asce 7-05 drifting and sliding snow

[tpmanzi]

I have a long (40' per side)shallow pitched (4:12) gable roof that drops down (8 ft) to a 30' long flat roof. I am developing a truss loading diagram for the flat roof area. The question is do you combine balanced snow + drifting snow + sliding snow or do you treat balanced + drifting as one case and balanced + sliding as another and take the worse case?

 

[SteelPE]

I'm surprised no one has answered this question.

I would not consider balanced + drifting + sliding as one total combination. The last sentence of ASCE 7-07 section 7.9 states "Sliding loads shall be superimposed on the balanced snow load." It does not mention superimposing the sliding snow on the snow drift.

In this case, I would look at windward drift + balance snow, leeward drift + balance snow and sliding snow + balance snow. I would also not consider a mineral surfaced roof slippery (if you have one).

This is how I interpret this section you need to do the same.

 

[chicopee]

At the wall projection between the flat and gable roofs I would have a triangular load distribution. For the altitude of the triangle, I would have the height of that wall and the base of the triangle would be about 1/3rd of the flat roof; for snow density, I would use wet snow value.--My two cents.

 

[csellc1]

You should review ASCE - Guide to Snow Loads by Dr. Michael O'Rourke. He is the chairman of ASCE7 snow load chapter. Good reference.