Poor design and extra weight caused 35W bridge collapse, NTSB says

The final National Transportation Safety Board has concluded in its final report that under-designed gusset plates and the weight on the bridge deck caused the 35W bridge to collapse last August that killed 13 people, according to Minnesota Public Radio.

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Reports of poor gusset plates has been reported previously, but the weight factor is new. The report says "the two inches of pavement added to the bridge deck in the 1990s, and the presence of 270 tons of construction equipment and materials on the bridge the day of the collapse, triggered a 'cascading effect' that led to the failure of the span, in effect pushing the structure beyond its capacity, MPR says.

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NTSB investigators have also concluded that bridge engineers and inspectors did not have a system in place to double- and triple-check that all factors and safety features of the bridge were sound, MPR news also learned.

One MNDOT bridge inspector had noticed that gusset plate U10 was bowed. However, that problem was not part of the analysis of the bridge's safety by MnDOT and URS Corp., the consulting firm it had hired to help with bridge inspections.

NTSB investigators also ruled out one maintenance issue as a factor. The investigators have determined that corrosion on the bridge's steel did not play a significant role in the collapse.

The investigators' findings will be presented at an NTSB hearing in Washington DC, which begins tomorrow morning and will last through part of the day on Friday.

The report doesn't point out any blame. We'll let you know when there is response from local officials on the report.