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

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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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