the bridge while the team is recording the readings from strain gauges, monitoring deformations.
“If we know the strain that’s occurring due to that loading, we can calculate stress and then compare that to the expected/known material strengths,” Emerson said. “Eventually, we compare the results of these load tests [over the years] to see if the behavior or reaction by the materials is relatively constant.”
Besides the load tests, the team also conducts visual inspections, yearly crack mapping and monitoring, as well as continuous monitoring of the bridge during normal use.
“We’ve been monitoring the repair that they put in place because they want to have a warning if something becomes abnormal and potentially dangerous,” Emerson said. “The best outcome for this project and the state is we find out everything is going well. But if it’s not, we’ll come up with repair strategies to strengthen it.”
Although the monitoring isn’t complete, according to Emerson, the current situation is looking good.
“We haven’t seen any lengthening of the cracks already present and based on the first two load tests, we haven’t seen any real changes in behavior, either, ” Emerson said. “So that tells me that things are currently stable.”
While this project is helping to ensure the safety of drivers across this bridge — and others like it — it is also having an impact on students and faculty at OSU.
“This partnership has been very productive,” Emerson said. “It has helped ODOT and created opportunities for many faculty members. It continues to be a very symbiotic relationship.”
OSU currently houses an ODOT roadways division on campus, where six to 10 students receive support from and work for ODOT while getting their degrees.
“It’s a fantastic experience for students,” Emerson said.
Some students join Emerson when he collects data and conducts the load tests.
“As faculty, we’re here to do research, we’re here to teach students, and we can bring all the work out in the field back in the classroom. We extend and expand our knowledge through these type of partnerships,” Emerson said.
“This project is an example of what should be at the heart of all research at a public institution, solving problems that directly impacts people.”
Because of this long-standing partnership, ODOT can be confident OSU is a resource it can tap into to help improve Oklahoma’s infrastructure and enhance the safety of its roads and bridges.
Photos By: Gary Lawson, Robert Emerson and ODOT
Story By: Harrison Hill | harrison.c.hill@Okstate.edu