Bridge collapse could be due to vibration, cracking

By PAT DOYLE
Minneapolis-St. Paul Star Tribune
Thursday, August 02, 2007

Investigators looking into the cause of the Interstate Hwy. 35W bridge collapse are likely to focus on two primary causes -- vibration and fatigue cracking, the former chairman of the National Transportation Safety Board said in an interview early Thursday morning.

Jim Burnett said they should look at whether vibration from the construction work and from a train that was passing under the bridge contributed to the collapse.

"Vibration is one of things that cause cracking to propagate," he said. "They will be looking at that."

Burnett, who is in Minneapolis for a Republican National Committee meeting, was watching the scene at 5 a.m. with other onlookers.

He also said he was intrigued by a 2001 University of Minnesota study that found signs of "fatigue cracking" in the bridge supports, though he noted that a later report apparently concluded that the bridge was in no immediate danger and did not need major repairs.

"I think that decision is going to come under new scrutiny," he said.

A 2005 federal study found that the bridge was "structurally deficient."

"A structurally deficient bridge might be one not adequate for the traffic it takes, but not necessarily dangerous," Burnett said. "But a lot of structurally deficient bridges are dangerous."

Burnett said he believed it would be the state's responsibility to check on the bridge in light of those reports. But he conceded that there could be a federal role and an issue of whether fed standards are adequate.

Minnesota state Sen. Steve Murphy, DFL-Red Wing, chairman of the Senate Transportation Committee and a vocal advocate for more transportation funding in the state, said "I can't remember a year when somebody wasn't pounding their fist on the table and saying 'we can do better.'"

"I am hoping the Legislature and the governor can do better in the future."

Murphy said he has received calls since the collapse from people critical of the governor for not spending enough on transportation.

But, he said, "The responsibility should be shared between the executive branch and the Legislature."

Murphy also offered his analysis of the 2005 federal report that deeded the bridge to be structurally deficient.

He said the report was not decisive and that the bridge's scores on a deficiency scale were "not good, not terrible."

"Did it need repair? Yes. Many of our bridges in the state need major work done to them."

Lucy Kender, a Minnesota Department of Transportation spokeswoman, acknowledged this morning that the state does have primary responsibility for inspecting and maintaining bridges.

When asked about Burnett's concern about the fatigue cracking in the bridge, she said she didn't know whether MnDOT took any special action beyond the routine inspections.

"We're going to have a thorough investigation," Kender said.

Mayor R.T. Rybak, asked this morning about the report that the bridge was "structurally deficient" said "those sorts of issues should go to the governor and department of transportation."

Gov. Tim Pawlenty said this morning that MnDOT engineers were aware of the two reports and took them into account when they inspected the bridge in 2005 and 2006.

"While there were concerns about fatigue cracking, the engineers did not (determine) that dramatic action needed to be taken," Pawlenty said.

"We have to rely on engineers and inspectors," he said.