snow loads - drifting sliding

huangyhg

snow loads - drifting & sliding
i have a condition where a higher gable roof abuts a lower flat roof. there is a large enough height difference to allow drifts to form on the lower roof and to allow snow to slid off the upper. my qeustion is: do i have to combine both the sliding and the drifting loads on top of the balanced snow load on the lower roof? asce 7-02 says to sumperimpose the drift (or sliding snow) onto the balanced snow load, but makes does not mention what to do if both are present. any thoughts would be appreciated. thanks.
from my point of view, it seems even if asce 7-02 doesn't ask you to superimpose the sliding load onto the drift load, it wouldn't be too conservative to do so.  in reality, your design drift could happen.  if that happens (meaning you just got a whole lot of snow) the chances the snow slides off the roof onto your drift just got a whole lot better due to extra weight.  maybe you could talk yourself into at least a compromise or a percentage of the snow sliding off the roof depending on the slope.
canadian codes require us to incorporate both sliding snow and drifting snow, however, a discretionary call can be made to use a maximum of 3x ground snow load. this may help you to get an approximation even though codes may not attack this area.
derb:
i think your original question is clear to say that there exists the possibility of drifting and sliding.  i would say common sense says that they must be superimposed with all other loads that act upon the structure at the same time. i would not try to reach any compromise short of this simply because i think anybody would have a hard time defending that decision when the lives are lost.
think about under this light and see what you come up with. would you like yourself and your family to be sleeping under that roof when you know the engineer that designed it found a code that somewhat allowed a roof that is not designed for what you know will be imposed? i would not.
good lock
just a question to think about: if there is a snow load on both roofs and the snow on the gable roof begins to slide, doesn't the snow on the flat roof imped the sliding of the snow.  i'm thinking that using the full drifting snow load on the flat roof would be the maximum.  look at a cross section of the two roofs, if the flat roof is full of drifted snow, where can the snow on the gable roof slide too?  just thinking of another line of reasoning.
as geoffdale mentions, the 3x the ground snow load is often used as a maximum snow drift load.
asce-7 is vague on sliding snow, unfortunately.  in ubc-97, you did combine a sliding surcharge with the drift and balanced snow loads.  ubc may bo longer be legally binding in your area, but at least it gives a rational basis of design that you can follow when you have to think about sliding snow.  
add them all. if i remember correctly, the drift and sliding loads may not even extend all across your flat roof. so, it may just be more load where the flat roof it attached to the structure with the gable roof. calculate the total height of snow and if it goes above the gable roof eave, then you may be able to limit it to that level. also, check with your local building department. they are ususally helpful in that regard.