Snow Drift and Loading advice for short structures in open fields.

[R Naples]

A client has a product line, similar to a carport roof construction that is less than 10' tall installed in an open field in a heavy snow region. Does anyone have literature or could anyone direct me to a place to read up on the drift requirements for structures under 15' tall due to open terrain? The ASCE 7 code is developed assuming a 15' tall structure, and all the drift equations are based on roof lengths.

Project specifics - pg = 50-psf. The structure has a monosloping roof at 30 degrees. It is cold unheated, open structure with a slippery surface for sliding. The max height is 8'. Low height is about 3'.

Any help is appreciated.

Thanks,

 

[JAE]

Are you looking at drifting under the canopy? Or on the canopy roof itself?
I wouldn't expect to see "drifting" on the canopy at all, only a uniform snow load as there's no high-low roof condition to create drifts.

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[R Naples]

JAE,

The Code agrees with you, but the concern is there was a site where drifting may have occurred from the snow in the adjacent field. I'm looking for a standard or study that we can reference to help the company improve their design as their structures are between 3' and 10' above grade maximum. Another way to consider it is the high roof is the roof and the low roof is the grass field.

Thanks,

 

[JAE]

...so a high drift on some grass is bad because???

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[R Naples]

LOL

Because it piles on the roof-like structure supporting some solar panels and the spindly stuff failed. LOL

 

[JAE]

So could you simply raise the panels up a bit to avoid drift piling on the panels?

This might be the root of your question that I'm having trouble getting into my head (be patient with me!).
...in other words, how high would we need to raise it?

The only thing that we are aware of that might be applied is that ASCE 7-16 now provides some language dealing with roof top equipment where if you raise the item up a foot above the balanced snow level you can assume no drifting.

I'm not sure what the section is but it is in there. (Not in ASCE 7-10 though).

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[R Naples]

Thanks JAE. I'll get into the language in 7-16 and then get with my client. In this case, had it been 1 - 2 feet higher, the snow wouldn't be an issue, but the wind loading would likely drive the main column size up a bit. It will come down to a business decision for them no matter what I tell them.

Thanks for all the help!

 

[SteelPE]

JAE, ASCE 7-16 really has this? I am probably still a ways away from purchasing that code. Raising the equipment above the structure is something that I was taught to do. I was told 2'-0" above the roof but always liked 2'-6" above the roof better. Our hb is typically 1'-9" here. The point always was to get the height above hb so the wind can "eat away" at the snow and not allow drifting.

 

[JAE]

Yes, section 7.8 has an exception that reads:
"Drift loads shall not be required where the side of the roof projection is less than 15 ft (4.6 m) or the clear distance between the height of the balanced snow load, hb, and the bottom of the projection (including horizontal supports) is at least 2 ft. (0.61 m)."

So I mentioned 1 ft. above - it appears that it is really 2 ft clear that is required.

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