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Re: Seismic and snow

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This takes some judgement.  I personally view the definition of as being both vertical and horizontal. Therefor if a mass is included for lateral loads, it is also there for vertical loads.  How can a load contribute to lateral and magically not be there for vertical?  The .6 in front of "D" in my eyes is conservative enough.  

You could also check a second condition.  Analyze the building without any snow in seismic (lateral or horizontal) and see if that governs any elements.  This is where the judgement comes into play.  Also, is the snow in a place where it may slide off of your loaded element during the first cycle and remain in place on the rest of the roof?

Some background may also be helpful.  The odds that *any* snow is present during an earthquake are very long, even when you have high snow loads like this.  I have read at least one statistical study (can't remember a name at the moment though, sorry) which looked at roof snow loads in place during earthquakes.  Your odds of keeping your money in Las Vegas are much better.  

Now for the bad news,  if you eliminate all the cases where there isn't snow, the snow in place during an earthquake can be fairly high, as much as 50% of the design load.  This can happen in places where snow will be on the ground a significant portion of the year.

I will see if I can find that report....

Jake Watson, P.E.
Salt Lake City, UT

P.S. If you think that is high, try 300 PSF or more and in an avalanche path.  Gives you a new respect for lateral loads.


---------- Original Message ----------------------------------
From: "Jason Christensen" <jason.christensen(--nospam--at)es2eng.com>
Reply-To: <seaint(--nospam--at)seaint.org>
Date:  Tue, 28 Jun 2005 09:14:13 -0600

>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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