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Re: AP: Wash. State bridge

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Thor:

I guess I missed something some place. I haven't heard that the bridge was designed (originally) in a fragile condition. I don't think that a computer program would actually design the bridge to much better than what I might do with my old slide rule. Basically in the 50's we weren't checking for secondary moments. But I have checked bridges of that era for very heavy (purple) loads, using the standard axle spacing with a FEM program and found that they are still robust and within the AASHTO code.

What we might argue about then would be maintenance. Or if you are saying that the bridge should be able to resist any kind of impact from any truck at any point on the bridge, then that is a different matter.

I once investigated (for Transport Indemnity) a dropped gantry on the Golden Gate Bridge. The next day our office investigated a dropped missile at a large aircraft company. Both preliminary (guesses) were operator error. The discussion at that moment with the adjuster was that these events usually happened in three's. A couple of days later the adjuster informed me that there had been another accident in Mare Island. Probably operator error. Whatever happened at the Washington bridge appears to be at this time - operator error.

So are we going to make all of our bridges for every condition of possible event that can happen?

Neil

On 5/26/2013 8:01 PM, Thor Tandy wrote:
Agreed.  None should drive an over height/size anything onto a bridge
(without supervision), however, Joe public is not unreasonable asking why
the bridge was designed in, what is effectively, a fragile condition. From this point in time, the answer is almost rhetorical and the real question is, maybe, are we doing anything about it.

Many ages ago, I was reminded by a peer that, (engineers) are remembered,
not for finding problems but for solving them ...

Bridges are not the only items that suffer from this ailment when you
consider that a lot of pre-computer era structures were conceived and
designed with only the power of, eg, the slide rule.  I can't help
remarking that these issues only really come to light when there is a
failure, especially if there is also a body count.

So I wonder if the (bridge) rating system is failing to include the
(potential) impact of fracture-critical (and other?) conditions ... another example of low probability but high impact ...? But maybe that's a political issue.


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