Thursday, August 09, 2007

Bridge investigation update

Call me a geek but the bridge investigation continues to be intriguing. NTSB Chairman Mark Rosenker, now back in DC, gave CNN a progress report last Saturday:
Rosenker said nothing significant was discovered on the south side of the bridge, and attention was now shifting to the north, which buckled vertically. The FBI images will be a great aid, he said.
Just a day after the crash they were focusing on the south side due to lateral shifting, then said nothing was wrong and went north, but after examining that side they announced Wednesday there was nothing overtly wrong there, either. Instead they've now focused attention on the 'gusset plates' that help tie the truss beams together while simultaneously pointing towards heavy construction equipment parked on the bridge. Not surprisingly the construction company quickly denied any wrongdoing.

While not an official finding it seems strange to suggest the gusset plates were at fault so early in the game. Of course, pinning blame is what everyone wants--not only a curious public but the pack of lawyers circling on the periphery. Most families will likely sue the city/state/feds or whomever else looks to be at fault, and that might not be limited to just the families. Commuters not even present at the crash might try to sue for lost revenue from having longer drives, etc.

In that vein, one of the construction crew members on the bridge was quoted as saying the structure was "wobbling" in the days before the fall. Now a spokesman for the contractor says:
..the company had interviewed every employee on the bridge when it collapsed and had not found anyone who could substantiate reports from a first responder who said workers told him the bridge was wobbling before it fell.
Too bad the specter of lawsuits has to hang over the investigation like a black cloud, but such is life.

They still haven't explained the loud sound heard on the south side by eyewitness Lori Patterson. Some are speculating about the listing north pier seen in the photo above, which were supposedly anchored pretty deep. We've been reminded of the uniqueness of the design; that those piers weren't sitting in the channel--true--but the north pier was placed at water's edge. Any significance?

As to the tilt, did it happen before or was it a result of the collapse? Common sense would suggest both are possible. If it suddenly shifted a few feet that might explain the loud sound heard on the south end as the superstructure failed at the steel joints. It might also explain the lateral shift upon collapse. The video released to the public showed the south end go down first, also consistent but we've not been privy to all the available video. One might also deduce the north pier was pushed forward by the sheer force of the collapsing approach deck after the main span failed. All speculation of course, and all without the benefit of a night's stay at a Holiday Inn Express, so take it for what it's worth.

Anderson Cooper posed several questions to a seasoned bridge engineer a few days after the collapse and posted them on his 360 blog:
I asked all the questions: Cracks in steel? Construction accident? Salt corrosion?

Galambos says anyone of them can bring down a bridge-but not like this. It was complete and instant failure. Cracks and corrosion give warnings, construction accidents can be witnessed. There is no evidence, he says, of either.

The surveillance video seen on CNN shows an event he calls one-in-a-billion; it just doesn't happen.
Not sure what to make of that--maybe just the emotional words of a proud engineer after a long career designing bridges. Or perhaps words of wisdom.

MORE 8/10/07

This is quite an interesting feature on bridges, for all those closet engineers out there.

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