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RE: Seismic and snow

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For what it's worth we recently requested a code opinion from ICC on
this very question. What we got back indicated that the load
combinations, as written, don't allow you to use snow to resist seismic
overturning or uplift. This isn't an official interpretation, so it's
not written in stone, but it may offer you some guidance. 

-Matt

-----Original Message-----
From: Haan, Scott M POA [mailto:Scott.M.Haan(--nospam--at)poa02.usace.army.mil] 
Sent: Tuesday, June 28, 2005 9:11 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Seismic and snow


I think you can use the snow included in seismic weight to resist
overturning
with the load combinations that exist.  I have had people argue with me
if
you have to use snow in seismic weight then you should be able to use
that
portion being included in seismic weight to reduce overturning.  It
seems
rational.

.6D+.6*(.2*S)+.7*E 

The E has a .2*Sds*(D+.2*S) vertical reducing the .6D.   


-----Original Message-----
From: Eli Grassley [mailto:elig(--nospam--at)psm-engineers.com] 
Sent: Tuesday, June 28, 2005 8:15 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Seismic and snow

It makes perfect sense. 

It's not conservative though. The bottom line is that it's not allowed
per
the code load combinations however. Jason is proposing 2 new load
combinations, and I for one think we already have plenty.

0.6D + 0.2S + 0.7(E w/ 0.2S)
0.6D + 0.7E (w/ no snow)


~~ Eli ~~


-----Original Message-----
From: Jordan Truesdell, PE [mailto:seaint1(--nospam--at)truesdellengineering.com] 
Sent: Tuesday, June 28, 2005 8:38 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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