High-tech Tools Examine Minnesota Bridge Collapse
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 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."
Even the dive recovery teams are turning to technology for help, using underwater video cameras to look under dangerously unstable debris. Local teams have also requested help from FBI and Navy dive teams in the search for the eight people feared dead in addition to the five confirmed victims.
State transportation officials said they would begin removing bridge debris from the Mississippi River later this week. Besides 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. Officials offered no timetable for how long it would take.
Investigators caution that it could take as long as 18 months to complete their exhaustive probe into why Minnesota's busiest bridge collapsed and fell into the river 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 that can examine the debris in precise detail for any troublesome signs. The camera is kept steady by a gyroscope - which is how Hollywood crews get smooth footage while filming from a vibrating 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 from around the country are in Minneapolis, working out of trailers, hotels and command posts. They will be working with investigators in Washington who will be putting in long hours in front of computers.
That will include re-creating various bridge collapse scenarios with high-tech software in what is called a "finite element analysis." In this analysis, investigators can simulate removing a key support structure from the bridge and see how the bridge reacts.
"If they remove a piece and it falls down the way they saw it, that's a pretty good indication they found the right piece and there's all sorts of ways of doing that," said W. Gene Corley, senior vice president of CTL Group, an engineering firm.
Corley, who has helped investigate bridge collapses, as well as disasters such as the attacks on the World Trade Center and the Oklahoma City federal building, predicted that the NTSB should have a pretty good idea within a few weeks of the cause.
During his weekend news conferences, NTSB Chairman Mark Rosenker repeatedly stressed the value of the computer modeling program. He said a range of data goes into the analysis, including weather, the number of cars, the speed of the vehicles, and the weight of construction equipment that had been on the bridge when it collapsed.
"This is a very accurate and a very complex model. This isn't something that most of us at least deal with on our computers," Rosenker said Sunday.
So far, investigators haven't been able to pinpoint a cause, but they've also managed to rule out a variety of 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," Rosenker said.
The early clues and advances in technology could make for a far speedier investigation than after past bridge collapses.
The investigation into the 1967 collapse of a bridge linking Gallipolis, Ohio, and Point Pleasant, W.Va., took more than four years. It found that the cause "was a crack no bigger than a fingernail," said Galambos, the engineering professor.
"It was a very long and involved process. It reads like a detective story," he said.
After a portion of a bridge on I-95 in Greenwich, Conn., collapsed in 1983, investigators hauled much of the debris to a nearby state maintenance yard, recreating the scene and trying to decipher the clues, said Jim Loersch, retired as the state's manager of bridge safety and evaluation.
Investigators cut out samples of suspect steel, placing them under electron microscopes to study the grain, he said.
"If you have absolutely no idea (what happened), you'll just have to pull out every piece and reassemble the bridge just like you would in an aircraft disaster," Galambos said.
In the end, NTSB investigators determined that the Connecticut collapse was caused by loosening in the pins and hangers holding the slab of bridge in place. That was due to inadequate inspection and maintenance by the state Department of Transportation, federal investigators said. The state came up with its own findings, and the whole process took several years.
The NTSB says the Minneapolis bridge will not have to be fully reassembled, largely because of all the technology they have.
Investigators can instead focus on the part of the bridge that failed and not the entire 1,900-foot span. They also will inspect the destroyed cars to better understand how people died - or escaped. Source: Associated Press, August 5, 2007.