ASCE 7-02 Snow Sliding

[cblanton]

I am currently working on a project where this a roof 8 feet above the lower roof. The lower roof is flat and the upper roof is a gable with an 8:12 pitch. I've looked in the commentary but can't find an answer as to whether or not I must add the drift load to the sliding snow or whether I only consider one at a time. Any idea? It seems a bit conservative to consider both simultaneously. Thanks!

 

[JAE]

You add the two....at least that is what has been in past codes.

 

[chichuck]

cblanton,

I don't know what code you are using. But in ASCE 7-02, commentaty section C7.9 I find this in the second paragraph:

"....Distribution of slidking loads might vary ... (at the end of the paragraph)....or where snow drifts on the lower roof create a sloped sufface that promotes lateral movement of the sliding snow."

This implies that sliding snow loads should be superimposed over drift loads. It also implies that the distribution is not uniform 5' wide, not uniform 20' wide, but does not give a clue as to how it is distributed.

Also, in the code body, Section 7.9 discusses reduction of sliding snow loads from the "normal" levels. If the drift on the lower roof is big enough, and the drop from higher roof to lower roof is small enough, then some of the sliding snow is blocked and will not slide. But I don't see anything here that tells you how to reduce the sliding snow loads.

I hope this helps.

Regards,


chichuck

 

[ajh1]

The last sentence in Section 7.9 clearly states:
"Sliding loads shall be superimposed on the balanced snow load."
There is no mention of drifting snow in that paragraph.

 

[JAE]

Yes, but....

all previous codes have even diagramed the sliding snow superimposed on the drift.

and...

per the ASCE 7-02, the code REQUIRES the balanced load, REQUIRES the drifting load and REQUIRES the sliding snow. In each of these there is no direct mention of not superimposing them...only that you do not superimpose sliding snow on UNbalanced snow load.

and....

The commentary section C7.9 also discusses the sliding snow being distributed when it falls ON the drift below.