Snow Drift Load on Adjacent Sloped Roof

[rabroke]

We are designing a new 70' tall building adjacent to an existing 2-story house. The edge of our new building is about 4' from the existing home (physical gap between the buildings), and nearly 50' higher than the eave height, so it is well within the requirements of ASCE 7-16 section 7.7.2 requiring snow drift on the adjacent structure.
The slope of the existing home's gable roof is approximately 12/12. In the previous section 7.6 regarding unbalanced loading, the unbalanced loads do not need to be accounted for gable roofs with a slope exceeding 7 on 12. Within the commentary of section 7.6, the second paragraph talks about this upper limit "is intended to exclude high-slope roofs on which significant unbalanced loads have not been observed" followed by "the upper bound for the angle of repose of drifted snow is about 30 degrees."
So my question is, is it reasonable to use that same upper limit on the unbalanced snow load on the snow drift calculation. I feel any snow blowing off the upper roof would be subject to the same "sliding" action that the unbalanced snow would be subject to, and fall between the gap between the existing and new building. And if that does make sense, is there any documentation I could present to a code reviewer other than saying my "engineering judgement based on the unbalanced snow load exception" as I know most reviewers would say this doesn't apply, even if it is sound engineering.
Any thoughts/help would be appreciated.

 

[dik]

I don't know what the requirements are for ASCE 7, but in Canada, you would have to reinforce the adjacent structure to accommodate the added snow load.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[HTURKAK]

If the roof slope larger than 7 on 12 , unbalanced or drift loading is not expected , as per Section 7.6.1 ASCE 7-16 Standard.

 

[rabroke]

HTURKAK, that's the section I'm seeing, but it's specific to the unbalanced loading. I know from an engineering standpoint it makes sense that it also applies to the drift, however I'm trying to think like a code reviewer that would say "but this exception doesn't specifically say it applies to drift loading". So I'm trying to see if I missed something else in the code that references drift from an adjacent building on a steep slope roof.

 

[ProgrammingPE]

I would tend to agree with your judgement for the drift on the "far" side of the gable roof since the triangular load would require an even steeper slope or "angle of repose" for the snow. You can also point reviewers to the commentary on the Drifts on Lower Roofs section where it says:

The leeward drift height limit of 60% of the lower roof length applies to canopies and other lower level roofs with small horizontal projection from the building wall. It is based upon a 30° angle of repose for drifted snow (tan (30°) = drift height/drift length = 0.577 ∼ 0.6) and is consistent with provisions in Section 7.6.1, which exclude roofs steeper than 7 on 12 from the gable roof drift (unbalanced load) provisions.

Since the commentary on snow drifts also refers to the 30° limit then I would think that it applies here as well and not just for unbalanced snow loads. Of course, it ultimately comes down to the discretion of the reviewer since the code does not formally say that the drift can be ignored here.

Where I'd disagree with you, though, is for the drift on the "near" side of the gable roof. The triangular snow load actually results in the slope of the snow being decreased since it is sloped in the opposite direction of the roof.

The other possible situation is where the gable roof is rotated 90 degrees from the image show below. For that case, the building section would have a flat roof and so drift would still apply since the drift does not change the required slope of the snow.

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[HTURKAK]

I agree with you that the clause Section 7.6.1 does not explicitly state for drift but if roof slope larger than 7 on 12 , unbalanced is not considered, using engineering judgement it is also valid for drift..

I will suggest you to look ( ASCE , Guide to snow Load Provisions of ASCE 7-10, by Michael O’Rourke)
There are worked examples useful to get the concept.

One of them ,

Copy and paste of the relevant statement,

(..Finally, note that if the mansard slope was 8 on 12 (i.e., larger than 7 on 12), one would not expect unbalanced or drift loading, as per Section 7.6.1 of the Standard ..)

 

[Brad805]

I assume the adjacent building is on your client's property.