Snow Drifts on Lower Roofs (in End Zones?)
Snow Drifts on Lower Roofs (in End Zones?)
(OP)
I have a small canopy that I am designing (10x17) that will be located just outside of a strip-mall supermarket (high walls, deep roof (upper roof length = 115')). The canopy is tight against the building, so I have drift loads on the canopy. Because of the depth of the supermarket, I am getting huge drift loads on my canopy (flat roof = 25 psf, total drift (surcharge plus flat) = 83 psf) and the width of drift is wider than my canopy (10').
Now to the question. Is there any documentation for reducing the snow loads in end zones? This is placed on one side of the building at the corner of, let's say, a 800'x115' plan view building. Higher end zone wind pressures to whip that snow up and a path to dump that snow "around the corner" seems like there should be something, somewhere that affords a break.
I normally wouldn't be concerned about the higher loads, but this is a cantilevered canopy (17' cantilever) so my foundation is getting ridiculous.
Now to the question. Is there any documentation for reducing the snow loads in end zones? This is placed on one side of the building at the corner of, let's say, a 800'x115' plan view building. Higher end zone wind pressures to whip that snow up and a path to dump that snow "around the corner" seems like there should be something, somewhere that affords a break.
I normally wouldn't be concerned about the higher loads, but this is a cantilevered canopy (17' cantilever) so my foundation is getting ridiculous.
Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant






RE: Snow Drifts on Lower Roofs (in End Zones?)
RE: Snow Drifts on Lower Roofs (in End Zones?)
That is currently what we are doing. It helps for the beams of the canopy, but does nothing for the overturning moment at the base of the column. I'm not saying I won't design for the loads I am calculating, but I thought maybe being in the endzone would help me.
Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant
RE: Snow Drifts on Lower Roofs (in End Zones?)
For a canopy - snow always tends to drift up on them a lot more than you might guess. Trying to make less conservative assumptions beyond the basic schemes provided in ASCE 7 would not be a good approach unless there was some other research to back me up...which I guess is your main question - is there any such research? I've never seen any.
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RE: Snow Drifts on Lower Roofs (in End Zones?)
RE: Snow Drifts on Lower Roofs (in End Zones?)
The canopy is architectural already which means it has curved main beams. The curvature doesn't exceed 30 degrees at any point though, so there is not slope factor to be applied.
This is going to Chicago which uses the ANSI A58.1-1982 for drift snow. I got this code yesterday and it seems the drift height is not a function of the upper roof length, so it produces somewhat better drift loads. I am comfortable using those lower drifts even though I know the newer code produces higher loads.
As always, thanks for your input guys/gals.
Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant
RE: Snow Drifts on Lower Roofs (in End Zones?)
So you are essentially faced with only being able to reduce the snowdrift value at maximum (90 mph, presumably) winds. The worst case scenario for snowdrift is during lower speed winds, which are a much more common occurrence.
CF
RE: Snow Drifts on Lower Roofs (in End Zones?)
Section 7.7.1
"…the upper level roof , however the drift height need not be taken as larger than 60% of the length of the lower level roof."
"If the drift width, w, exceeds the width of the lower roof, the drift shall taper linearly to zero at the far end of the lower level roof"
RE: Snow Drifts on Lower Roofs (in End Zones?)
Dik