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SNOW LOAD DRIFT + SLOPE FACTOR

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ReverenceEng

ReverenceEng

Structural
Feb 18, 2016
81
Greetings!

Question on low roof snow.

Background: We apply flat and drift snow loads on a low roof such as a canopy or awning. As the slope of the low roof (such as for an awning) increases, the flat roof snow load becomes a sloped roof snow load which starts to decrease after about 15 degrees until the slope is so steep that no snow load needs to be applied (this is all for cold slipper roofs).

Question: Can we apply the same reduction to the drift loads? Naturally, as the slope increases, the same slope factor and effects would apply to drifting snow as well, but I don't see to see anything specific in the code about this. I am thinking this might fall under interpretation and judgment. Curious to hear others' thoughts...


 
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PS to previous answer.

The standard calculation for Snow Drift already contains a reduction of Snow Load on a lower roof built in.

Jim


 
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When roofs have high slopes, the sloped roof (balanced) snow load is decreased because the Cs factor goes down (ps = Cs*pf). The drift load is a surcharge that is superimposed on the balanced snow load. (In the graphic below, hb is due to the balanced snow load and hd is due to the superimposed drift load.)

This means that the roof slope is already being accounted for over the entire area of the roof because the balanced snow load already applies everywhere.

(I do understand your point, though. The snow drift provisions were designed with flat roofs in mind. There's some examples of snow drift on sloped roofs where they take the total amount of drift snow calculated and spread it out so that the 1:4 slope that the code uses is maintained. This method wouldn't really be possible once your roof is steeper than 3:12. Also, unbalanced snow loads get ignored once the slope exceeds 7 on 12, which is kind of similar. At this point, though, the only thing in the code that adjusts for drifts on sloped roofs is the balanced snow load that I mentioned above. Adding additional provisions, though, that address this specifically may not be worth the additional complexity.)

Snow_Drift_Dimensions_idlhk1.png


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