Snow Drift
Snow Drift
(OP)
Hi all,
I have an existing building with a slope roof and a new building with a flat roof. The existing building is about 20' higher than the new building. I calculated the snow balance and the snow drift based on windward direction on the new flat. Then, I calculated the sliding snow from the existing slope roof on the new flat roof. Let say my balance snow is 20 psf, drift is 80 psf, and sliding snow is 30 psf. My question is how can I put them all together? Do I use balance + sliding + drift or balance + drift? I know that the sliding snow will be controled over the balance on the portion of the flat roof. I couldn't find the clear explaination in the ASCE-05. Thank you all in adveance. Below is a simplified sketch.
-------------\
Slope Roof \
\
----------------
I \Drift (7' per calc.)
I \
I-------Sliding Snow (15' per code)
I-----------------------Balance Snow (100')
I-----------------------Flat Roof
I have an existing building with a slope roof and a new building with a flat roof. The existing building is about 20' higher than the new building. I calculated the snow balance and the snow drift based on windward direction on the new flat. Then, I calculated the sliding snow from the existing slope roof on the new flat roof. Let say my balance snow is 20 psf, drift is 80 psf, and sliding snow is 30 psf. My question is how can I put them all together? Do I use balance + sliding + drift or balance + drift? I know that the sliding snow will be controled over the balance on the portion of the flat roof. I couldn't find the clear explaination in the ASCE-05. Thank you all in adveance. Below is a simplified sketch.
-------------\
Slope Roof \
\
----------------
I \Drift (7' per calc.)
I \
I-------Sliding Snow (15' per code)
I-----------------------Balance Snow (100')
I-----------------------Flat Roof






RE: Snow Drift
DaveAtkins
RE: Snow Drift
Mike McCann
McCann Engineering
RE: Snow Drift
RE: Snow Drift
"Distribution of sliding loads might vary from a uniform load 5 ft (1.5 m) wide, if a significant vertical offset exists between the two roofs, to a 20 ft (6.1 m) wide uniform load, where a lowslope upper roof slides its load onto a second roof that is only a few ft (about 1 m) lower, or where snow drifts on the lower roof create a sloped surface that promotes lateral movement of the sliding snow."
RE: Snow Drift
Drifting snow is to be added to the balanced snow load.
http://eng-tips.com/viewthread.cfm?qid=107700
RE: Snow Drift
7.7.1: "Drift loads shall be superimposed on the balanced snow load."
7.9: "Sliding loads shall be superimposed on the balanced snow load"
Having said that, I have always assumed the worst condition by adding them. I think it is reasonable to believe that drifting snow could occur, followed by snow sliding on top of it. My exception to this would be if the drift height on the lower roof were as high as the snow depth of the upper roof. (As permitted by ASCE 7-7.9)
The old BOCA codes had a diagram specifically showing the sliding surcharge superimposed on the drift surcharge. But back then, they also simply specified the sliding height as 40% of the drift height.
RE: Snow Drift
Also, using balanced + drift + sliding would be conservative but probably over-conservative as the likelyhood of seeing the design values for all three simultaneously is unlikely.
Perhaps balanced + 0.75*drift + 0.75*sliding may be a good thing to use if you feel the need to combine all three...
RE: Snow Drift
After seeing that, I chose to err on the conservative side. The Codes do not have all the answers. That's the reason they're always changing.
Mike McCann
McCann Engineering
RE: Snow Drift
RE: Snow Drift