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

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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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