Wednesday, August 29, 2007

More on the 35W Bridge Collapse

In no particular order,

1) Removal of the bridge pieces is now going more rapidly since 1) all the bodies have been recovered, and 2) the NTSB has taken whatever samples they have deemed necessary.

2) It already known that the collapse was particularly "catastrophic" (i.e., the whole thing fell all at once) as it was built in a time period were a lack of redundancies in the design were common. Bridges prior to that period (sorry, I can't define the period exactly) and since that period all have redundancies so that if a part of the structure fails, the rest of the structure is strong enough to support the load.

3) The media's attention at this point is on the gusset plates which connect the girders together. MnDOT had looked into strengthening them earlier this year by adding additional rivets, but this was not done as concerns were raised that the additional holes would weaken the structure more than the rivets would help.

What I want to know: bridges are all rated on a scale of 0 - 120. This particular one had been most recently at 50, but bridges with far lower ratings are still standing. Obviously the rating is not deterministic, but rather statistical. So, what's the standard deviation for any bridges rating? Is it constant, or does it vary with the rating? How accurate is a rating given that they are based on visual inspection? How well do ratings correlate with reality?


Vithar said...

In regard to the rating system, supposedly when a bridge is rated 49 its it required to be schedualed for replacement. The schedualing can be years into the future depending on the bridge.

There is some controversy as to whether this bridge received the 50 and not 49 so the expense of a new bridge could be avoided.

It would be interesting to learn the statistical information you speculate on.

John said...

This bridge was listed for complete repalcement in 2020 - basically sometime in the distant future when the current leaders are long gone - it's someone else's problem. But as bad as a 50 is, it no one was expecting it to fall down.