ASCE 7-16 Snow Drift on Adjacent Buildings

[CN-EIT]

There was a profoundly informative discussion previously had on this topic some years ago (see thread: thread507-443423) that relates to snow drifts forming on adjacent roofs; particularly with respect to leeward drifts. I seem to be running into a lot of the same issues that were previously mentioned but would appreciate further discussion on it.

Essentially, it seems to me that for leeward drifts, the building separation only has a minimal impact on drift width but does not necessarily impact the drift surcharge load. As previously mentioned, the drift width for adjacent buildings seems to exceed the drift width of the "equivalent" drift that would form if there there was a continuous lower roof (read: no building separation). This makes very little sense to me since I would expect that as the building's separation increases, the leeward drift would decrease in both magnitude and width. I created an excel program which calculates the drift (and honestly probably does a better job of explaining the calculations better than I can here) but seems to produce unrealistic leeward drifts.

Attached is the file. User inputs are in blue shaded cells.

My questions are:
1) Do the code provisions really dictate that a full size leeward drift will form on the lower roof regardless of building separation? Or am I simply misinterpreting the code?
2) Does the spreadsheet attached accurately reflect the code provisions with regards to equations?

Thanks in advance

 

[RWW0002]

CN-EIT
I do not have time to look at your spreadsheet now, but to my knowledge the provision has not changed from 2010 to 2016 and we are still stuck with this odd (and in my opinion very inconsistent) approach to snow drift across buildings with a gap. See examples in the referenced thread.

 

[J1A1]

If it makes you feel any better, I just ran a calc through a different spreadsheet got a 52.6 psf design surcharge with a 20 psf balanced snow load making the drift loading 72.6 psf.

Edit: I don't have time to review your calcs but I have reviewed the spreadsheet we use many times and it's always accurate. I'm sure the numerical justification behind higher surcharges with a building separation is rooted in some sort of testing, but I couldn't tell you why myself.

 

[CN-EIT]

Seems to be the conclusion I came to as well. The windward drift provisions still allow the truncated drift which makes sense since wind would blow snow from the low roof into the space between buildings. Talking with more senior engineers the leeward drift provisions seem to rely on the blowing from the higher roof and carrying across the building gap (6 to 1 angle vs. 4 to 1 for a leeward drift with no building separation). At least visually, that would seem to match what we see in the Dakotas with snow blowing off of higher roofs and not immediately settling to the ground at the roof edge. Guess we are stuck designing for large drifts.