An engineering firm that studied the Interstate 35W bridge should have found the crucial flaw that led to the collapse of the span in downtown Minneapolis, the state claims in a lawsuit filed just ahead of the two-year anniversary of the disaster.
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From wikipedia ...
In 1990, the federal government gave the I-35W bridge a rating of "structurally deficient," citing significant corrosion in its bearings.
Since 1993, the bridge was inspected annually by Mn/DOT, although no inspection report was completed in 2007, due to the construction work.
According to a 2001 study by the civil engineering department of the University of Minnesota, cracking had been previously discovered in the cross girders at the end of the approach spans.
In 2005, the bridge was again rated as "structurally deficient"
The I-35W bridge ranked near the bottom of federal inspection ratings nationwide. The scale used was a "sufficiency rating" which ranges from the highest score, 100, to the lowest score, zero. In 2005 the bridge was given a rating of 50, indicating that replacement may have been in order. Out of over 100,000 heavily used bridges, only about 4% scored below 50. On a separate measure, the I-35W bridge was rated "structurally deficient," but was deemed to have met "minimum tolerable limits to be left in place as it is."
In internal Mn/DOT documents, bridge officials talked about the possibility of the bridge collapsing and worried that it might have to be condemned.
Yeah, it's the firm's fault.