Windward Snow Drift Adjacent Building with Gap

[L212]

I'm designing a new building that is next to an adjacent existing building. My new building is going to be almost 10' taller than the existing building, so clearly snow drift is something I have to consider for the existing lower building. (Buildings are both the same owner and so it is part of my project scope to check and possibly reinforce the existing framing). My thought was to separate the buildings by 2 feet or so (assuming arch approves). I know this would not eliminate the leeward drift since the existing building is <20' separation and within the 1:6 wind shadow. But I thought this would totally eliminate the windward drift (which is higher than the leeward when I run calcs as if the buildings are not separated). I thought the snow would just essentially fall between the buildings when the wind blew across the existing and towards the face of the new. However, the commentary in ASCE 7-16 (Section C7.7.2) says otherwise. It looks like I have to still run the same windward drift calc to come up with the typical triangular drift and can only eliminate the portion of drift that would be between the buildings. Figure C7.7-3 shows this and shows the lower building having a parapet. Does this still apply if the lower building does not have a parapet? (See image) If the new building is the green outline, with no parapet, how would the windward snow drift build up on it? Does anyone know of anything in the code that specifically addresses this that I might be missing?

 

[SWComposites]

 

[KootK]

Does this still apply if the lower building does not have a parapet? (See image)

I feel that it would still apply. And, in general, I agree with ASCE's take on it.

If the new building is the green outline, with no parapet, how would the windward snow drift build up on it?

When the windward air flow hits the taller building, it will create a mass of air in front of it that will be disturbed and slowed down. Like sediment carried by a river, snow will settle out of this slow moving air mass. If that slow moving air mass extends across the gap and over the low building, then you'll see some snow drift there. This is what informs ASCE's take on this.

 

[JLNJ]

Drift isn't about having a flat surface to drift against, as KootK said, it's about pressure.

Is it more complex than just truncating the drift? Certainly. But I credit the ASCE 7 Snow People for keeping their section of the code relatively simple.

 

[DaveAtkins]

The only thing I would add is that ASCE 7-16 has added provisions for snow drifting on canopies, which indicate the angle of repose for drifted snow is 30 degrees. While that is not directly applicable to your situation, I think one could argue that the windward (or leeward) drift on the existing lower building does develop, but once it gets near the gap, the drift tapers back down at a 30 degree angle to the top of the balanced snow. Normally, this is overly complex and not worth considering, but if it means not reinforcing an existing building roof, you might want to consider that.

DaveAtkins