Snow drift on very small lower roof 1

• Jul 22, 2008

• #2

msquared48

Structural

Aug 7, 2007

14,745

I would consider impact!

Mike McCann
MMC Engineering

 

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• Jul 23, 2008

• #3

DaveAtkins

Structural

Apr 15, 2002

2,894

Per Code you must use the truncated shape.

DaveAtkins

 

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• Jul 23, 2008

• #4

Lion06

Structural

Nov 17, 2006

4,238

Yes, ASCE 7 is pretty clear on this. You MUST use the truncated shape.

 

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• Jul 23, 2008

• Thread starter
• #5

structurebeton

Structural

Apr 24, 2003

88

I just find it hard to justify the truncated loading pattern. Please take a look at the illustration, and you are suggesting I use option 1...What about option 2?

 

 http://files.engineering.com/getfile.aspx?folder=7f38198e-7a87-41f3-98c5-d6f9867db83c&file=SNOW_DRIFT_OPTIONS.pdf

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• Jul 23, 2008

• #6

miecz

Structural

Sep 30, 2004

1,400

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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• Jul 23, 2008

• #7

CTSeng

Structural

Jan 21, 2003

125

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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• Jul 23, 2008

• #8

Lion06

Structural

Nov 17, 2006

4,238

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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• Jul 23, 2008

• #9

COEngineeer

Structural

Sep 30, 2006

1,186

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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• Jul 23, 2008

1
• #10

miecz

Structural

Sep 30, 2004

1,400

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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• Jul 23, 2008

• #11

PEinVA

Structural

Nov 15, 2006

321

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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• Jul 23, 2008

• #12

COEngineeer

Structural

Sep 30, 2006

1,186

75 psf LL from what?

Never, but never question engineer's judgement

 

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• Jul 23, 2008

• #13

jike

Structural

Oct 9, 2000

2,160

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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• Jul 23, 2008

• Thread starter
• #14

structurebeton

Structural

Apr 24, 2003

88

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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• Jul 23, 2008

• #15

PEinVA

Structural

Nov 15, 2006

321

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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• Jul 23, 2008

• #16

COEngineeer

Structural

Sep 30, 2006

1,186

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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• Jul 23, 2008

• #17

PEinVA

Structural

Nov 15, 2006

321

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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• Jul 23, 2008

• #18

PEinVA

Structural

Nov 15, 2006

321

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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• Jul 23, 2008

• #19

COEngineeer

Structural

Sep 30, 2006

1,186

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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• Jul 23, 2008

• #20

JAE

Structural

Jun 27, 2000

15,591

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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