Why did the bridge collapse?
Minnesota officials were warned as early as 1990 that the bridge that collapsed into the Mississippi River was "structurally deficient," yet they relied on patchwork repairs and stepped-up inspections that unraveled amid a thunderous plunge of concrete and automobiles.
"We thought we had done all we could," state bridge engineer Dan Dorgan told reporters not far from the mangled remains of the span. "Obviously something went terribly wrong."
Questions about the cause of the collapse and whether it could have been prevented arose Thursday as authorities shifted from rescue efforts to a grim recovery operation, searching for bodies that may be hidden beneath the river's swirling currents.
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.
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.
Gov. Tim Pawlenty responded Thursday 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.
Several officials, including former State Transportation Commissioner Elwyn Tinklenberg, took pains to say that "structurally deficient" sounds worse than it really is.
"We thought we had done all we could. Obviously something went terribly wrong."
"'Structurally deficient' shouldn't be interpreted as meaning imminent danger. It means there are some things we need to monitor to look at. In this case, there was some fatigue that needed to be looked at and monitored," said Tinklenberg.
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 theorized it's 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 Infrastrucure Tecchnology Institute at Northwestern University. "I think it's a major factor."
The collapsed bridge's last full inspection was completed June 15, 2006. The report shows previous inspectors' notations of fatigue cracks in the spans approaching the river, including one 4 feet long that was reinforced with bolted plates. A 1993 entry noted 3,000 feet 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.
"A bridge of that vintage you always have to be concerned about that," said Richard Sause, director of the Advanced Technology for Large Structural Systems Center at Lehigh University. "In a steel bridge of that age, sure you'd be concerned about those kind of things and be diligent about looking after it. And it seems like they were."
It takes time for a fatigue crack to develop, but a crack can then expand rapidly to become a fracture, said James Garrett, co-director of the Center for Sensed Critical Infrastructure Research at Carnegie Mellon University. "If you get a crack that goes undetected it would be something that appears to happen more rapidly."
There's a good chance that investigators will find out what went wrong. The chairman of the National Transportation Safety Board says the investigation is underway.
Mark Rosenker says investigators are studying every possible detail -- ranging from interviewing the men and women who were working on the bridge to looking for faults in the structure's design, as well as how workers built it. But Rosenker says authorities have police surveillance video that shows the bridge falling.
"That's the equivalent of getting a cockpit voice recorder or flight data recorder. Without this video, we would be working in a real-time blind. This is going to show us what happened in the amount of time, where it began, where this failure began," said Rosenker.
The I-35 bridge was a truss bridge. A single 458-foot-long steel arch supported the bridge to avoid putting piers in the water so as not to interfere with river traffic.
Abolhassan Astaneh, a civil engineering professor from the University of California, Berkeley, says truss bridges are not as reliable as suspension and other bridge types.
"One of these four arches that they had in this bridge -- if one member of one of these four arches cracks, the bridge is gone. So it is not one of the very resilient systems," said Astaneh.
Another expert says corrosion is also major problem. Gene Corley is an expert on why bridges and buildings collapse. He was lead investigator in the World Trade Center disaster, the Murrah building collapse in Oklahoma City, and closer to home -- the Minneapolis Lake St./Marshall Ave. bridge collapse in 1989.
Corley says the bridge design left it susceptible to rusting of the steel in the bridge.
"It's rusting of the reinforcing bars that are in the concrete. That's not a problem that's likely to cause collapse, but the type that IS likely to cause collapse is the rusting of the main members of the bridge, the truss members that were supporting the bridge," said Corley.
Corley says if the bridge did have redundant supports, it likely would not have collapsed. Nevertheless, he says bridges like these that carry some risk are not inherently dangerous as long as are close inspections.
"We've got lots of bridges that get inspected all the time because there is some concern about them, and they go along and serve out their useful life before they're replaced, and don't collapse. It's certainly reasonable to handle a bridge that way," said Curley.
Others have raised the question of whether the latest bout of 90-degree days could have contributed to the collapse. Henry Petroski, a structural engineering professor at Duke University, says probably not. Petroski says given Minnesota's wide temperature swings, the engineers designing the bridge would have taken that into account. He says expansion joints allow for that movement with the shifts in temperature.
It's conceivable that the joints could have stuck or pushed against the bridge. But he says in general, bridges in areas with wide temperature ranges are more likely to have problems in cold weather, not hot.
"When it gets very cold bridges can fail, because the metal gets very brittle when it's cold. So if there are existing cracks, that could be aggravated," said Petroski. "But in this case because it was a very hot day, that's not going to be relevant."
The National Transportation Safety Board says it could be a year before it issues its final report.
(The Associated Press contributed to this report)