Snow loads at or below ground level?

[AaronMcD]

For loads at a ground level roof, or near ground level deck, do you use ground snow loads for the balanced load? Should they be increased by exposure and temperature factors (for instance an unheated crawl space)?

Edit: Using ASCE

http://www.hellodwell.design

 

[AaronMcD]

Also, what do you do for drifts at ground level? What do you use for windward fetch?

http://www.hellodwell.design

 

[Rabbit12]

I just had this scenario for drifting. Theoretically, the fetch could be essentially infinite. I ended up designing for a snow depth of like 6 or 7' which I thought was reasonable. I was born and raised in the area I did this so pretty familiar with potential drift sizes against structures.

 

[r13]

Do you mean roof for single storey building (first level roof)? A roof is a roof, follow code to calculate snow load. For exposed ground level deck, since it posses low risk to human life, ground snow should suffice.

 

[KootK]

I dug around a bit on this out of curiosity and didn't come up with much. I think that it comes down to the root question of "why is flat roof snow load less than ground snow load?". I didn't find a definitive answer to that but, rather, just some musings to the tune of "stuff up off of the ground blows away more easily". Were that to be all that there is to it, then the answers to your questions would logically have to be:

1) Ground snow load for stuff at ground level.

2) Windward fetch on the ground for drift is... infinity? To the building across the street?

My money says there's a code commentary someplace that would elucidate this if only we knew where to find it. It kind of begs the question: how high does a level have to be before the snow load is not the ground snow load.

 

[phamENG]

I think KootK is onto something with the "stuff up off the ground blows away more easily" line of thought. Flat and low slope roofs with p_g less than or equal to 20psf have a minimum load of p^g. Since the snow load and accumulation are directly proportional, it would follow that "taller" accumulations are more prone to be blown off, thus reducing the minimum load.

I think you can justify something with the exposure and thermal factors. If you consider the ground to be "sheltered in terrain B" and the ground is unheated and open air, then you pf=0.7*1.2*1.2*Is*pg=1.008*Is*pg...so essentially ground snow.

 

[dauwerda]

From the ASCE 7-10 commentary:

C7.3 FLAT-ROOF SNOW LOADS, pf

The live load reductions in Section 4.8 should not be
applied to snow loads. The minimum allowable values
of pf presented in Section 7.3 acknowledge that in
some areas a single major storm can generate loads
that exceed those developed from an analysis of
weather records and snow-load case studies.

The factors in this standard that account for the
thermal, aerodynamic, and geometric characteristics
of the structure in its particular setting were developed
using the National Building Code of Canada as a
point of reference. The case study reports in Peter et
al. (1963), Schriever et al. (1967), Lorenzen (1970),
Lutes and Schriever (1971), Elliott (1975), Mitchell
(1978), Meehan (1979), Taylor (1979 and 1980) were
examined in detail.

In addition to these published references, an
extensive program of snow load case studies was
conducted by eight universities in the United States,
the U.S. Army Corps of Engineers’ Alaska District,
and the United States Army Cold Regions Research
and Engineering Laboratory (CRREL) for the Corps
of Engineers. The results of this program were used to
modify the Canadian methodology to better fi t United
States conditions. Measurements obtained during the
severe winters of 1976–1977 and 1977–1978 are
included. A statistical analysis of some of that
information is presented in O’Rourke et al. (1983).
The experience and perspective of many design
professionals, including several with expertise in
building failure analysis, have also been incorporated.