CHATTANOOGA — Tennessee Department of Transportation engineers say they believe they've figured out what caused a concrete barrier to collapse on I-75 in Chattanooga.
The chief engineer with TDOT, Paul Degges, told ABC News that he believes a truck carrying a load above federal limits hit the 75 south bridge from underneath.
[PHOTOS: Bridge collapses on I-75 in Chattanooga]
They believe the load hit one of the steel beams that supports the bridge, which caused it to collapse yesterday.
Degges says the steel strands were broken at the same location.
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Degges said they have not seen and surveillance video and, at this time, do not have any video evidence to support their theory.
Degges says it could have happened an hour, a week or even a month before the railing collapsed.
TDOT says that about 50 bridges are hit by large semi trucks each year in Tennessee.
Crews are still working to repair the bridge. They plan to replace it this fall.
The driver who collided with the concrete barrier after it fell is listed in critical condition at a Chattanooga hospital.
Cox Media Group