Lateral snow load

[whymrg]

I need to design 5 feet tall parapet in area of ground snow load equal to 90 psf. Snow drift is more than 5 feet. How can I find lateral snow load on parapet? Any ideas?
Thanks.

 

[apetr26542]

I don't believe there is a provision for lateral snow load, the foundation for the calaculation of snow drifts I use is in ASCE 7.

 

[whymrg]

But what is the load on PARAPET (not on roof) from snow drift?

 

[apetr26542]

If, by parapet, we are talking about a vertical projection above the roof, I would think your wind psf would be more of an issue than snow piling against it. What elevation is this parapet?

 

[JAE]

There is no lateral snow load in the US codes. Only wind and seismic. Snow is only a vertical load.

 

[sundale]

I agree that wind or seismic would likely control the lateral design of the roof parapet. Per ASCE 7.7.1, you only need to consider 75% of the drift height at a parapet.

You could perhaps calculate the snow density and treat it like a fluid (no internal shear strength) and see what kind of lateral load this theoretical fluid imposes. This is not a code requirement that I know of and is probably pretty conservative. Namely, if the roof parapet can withstand the wind that deposits a leeward drift, then it can probably take the indeterminate lateral load of the snow too.

The only lateral snow loads (that I know of) that the code says to consider is per ASCE 7.9. At the higher elevations here in Colorado, alot of the local building departments specify snow guards to prevent a big chunk of snow or ice from sliding off a metal roof and hitting a person below, typically over ingress/egress doors. These lateral forces can be quite large.

 

[whymrg]

With wind suction on one face and snow pile on other, treating snow as fluid, I got much bigger force than wind alone per example 3.2 of "Guide to the use of the wind load provisions of ASCE 7-02".
Thanks guys, I was hopping someone knew internal friction angle of snow, to reduce fluid pressure. Thanks.

 

[apetr26542]

Using ASCE 7 you can determine the density of the snow drift, I think its about 20 pcf, but you'll have to verify. This would also be a triangular load. If you used snow as fluid, = 62.4 pcf that is going to give v. high pressure.

 

[sundale]

Avalanche slopes are typically assumed to be between something like 30 and 45 degrees. Anything less than 30 degrees is thought to be too shallow to slide and anything more than 45 degrees has already sliden... (Is "sliden" a real word?)

This implies some sort of internal friction (phi) angle for snow. The internal friction angle is probably quite high (nearly 90 degrees) with any thaw freeze cycling. Would this be analagous to cohesion?

Anyway, you could use some sort of phi angle to calculate an "at rest" lateral coefficient using a unit weight of about 20 pcf.

You could add this lateral fluid pressure due to the windward (not leeward) drift at the parapet to the wind suction. I have never done this, and the code does not prescribe this, but there are instances when there might be some sort of fluid bin pressure due to loose snow, I think.