Canopy Drift Loads 1

[rjstubbl]

I am designing a precast building with a small (cantilevered 5'-0") canopy at one end. The building is approximately 1300' long and 40'-0" tall.

The drift load would be around 200 psf and 40' long, then truncated to the 5' canopy according to what I can find in the ASCE. This would usually not cause any problems with the PC, but the canopy is on a 2'-6" spandrel panel which has a large amount of torsion due to the drift.

It doesn't make sense to me that the snow will stack up perfectly 11'-0" up the side of the building. Is there a maximum angle the snow will accumulate from the front edge of the canopy? Or any other way around using the 200 psf?

Thanks for the information.

 

[paddingtongreen]

Visualizing from your description is difficult. If the canopy is at one end, is the length and height significant? It would be better if you provided a diagram.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[rjstubbl]

Let me know if you can see the file. If not, I could email it to you.

Thanks for your reply.

 

[mijowe]

I think the consensus here in the past is that the snow load that you have shown, the truncated triangle, is the only direction that the code gives you to design the canopy. It does not seam reasonable, but there are not provisions for relief.

Your 29' drop makes it even more unreasonable to stack that high.

How wide is your canopy?

 

[mijowe]

a previous thread

http://www.eng-tips.com/viewthread.cfm?qid=222195

 

[rjstubbl]

The canopy is about 30' long and cantilever's out 5'-0"

I remember seeing a max angle the drift will go up in the old BOCA codes but I can't find anything in the new codes about it.

 

[ajk1]

Have you tried calculating by the procedure given in the National Building Code of Canada (you need also the Users Guide and the climatic data for your area), so if you are not in Canada, it may be a problem, but it will show you how much the parapet on the high roof reduces the snow drift. If there is a parapet on the high roof, you can use that to reduce the snow load on the canopy. Whether people have seen a drift on this type of canopy or not is perhaps not all that relevant, because our duty as professional engineers and our responsibility to the public is to design in accordance with the Code, unless we can prove that the Code provisions do not apply to our case. Different areas of the continent tend to have different types of snow (light dry, heavy wet, etc.)as well as wind exposure factors, so it may be that drifts on short canopies are more likely in some areas of the continent than others, and that ios why some have said they have seen it and others have said that they have not. Sorry that I can't be of more help.

 

[rjstubbl]

I understand that I cannot deviate from the code. But, the code is very vague when it comes to canopies. I was looking for something I may have missed in the code.

 

[structSU10]

This is not codified, but it is a way I have thought about potential for snow accumulation on a conopy in the past - see attached (hopefully somewhat clear).

Basically the projected area cut off by the wall above the canopy would accumulate a certain volume of snow during the storm - this snow is in turn dumped onto the canopy. I imagine it as the snow coming down at an angle and hitting the face of the wall and droppins down to the canopy. The snow accumulation would look like a drift, forming an angle due to the angle of repose of the snow.

I think in many circumstances this accumulation could tend to be very high - if the conditions permited. This all assumes wind doesnt disturb it, the snow falls in the worst case direction, etc.

just my thoughts that, again, are not spelled out in any code that I know of...

 

[mijowe]

What is the code defined snow fall angle?

 

[miecz]

It would be helpful for those of us who have not seen 11 foot drifts on little canopies if some engineers could post some pictures of these occurrences.

 

[jimstructures]

I will try to add this picture of a snow drift at the end of a building. It is from the USACE publication "Minimizing Snow Effects on Roofs" MP-01-5663.

Hope this worked.

Jim

This is not quite an 11 foot drift on a 5 foot canopy but I guess the dormitory building is not 1352 feet long either.

 

[mijowe]

Impressive picture. It definitely shows the high end of the drift, but it also shows a quick dropoff to the perimeter, and not the truncated drift that the code requires.

 

[jimstructures]

I have rarely seen a clean truncated drift, usually they are more like the one in the picture. Tapered down to the edge of the roof.

I had always assumed that the truncated requirement was due to code conservatism, but it does happen now and then, I just don't any clean photographs of a nicely truncated drift.

Jim,

 

[miecz]

When I asked for pictures, I wasn't thinking picture out of a code book. I was thinking - pictures of buildings taken with somebody's smartphone today. It's January. There should be dozens, maybe hundreds of these across the northern hemisphere.

 

[rjstubbl]

I am not saying that the drifts cannot happen as shown in the attached picture above. But, that is a different situation than I originally posted. My panels are over 40' feet tall and the canopy is about 30' down, not a 5' elevation difference as shown in the picture. I understand how the drift accumulates in the picture but it would not act like that in my case.

With that said, by ASCE, I believe I'm stuck with the ridiculous 11' drift load.

I too would like to see some current pictures of canopies loaded with that much snow. We have gotten about 20" of snow in the last week along with strong winds but I have not seen any to document.

 

[azcats]

Is drifted snow considered cohesive? Is there a maximum internal angle of friction it can support before a chunk slides off?

 

[azcats]

Is drifted snow considered cohesive? Is there a maximum internal angle of friction it can support before a chunk slides off?

 

[jimstructures]

Check out this picture of a South Dakota snow fall. Look at the right side of the building where he hasn't shoveled the roof yet.

 

[miecz]

Fascinating shot. Does it show drift on a canopy?