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Snow Drift on Existing Building

Snow Drift on Existing Building

Snow Drift on Existing Building

(OP)
I'm working on a project where a new building is adjacent to an existing building and is approximately 13' higher than the existing. I'm in the process of analyzing the existing roof structure for the potential snow drifting introduced by the new building. The issue I'm having is determining the correct "lu" (length of roof upwind of the drift) to use. I've attached a sketch showing the new and existing roof profiles. Should I be using 500' as my length or does the 3' step justify using 180' as the length? Typically, I would just go with the worst case scenario, but, if I do this, the snow drift load on the existing roof structure becomes extremely large and will require very costly reinforcements. Has anyone come across this type of situation before? Any help or guidance would be very much appreciated. Thanks in advance!

RE: Snow Drift on Existing Building

I would say the 3' step could be considered justification for using 180'. However I don't have experience using the IBC (I'm assuming that's the code this is under). When determining snow drifts via the NBCC the lower roof dimensions do not come into play but rather the upper roof dimensions and the step height.

RE: Snow Drift on Existing Building

Refer to figure 7-8 in the ASCE 7-10. Lu is the length of your new roof from which the snow blows off and loads your lower roof. It is not your existing, lower roof length.

RE: Snow Drift on Existing Building

(OP)
jayrod - that's what I was thinking as well. If the change in height was less than the theoretical depth for flat snow, then I don't think you could bring that into effect.

bigmig - I believe that is only for checking leeward snow drift. When checking for windward drifting you substitute the length of the lower roof and use 75% of the drift height determined from Figure 7-9 of ASCE 7-05.

RE: Snow Drift on Existing Building

Can your total snow depth including windward drift reach 3' at the 3' step? If so, then snow can start to move across the entire 500' roof length.

RE: Snow Drift on Existing Building

I'd used the longer length. The little upwind intermediate step won't keep the full width of roof snow from drifting further upwind.

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RE: Snow Drift on Existing Building

Full 500 foot length of the windward drift.

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