drifting snow on exterior decks

[SLTA]

ok folks. this has been asked on here before but I haven't seen a concise answer.

For a commercially-loaded exterior deck at a restaurant, does one consider drifting snow as well as flat snow? We're still using ASCE 7-05, and the snow section is all about drifting onto lower roofs. This is definitely not a roof, and honestly, no one would be using the deck at a restaurant when it's cold enough to have drifting snow on it. Thoughts?

 

[msquared48]

If the deck is ledgered to the building ... definitely considering the shear.

Mike McCann, PE, SE (WA)

 

[msquared48]

Also depends if the restaurant is in a ski area such as Stevens Pass in Washington where the design snow load is 400 psf. Definitely put sliding and drifting snow on that then. Hopefully the Architect will not let the snow slide onto the customers.

Mike McCann, PE, SE (WA)

 

[JAE]

I'm not sure I understand what you are asking - drifts are always added to the flat snow - additive - are they not?

If you are asking whether drifts/snow should be added to live load - per ASCE 7 I believe that Chapter 1 includes load combinations with both live load and snow.

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[msquared48]

Not directly additive JAE.

As you move away from the vertical projection, the snow load decreases, eventually to a level less than the design roof snow load... In other words, where the design flat snow may be 40 psf at the foor as an example, the drift load may vary from 80 to 20 or 30 psf over several feet, then level off at the 20 or 30 psf value, below the design level for the roof.

Mike McCann, PE, SE (WA)

 

[msquared48]

I guess what I am saying is that, neglecting sliding snow, approximately the same amount of snow is deposited on the roof in a drifting situation as opposed to non-drifting. It's just distributed differently, increasing the shear more than the moment to design for...generally.

Mike McCann, PE, SE (WA)

 

[JAE]

I don't think that is true, Mike. If you have a high roof next to a lower roof - with no wind - you get a base snow on both roofs.
With wind you could get the base snow on the lower roof with the additive drifting on the lower roof from higher roof snow blow-off.
Additive.

I don't have ASCE 7-05 with me but I believe it actually states that you add drifting to the base snow.
What you don't do is use the minimum Pf base snow with the drift. You always add the drift to the actual calculated Pf independent of the minimum snow.

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[msquared48]

JAE:

Correct for a higher roof next to a lower one. Different for a parapet condition I was thinking of.

Mike McCann, PE, SE (WA)

 

[Ron]

I haven't had to do snow loading in over a year, but used to do a fair amount. My interpretation is the same as JAE's...add drift to base.

slta....yes, drift should be considered. Even though the deck might be "wind permeable" with spaces between deck boards, snow will bridge those easily and once the base snow is there, drifts will occur against building. My former partner and I did a failure investigation of a snow load collapse of a low roof adjacent to wall below high roof. Drifting was significant culprit of progressive collapse.

 

[SLTA]

Thanks, folks. It just wasn't clear in the code... Section 1604.8.3 in the NC 2012 code talks about designing the connections for either live load OR snow load. Given that our snow load is 15psf ground, and that the live load on this deck is 100psf, I doubt that the drift would get to the live load. Maybe I'll call on Monday to make sure.

cheers. Now go enjoy Saturday night.

 

[JAE]

Depending on where your structure is - and the size of structure, drifts can get pretty high. We've had some cases with 160 psf + drifting.



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[steellion]

Design for drifting snow on the roof, and for live load of the dining patrons, but it is probably not necessary to consider both simultaneously. The ASCE load combinations are generally written to include roof live load OR snow load. Perhaps I would add 20 PSF to the snow load to account for tables, chairs, decor, etc? You are correct, it is unlikely patrons would be dining on the deck during a design-level snow, unless perhaps it is a ski resort.

 

[JAE]

I would agree that patrons sitting at tables and code-high snow loads probably don't go together.

But ASCE Load combinations DO include both snow and live load together. Note combination 4 below:
1. D + F
2. D + H + F + L + T
3. D + H + F + (Lr or S or R)
4. D + H + F + 0.75(L + T ) + 0.75(Lr or S or R)
5. D + H + F + (W or 0.7E)
6. D + H + F + 0.75(W or 0.7E) + 0.75L
+ 0.75(Lr or S or R)

Similar case for the LRFD combinations.
And I'd just like to point out that the definition of "S" in ASCE 7 is "snow"....NOT "roof snow".
So if the snow falls on an exposed floor you still have to treat it the same as a roof.



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[SLTA]

Just heard back from the NC Code people. They agreed that there's no way I'd have 100psf occupancy live load AND drifting snow, and in our area, the drift load won't get to the live load levels, so I don't need to include it. (Please note the disclaimer: this won't likely be true in areas with greater snow than we have, so don't go using this as an excuse to not do proper design work for your area.)

cheers everyone for the good debate!

 

[steellion]

JAE, depends on your definition of a roof, I guess? Each of the combinations you list has (roof live load or snow load). You can have a roof that is publically accessible. I believe the intention and spirit of the code is generally to not require live load caused by occupancy to be considered simultaneously with a design snow load.

 

[TLHS]

Just for comparison, the National Building Code of Canada has a clause that says occupancy live load and snow load in a pedestrian access area need not be considered simultaneously.