asce 7-05 sliding snow provisions

huangyhg

asce 7-05 sliding snow provisions
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.

i can't say that i've ever hear of this situation before and i am not away of any code provisions for it.  i would approach it in one of two ways.
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
i am not aware of any code provisions for it, sorry for the typo.
drift provisions in the canadian code, cf, take this case into account.
if it is not able to slide off of the roof because of the obstruction, then you should not use any reductions that consider that the snow is able to shed off of the roof.
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.
according to asce7 the cs for a 3.5:12 slope is 1.0 unless you have a "unobstructed slippery" roof surface.  use cs=1.0 for your snow load as sliding snow probably is not applicable.
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.
i think there can be additional load on the parapet when it is acting as an obstruction to prevent sliding. adding the standard parapet drift plus the roof snow without reducing for sliding would be a minimum i would design the roof for in this case.