Monday, August 20, 2007

It wouldn't have been less expensive

"They expressed concerns about the bridge, but at no point, as far as I know, did anyone ever say 'close the bridge'," stated Gov Tim Pawlenty.

The search continues at the site of the I-35W bridge collapse for the eighth and last person listed as missing, Greg Jolstad, 45, who had been part of the construction team doing surface repairs to the bridge when it collapsed. While no cause for the collapse has been found yet, more details have been uncovered in the bridge inspection documents. These discoveries may shed light on the cause and lead other states to seriously review their own bridge repair plans.

Much of the attention has been on the consultant reports from 2006 and 2007 that had expressed serious reservations about the bridge. The Star Tribune has reviewed the older reports from inspectors and they show that concerns about that bridge had been growing since the 1990's. In most cases, the reports simply list a growing list of problems found with the bridge but don't rate their severity.

By the 1996 report though, the inspectors began to use a more urgent tone in their reports. They had noted that a pier supporting steel spans had titled towards the north and they warned that it should be an area that is inspected closely as it wasn't going to be repaired in the near future.

In 1998, they wrote that they had found numerous fatigue cracks in the three decades old north and south approach spans. The report did note that the cracks had been drilled out and the fractured beams had been reinforced with steel plates. Each susequent report mentioned the cracks found in 1998 and by 1999, they reported that the cracks were a major concern and so widespread that they would need to be inspected every six months.

No further cracking was noted by 2000, so the inspection cycle was increased. A state sponsored study performed by the University of Minnesota concluded in 2001, that fatigue cracking of the deck truss is not likely and the bridge shouldn't have any problems in the foreseeable future. MnDOT concluded as a result of that study, that they did not need to prematurely replace the bridge because of fatigue cracking and could then avoid the high cost of such a project. The earliest possible date for replacement suggested by the MnDOT was in 2020.

The state bridge inspectors almost a decade ago were stating that the I-35W bridge had severe and extensive corrosion of it's beams and trusses, widespread cracking in it's spans and missing or broken bolts. They noted that the superstructure was in poor condition and that certain components were beyond tolerable limits. By 2000, they had written that replacement of the entire structure would be preferable to redecking the bridge but if replacement was significantly delayed, redecking should be done.

That particular recommendation was repeated in subsequent reports but had not been done. The MnDOT inspectors were still concerned, enough to ask an engineering consultant URS to review the bridge's condition. In 2006, URS expressed concerns that a serious fatigue crack might go undetected because of the difficulty in inspecting parts of the bridge. They had recommended steel plating as a fix for the problems.

Again it seems that cost was the largest factor involved because MnDOT asked URS to come up with other solutions and the department chose what URS called the "cost efficient" alternative that included increased inspections and repairing visable faults. URS had warned though that the critical issue with that method was to insure that inspectors didn't miss any measureable flaws.

Chief bridge engineer Dan Dorgan stated that previously, MnDOT had chosen an option that included drilling to add plates that he felt might weaken the structure. Roberto Ballarini, a civil engineering professor at the University of Minnesota, stated that a crack could grow very little in a year or it could grow rapidly almost overnight and Dorgan agreed that there was a potential for that to happen. I would have thought that what had happened to the Mianus River Bridge in CT was warning enough of what will happen when difficult to view parts of a bridge structure fail.

As tragic as this collapse is, I would hope that towns and states look to the I-35W bridge collapse as a serious warning. Not the fact that our nation's bridges are aging but that what seems to be the view of cost effectiveness versus the correct fix could have led to this collapse. The inspection reports detail an older bridge that continued to develope additional, serious problems, problems that would not just disappear on their own. I do not understand the logic of saving money now on replacement..... did the state actually think that replacing a bridge was going to get any cheaper, twenty years into the future?

For my previous post, go here.

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