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Atlanta bridge collapse shows how fire defeats concrete, steel

USA TODAY logo USA TODAY 3/31/2017 Bart Jansen
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The bridge collapse in Atlanta from a raging fire illustrates the damage intense heat inflicts on concrete and steel, and the massive disruption that results when a vital transportation link is broken, experts said Friday.

Investigators are still searching for the cause of the fire that erupted Thursday in a storage area beneath the Interstate 85 bridge over Piedmont Road. Construction equipment such as plastic conduit for fiber-optic networks was stored in the area, according to Russell McMurry, Georgia transportation commissioner.

The collapse severed the northbound highway that typically carries 250,000 vehicles a day, but southbound lanes will also be closed for months to replace a 350-foot stretch of the bridge, McMurry said.

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Intense fire can damage both the concrete that serves as the road deck and the support piers for the bridge, as well as the steel that reinforces the concrete. Even if a bridge doesn’t collapse in a fire, the damage can leave concrete crumbling and the steel warped and weak.

“The heat can damage the concrete and cause the reinforcing steel to lose strength, and that can cause the bridge to fall down,” said Andy Herrmann, a past president of the American Society of Civil Engineers and a 40-year consulting engineer in New York City.

Concrete starts to crack at 500 degrees, suffers large cracks at 1,000 degrees and melts at 2,500 degrees, he said.

Larry Petrick, deputy director for occupational safety and health for the International Association of Fire Fighters, said steel is unprotected in bridge construction and isn't immune to fire.

A 50-foot beam could expand 4 inches when heated to 1,000 degrees, he said. After cooling down, the beam loses 50% of its strength, he said. The weight of concrete loaded on top of the steel adds additional stress, which leads to failure and collapse, he said.

“The steel may begin to twist or warp,” Petrick said. “This was a large, high-temperature fire under the bridge, with its structural members directly exposed to the steel.”

Inspectors will likely spend some time determining how much of the structure must be repaired or replaced.

Herrmann said concrete will change color at high temperatures, turning purplish-gray at 1,000 degrees. Inspectors may hit the concrete with a hammer, to see if it crumbles, he said.

Later, inspectors may put weight, such as a truck, onto the bridge, to see how much it moves.

“It’ll start sagging under the weight. You can’t make a totally stiff bridge,” Herrmann said. “Depending on how much it sags, you can say whether it’s normal or not, or it’s damaged and it’s sagging more than it should.”

The Metropolitan Atlanta Rapid Transit Authority added extra trains to accommodate additional passengers. “This is about as serious a transportation crisis as we can imagine,” Atlanta Mayor Kasim Reed said.

While construction and safety officials frequently warn about deteriorating bridges nationwide, this I-85 bridge was rated in good shape before the fire.

The bridge was built in 1953 and last renovated in 1985. The bridge scored 94.6 out of 100 “sufficiency rating” when last inspected in 2015, according to the Federal Highway Administration.

“That’s a pretty high sufficiency rating, so it was in good condition,” Herrmann said. “This was an extreme event to happen at the bridge site, and it’s very rare across the country. It doesn’t happen very often.”

The cause of the fire remains under investigation. A witness, Bobby Barnhart, who works in sales for a financial technology company near the highway, said he was 60 yards away from the fire when he heard several explosions and a slow rumbling before the collapse.

“It was a loud, muted rumble,” Barnhart said. “You could feel the vibrations."

The collapse served as a reminder of how many motorists depend on busy bridges in the middle of cities.

In Minneapolis in August 2007, the Interstate 35 bridge collapsed from too much weight being placed on the span during a construction project, according to the National Transportation Safety Board. Thirteen people died and 145 were injured when about 1,000 feet of the bridge plunged 100 feet into the river.

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While rare, intensely hot fires occasionally cause bridge collapses.

In Hazel Park, Mich., in July 2009, a tanker truck carrying 9,000 gallons of gas jack-knifed while avoiding a passenger car on northbound Interstate 75. The resulting fire severely damaged a bridge that carries traffic on 9 Mile Road above the highway, causing the bridge to collapse.

In Oakland, Calif., in April 2007, a tanker truck carrying 8,600 gallons of gas rolled over at the interchange of interstates 80, 880 and 580. The resulting fire caused the collapse of an elevated highway ramp above where the truck came to rest.

Despite the good condition of the Atlanta bridge, nearly 56,000 bridges nationwide including 13,000 interstate bridges are structurally deficient, according to an analysis of federal inspections by the American Road and Transportation Builders Association. Vehicles cross the deficient bridges 185 million times daily, according to the group.

Contributing: The Associated Press

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