Northern end of I-35W bridge is now focus of probe

The search for clues
Mark Rosenker, chairman of the National Transportation Safety Board, explains to reporters at a news conference on Saturday how the I-35W bridge fell.
MPR Photo/Sea Stachura

NTSB Chairman Mark Rosenker put it plainly.

"We don't believe the southern portion of the bridge is where the accident began," he said Saturday afternoon.

Rosenker's agency focused on the the southern end because that part of the bridge shifted not 50 feet to the east as originally suspected, but 81 feet. Rosenker now says he believes that movement was the result of the collapse, not part of the cause.

Now, his investigative team is training its gaze on the northern portion of the bridge. Teams were quickly able to clear the south end as a possible cause, but Rosenker says the investigation on the north end may take longer.

"The northern side, as it came down, a lot of the superstructure was actually pushed up. So it's difficult for us to get a visual inspection on it. And because of that we're going to do is get a helicopter with a high-resolution camera on it," he said.

He says the camera will zoom in on the bridge and its superstructure. One of the points of interest for inspectors is the way the metal on the bridge broke. Rosenker says there are several types of breaks, including compression and tension fractures that may help experts map the forces at play. Rosenker says if his team finds anything at the north end, it will ship pieces of the bridge back to Washington, otherwise, crews will move onto the center portion of the bridge, which lies in the Mississippi.

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Rosenker emphasizes that an investigation of this magnitude will take a long time. He says his agency took a year to investigate just 200 feet of a collapsed tunnel that was part of Boston's Big Dig. The 35W bridge is a quarter of a mile and, he adds, far more complicated.

Rosenker says much of the investigation will move to Washington in the coming days, but members of his crew will remain here until a possible cause is found. He says the team has documented enough about the bridge that portions can now be removed.

"We are giving the center section over to MnDOT, to begin their excavation to begin removing the cars and two to begin removing the deck," he said.

That may help divers reach vehicles crushed under the decking.

The bridge is not the only focus of the analysis. Rosenker says investigators will also examine the cars that went down with the bridge. He says they'll be looking for clues to explain why people survived a 60-foot drop into the Mississippi.

"Our survival folks will be looking at that to understand why we were so fortunate here, in Minneapolis, to have so many people survive this tragic collapse."

While MnDOT gears up to remove vehicles and concrete slabs from the area, it's also developing plans for a new bridge. MnDOT's Assistant to the Commissioner Bob McFarlin says the department took the first steps of the bidding process for a new bridge. He emphasizes that the selection of a plan will be based not on the lowest cost, but on best proposal.

"Wwe're going to be working with those teams to see how quickly they think the bridge can be done," he said. "We are very hopeful, very hopeful, that the bridge could be open for service late next year."

MnDOT also identified a fifth bridge that will be inspected because of a design similar to the I-35 bridge. After the Minneapolis bridge collapsed, the department launched inspections of four similar bridges. McFarlin says all of those bridges are functioning as they were designed. On Saturday, the department added a fifth bridge: the Gooseberry River bridge on the North Shore. The structure was built in the 1990s, but it fits the federal criteria for inspection. It, too, is a steel, arch truss bridge.