Snow Drift on Small Extension
Snow Drift on Small Extension
(OP)
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.
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.






RE: Snow Drift on Small Extension
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
RE: Snow Drift on Small Extension
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.
RE: Snow Drift on Small Extension
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.
RE: Snow Drift on Small Extension
RE: Snow Drift on Small 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.
RE: Snow Drift on Small Extension
RE: Snow Drift on Small Extension
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.