MnDOT feared cracking in bridge but opted against making repairs

Structural deficiencies in the I-35W bridge were so serious that MnDOT last winter considered bolting steel plates to its supports to prevent cracking in fatigued metal.

This view of the I-35W bridge collapse is from the south side
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This view of the I-35W bridge collapse is from the south side; the overpass in the background is University Avenue SE.

Photo: Bruce Bisping, Star Tribune

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Structural deficiencies in the Interstate 35W bridge that collapsed Wednesday were so serious that the Minnesota Department of Transportation last winter considered bolting steel plates to its supports to prevent cracking in fatigued metal, according to documents and interviews with agency officials.

The department went so far as to ask contractors for advice on the best way to approach such a task, which could have been opened for bids later this year.

MnDOT considered the steel plating at the recommendation of consulting engineers who told the agency that there were two ways to keep the bridge safe: Make repairs throughout the 40-year-old steel arched bridge or inspect it closely enough to find flaws that might become cracks and then bolt the steel plating only on those sections.

Fears about bridge safety fueled emotional debate within the agency, according to a construction industry source. But on the I-35W bridge, transportation officials opted against making the repairs.

Officials were concerned that drilling thousands of tiny bolt holes would weaken the bridge. Instead, MnDOT launched an inspection that was interrupted this summer by unrelated work on the bridge's concrete driving surface.

"We chose the inspection route. In May we began inspections," Dan Dorgan, the state's top bridge engineer, said. "We thought we had done all we could, but obviously something went terribly wrong."

Dorgan said there was enough money in the agency's budget to pay for construction work on the underside of the bridge. But he and Gov. Tim Pawlenty acknowledged that transportation officials will face tough questions about the state's upkeep of the bridge, which has had known deficiencies since 1990.

"We will absolutely get to the bottom of this," Pawlenty said. "There were a lot of decisions made, a lot of judgment calls made, and they're all going to have to be critically reviewed."

Pawlenty said an independent consultant will be hired to scrutinize MnDOT inspection practices meant to safeguard the state's 13,026 bridges. In the case of the I-35W bridge, MnDOT inspections convinced officials that the bridge wouldn't need to be replaced or overhauled until 2020, the governor said.

Was there an internal debate?

State and federal officials said it was too early to speculate on what caused the eight-lane bridge to collapse during Wednesday's evening rush hour, but Dorgan said the focus of the investigation is on the bridge's superstructure, or steel underside.

He said inspectors have long been on the lookout for metal fatigue and cracking in the bridge because it was designed before engineers learned about dangers to bridges from fatigue cracking.

"Up until the late 1960s, it was thought that fatigue was not a phenomenon you would see in bridges. Unfortunately that was a wrong assumption," Dorgan said.

According to a source with knowledge of the state and federal investigations, MnDOT is focused on the east side of the northbound section of the bridge past the Washington Avenue entrance as the likely spot where the bridge first gave way.

Bob McFarlin, assistant to the commissioner at MnDOT, dismissed speculation that the collapse was somehow linked to corrosion from de-icing chemicals automatically dispensed on the bridge.

The National Transportation Safety Board is conducting the official investigation and Minnesota has hired its own forensic engineering firm to conduct a parallel study.

A construction industry official who met with MnDOT about shortcomings on the I-35W bridge told the Star Tribune that there have been ongoing concerns among some MnDOT employees about the safety of this and other similar bridges.

"There were people over there that were deathly afraid that this kind of tragedy was going to be visited on us," the industry official said. "There were people in the department that were screaming to have these replaced."MnDOT has been trying to move these 'fracture critical' bridges up in their [budget] sequencing so something like this wouldn't happen," the source said.

"The Lexington Bridge [I-35E over the Mississippi River], that was a fracture-critical bridge. MnDOT moved that up pretty aggressively. What was happening on the Lexington Bridge was crack migration in the steel in the I-beam."

Dorgan said no open dissension existed.

"It was talked about whether replacement was needed, whether we could keep it in service," Dorgan said. "That was the whole point of those studies. There was engineering discussion of that, but I'm not aware of anyone who was rankled, or a heated discussion."If there was a strong opposition, it was not voiced," he said.

MnDOT said Thursday that about 8 percent of all bridges in Minnesota, including the I-35W bridge, have been listed by the federal government as "structurally deficient," compared with 13 percent nationally. The label doesn't necessarily mean a bridge is unsafe, but in the case of the 1,907-foot bridge, inspections were increased from once every two years to once every year, officials said.

According to findings from the most recent inspection in June 2006, inspectors noted various cases of corrosion and cracking, but found no evidence of growth in pre-existing cracks, Dorgan said. Another inspection began early this year but was put on hold when work began on a $9 million contract to patch and improve the bridge's driving surface. Dorgan said he has seen no link between the surface work and the collapse.

"We considered the bridge fit for service," he said.

Still, as recently as December, MnDOT indicated a desire to reinforce the bridge by 2008 with steel plates. According to a newsletter distributed in January 2007 by the Minnesota chapter of the Associated General Contractors, MnDOT was intending to take bids in late 2007 on a project that would "retrofit some of the chord members on the steel deck truss of [the I-35W bridge over the Mississippi River]."The Department is looking for feedback and advice from contractors regarding the project staging and constructability," the newsletter said.

But Dorgan told the Star Tribune Thursday that plans changed. "We decided to handle it with inspections instead," he said. Gary Peterson, MnDOT's assistant bridge engineer, said plating would have required drilling thousands of holes in the bridge.

"If you take a look at drilling all of those holes in a bridge that is already fracture critical you could initiate flaws that might initiate a fracture," Peterson said.

The option to monitor through inspection was one of two suggestions given to the department in 2006 by URS Corp., a San Francisco-based construction management consultant.

Some close observers of MnDOT continued to speculate Thursday that the decision to monitor instead of fix deficiencies in the bridge was driven by financial concerns. Dave Semerad, CEO of the Minnesota chapter of the Associated General Contractors, said everything MnDOT does is based on cost-benefit analysis.

"Let's face it. They don't have any money," Semerad said. "At the end of the day, that's the issue. This is indicative of a long-term pattern."

Asked whether a lack of money was behind MnDOT's decision not to reinforce the bridge, MnDOT Metro District Engineer Khani Sahebjam said: "No, we would never do that because of money."

 

Laurie Blake • 612-673-1711 lblake@startribune.com Paul McEnroe • 612-673-1745 pmac@startribune.com Pat Doyle • 651-222-1210 pdoyle@startribune.com Tony Kennedy • 612-590-5973 tonyk@startribune.com

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This view of the I-35W bridge collapse is from the south side