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Re: ASCE 7-05 Section 12.14.8.1 (#4) clarification needed

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I have found that it is somewhat of a moot point...because as you noted,
local jurisdictions tend to "modify" (to something more severe typically) in
my experience.

Regards,

Scott
Adrian, MI


On 7/21/08 12:35 AM, "Terry Weatherby" <terry(--nospam--at)wrfed.com> wrote:

> Hi Dennis and Scott;
> Wow!  I never saw the portion of the note that said ³regardless of
> actual roof slope².  That makes absolutely no sense to me.
> If you have a basic flat roof snow load of 40 psf (as Scott says
> below) that reduces to 24 psf due to slope, why wouldn¹t you take the 20%
> based on the actual design roof snow load of 24 psf?  This would mean that
> you would add 4.8 psf to your roof dead load for seismic lateral design.
> The steeper the roof, the less snow that would need to be added into the
> seismic lateral load because there would be less snow actually on the roof.
> 
> As someone who designs with Snow on almost every single project I
> would say we have been doing it this way for years.  The only difference is
> in what percentage of snow to put into the seismic load.  This is usually up
> to the local jurisdiction and regulated by ordinance.  Some examples that I
> am aware of: Alpine County uses 50%; Calaveras County uses 25%; and Mono
> County uses 33 1/3%.
> If you were using 8 psf (based on the 40 psf from a FLAT roof), that
> would be 33 1/3% of the snow load so why not just say 33 1/3% instead of
> 20%?
> Take care,
> 
> 
> Terry Weatherby
> Weatherby-Reynolds-Fritson
> Engineering and Design
> Jackson, CA
> 
> ________________________________________
> From: Scott Maxwell [mailto:smaxwell(--nospam--at)umich.edu]
> Sent: Sunday, July 20, 2008 7:06 AM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: ASCE 7-05 Section 12.14.8.1 (#4) clarification needed
> 
> I interpret this as if the flat roof snow load (Psubf) exceeds 30 psf, then
> you take 20% of the FLAT roof snow load (Psubf) and add that to the seismic
> mass.  You do NOT take 20% of the SLOPED roof snow load (Psubs)...that is
> the "regardless of actual roof slope" part.  And you do NOT take 20% of the
> portion that exceeds 30 psf.
>  
> Thus, if you have a 40 psf flat roof snow load and a 12:12 roof pitch on a
> non-slippery roof such that the sloped roof snow load is 24 psf, then you
> would add 8 psf (20% of 40 psf) to the seismic mass...NOT 4.8 psf (20% of 24
> psf).
>  
> Regards,
>  
> Scott
> Adrian, MI
> -----Original Message-----
> From: Dennis Wish [mailto:dennis.wish(--nospam--at)verizon.net]
> Sent: Saturday, July 19, 2008 9:56 PM
> To: seaint(--nospam--at)seaint.org
> Subject: ASCE 7-05 Section 12.14.8.1 (#4) clarification needed
> I need an interpretation to note number 4 of ASCE 7-05 Section 12.14.8.1
> which states:
> ³Where the flat roof snow load, Pf, exceeds 30 psf (1.44 kN/m^2), 20 percent
> of the uniform design snow load, regardless of actual roof slope.²
> 
> This seems to conflict with itself. My first interpretation is that if the
> snow load for a flat roof (say 1?4² in 1-foot) slope or a shed roof exceeds 30
> psf then you would add 20% or 6 psf to the roof dead load. However, the last
> section of the statement reads ³regardless of actual roof slope² in which
> case I would interpret the 20% of the Snow Load exceeding 30-psf to be added
> to any sloped roof.
> 
> How do others interpret this statement. If the roof is flat should 20% of
> any Snow Load be considered whereas only 20% of the Snow load exceeding
> 30-psf be used if the roof is pitched?
> 
> Dennis S. Wish, PE
> 
> Dennis S. Wish, PE
> California Professional Engineer
> Structural Engineering Consultant
> La Quinta, CA 92253
> dennis.wish(--nospam--at)verizon.net
> http://structuralist.wordpress.com
> http://www.structuralist.net
> 
> 
> 
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