# RE: Seismic and snow

• To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
• Subject: RE: Seismic and snow
• From: "Lutz, James" <James.Lutz(--nospam--at)earthtech.com>
• Date: Tue, 28 Jun 2005 09:56:21 -0700
Although it makes intuitive sense that snow mass used in lateral force
calculations should also be counted in the resisting gravity load, I don't
think the code permits you to do it as written. It seems arbitrarily
conservative, but then counting only part of the snow load in the mass is a
little arbitrary, too.

I think there are probably some parts of the country where  even though you
don't have to, I would include all the snow load. I used to live in a place
where the temperature hovered around freezing most of the winter and we went
through constant cycles of snow rain  freeze melt snow, etc. The roof load
was probably never high enough to trigger the code requirement to include
snow load in the seismic mass, but the added weight was up there on the top
of the structure just about all winter long in a Southeast Alaska seismic
zone.

The point is, don't lose sleep over the conservatism in the load combo
formula.

-----Original Message-----
From: Jason Christensen [mailto:jason.christensen(--nospam--at)es2eng.com]
Sent: Tuesday, June 28, 2005 8:14 AM
To: seaint(--nospam--at)seaint.org
Subject: Seismic and snow

Food for thought.

I just designed a building that had a 100psf flat snow load, with a high
seismic area (SDC = D).  The building was of light frame construction with
wood shear walls.  I added the twenty percent of the snow into the design
base shear as prescribed by code.  Then when I designed my shear walls the
controlling load combo for uplift was 0.6*D + 0.7*E.  Here is my question,
another engineer I talked to mentioned using twenty percent of the snow load
in addition to the dead load for the uplift design.  His reasoning was 'if
the snow has been added to the seismic base shear then it will be there for
the uplift also'.  This makes sense to me, however I cannot find anything in
the code that would say this is ok.

Jason

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