Technology to help explain Minneapolis bridge collapse

Investigators are probing the cause of Wednesday's bridge failure in Minnesota.

from the August 6, 2007 edition

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Officials used the same sort of device after freeway collapse in Oakland, Calif. this past April to get highly accurate measurements of the road gap to be retrofitted.

Municipalities are buying these scanners with the help of Homeland Security funds to produce maps of potential soft targets, says Tony Grissim with Leica Geosystems. The Swiss company made the scanners used in the Oakland repairs. Its latest generation system, can measure with an accuracy of 6 millimeters from a distance of 50 meters, he says.

With the wreckage mostly mapped, the NTSB will move segments of the bridge by barge to a nearby field. There, a collection of pieces of particular interest will be laid out much like the agency's postcrash airplane reconstructions in hangars.

The next step, say engineering experts, is to make a computer model of the bridge (also known as finite element analysis), dividing it into 52 separate pieces, each critical to the bridge's stability. The more data the better: including original drawings, steel and concrete strengths, temperatures, and the pattern of traffic.

Then, like pulling the crucial block out of a Jenga tower, engineers will remove a piece to see how the bridge would fall. Each collapse scenario will be compared against the maps from the 3-D laser scanner, and the fortuitous video of the event.

Once a match is found, the focus turns to why that particular piece failed. Investigators will be on the lookout for indications that the metal had a slow-growing fatigue fracture, says Abolhassan Astaneh-Asl, an engineering professor at the University of California at Berkeley.

If such a fracture brought down the bridge, controversy over whether the bridge had been properly inspected and maintained may intensify. The head of the state transportation department on Friday angrily denied insinuations that the agency skimped on maintenance following inspections that revealed fatigue stresses.

One longtime bridge collapse expert says he doubts a fracture was the cause. "Brittle fracture problems generally occur in cold weather conditions," says John Hanson, a structural engineering consultant. "A defect of this size that didn't fail when it was 30 below zero would have had to grow [significantly] size since last winter."

Instead, Dr. Hanson says, he would first focus on the construction work that was being done on the deck when the bridge fell. He suspects that the deck, or concrete roadway, played a larger-than-intended role in making the bridge sturdy. Rosenker said in a press conference Sunday that employees of Progressive Contractors Inc. are being interviewed, noting that the company was experienced and had worked on the bridge before.

Mr. Astaneh-Asl also noted that jack-hammering and a heavy asphalt truck on the bridge could have played a role. A respected structural engineer, he shares some of the public frustration with the bridge inspection ratings.

"[Officials] are at such pains to say that 'structurally deficient‚' doesn't mean unsafe," he says. "We should really not have inspections using these terms ... [Just] tell me: Is the deterioration enough to cause someone to become injured or killed?"

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