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Re: Snow Load + Seismic

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I think there was more the designers should be aware of:
 
"The Hood Canal Bridge suffered catastrophic failure during the February 13, 1979 Windstorm. During the night the bridge had withstood sustained winds of up to 85 mph and gusts estimated at 120 mph, and finally succumbed in the early morning of February 13. The western drawspan and the pontoons of the western half had broken loose and sank.

Fortunately the bridge had been closed to highway traffic, the drawspan opened beforehand to relieve lateral pressure, the tower crew had evacuated, and no casualties resulted. There is considerable evidence that the failure was caused by hatches blowing open and water flooding in. In addition, a complete investigation seemed to be blocked by the eventual settlement between the state and the insurance company -- too many facts might have gotten in the way of a settlement." (WA DOT info, the emphasis is mine)

More info at http://www.historylink.org/essays/output.cfm?file_id=5501

Also interesting (although practically irrelevant to the subject) is the story of the floating bridge across Lake Washington http://www.historylink.org/essays/output.cfm?file_id=2002.

Steve Gordin SE
Irvine CA

---- Original Message -----

Sent: Saturday, July 22, 2006 10:36 PM
Subject: Re: Snow Load + Seismic

In a message dated 7/22/06 10:22:58 PM, ad026(--nospam--at)hwcn.org writes:
Is it reasonable to assume the maximum snow gravity effect (1 in 50 yr),
as either gravity mass or horizontal inertial mass, to occur at the same
time as the maximum seismic effect (1 in 500 yr)? Doesn't this blow the
statistical basis into microscopically small probability ranges?

Knowing nothing of statistical theory I've always wondered how the wind managed to be strong enough, for a long enough time, precisely aligned with the very long and narrow Hood Canal, right at low tide ... to sink the Hood Canal Floating Bridge (in Washington State, in the early 1970s).  My understanding is that without all of those factors it wouldn't have failed.  I can just picture the original designers saying "What are the chances of all that happening simultaneously -- nil."  But I'll bet the designers of the planned new bridge will!

Just my 2 cents, as they say.

Ralph Hueston Kratz, S.E.
Richmond CA USA