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Re: Load probability (was Snow Load + Seismic)

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> From: Rhkratzse(--nospam--at)aol.com

> 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!

I got through Statistics 101.

Consider: low tide occurs daily and is relatively consistent rather than
once a year and highly variable. The long, narrow canal may serve to
direct the wind despite minor wind directional variances and this may be
well known by the local population but not the spec writers or design
engineers.

So, there may have been unexpectedly high probabilities and, possibly,
unanticipated failure modes. Where did they point the finger in the end?

We know snow relatively well. Seismic is a crap shoot but the code
writers have made choices.

The code enforcers don't necessarily have better understanding of the
code intent than we practitioners. However, we follow their potentially
misguided regulations because they set the minimum standard by law. The
cost is to the owner.

So, why do we use different snow mass for gravity vs. seismic in the
same load case? Especially, if we use a higher value for seismic
contribution than gravity ... this is a logical inconsistency.
   
-- 
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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