Adjacent Buildings, Drift Snow 1

[ToadJones]

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 ?

 

[RHTPE]

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

 

[ToadJones]

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

 

[RHTPE]

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

 

[dhengr]

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.

 

[ToadJones]

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

 

[dhengr]

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.

 

[ToadJones]

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.

 

[RHTPE]

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

 

[ToadJones]

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

 

[RHTPE]

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

 

[ToadJones]

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

 

[RHTPE]

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

 

[ToadJones]

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

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

 

[RHTPE]

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



Ralph
Structures Consulting
Northeast USA

 

[ToadJones]

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

 

[RHTPE]

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

 

[ToadJones]

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

 

[RHTPE]

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

 

[ToadJones]

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

Where can I find that Errata?