Snow Drift on Fabric Awnings

[rgerk]

Is there an appropriate time to neglect snow drift on a fabric awning? I'm looking at an aluminum frame supporting a fabric awning attached to the side of a restaurant. Per the code, there is a 42 psf snow drift (10psf ground). I don't really believe the fabric awning can support this load and so I'm left wondering if the frame should be designed for it.
Thanks.

 

[dgkhan]

Snow drift, unbalanced snow loading etc are caused by transportation of snow by wind activity. As the awning is not a rigid base, as wind blows, it will also move the awning fabric. You have to think yourself about his process. Normal formulae from ASCE snow code are not apllicable as it is as base is also moving.

 

[cvg]

as wet snow sticks to the fabric and then freezes, it will become rigid. snowdrift will then occur on the awning.

 

[BAretired]

You should design the awning for the same snow load you would use to design a permanent roof. If the fabric is incapable of carrying the snow, there is a potential for injury from falling snow, so you should ensure that it is capable of carrying the snow.

BA

 

[msquared48]

I agree with BA here.

nd don't forget to mix in the fabric degredation due to UV exposure over time.

Mike McCann
MMC Engineering

 

[rgerk]

Thanks guys. I was thinking along the same path. Sometimes, the loading just seems illogical with what I see being put up around here. I would imagine what I'm seeing isn't being designed correctly.

 

[TTUengr51]

I agree that drift must be considered. However, does anyone know of a slope parameter that may negate the need to apply snow drift. In my opinion, if the awning is at a 12:12 pitch slope, I cannot see snow drift being as much of an issue as if it were sloped at 1:12. To my knowledge, snow drift in ASCE 7 does not have a slope parameter to consider.

Nick Deal, P.E.
Michael Brady Inc.

http://www.michaelbradyinc.com

 

[trainguy]

It sounds like you could have problems with ponding instability (fabric changing shape and attracting more load, and so on...)

You should make sure that the fabric portion of the design is clearly out of your scope, or analyze it properly.

tg

 

[BAretired]

Don't know about ASCE 7. In the National Building Code of Canada (NBC) where alpha is less than or equal to 30 degrees, the slope factor is 1.0. Above 30 degrees, it is (70 - alpha)/40.

For unobstructed slippery slopes greater than 15 degrees, the slope factor is (60 - alpha)/45.

BA

 

[JedClampett]

Back in Chicago, our house had an awning that was retractable. During the winter, we would pull it up. We weren't worried about snow load (maybe we should of been), but wanted to get as much sun as possible in the house in the winter.
Is your awning of a size where retraction is an option? The restaurant might save a few bucks on their heating bill as an added benefit.

 

[miecz]

Twenty years ago, I designed the support for a fiberglass awning attached to the garage where I park my car daily in the winter. I designed the support for snow (plus drift) and the result was. um, beefy. The owner (the city) didn't like the cost and put up a fabric awning with some inexpensive flimsy supports. I thought: "they'll learn when this thing comes crashing down." Twenty years later, that awning is still there. I've never seen more than a few inches of snow on it, though we've had some winters with well over 100 inches. The snow seems to slip right off. The fabric is sloped at around 1 to 4. The fabric is pretty ragged, but the teeney frame with its tiny attachment screws is fine.

 

[rgerk]

The awnings are 11 ft wide, 3'-11" tall and project away from the building 6'. The awning is supported by aluminum tube frames approx 2' o.c. Slope ends up being 7.84:12. Fairly steep. That's why I was asking. I just didn't see any code provision in ASCE 7 or IBC 2006 that would allow a reduction or deletion of snow drift. Our customer will just have to increase their frame size for the loading. Thanks again.

 

[beton1]

Agree with comments, seems to be Murphy's Law. If you (miecz) had specified that flimsy awning, all the factors of nature would likey have ganged up on you the 'engineer'... but the city or homeowner or other seems to luck out, but not always. Keep a eye on it. We work with science and a code, yet sometimes nature is inexact,
or is forgiving? Seen some wierd stuff still standing.

 

[csd72]

rgerk,

Most small fabric structures that I have seen were not designed correctly.

Although sometimes they are designed based on the failure of the fabric rather than full loads.