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RE: Snow Load + Seismic

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Dennis:

A slight correction...the provisions that I quoted from section 1630.1.1
is NOT an exception in that provision in the code.  It is "applicable
portion" of "other load" #3 when section 1630.1.1 sates "Seismis dead
load, W, is the total dead load and applicable portions of other loads
listed below".  I quoted item #3 of that list.

It so happens that what I quoted is almost  identical to exception #2 of
section 1612.3.2, which is the section that provides the alternative ASD
load combinations.

The point is that to me section 1630.1.1 means that by the 1997 UBC code,
you must use 100% of the design snow load as part of the seismic dead load
if that seismic design load is greater than 30 pdf UNLESS the builing code
offical gives you the approval to use something less that 100% of that
load, but even the building official cannot let you use less than 25%.
That deals with the seismic dead load that is used to determine the
seismic loading (i.e. V).

Then if you choose to use a load combination from section 1612.3.2 (i.e.
an alternative ASD load combination) that has snow load in combination
with seismic (i.e. equation 12-16), then the exception allows you to use
zero (0) psf for the S part of the combination if the design snow load is
30 psf or less.  If the design snow load is greater than 30 psf, then you
must use 100% of the snow design load for the S part of the load
combination, UNLESS the building official give you the approval to use
something less than 100%, but again the builing official cannot let you
less that 25% of the snow design load.

So, in Gerard's case, I would say that unless he gets permission from the
building official (either by some written letter or something indicated in
writing on the jurisdiction's website or such...at least that would be my
suggestion), he must include 40 psf as part of the seismic dead load/mass
_AND_ use a 40 psf gravity load for S part of the load combination if he
uses alternative ASD load combinations.  At most, the building official
could allow him to only use 10 psf of the 40 psf design snow load as part
of the seismic mass and/or allow him to only use 10 psf for S as part of
the load combination.  Or it could be somewhere between those two
extremes.  But again, one needs to get something official from the
building offical...if the building official is silent on the issue, then
it is the full 40 psf for both the seismic mass and gravity load S value
in the load combination.

Thus, if you are updating your spreadsheet, then by default it should use
100% of the design snow load if it is greater than 30 psf, but you could
allow a modification of that to somewhere between 25% and 100% for
situations where a building official allows it.  I suspect that you don't
do anything with load combinations so you don't need to worry about the
section 1612.3.2 exception, which technically is completely seperate and
the building official could use a different allowable reduction than the
section 1630.1.1 value for inclusion in the seismic mass.

Regards,

Scott
Adrian, MI


On Fri, 21 Jul 2006, Dennis Wish wrote:

> 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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