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How fire destroyed the I-85 steel and concrete bridge

During rush hour traffic in Atlanta on Thursday evening, a massive fire broke out under one of the bridges on Interstate 85. The fire burned underneath the bridge for approximately two hours before a portion of the bridge collapsed.

READ MORE: Interstate collapse: I-85 closed after fire and partial collapse

"This was a reinforced concrete bridge structure. They are also susceptible to fire damage and potential collapse," said UT Professor and Highway Design and Safety Engineer Dr. Stephen Richards.

Richards is an expert in safety and traffic, and understands the conditions needed to take down an interstate bridge.

"Not only does the extreme heat reduce the strength of the steel put inside the concrete for reinforcement, but it can also reduce the strength of the concrete itself," said Richards.

VIDEO: Interstate 85 collapses in massive fire in Atlanta

Richards said different types of concrete can handle different temperatures of heat.

"It has a lot to do with the type of coarse aggregate they use and that's the rock they mix with water to make concrete. If it's a silicon-based aggregate material then that's a little bit more susceptible to heat problems," said Richards.

Richards said the heat index can be as low as 800 degrees, but for other types of aggregate it can be around 1,200 degrees.

"We're still talking about very high temps but some type of concrete mixtures are more susceptible," said Richards.

Georgia Governor Nathan Deal told WBIR's Atlanta sister-station WXIA that authorities were focusing on the possibility that polyvinyl chloride (PVC) piping commonly used in construction may have ignited for some reason. WXIA reported that large amounts of PVC pipe were stored under the bridge.

"PVC is not that flammable. It's not what we could consider combustible. We would consider it stable," said Stephens.

TDOT spokesperson Mark Nagi said nothing is allowed to be permanently stored under Tennessee bridges. However, construction material - like PVC pipe - may be stored there temporarily.

"We don't allow anything to be stored underneath our bridges on a permanent basis. If there's a construction project, a bridge repair project, widening or something like that construction materials may be left under the bridge on a temporary basis," said Nagi.

MORE: How to get into Atlanta after I-85 fire & collapse

The exact cause of the fire and the bridge's condition are under investigation.

In Tennessee, bridges are inspected once every two years. Older bridges are inspected once a year.

"We wont allow any bridge, any roadway to be open if it presents any danger to the motoring public," said Nagi.

The I-85 bridge in Atlanta is expected to be closed for several months.

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