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Adjacent Buildings, Drift Snow

Adjacent Buildings, Drift Snow

Adjacent Buildings, Drift Snow

(OP)
What approach do you think is best for determining drift snow loads between two adjacent parallel gable roof buildings?

Should the drift surcharge height be taken run flat between the two roofs ?
 

RE: Adjacent Buildings, Drift Snow


By parallel gable roof buildings I assume you mean that the ridges are parallel, and that they are positioned gable end to gable end.  What is the height difference between the two and what is the separation?  Both influence drift surcharge.  What is the roof pitch?  If they are positioned eave to eave, the height difference, separation and pitch will still influence.

 

Ralph
Structures Consulting
Northeast USA

RE: Adjacent Buildings, Drift Snow

(OP)
Assume (for the questions sake) that they are identical buildings. Eave-to-eave with ridges parallel. There is no separation.  

RE: Adjacent Buildings, Drift Snow


To be really conservative, I would assume level, with the top of the snow equal to the height of flat roof snow load at the ridges.  I would also consider the leeward side drift loading on downwind roof as well.  I would think that the lemgth of the windward surface of the upwind roof would somewhat limit the volume of snow that could accumulate between the two ridges.

All would be tempered by, or increased by, your knowledge of the snow accumulation patterns in the buildings' geographic area.
 

Ralph
Structures Consulting
Northeast USA

RE: Adjacent Buildings, Drift Snow

Toad:
Seems that this condition would be akin to a folded plate roof system, or a multiple gable roof system (multi-bay bldg.) which are quite common in the industrial buildings you deal with.  I'm sure ASCE 7 covers this in excruciating detail, although I don't have the latest couple editions of that, I'm sure you do.
 

RE: Adjacent Buildings, Drift Snow

(OP)
Yea, you'd think it would cover it in detail as you describe, however, I'm not having much luck with it.  

RE: Adjacent Buildings, Drift Snow

Toad:
Sorry I can't help you because I don't have the latest Eds. of ASCE 7, but I know I've seen some material on that roof system and snow loading.  I followed that issue fairly closely back in the mid 70's to the mid 90's; actually a good deal of our code material originated from work done in Canada on the snow loading subject.  Maybe their code/commentary and work is where I saw it.  Maybe Dik, BA or one of those guys will chime in here, with their take on the matter.

RE: Adjacent Buildings, Drift Snow

(OP)
I need to print out the commentary (cant stand reading PDF's).
Drift snow loads for "adjacent structures" is covered under ASCE 7-05 section 7.7.2 but it seems quite vague as it simply states the same procedure should be used for adjacent structures as that which is used for "lower" structures and gives some provision for separation.

I will read commentary.  

dhegr-
You are correct about this being multibay industrial facility.
One knows intuitively that snow must pile up in between these roofs.  

RE: Adjacent Buildings, Drift Snow


From ASCE 7-10, 7.6.3 (sawtooth roofs, unbalanced):

at ridge - 0.5pf;
at valley - 2pf/Ce
snow at valley no higher than snow at rdige.
see Figure 7-6

For balanced, ps = pf
 

Ralph
Structures Consulting
Northeast USA

RE: Adjacent Buildings, Drift Snow

(OP)
Ralph-
thanks, I was going that route, but my confusion is not with unbalanced vs balanced, but rather with drift.

RE: Adjacent Buildings, Drift Snow


Toad - I think drift will be affected by the relative heights of the ridges.  If the ridges are the same height, you are not going to see snow depth much greater than 0.5pf at the ridge, and relatively level snow between the ridges.

As I said previously, you have to have enough snow falling on the upwind roof surface to fill the valley between.  Some of the snow picked up from that upward surface could be transported past the second ridge resulting in a drift condition diagrammed at the bottom of Figure 7-5.

All will be affected by the aerodynamics of your 2 gable roofs based their geometry.

JMHO

Ralph
Structures Consulting
Northeast USA

RE: Adjacent Buildings, Drift Snow

(OP)
Ralph-
When you say "level" you are not suggesting that I simply fill in the whole valley up to the ridge height, are you?  

RE: Adjacent Buildings, Drift Snow

Toad,

Yes, that is the maximum possible condition, but again, I believe that depends on the total picture.

You could conceivably have something like two Figure 7-5 (bottom) drift conditions - it will depend on the aerodynamics of the roof profiles.

Knowing the geopmetry would give a better feel for the possibilities.
 

Ralph
Structures Consulting
Northeast USA

RE: Adjacent Buildings, Drift Snow

(OP)
the eave-to-ridge height of these buildings is about 20ft !

That would mean 20' of snow piled in the valley.
 

RE: Adjacent Buildings, Drift Snow


And:
Width of each building (eave-to-eave) = ?
Pitch of the roof = ?
Flat roof snow load, pf = ?

 

Ralph
Structures Consulting
Northeast USA

RE: Adjacent Buildings, Drift Snow

(OP)

Width of each building (eave-to-eave) = 100'
Pitch of the roof = 5:12
Flat roof snow load, pf = 15 psf
 

RE: Adjacent Buildings, Drift Snow


ToadJones - What is your ground snow load, pg?  It's required to determine snow density which is needed in the drift assessment.  I'll have a sketch for you later on.
 

Ralph
Structures Consulting
Northeast USA

RE: Adjacent Buildings, Drift Snow

(OP)
20 psf....I'm quite familiar with determining snow loads....this is just one area I have yet to encounter

RE: Adjacent Buildings, Drift Snow


ToadJones - I'm sure you are.  It just gives me a chance to exercise my spreadsheet and to compare my approach to that of others.  I found an error in ASCE 7-10 Ch 7 last night.  I've since discovered that there are 19 pages of errata.
 

Ralph
Structures Consulting
Northeast USA

RE: Adjacent Buildings, Drift Snow

(OP)
I'll be interested in seeing what you come up with for comparison.

Where can I find that Errata?

RE: Adjacent Buildings, Drift Snow

ToadJones,

I've attached my interpretation of ASCE 7-10, Ch 7, for your case.  Note that I had to make some assumptions for Ce & Ct.  Page 1 is my summary, Page 2 is my loading diagram.  I'd be interested in hearing from others as well.

I'll post the errata next.  It was really buried in ASCE's web site.
 

Ralph
Structures Consulting
Northeast USA

RE: Adjacent Buildings, Drift Snow

(OP)
Ralph, I basically came up with the same thing.
I had 7.7 psf at ridge and 30 psf at valley.
I like this approach. Very reasonable.

Did you mean for the valley load to be 29.6psf rather than 26.9 psf?



Thanks for taking time out your day Ralph.  

RE: Adjacent Buildings, Drift Snow

TJ, The valley load is 2x pf, then DIVIDE by Ce.  If Ce >1 then the valley load is less than twice pf, or in this case 26.9 PSF.

ASCE 7-10 is not too clear about applying the drift surcharge just downwind of the ridge.  I feel it belongs there as everything I see after a windy snowstorm seems to confirm.

The snow accumulation in the valley also makes sense.  For your case (with my assumptions) I came up with a snow density of 16.6 PCF, so the valley snow depth is about 20" and the depth at the ridge about 5".  I finished looking at a project here in CT and the snow density calc'd at 18.6 PCF.  Seems a bit high from what I observed last winter.
 

Ralph
Structures Consulting
Northeast USA

RE: Adjacent Buildings, Drift Snow

(OP)
yea, I'm looking at your calcs now....my factors vary a little form yours ....making the difference.  

RE: Adjacent Buildings, Drift Snow


ASCE 7-10 Chapter 7 seems pretty straightforward once you've run through a few.

Now Chapters 26 thru 30 on the other hand ...
 

Ralph
Structures Consulting
Northeast USA

RE: Adjacent Buildings, Drift Snow

(OP)
I agree...snow loads have always been very practical (although they can really make for numerous load combos).
I have just never had to check this exact situation.

Thanks again for the input.  

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