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Engineer examines 2020 I-40 bridge inspection report

Updated: May. 13, 2021 at 9:58 PM CDT
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MEMPHIS, Tenn. (WMC) - WMC Action News 5 talked with an engineer who worked on the Hernando DeSoto Bridge, installing seismic sensors.

He interpreted the most recent inspection report for us, then explained why the iconic M-shape we love so much about the I-40 bridge likely contributed to the damage.

After reviewing the September 2020 inspection report on the bridge, University of Memphis engineering professor, Adel Abdelnaby, found no glaring problems. But one thing did stand out.

“When I looked at the cracking the inspectors identified some cracking issues. But they didn’t list any cracks in the Category 4, the most serious cracking, and that is something that surprised me, you know,” he said.

The ARDOT inspection, performed just eight months before a major fracture was discovered this week, reported no sign of severe cracking. Abdelnaby says the issue could have started inside the steel beam box.

“The crack propagates faster and faster,” he said. “And a crack can propagate where you can’t see it, from the inside of the box, so you can’t see it.

The major problem, he says, is the unique design of the bridge, the truss system and the two arches that form the M-shape. Abdelnaby says weight isn’t distributed evenly on the cables.

“There is a high stress concentration at this point,” he said of the steel box below the shorter cables where the fracture occurred, “because of the uneven stretching of the cables. The structural system used in the bridge is not the ideal system to use in a span that long.”

Abdelnaby created a computer program that shows how weight is actually distributed across the bridge.

“Back in the 1960s when that bridge was designed, computer models wouldn’t capture that kind of behavior,” said Abdelnaby.

He says the existing fracture could be repaired in two days. But his computer model shows the load path would then shift to other weak spots. The big question now is how many weak spots are there?

“So, it’s a very dangerous situation,” he said.

Inspectors have to scan every inch of the I-40 bridge, including using X-rays to find any other cracks, and that’s going to take time. The bridge remains closed to cars up top, and the Coast Guard says more than 700 barges remain stranded below, unable to pass beneath the bridge.

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