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

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Hi Dennis and Scott;
	Wow!  I never saw the portion of the note that said ?regardless of
actual roof slope?.  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 wouldn?t 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:
?Where 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.?

This seems to conflict with itself. My first interpretation is that if the
snow load for a flat roof (say ¼? 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 ?regardless of actual roof slope? 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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