Snow Drifts on Lower Roofs (in End Zones?)

[P1ENG]

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.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[jayrod12]

Our office has made some good money fixing canopies like this that weren't properly designed for the buildup they're going to see. If it's the cantilever that's causing you grief, see if you can talk the owner/architect into tie rods from the outside of the canopy beams back upwards to the columns.

 

[P1ENG]

jayrod,
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

 

[JAE]

I don't ever see that ASCE 7 snow loading (or other codes for that matter) are that "mature" in how snow drifting is derived. We usually use our best engineering judgement with cases like this.

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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[ajh1]

The new ASCE7-16 provisions cap the snow drift depth to no more than 60% of the lower roof length to compensate for this somewhat. You are not up to that point yet since your drift is not going to be 10' tall but it is a new provision to remember that might help down the road. Can you put a pitched top to the canopy so the snow will slide off rather than drift and collect?

 

[P1ENG]

ajh1: That is good information. I was just telling another engineer yesterday that is was weird there was no limit on the upper roof length. An upper roof length of infinity would produce a drift load of infinity. You would think the wind couldn't carry an infinite amount of snow.

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

 

[26sig]

It's my opinion that the OP should design for the full calculated snowdrift. While it's true that the wind pressure is going to be higher at the building ends, and that will have an effect on the snow, this is only during a design-level wind event. At extremely high wind speeds the snow might well blow off of the canopy, but at lower than design wind speeds, the drifts will accumulate normally.

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

 

[DETstru]

This is in the new ASCE 7:

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"

 

[dik]

If it's a small canopy, I usually treat it as a load adjacent to a high wall with a coefficient of approximately 3. I've seen this type of accumulation in real life, and have on numerous occasions seen snow build up beyond the 'free edge' for a couple of feet 'rolling over the edge'. Last winter at the local Ikea they had a crew removing an accumulation that likely exceeded the design loading on a canopy over the loading dock.

Dik