High-tech arsenal used to dissect span collapse

MINNEAPOLIS — A helicopter with a camera like those used to film Hollywood movies will soon peer into the wreckage of the collapsed bridge. Laser-guided surveying equipment already has helped produce a detailed map of the debris. Software re-creating the disaster on a computer screen may even pinpoint the exact piece of bridge that gave way.

Even with the water still filled with debris, investigators are already using this powerful technological arsenal to get answers about why the bridge collapsed into the Mississippi River last week. It is a quantum leap ahead of investigations of previous eras, when crews literally had to put the pieces of a fallen bridge back together.

"Computers and modeling techniques are just light years from what was available 40 years ago," said Ted Galambos, a professor emeritus of structural engineering at the University of Minnesota and an expert in the stability of structural steel. "Now we can have an idea, and we can test that on a computer in a few hours."

Searchers Sunday concluded a fourth day in the water without finding any of the eight people still missing, the Hennepin County Sheriff's Department said.

Investigators also said they had interviewed officials and workers at Progressive Contractors, which had been working on a $2.4 million project that included resurfacing the bridge.

State transportation officials said they would begin removing bridge debris from the river later this week. In addition to helping with the recovery operation, one goal of clearing the wreckage is to open a channel at least 56 feet wide to accommodate barge and boat traffic.

Investigators said it could take as long as 18 months to complete their exhaustive probe into why Minnesota's busiest bridge collapsed and fell Wednesday. But they already have begun zeroing in on clues.

On Friday, they were focusing on the south section of the bridge, where they quickly found that the span shifted 81 feet during the collapse.

On Saturday, the north side became the focus. That's where they plan to use a helicopter equipped with a high-resolution camera to examine debris in precise detail. The camera is kept steady by a gyroscope, which is how Hollywood crews get smooth footage while filming from a helicopter.

Investigators also plan to watch frame-by-frame enhancements of video of the collapsing bridge. In addition, the FBI used laser-guided surveying equipment to complete a detailed 3-D map of the wreckage and quickly provided the data to the lead investigation agency, the National Transportation Safety Board.

Nineteen NTSB investigators are in Minneapolis, working out of trailers, hotels and command posts. They will be working with investigators in Washington who will be re-creating various bridge-collapse scenarios by computer.

So far, investigators haven't been able to pinpoint a cause, but they've also managed to rule out some scenarios. "Every day we make progress in understanding at least where the failures are not. Where they are is where we're going to have to work a lot harder," said Mark Rosenker, NTSB chairman.

For now, the bridge-construction project is only one item on a long list of possible causes that includes aging steel, deteriorating welds and vibrations from adjacent train tracks.

Employees of Progressive, based in St. Michael, Minn., have helped investigators map out the locations of its equipment, vehicles and materials at the time of the accident, and how much each piece weighed. The company's work at the time was concentrated on a section of bridge over the river toward the southern end, Rosenker said.

The contract required Progressive to repair the bridge deck, replace the concrete surface and expansion joints and work on the anti-icing system, said Minnesota Department of Transportation spokesman Kevin Gutknecht.

Company officials said the crew was preparing to pour a 2-inch layer of concrete when the span gave way.

David Lillehaug, a lawyer for Progressive, said there is no reason to think the project had anything to do with the collapse. The most recent jackhammering of deteriorating concrete had occurred the day before, and none was performed the day of the accident, he said.

One question for the investigators is whether the diversion of traffic caused instability among the supporting structure's steel-arch ribs. The project took up four lanes of the eight-lane bridge.

Bridge experts have a range of theories about whether such construction could have played a role in the collapse.

One who discounts a connection is structural engineer Stuart Sokoloff, owner of CTS Group in New York, a company that performs engineering-failure analysis.

"If indeed the work that was going on was only patching or resurfacing, even if they're using jackhammers, I can't see how that would cause a major catastrophic failure of these massive [steel] trusses," Sokoloff said. "I just don't put the two of them together."