Why did the Pittsburgh bridge collapse? Experts weigh in

Ed Blazina
The Pittsburgh Post-Gazette (TNS)

PITTSBURGH — For experts, it’s way too early to reach a conclusion on the cause of the Forbes Avenue bridge collapse on Friday.

The National Transportation Safety Board arrived at the scene hours later to launch an investigation that could take 18 months or longer.

But some engineering experts interviewed by the Pittsburgh Post-Gazette who reviewed prior inspections and saw news media photos of the wreckage have theorized about what went wrong in the morning hours when 10 people were injured and seven vehicles, including a Port Authority bus, were left in the rubble.

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To Hota GangaRao, the repeated poor ratings and deterioration of the 52-year-old bridge deck are definite warning signs. GangaRao, an engineering professor and director of the Constructed Facilities Center at West Virginia University, said a bridge with proper maintenance should last much longer than 50 years.

“A deck is the first line of defense in protection of the superstructure,” GangaRao said. “If you properly take care of it, a bridge can survive 70 to 80 years.”

Cracks and spalling, where small pieces of the surface break away, allow water and road salt to penetrate the deck, expansion joints and superstructure below it, the professor said. If that isn’t corrected, it leads to corrosion that could cause supports to fail, he said.

The way the bridge collapsed, the middle section fell downward, forcing up the end near South Braddock Avenue.

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That’s where the deck is attached to the abutment at street level using plates and bolts, and the movement could indicate a problem with the expansion joints at the end and in the middle of the structure, GangaRao said.

“The way it is looking, I think the Achilles heel, the last straw, is the joints,” he said. “The way it collapsed makes it look like the bridge came loose from the abutment.”

Kevin Heaslip, a professor in the Department of Civil and Environmental Engineering at Virginia Tech, said continuing poor ratings could indicate growing deterioration on the bridge, which had last been inspected in September.

“This is something in the maintenance component,” he said. “If I had to guess, something happened between that September inspection and the collapse.”

Roberto Leon, the Burrows Professor of Construction Engineering at Virginia Tech, said the repeated poor ratings show “the bridge was never well taken care of.” He called it “unusual” that cables were used to buttress a bridge support.

“An actual collapse of a bridge this size is unusual,” Leon said. “From a policy standpoint, we should be worried. Maintenance is an investment.”

Kent Harries, a professor of civil and environmental engineering at the University of Pittsburgh, regularly includes a lesson in his classes on the role of failure in engineering. That lesson occurred last week.

What he teaches, Harries said, is that as long as there is any probability above zero, failures like bridge collapses will happen.

“We can’t possibly prevent all bridge failures,” he said. “The important thing is we learn from that going forward, and I have no doubt we will learn from this, too.”