I-85 collapse likely caused by softened steel, overheated concrete

ATLANTA — Engineers at the Georgia Institute of Technology say the fiery collapse of a section of Interstate 85 during rush hour Thursday was likely the result of superheated steel rebar supports and concrete that expanded and cracked.

"Clearly, we are looking at high-temperature exposure that led to changes in material properties so that the structural elements which look to reinforce concrete are no longer able to carry the load that they are designed to do," Dr. Kim Kurtis, a professor at the School of Civil and Environmental Engineering at Georgia Tech said.

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Kurtis told Channel 2's Tom Regan that temperatures from a fire underneath the bridge exceeded several hundred degrees and the melting point of steel is 2,500 degrees Fahrenheit.

"Once we get a third or half of the melting point of steel, we see very serious decreases in the strength of steel. Concrete is hydrated. It has water in it's structure, so it's literally boiling. It's trapped within the concrete and the only way to escape it to create cracks," Kurtis said.

A man has been arrested and charged with arson in connection with the fire burned underneath the bridge.

The fire stranded thousands of  motorists on roadways for hours as police detoured traffic off north and southbound I-85.

Regan got a tour of Georgia Tech's Structural Engineering and Materials Laboratory. The facility does full-scale tests on bridges, buildings and other structures. Tests include simulating earthquakes and bomb blasts. The director of the laboratory said scotching fire can be just as damaging.

"This definitely was not a typical event. You don't see this happening very often in our world. It's something we can learn from, and try to improve safety," Georgia Tech engineering professor Lauren Stewart said.

Stewart said it will probably take several months for the highway to be rebuilt, but added there is reason to be thankful.

"I think it's a good thing nobody got hurt, which was great," Stewart said.

Cox Media Group