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

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I believe (I reserve the right to be wrong) that the IRC span charts are
"keyed" to the ground snow load.  Since the IRC is supposed to be a
prescriptive design and no "engineering" is supposedly involved, I believe
the intent is to not "confuse" the issue with modifications to the ground
snow load, which result in the flat roof or sloped roof snow loads (i.e.
you don't do "engineering" calculcations to adjust the ground snow load).
In otherwords, I believe the IRC (largely slightly educated speculation on
my part) that the span charts "account" for average adjustments from
ground snow load to a roof snow load.

If this is true, then I would assume that if you go to a full engineered
design (i.e. don't use the IRC) for a residential structure then most
building officials would permit the use of the typical adjustment factors
to get to roof snow loads.

All I know for sure is that we tend to just use the ground snow load as a
minimum roof "live" load for residences that we design, which are
engineered as SIPs and timber frames don't fall under the IRC.  The
exception is when there are REALLY high snow loads (i.e. had project where
the ground snow load was 266 psf in a mountain pass...used ASCE 7 to get
roof snow loads).

Regards,

Scott
Adrian, MI


On Tue, 20 Sep 2005, 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
>
> --- Drew Morris <dmorris(--nospam--at)bbfm.com> wrote:
>
> > Jim Wilson wrote:
> >
> > >I'm under the impression that typical roof span
> > charts
> > >for wood rafters, beams, etc. are based on ground
> > snow
> > >load.  At least that's what the tables say.  That
> > >seems misleading since using the ASCE formula, one
> > >could reduce that by 0.7 or more for roof loading.
> > >I'm guessing that the prescriptive code takes the
> > >conservative route by making no adjustment for roof
> > >snow factors.
> > >
> > >Similarly, if "engineering" a wood roof, should the
> > >same adjustment factors be ignored to maintain a
> > >conservative consistency?  Maybe there is a
> > commentary
> > >on this some where.
> > >
> > >Just curious...
> > >
> > >Jim Wilson, PE
> > >Stroudsburg, PA
> > >
> > >
> > >
> > In Alaska, Anchorage and Juneau specify a ground
> > snow load and a
> > minmimum roof snow load.  The roof snow load is
> > calculated using ASCE 7,
> > Section 7.
> > In Anchorage, the ground snow load (Pg) is 50 psf
> > and the minimum design
> > roof snow load (Pf) is 40 psf.
> >
> >
> >
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