Minn. structure passed inspections for years
But support got rating of 'poor' in 2006 report
NEW YORK -- The eight-lane bridge in Minneapolis that collapsed on Wednesday had been inspected diligently for years and had always passed, Minnesota officials said yesterday.
It did not, however, get stellar grades. Officials said the bridge's design had been considered obsolete for decades because a single failure of a structural part could bring down the whole bridge.
About 11 percent of the nation's steel bridges -- mostly from the 1950s and '60s -- lack the redundant protection to reduce these failures, federal officials said.
Overall, the bridge had been rated a four on a scale of zero to nine, with nine being perfect and zero requiring a shutdown.
An inspection report last year said the supporting structure was in "poor condition."
That is far from the lowest category, and hundreds of other working bridges are in similar shape, but it did indicate that the bridge had possible issues that needed to be regularly inspected.
Those inspections had occurred every year since 1993, but independent engineers acknowledged yesterday that there are well-known limits to how useful an inspection can be.
Bridges, they said, are prone to a variety of problems, and some of them are hard to spot.
At the Minnesota Department of Transportation, shaken engineers made it clear that they knew something crucial had somehow been overlooked.
"We thought we had done all we could," said Daniel L. Dorgan, head of the department's bridges division. "Obviously, something went terribly wrong."
Yesterday, the US Department of Transportation said it had told all the states to inspect bridges similar in design and construction to the one that collapsed, or to review inspection reports closely. The department said there were 756 such bridges.
The cause of Wednesday's failure of the Interstate 35 west bridge will probably become clear through metallurgical examination of the wreckage, specialists said, but recovering those parts will be delayed by the search for human remains and the need to keep investigators safe in the swirling waters of the Mississippi.
Schulz said he would be stunned if the bridge repair work being done at the time had not played a major role in the collapse.
"It's too much of a coincidence," he said.
But Dorgan said he saw no connection between the repair work -- mostly taking place on the roadway -- and the collapse of the steel support structure far below.
The Minneapolis bridge had been inspected annually since 1993, Dorgan said, and the structure had been thought to be in good enough shape to last until 2020, when it was due for an overhaul or replacement.
Parts of it were considered structurally deficient because of corroded bearings and tiny, metal cracks that had been spotted years ago but were considered stable.
The rating of "deficient" is a common one that indicates the need for regular inspection and does not mean the bridge was dangerous, said Thomas D. Everett, a top bridge official with the Federal Highway Administration.
The most visible threat to a bridge is usually corrosion.
But metal fatigue -- the weakening of steel by the repeated weight of heavy trucks bouncing across the bridge surface -- is harder to see.
Bridges in northern climates are particularly vulnerable to metal fatigue because steel becomes more brittle, and prone to crack, when it is cold.
"A crack is very difficult to observe visually," said Steven J. Fenves, a guest researcher at the National Institute of Standards and Technology, part of the Commerce Department, and a professor emeritus of civil engineering at Carnegie Mellon University.
"There may be paint over it or maybe many layers of corrosion over it. It may be in an invisible place, in the second plate, not the outermost plate."
The possibility that metal fatigue could cause a bridge to fail was not even considered by bridge engineers in the 1950s and '60s, when the Minneapolis bridge was designed and built, Dorgan said.
Mark Rosenker, chairman of the National Transportation Safety Board, said yesterday that his agency would determine whether the criteria for inspections were adequate.
"They may well not be enough," he said, speaking from a riverside park near the wreckage. Or the procedures may be adequate but may not have been followed, he said.
NTSB investigators will use computer modeling to study the failure, he said, and will also reconstruct parts of the bridge from the wreckage to determine the cause.
The board's final report could be many months away, he said.