Officials in the US were warned as early as 1990 that a bridge which collapsed into the Mississippi River was “structurally deficient”.
But Minnesota officials relied on patchwork repairs and stepped-up inspections that unravelled amid a thunderous plunge of concrete and cars during Wednesday evening’s rush hour.
“We thought we had done all we could,” state bridge engineer Dan Dorgan said yesterday. “Obviously something went terribly wrong.”
Questions about the cause of the collapse and whether it could have been prevented arose as authorities shifted from rescue efforts to a grim recovery operation, searching for bodies that may be hidden beneath the river’s swirling currents.
The official death count from the collapse stood at four, with another 79 injuries.
But police said the death count would surely grow because bodies had been spotted in the water and as many as 30 people were still reported missing.
The Army Corps of Engineers lowered the river level a foot (30 centimetres) to help recovery efforts, said agency spokeswoman Shannon Bauer.
In 1990, the federal government gave the I-35W bridge a rating of “structurally deficient”, citing significant corrosion in its bearings.
The bridge is one of about 77,000 bridges in that category nationwide, 1,160 in Minnesota alone.
The designation means some portions of the bridge needed to be scheduled for repair or replacement, and it was on a schedule for inspection every two years.
White House press secretary Tony Snow said while the inspection did not indicate the bridge was at risk of failing, “if an inspection report identifies deficiencies, the state is responsible for taking corrective actions”.
Gov Tim Pawlenty responded by ordering an immediate inspection of all bridges in the state with similar designs, but said the state was never warned that the bridge needed to be closed or immediately repaired.
“There was a view that the bridge was ultimately and eventually going to need to be replaced,” he said. “But it appears from the information that we have available that a timeline for that was not immediate or imminent, but more in the future.”
Federal officials alerted states to immediately inspect all bridges similar to the one that collapsed.
Dorgan said the bearings could not have been repaired without jacking up the entire deck of the bridge. Because the bearings were not sliding, inspectors concluded the corrosion was not a major issue.
During the 1990s, later inspections found fatigue cracks and corrosion in the steel around the bridge’s joints. Those problems were repaired. Starting in 1993, the state said, the bridge was inspected annually instead of every other year.
A 2005 federal inspection also rated the bridge structurally deficient, giving it a 50 on a scale of 100 for structural stability.
The eight-lane Interstate 35W bridge was Minnesota’s busiest bridge, carrying 141,000 vehicles a day.
It was in the midst of mostly repaving repairs when it buckled during the evening rush hour.
Dozens of cars plummeted more than 60 feet into the Mississippi River, some falling on top one of another. A school bus sat on the angled concrete.
Engineers wondered whether heavy traffic might have contributed to the collapse. Studies of the bridge have raised concern about cracks caused by metal fatigue.
“I think everybody is looking at fatigue right now,” said Kent Harries, an assistant professor of civil and environmental engineering in the University of Pittsburgh’s School of Engineering. “This is an interstate bridge that sees a lot of truck traffic.”
After a study raised concern about cracks, the state was given two alternatives: add steel plates to reinforce critical parts or conduct a thorough inspection of certain areas to see if there were additional cracks.
They chose the inspection route, beginning that examination in May.
Dorgan said officials considered the cracks on parts of the bridge to be stable and not expanding.
When conducting inspections, Dorgan said, inspectors get within an arm’s length of various components of a bridge. If they spot cracks, that leads to more hands-on testing to determine the depth and extent of the fissures.
Although concern was raised about cracks, some experts theorised that it is no coincidence the collapse happened when workers and heavy equipment was on the bridge.
The construction work involved resurfacing and maintenance on guardrails and lights, among other repairs.
“I would be stunned if this didn’t have something to do with the construction project,” said David Schulz, director of the Infrastructure Technology Institute at Northwestern University. “I think it’s a major factor.”
The collapsed bridge’s last full inspection was completed on June 15, 2006.
The report shows previous inspectors’ notations of fatigue cracks in the spans approaching the river, including one 4 feet (1.2 metres) long that was reinforced with bolted plates.
A 1993 entry noted 3,000 feet (914 metres) of cracks in the surface of the bridge; they were later sealed.
That inspection and one a year earlier raised no immediate concerns about the bridge, which wasn’t a candidate for replacement until 2020.
In a 2001 report from the University of Minnesota’s Department of Civil Engineering, inspectors found some girders had become distorted. Engineers also saw evidence of fatigue on trusses and said the bridge might collapse if part of the truss gave way under the eight-lane freeway.
At the scene, about 15 divers and a dozen boats were in the water, but the search was proceeding slowly because of strong currents and low visibility.
By mid-afternoon, they had located four submerged cars besides the dozen or so visible from the surface.
“We have a number of vehicles that are underneath big pieces of concrete, and we do know we have some people in those vehicles,” Police Chief Tim Dolan said. “We know we do have more casualties at the scene.”