November 2, 2007

Working Theory On Bridge Collapse Is Just That

The NTSB's working theory on the St. Anthony Bridge collapse involves design flaws and overloading, according to comments by Transportation Secretary Mary Peters. That prompted questions by two state legislators about the role of maintenance and whether a lack of it didn't also have some role to play in the collapse, but Peters said that the legislators have misinterpreted her remarks (via Mitch):

The top federal transportation official said that investigators have a "working theory" of why the 35W bridge collapsed in August: a poorly designed metal component called a gusset plate and excessive weight on the bridge that day.

U.S. Transportation Secretary Mary Peters' comments Thursday mirrored statements she made in August, a week after the collapse, and like her previous comments immediately led to controversy. The National Transportation Safety Board, which is investigating the collapse, has said a formal finding will not be available for at least a year.

Sen. Steve Murphy, chairman of the Senate Transportation Committee, said Peters told a gathering Thursday in Washington, D.C., that he attended that "a finding of fault was not going to be lack of inspection or lack of maintenance" by state officials.

"I think it taints the findings," he said.

The Senator jumped to a conclusion, although Peters may have inadvertently pushed him along. In any investigation of this kind, the team gathers evidence and starts testing hypotheses to see whether the evidence supports them or rules them out. A "working theory" means that the investigators have settled on one for the moment that fits the evidence seen thus far. It does not rule out that other theories will be tested, perhaps either replacing the working theory or added to it.

The NTSB says it still hasn't ruled out any hypothesis at the moment, including maintenance or de-icing fluid issues. However, it sounds as if Senator Murphy has; any result that rules out maintenance and testing will "taint the findings". He wants a finding of some form of incompetence with which he can beat the Pawlenty administration. A poor gusset-plate design won't allow him to do that.

However, gusset plates seem a likely cause of the collapse. A similarly designed bridge in Cleveland had a partial collapse in 1990, and the gusset plates failed in that incident. As it turned out, the plate design was insufficient for the size and weight of the bridge. Given that history, using gusset-plate design as a starting point doesn't seem unreasonable.

It will take several more months before the NTSB can make its decision on the cause of the collapse. Peters should probably keep her updates less specific until the investigators have their report ready. State politicians should keep their mouths shut as well, lest their desires to warp the process for their own political ends get too obvious to Minnesotans.


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The NTSB is thinking design flaws and overloading as to why the bridge went down. But the agency hasn't ruled other causes. Ed Morrissey is speculating that faulty gusset plates make sense at least part of the reason that the bridge collapsed. vadkinsQ... [Read More]