Shift to the east on south side of bridge collapes draws attention

CNN:

Investigators trying to figure out what caused Wednesday's massive bridge collapse are focusing on the southern end of the span, which "behaved differently" as it fell, the National Transportation Safety Board said Friday.

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"It appears that it has shifted approximately 50 feet to the east and when we compare that to what we've seen in the rest of the bridge -- the rest of the bridge appears to have collapsed in place, " said NTSB Chairman Mark Rosenker, who is leading a 19-member team charged with determining the cause of the accident.

"We're going to take a look at any unique design factors that could have created that shift...for whatever reason."

It's not the answer to the mystery of why the bridge fell, but a step forward, Rosenker said.

The NTSB will also examine three more videos provided by the Army Corps of Engineers, one of which may show another view of the bridge falling, Rosenker said. Investigators aren't optimistic the other two will show anything because they weren't pointed at the bridge.

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"The bridge's deck truss system has not experienced fatigue cracking, but it has many poor fatigue details on the main truss and the floor truss system," said a report conducted for the Minnesota Department of Transportation in 2001.

The 40-year-old bridge is of a type known as deck steel truss. It has three parts: a deck, superstructure and substructure.

The deck is made of concrete and rebar, the superstructure is made of steel and the substructure is made of steel and concrete footing, according to Rosenker.

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There has been some speculation that the footing supporting the bridge structure may have lost some of its surrounding soil which may have let the support shift causing a domino effect on the rest of the bridge.

CNN has done a good job with this story focusing on engineering and construction issues. The attempts by the Democrats to act like ambulance chasers passing out business cards at teh scene of the accident is avoided. It is a very fair story.