Twin Cities bridge collapse: Design flaw?

I missed this, it's from mid-October:

Federal investigators are trying to determine whether 91-degree heat caused expansion that put too much pressure on the corroded gusset plates that held the I-35W span together.

Ten weeks into their probe of the Interstate 35W bridge collapse, National Transportation Safety Board investigators have intensified their inspection of a long-corroded gusset plate that was located in the section of the bridge that fell first.

In addition, authorities are analyzing what role the 91-degree heat on Aug. 1 might have played in increasing stress on the already-weakened L-11 gusset plate, which connected four steel beams located near the bridge's south end.

In 1993, a state inspector found that the half-inch gusset plate had lost nearly half of its thickness in some spots due to corrosion along an 18-inch line, but no repairs were ordered, according to Minnesota Department of Transportation records.
...
...two of the three damaged gusset plates that appear to be of primary interest to the NTSB are half an inch thick. The thickness of gusset plates used in the bridge varied between half an inch and 1 inch. That could be an important issue, because a consulting firm hired by the state has said that some half-inch gusset plates may not have been strong enough to hold the bridge up.-- Star-Tribune, October 17, 2007

(More details in the original article.)

Now there's this:

U.S. Transportation Secretary Mary Peters said that investigators have a "working theory" of why the I-35W bridge collapsed: a poorly designed gusset plate and excessive weight on the bridge that day.
...
Peters' comments Thursday mirrored statements she made in August, a week after the collapse...

but...

NTSB spokesman Peter Knudson said. "We're also looking at the maintenance and repair history. We're looking at the de-icing fluids -- any role they may have played. We basically haven't ruled anything out yet."

So: It appears they know the point of failure, the question now is the reason.

The Star-Tribune's bridge coverage homepage is here.
November 1 story via buzz.mn.

Previously:

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