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Re: ground snow v. roof snow - which is it?

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I believe that the requirement to use ground snow would have to be proposed and included in the legislation adopting the code, or in subsequent published addenda.  Code officials, as far as I know, do not have the right to change the building code, though some feel that they do. That's the whole idea behind using model codes. Otherwise, we're back to pre-national code times when every jurisdiction makes it own rules. It may not be worth the fight - its not our money we're spending by upsizing roof structures - but the courts would likely side with the letter of the law, and in this case the building code is extremely clear. "Shall be determined in accordance with" doesn't leave much wiggle room.

Jordan

Scott Maxwell wrote:
Jordan,

Not so quick.

You have a point, but then Jim does as well as local code officials or
states can "modify" the plain vanilla IBC (i.e. adopt it with
modifications/additions).  From my experience, some local juridictions
will require the use of the full ground snow load as the roof snow load
(i.e. no modifications to flat roof snow load or sloped roof snow load).
Now, it is my belief that this is largely due to the IRC (which I believe
REQUIRES the use of ground snow load and does not PERMIT modifications per
ASCE 7) and local code officials not realizing that such it not "required"
if the design is an engineered design (i.e. IBC is used rather than IRC).

Scott
Adrian, MI


On Wed, 21 Sep 2005, Jordan Truesdell, PE wrote:

  
Jim, I think you've got is bass ackwards ;-)

I quote, "Design snow loads shall be determined in accordance with
Section 7 of ASCE 7..." This is the first sentence of section 1608 of
the 2000 IBC (Snow Loads).  The ellipses referes to the requirement that
the roof load must not be less than the roof live loads specified in the
Live Load section, 1607.

That's pretty cut and dried - engineers (actually, anybody) are
specifically allowed by the code to use any and all reduction (or
amplification factors) they deem appropriate, as listed in ASCE 7.  The
code writers appear to have specifically allowed the reduction of loads
where allowed by the ASCE.

As has been mentioned, the span tables are probably based on ground snow
loads, and should not be altered without analysis by a registered
engineer.  Ground loads are convenient, because that's what's in the
table.  I would suspect that the tables also assume partial exposure,
ventilated roof over thermally isolated living area, Terrain Categories
A, B or C, an importance factor of 1.0, and no drifting conditions or
sliding surcharges, and - of course - no unbalanced loads.  Prescriptive
design has its place, but can always be overridden by proper analytical
techniques. (That doesn't always mean reduced loads, btw)

Jordan

Jim Wilson wrote:

    
It seems as if the code writers have eliminated the
option of reducing snow load when using prescriptive
design.  Subsequently, it is up to the local enforcers
to decide if an engineered design is permitted to
include the reduction factors.  Does that sound
correct?

Jim






      
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