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RE: Seismic and snow

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Sorry I left part of his argument out; his thought was the seismic load has
been increased so the design is greater.  If the earthquake occurs without
the snow load the design will be adequate, since it was designed with the
higher force.

Jason

-----Original Message-----
From: Jordan Truesdell, PE [mailto:seaint1(--nospam--at)truesdellengineering.com] 
Sent: Tuesday, June 28, 2005 9:38 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Seismic and snow

That makes no sense.

If you have a high snow load, it is reasonable to assume that a seismic 
event is likely to occur while snow is on the roof (20% is what is 
assumed to be the intersection of the probability curves).

It is not reasonable to assume that seismic events may only happen while 
snow is on the building.  I have always assumed that the 0.6D number was 
there because we (SEs) traditionally overestimate the dead loads in 
order to account for the worst case loads (sprinklers, MEP, finishes, 
two layers of asphalt shingles, etc.).  Since the conservative approach 
is to UNDERestimate the dead loads in the case of uplift, the code makes 
it easy on us and lets us take a factor.  Adding a seasonal snow load to 
offset a seismic uplift seems non-conservative.

Jason Christensen wrote:

>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.
>
>Your comments would be helpful.
>
>Jason
>
>
>
>
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