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Snow drift on very small lower roof 1

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structurebeton

structurebeton

Structural
Apr 24, 2003
88
US
What is your opinion on a very long Lu and very tall upper roof, against a very small lower roof and in which the calculated drift width W ends up being a LOT larger than the lower roof width?

Would you then use the drift width W the same as the lower roof itself or would use a truncated snow drift shape? (Trapezoidal shape) Doing that would increase the snow drift load by a factor 6 or 7.

 
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Yes, ASCE 7 is pretty clear on this. You MUST use the truncated shape.
 
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The code says you're stuck with the truncated shape. Seems silly to me, though. I've never seen six feet of snow on a four foot wide roof. Perhaps this is a place for engineering judgement to overrule the code.
 
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We see this situation a lot on warehouse type buildings with small canopies and you'd be amazed how the snow actually does pile up on these small roofs.

Use the truncated drift.
 
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I don't know why you find it so hard to imagine 6' of snow on a 4' wide roof. Additionally, if you ignored it and it ended up failing you will have a lot to answer for.
Use the truncated shape. ASCE is very clear on this and I don't believe it's open to much interpretation.
 
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I agree.. but I think ASCE needs to improve it somehow. Lets say you have a skyrise and you have a small canopy on the 1st floor, I dont see how snow from the penthouse can drift to the lower roof. Only leeward drift makes sense.

Never, but never question engineer's judgement
 
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I've lived for 58 years in New England, and I've never once seen it. It's something that I look for, as I once designed a canopy to attach to a parking garage. I used the truncated load to design the support. The city put up some cheap, flimsy canopy instead. Twenty years later, and it's still there. It's never had even close to the snow drift I had designed for.
 
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For such short members, scaled to 3' or so, is it going to make that large a difference? You also have a 75 PSF LL you need to apply, so the snow still might not control with the appropriate load factors.

Is there an angle of friction for the snow that you cna use to justify reducing the snow load?

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 
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Depends on many variables. I live and work in Wisconsin and have seen drift loads in the 1st year after construction larger than the code specified loads.
 
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Per ASCE 7 the build-up would be 10 ft high of snow drift...the lower roof is only 3 ft wide...Pretty crazy...
 
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My mistake. I thought this was a canopy, not a lower roof. Canopy LL is equal to 75 psf in ASCE.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 
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I am still confused with the 75 psf. I always design canopy with snow+drift load.

Never, but never question engineer's judgement
 
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If you look at Table 4-1 it states "Marquees and canopies" it is listed as 75 psf design LL.



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 
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Am I missing something?

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 
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Hmm.. I never really used Marquees for loading. Isnt that mostly for non permanent cicus type roof structure?

Never, but never question engineer's judgement
 
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Use the truncated.

I've seen small eyebrow canopies with piles of snow like that many times in the midwest.

The thing is, whether the snow piles up or not depends on a lot of factors (prevailing wind direction, wet vs. dry snow).

For some conditions, it might not ever occur (per miecz's experience stated above), but it can happen (I've seen it) and therefore the code requires you to account for it.

 
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