Snow Load analysis OBC (NBCC) Part 9 v Part 4

[puszka]

Hi, fresh EIT here. I'm working on a few projects with complicated intersecting roofs and building volumes with exterior decks/roof decks and I am slightly confused about how in-depth the snow load analysis is needed for buildings which meet Part 9 in the code.

OBC 9.4.2 indicates the minimum uniform snow load required, and specifies that this is also to be used on exterior platforms and decks. My projects are small-to-large private homes for wealthy clients, and based on the geometry I can see how the snow loads could accumulate through drifting and sliding in a variety of ways. So many at times that it is a bit daunting.

Is 9.4.2 saying that the more complex analyses of part 4 (full sliding/drifting consideration) aren't required in favour of the simplified analysis where the uniform snow load is applied across the projects? Appendix A-9.4.2 even says: "The simplifies specified snow loads are also not designed to take into account roof configurations that seriously exacerbate snow accumulation. This does not pertain to typical projections above a sloped roof, such as dormers, nor does it pertain to buildings with higher and lower roofs. Although two-level roofs generally lead to drift loading, smaller light-frame buildings constructed according to Part 9 have not failed under these loads. Consequently, the simplified calculation may be used in these cases."

Now -- taking the common cases that I work with where there is a combination of (mostly) stick frame and some steel elements (portal frame around large window walls generally), I am understanding that I can apply the simplified uniform load across the roofs/exterior area and do away with the more complicated analyses of Part 4.

Based on everyone's experience, is this a reasonable interpretation?

 

[skeletron]

I apply judgement based on the location's snow load (Ss, Sr) and if there are any arrangements that may exasperate uniformity in the snow accumulation (ie. valleys b/w roofs, large steps in roof, big flat roofs). I'll use Part 9 for most single-family homes, stick-built, relatively boxy structures with some sloped and intersecting roofs but nothing that is out of the ordinary. I'll boot over to Part 4 if I can suss out that any of the above are fouled.

Here is a good practice advisory from EGBC on snow loading Part 9 vs. Part 4:
Practice Advisory - STRUCTURAL DESIGN OF PART 9 BUILDINGS IN HIGH SNOW LOAD REGIONS
I don't think that PA makes it any clearer even after 17 pages, but it definitely shows the considerations for when judgement is correctly/incorrectly applied.

For decks/balconies in my area (25psf - 35psf) I generally don't account for drift in a SFH because the deck/balcony is already going to be designed for 50psf live load. The caveat being in high-snow load areas and if it's above grade or super wide.

 

[jayrod12]

I agree with Skeletron. I have on occasion needed to consider snow build-up in a part 9 structure just due to geometry and expectation that the build-up will occur. However when doing so I've generally used a Cb=0.55 to match the part 9 code snow load provisions. That way I am designing for increased snow loads where anticipated, but also not fully punishing the structure with the part 4 building provisions.

 

[puszka]

Thanks for the input! I will use my judgement but that input really helped.