ASCE 7-05 Sliding Snow Provisions
ASCE 7-05 Sliding Snow Provisions
(OP)
I need to calculate the snow load for a gable roof with a 3.5:12 pitch and 4'-0" high perimeter parapet. How do you calculate the amount and distribution of sliding snow that accumulates up against the parapet?
The "Cs" factor in section 7.4 reduces the the magnitude of "Pf" to account for the snow that slides off. However, the only specification section that specifically addresses sliding snow is section 7.4, however, it is specifically for calculation the accumulation of sliding snow on lower roof and therefore doesn't appear to be applicable.
Has anyone else run into this? Also, should the sliding component be superimposed on the the balance and unbalanced snow loads? Any help with this would be greatly appreciated.
The "Cs" factor in section 7.4 reduces the the magnitude of "Pf" to account for the snow that slides off. However, the only specification section that specifically addresses sliding snow is section 7.4, however, it is specifically for calculation the accumulation of sliding snow on lower roof and therefore doesn't appear to be applicable.
Has anyone else run into this? Also, should the sliding component be superimposed on the the balance and unbalanced snow loads? Any help with this would be greatly appreciated.






RE: ASCE 7-05 Sliding Snow Provisions
1. Equation 7-3 in ASCE 7-05 provides an equation for the density of snow. You would have to come up with some way to estimate how high the snow would pile up behind the wall, and maybe look at impact, but design the parapet as a retaining wall.
or
2. Prevent sliding from occuring in the first place by placing stops set behind the parapet. Similar to when angle like pieces are put above doorways on the roof with metal roofing to prevent the snow from falling on people traffic in and out of the building. (I found a nice website demonstrating this here http://www.snojax.com/ but there are other available products or solutions). This would prevent you from designing for this situation in the first place, but you might want to check that your snow loads are appropiate for that situation.
RE: ASCE 7-05 Sliding Snow Provisions
RE: ASCE 7-05 Sliding Snow Provisions
RE: ASCE 7-05 Sliding Snow Provisions
As far as additional load because of the parapet blocking sliding, I am not sure about that one. It is essentially like a snow guard. The commentary on page 329 just says the following: "When snow guards are added to a sloping roof, snow loads on the roof can be expected to increase".
Here is some useful information on your situation.
http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd228_e.html
RE: ASCE 7-05 Sliding Snow Provisions
Use the drift provisions from section 7.8 - Roof Projections (ASCE7-05) for the drift loading at the parapet wall. The drift at the parapet is 0.75Hd and you use Lu equal to the upwind roof length.
RE: ASCE 7-05 Sliding Snow Provisions