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# RE: Snow Load + Seismic

• To: <seaint(--nospam--at)seaint.org>, <seaint(--nospam--at)seaint.org>
• Subject: RE: Snow Load + Seismic
• From: Neil Moore <nma(--nospam--at)omsoft.com>
• Date: Tue, 25 Jul 2006 12:12:22 -0700

I retrieved my calcs for this particular project and the snow load was 16 kips . The seismic coefficient at the roof was 1.06g. This was with 100 per cent snow load participation. The snow load density was 45% per Sierra Hydrotech, which provided about 28 pcf. From there we came up with an equivalent water design height and approximate centroid of the snow mass.
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Again, this snow mass was surrounded by six foot walls and the basic snow load was 150 psf with a seismic coefficient of 1.00. Even if you reduce this down to 33% or 25%, it's a force that has to be accounted for. The code used was the 97 UBC (the IBC came into force a few months later).
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The result of this exercise was that the client decided to install his new equipment in this area on the ground. That equation (32-2) is sometimes a project killer.
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Neil Moore, SE, SECB

At 11:27 AM 7/25/2006, Polhemus, Bill wrote:
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```-----Original Message-----
Sent: Tuesday, July 25, 2006 12:31 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Snow Load + Seismic

So the question is what percentage of the mass can move freely and
how much of it will rest on the structure to add to the W.

-----/Original Message/-----

I think that snow on the roof has got to be considered an active mass
for seismic analysis. It just seems to me that the calculation of the
roof snow load defined by the code(s) takes into account the probability
of its presence, and therefore what would need to be accounted for would
be the probability of its presence in a seismic event.

Thirty-three percent might be too simplistic--but I keep going back to
the idea that you've already accounted for all the other probabilities,
except perhaps a "risk/importance factor" (i.e. by how much does the
danger of exceeding the allowable response increase if you DO happen to
hit on a seismic event with lots of lovely wet stuff on top of your
structure).

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