Snow Drift

[zestructural]

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

 

[DaveAtkins]

I don't have my ASCE 7 in front of me, but I am almost certain that sliding snow is superimposed on the BALANCED snow load. This means you don't add balanced plus drifted plus sliding snow. Compare balanced plus drifted to balanced plus sliding, and use the worse case.

DaveAtkins

 

[msquared48]

I would do it regardless of any ASCE 7 requirements as the combination is logical from what I have seen in the field.

Mike McCann
McCann Engineering

 

[zestructural]

Thank you Mike McCann & DaveAtkins.

 

[SteveGregory]

In ASCE 7-02, the commentary section C7.9 on Sliding Snow indicates that sliding snow can occur with drifting. So, we should be adding balanced+drifting+sliding. Here is the quote:

"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."

 

[strguy11]

There have been many debates on this in this website, but I think they need to be superimposed on each other.

Drifting snow is to be added to the balanced snow load.

http://eng-tips.com/viewthread.cfm?qid=107700

 

[JKW05]

The way ASCE is written, it would appear that they are not required to be superimposed:
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.

 

[GoCyclones]

I agree with JKW05 on the interpretation of ASCE.

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...

 

[msquared48]

I've seen the aftermath of a roof that was not designed for the full load described above. Lucky no one was killed.

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

 

[strguy11]

What about the dynamic effects of the sliding snow falling onto the roof below? If this is a large drop, this force could cause damage to the roof below.

 

[GoCyclones]

msquared48: I am not trying to start an argument against being conservative, but I am curious about the failure you speak of (I always want to learn). Was it determined that the roof was properly designed and constructed to the current code (worst case of the balanced + drift and balanced + sliding) and that the actual snow loading exceeded that causing the failure?