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RE: Snow Load + Seismic

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I've been updating the MutlLatT spreadsheet (rewriting it actually) and I
came upon this same issue. I would agree with Gerard and Scott - section
1630.1.1 lists the lateral use of the snow load as an exception. This does
not mean that it is to be ignored - it is up to the building official to set
the lateral standard for use of snow loads. Probably why it is not published
is because the snow load may vary by elevation and also by the amount of
overhang on the roof. 
For the moment, I have written the section 1630.1.1 exception #3 into the
spreadsheet. I believe that in past codes it was typical to use 25% of the
snow load as an addition to the dead load for lateral. Once again, this is
determined by municipality and needs to be verified with the local building
official.

Best Regards,
Dennis
-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu] 
Sent: Friday, July 21, 2006 2:08 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Snow Load + Seismic

Gerard:

Look at the definition of W (seismic dead load) in section 1630.1.1 of the
1997 UBC.  It states:

"3. Design snow load of 30 psf or less need not be included.  Where design
snow loads exceed 30 psf, the design snow load shall be included, but may
be reduced up to 75 percent where consideration of sitting, configuration
and load duration warrant when approved by the building official."

To me, this means that in your case (since it appears that you design snow
load is 40 psf) you must include the snow load as part of the seismic
mass.  And since there does not appear to be anything on the building
department's website allowing a reduction, I would say you must use the
full snow load as seismic dead load unless you actually call the building
official and get approval for a reduction (preferably in writing).

And that section above does NOT deal with the snow load as a gravity load
in the load combinations, except that there is the exception to the
alternative ASD load combinations that is basically ideatical (except for
difference purposes) to the section I quoted above.

HTH,

Scott
Adrian, MI


On Thu, 20 Jul 2006, Gerard Madden, SE wrote:

> 2001 CBC section 1612.2.1 (LRFD)
>
>
>    - Load Combo 1.2D+1.0E+(f_1L+f_2S)
>
>
>    - where f_2 = 0.2 for roofs that shed snow.
>
> Section 1612.3.2 (ASD Load)
>
>
>    - Load Combo D+L+S+E/1.4
>       - Exception 2 states that for Snow Loads 30 psf of less need not
>       be combined with seismic loads.
>
>
> I have a project with a 40 psf snow load. Therefore, the S value must be
> combined with the E/1.4 in the ASD value and I may reduce it up to 75%.
>
> What I am not clear about is if the mass of the snow is included in the E
> value. I read this as Snow is not a part of Seismic Dead Load.
>
> I have done projects in Alaska under IBC a few years back where I had to
> include 50% of the snow load as seismic mass, but the building department
> I'm dealing with does not state that in their webpage of code
requirements.
>
> what do you think? The structure is a residence with a simple 6/12 gable
> roof.
>
> TIA,
> -gm
>

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