Snow Drift on Low Roof from a Much Higher Roof

[JNEnginr]

Couldn't find anything on this subject, so here i am.

Picture this: High Flat Roof, 200ft long, no parapet. Lower Roof is 10ft lower, also flat, and only 10 ft long. Leeward caused snow drift would accumulate, as well as Windward.

My question: At what high roof - low roof height differential does leeward stop causing snow drift on a lower roof? 35ft? 100ft? Never?

Thanks!

 

[jayrod12]

Are we talking in reality, or by code? Because the two answers may be different.

I would say never though, as I thought the leeward buildup was caused by the wind speeding up over the high roof and picking up snow, and then losing speed therefore dropping snow on the leeward side.

 

[KootK]

I can't remember if it's Canada or the US but, someplace, I've seen a provision saying that canopies don't need to be designed for leeward drift when the differential height is very great. That kind of speaks to your intuition. At some point, the snow just flies off into space and can't reasonably be assumed to just plop down on a roof 50 stories below. I think it takes a pretty good differential though.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

We've had to repair a few canopies for just this reason. Although we're getting a snow study done on a 40 storey tower soon. It may shed some light on the idea..

 

[ajh1]

The 2010 Canadian Structural Commentaries has the situation that you are described. (Commentary G Paragraph 39)
The reduction is shown for lower roofs less than 25 sq. meters and at least 10 meters below the elevation of the upper roof. That paragraph provides a linear scale from normal drift at 10 m height differential to 0 drift at 20 m height differential.
To me the Commentaries have always been confusing because some parts are clearly code language while others are clearly commentary only and everything in between.
Both ASCE7 and the new 2015 Canadian code are silent on any adjustment due to large height differential.

 

[TehMightyEngineer]

I'm with jayrod and vote for never. My rational is that during a code level event with predominantly leeward wind any snow blown off the upper roof can be idealized to fall straight down after leaving the upper roof. In addition, the taller roof will still shield the lower roof and prevent the wind from clearing the added snow off; resulting in increased accumulation.

ajh's Canadian commentary makes sense, as obviously a small roof with a large height differential would result in any added snow from the upper roof likely missing the lower roof. However, I still think there would be shielding of the lower roof.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[dik]

and the Canadian code commentaries do not form part of the code, unless specifically included. The code, however, makes reference to them, even though they are not included (go figgur?)

Dik

 

[ajh1]

The Canadian code is cleaning up its act somewhat. The new Structural Commentaries for 2015 have not been issued yet, but the bulk of the old "code" stuff in the commentaries is now included directly in the Code proper (wind coefficients, snow drift equations, etc.). That will help immensely in understanding what must be done versus helpful comments only.

 

[spermar]

According to ASCE 7-10, the answer is never. The calculation to determine leeward drift height only takes into account your upper roof length and ground snow load. See section 7.7 in ASCE 7-10 "Drift on Lower Roofs".

When going through the entire calculation, the difference in roof elevation only comes into play when determining the drift width.

 

[jayrod12]

According to ASCE 7-10, the answer is never. The calculation to determine leeward drift height only takes into account your upper roof length and ground snow load. See section 7.7 in ASCE 7-10 "Drift on Lower Roofs".

When going through the entire calculation, the difference in roof elevation only comes into play when determining the drift width.

There has to be a minimum height difference at which the step fills to the top no? I'm not familiar with ASCE so I don't know.

 

[JNEnginr]

Spermar,

I was more thinking along the lines of "In real life", similar to what KootK was saying, where if the difference was 100ft from low roof to high roof, does the snow just "blow away"

 

[ajh1]

ASCE7: If hc/hb < 0.2 then you don't need to consider drift.
hc: height of step above balanced snow load
hb: depth of balanced snow load

 

[spermar]

Good point ajh1. "In real life", I would have to think that at least some of the snow does blow away, especially if there is a 100ft difference in roof elevations.