Stored materials appear to be likely culprit, officials say,

But those materials don’t typically ignite by themselves.

It wasn’t known late Friday what caused the massive fire that caused a portion of I-85 to collapse. But materials being stored under the bridge appeared the likely culprit, according to transportation and fire officials.

Those materials — including PVC and high-density polyethylene pipe which cover fiber optic cable — were under the bridge, but typically don’t ignite, experts said.

The Thursday night fire burned for about an hour before firefighters were able to extinguish it and clear the area when the bridge collapsed. No injuries were reported, but the fire and collapse forced numerous ramp and side street closures that remained in place during Friday rush hour.

“I’m not sure storing them under a bridge was the smartest thing to do, but you could store them there because these types of materials are not self-igniting,” Georgia Tech’s Lisa Detter-Hoskin said. “These things just don’t catch fire by themselves.”

Detter-Hoskin, principal research scientist of Tech’s Materials Analysis Center, declined to comment on whether the blaze appeared to be purposefully set.

A possibility, she said, was something flammable was being stored with the cables or conduits. Google Earth images show rolls of plastic conduit, which typically require some type of lubricant to pull fiber optic wires through, she said, and “those lubricants would likely be flammable.”

“So a cigarette or spark wouldn’t likely ignite it, but if a cigarette or spark caught the grass on fire, then maybe,” said Detter-Hoskin. “But I think there was something more flammable.”

Once ignited, the makeup of the high-density conduit makes it flammable right away, and because it’s made from an oil-derived plastic, it’s going to burn and have a lot of heat, she said.

But don’t point the finger at Google, the Internet giant said Friday. The materials under the bridge did not belong to the company, according to a spokesman.

“We are confident that the materials that caught fire did not belong to Google Fiber, and that the cause of the fire was not related to Google Fiber’s project in Atlanta,” a spokesperson said in an emailed statement.

Regardless of what caused the fire, repairs in the area could take months to complete, transportation leaders said. Possible damage to the bridge’s columns could complicate repairs because engineers must consider the overall stability of remaining structures, according to a Florida International University professor.

“When you have a fire of that intensity, the bridge is going to collapse. It’s not designed for something like that,” said Atorod Azizinamini, professor and chairman of civil and environmental engineering.

The damage may not be visible, but just because you can’t see it or because a portion of it didn’t collapse, doesn’t mean it’s safe, he said.

Azizinamini, through FIU’s Accelerated Bridge Construction center, has worked with Georgia’s transportation department on bridge projects before. GDOT hasn’t reached out to the center yet, but the professor said Georgia’s I-85 bridge has its advantages for repair: the main one being that no traffic runs under that portion. The area underneath is flat land.

Approximately six sections and 700 feet of the roadway — 350 feet northbound and 350 feet southbound — will be removed and replaced, including support columns, according to the Georgia Department of Transportation. The federal government said Friday it will contribute $10 million to the cost of repairs.