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 ?
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
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
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
RE: Adjacent Buildings, Drift Snow
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
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
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
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
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
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
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
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
Where can I find that Errata?
RE: Adjacent Buildings, Drift Snow
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
Errata dated 11 Jan 2011 for ASCE 7-10.
Ralph
Structures Consulting
Northeast USA
RE: Adjacent Buildings, Drift Snow
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
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
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
I have just never had to check this exact situation.
Thanks again for the input.