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Continued: MnDOT doubted a plan to bolster bridge

"It's likely that retrofit of main members may be postponed for 15-20 years once an overlay is placed next year," MnDOT assistant bridge engineer Gary Peterson wrote in commenting on the draft report.

Instead of a major replating project, Peterson added, maybe MnDOT could simply "purchase plates and bolts for one or two member repairs ... should a short unrepairable crack be discovered."

Peterson added that in-depth weld inspections would continue every five years on critical members, and a "visual, arms-length" inspection would occur every year.

'Zero-probability' possible

In earlier interviews, state bridge engineer Dan Dorgan said the replating project lost steam only following a January 2007 conference call when URS acknowledged that drilling required for the retrofit could in fact weaken the bridge.

More recently, Kender, MnDOT's spokesperson, said there were more factors in the agency's decision, with URS telling state officials that if inspections and testing showed no measurable cracks then their confidence in the bridge's structural integrity would increase.

When Peterson appeared to reject the replating idea, Flemming for one was taken aback. "Gary's comments seem more difficult to handle," Flemming wrote in an August 2006 e-mail.

"If we really experienced major crack propagation in a critical member that it may be too late to make such a repair in a timely manner," Flemming wrote, and then added that "I am very concerned that they are suggesting relaxing inspections to a five year cycle and that retrofit be postponed for 15 to 20 years."

Although Dorgan at the time stated that he remained committed to the replating, other comments by Peterson drew a concerned response from Zhou. Asked by Peterson at one point asked whether the bridge's "absence of crack[s] ... indicate a low probability of future cracking?" Zhou replied: "No one can explain why some fractures [on other bridges] happened after so many years," but added that "as we learned from probability and statistics, a zero-probability event may still happen."

As URS worked to finalize its report last November, Zhou was still committed to the retrofit plan. But e-mail exchanges show that by mid-December -- without explanation -- he had changed. "Based on all the results we have obtained, I strongly believe that doing a 2-million-dollar plating retrofit is not necessary," he wrote to Flemming on Dec. 13. Zhou did not return a phone call for comment.

Zhou instead recommended a plan to use electronic sensors to detect cracks. "After that, we should have the peace of mind just as good as doing the plating," he wrote. The idea was later rejected by MnDOT, which determined the technology was unproven.

In the end, URS recommended three options: the original replating plan; a plan to do some replating combined with inspections, and a plan to inspect the bridge and repair any cracks that were found. MnDOT chose the third option, which was described by URS as the "most cost efficient." And although they signed a contract to use URS in a May inspection of the bridge, state officials went ahead without the consultant.

Thirteen days before the collapse, MnDOT official Todd Niemann sent an e-mail to colleagues to tell them a meeting would be held in August "to determine if inspection can safely and adequately identify potential critical defects."

URS engineers continued to tinker with a final report on the bridge through e-mail exchanges on Aug. 1 -- just three hours before the 35W bridge collapsed.

Staff writer Dan Browning contributed to this report. mkaszuba@startribune.com • 612-673-4388 pdoyle@startribune.com • 651-222-1210

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