Snow Drift on Small Extension

[amecENG]

Please see the attached sketch.
This is in refernce to NBCC but it is probably similar to ASCE:
When calculating the snow drift on a lower extension should we use the characteristic length of the hatched portion of the upper roof or the entire upper roof?

Please notice that the section of the building that the extension is attached to sticks out 9m from the main portion of the building. What has me confused is that if this piece wasn't there we wouldn't have to account for snow drifting at all because the distance would be greater than 5m.

 

[msquared48]

I would certainly consider drifting there even considering the length you mention as wind blown snow will still tend to accumulate there in the area of less wind.

Depending on the slope of the higher roof, I would also consider impact of falling snow, possibly wet snow, on the lower roof, and additional drifting due to sliding snow.

Mike McCann
MMC Engineering

 

[jayrod12]

My grandma always told me that if "ifs and buts were candies and nuts it would be christmas everyday". If the extension wasn't there you wouldn't have buildup but it's there so you need to account for buildup.

I doubt using the larger characteristic length as opposed to the smaller hatched portion will make a huge difference in your actual buildup values. In the end with the lower building being so small the overall load on the structural will be a minimal increase.

 

[amecENG]

My point was that I don't understand how the rest of the size of the roof contributes to the snow shedding on the lower roof when it is 9m away. The code is not very clear on a situation like this. It just says you take the characteristic length of the upper roof. What if the offset was 20m or 50m?

Your right it doesn't add very much (1kPa or 21psf) in this situation which hardly provides justification for a snow shedding study but I was just curious from a theoretical point of view.

 

[amecENG]

I guess the snow can blow into the hatched portion and then blow onto the lower extension and hence use the characteristic length of the full roof. That's possible I suppose.

 

[ajh1]

Not sure I quite followed your statement that if it wasn't for the 9m extension, there would be no snow on the lower roof because the gap would be greater than 5m. Are you saying that the small lower roof is a separate building that would not meet up with the higher building except for the extension?
As to the characteristic length, no one has managed to teach the wind to blow at only 90 degrees to the face of a building. A quartering wind will potentially pick up snow from the outer portions of the upper building and deposit them on the lower building. The overall length of the building is probably not required in the calculation since the lower structure does not extend to the ends, but using only the 16m in the middle is probably unconservative as you will get some transfer from the back portions of the end areas through the center area onto the lower building. Perhaps look at a length that would be consistent with a 30 - 45 degree fetch angle that would give the greatest likelihood of snow actually making it to the lower roof across the upper roof.

 

[jayrod12]

I see your reasoning for only including the hatched area. If you wanted to trim some fat you could maybe take an additional 9m width on either side of the hatched area and use those dimensions (roughly 34m x 31.6m). But in reality that's probably only saving you 0.5kPa and so what's the point.

 

[RobertHale]

"Drift size is related to the amount of driftable snow as quantified by the upwind roof length and the ground snow load." ASCE 7-10.
Keep in mind just because that 9m segment is not there does not mean that drifting doesn't occur. It just means it occurs on the ground where it doesn't load your structure.