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Technology to help explain Minneapolis bridge collapse

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Throughout the weekend, residents trekked alongside a police perimeter and perched on grassy knolls near the defunct flour mills at the river's edge. Some brought flowers to cast; others, frustration, carrying "build bridges not bombs" placards.

"You can't put up infrastructure on the cheap, and if you are going to do that, something has got to give," says Eileen Johnson, who sells flowers in the city, echoing a common sentiment.

In its quest to pinpoint just what happened, and why, the NTSB got a boost from a 3-D laser scanner brought in by the FBI. The device sits on a tripod and shoots thousands of lasers to take a three-dimensional picture, replacing the tedious work once done one point at a time by surveyors.

"Because you have this mess of twisted metals, it would take you months to do point-by-point maps. This will allow us to do it in a day," says Mark Bagnard, an NTSB investigator.

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.

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