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Snow Drift in 5psf GSL Region

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Brooklyn2012

Structural
Mar 20, 2013
11
How are Engineers in the southern US handling snow drift in a 5 psf ground snow load region?
The commentary states that a 3 or 4 foot drift could occur in regions of low ground snow load.
My roof length is causing a 6 foot snow drift. That seems completely reasonsable here in the Midwest. But I am getting some questioning looks from my Southern counterparts.
 
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That seems unreasonable although I'm from north of the 49. I use 6 foot snow drifts here constantly, but our GSL is more like 40 PSF.
 
what does a 6 ft. deep snow drift weigh?
 
XR250,

Been awhile since I've done this calculation, given that I've lived/worked in an area with no snow for nearly the past decade. Anyway, if I remember correctly, aside from needing the geometry of the drift (to help get an equivalent psf) this involved (in part) calculating the density of the snow. I found the attached calculation online (see page four of the document; they ended up with a density of 17.25 pcf). Before even finding the document, 17 pcf was a number I had remembered running into on several occasions. Obviously you will need to specifically calculate it for your instance. If you're looking at ASCE 7-05 (for example; I didn't have a more recent version with me while typing this), you would need to look at section 7.7.1, equation 7-3. Additional detail/calculation can be found in the commentary, on page 331.

FWIW... this spreadsheet ( looked interesting, though you will obviously want to double-check the results.



 
@Jeffandmike;

I was just asking the OP what it weighed. I got better things to do on a Saturday than that calc :>
 
In the south, I don't even consider snow loading unless the ground snow load exceeds 10 psf. That being said, if I have a building where deep snow drifts can form, I will calculate the load and apply it if needed; but it very rarely controls for typical commercial buildings.
 
Older versions of the Code (I found a BOCA 93) state that drifts shall be calculated for ground snow loads greater than 10 psf. So I guess the answer is that they revised the code so that drifts must be considered even in 5 psf regions.
Large industrial and manufacturing complexes may have vast expanse of roof area and could cause a significant drift (I have 720 feet of upwind roof)
 
ASCE 7-10 Commentary

The leeward drift load provisions are based on
studies of snow drifts on roofs (Speck 1984, Taylor
1984, and O’Rourke et al. 1985 and 1986). Drift size
is related to the amount of driftable snow as quantifi
ed by the upwind roof length and the ground snow
load. Drift loads are considered for ground snow loads
as low as 5 lb/ft2 (0.24 kN/m2). Case studies show
that, in regions with low ground snow loads, drifts 3
to 4 ft (0.9 to 1.2 m) high can be caused by a single
storm accompanied by high winds.
 
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