Snow Drift on at Grade Deck w/o Canopy 2

[KootK]

I'm working on a project where the ground snow load is 175 PSF. That's about six times what I'm used to and has prompted me to consider snow drift on decks in a little more detail than I've done in the past. And it turns out that I have questions. How do we feel about the interpretations shown below? Consider the bits highlighted in yellow to be questions.

 

[Craig_H]

Part 9 is full of exceptions for code requirements if your joists are less than 600 mm above grade (distance from grade to the underside of joists). Generally, that exception only applies to decks and platforms less than 55 sq.m in area. That may or may not help you in this instance...

In my general experience:
- Good skiing is not found in high-wind areas. That doesn't stop people from putting ski resorts in such places (almost every Alberta ski hill), but in general, high snowpack areas have calmer winds. They generally occur where storms "pile up" against terrain and dump their precipitation, rather than where the storms are flying through en route to Saskatchewan.
- Below treeline, drifing effects are really minor. Big conifers are really good at killing the wind. At treeline and above, drifts rapidly show up. Near ridgetop, drifting effects are incredible. I am guessing that the ski getaway mansion is built in some fancy-pants development near the base of a ski hill. That should help keep the wind in check, barring giant clear-cut fetch areas.

Your thoughts on lateral loads are not misplaced. Deep snowpacks do exert creep and glide lateral forces. At your magnitude of ground snow load, these will begin to accumulate if you are on a sloped site, and are applicable to things sticking up above your roof. I haven't had a project where these are applicable to the building globally (like pushing the building off its foundation), but it definitely affects things like chimneys on sloped roofs. The Swiss SLF Institute is generally the reference that I use in estimating these. You should be able to find some design guides for snow restraining structures, and that's what I have turned to in the past. More for the architect too, but watch out for the roof-a-lanches and don't put the Porsche parking area under one.

 

[KootK]

Part 9 is full of exceptions for code requirements if your joists are less than 600 mm above grade (distance from grade to the underside of joists). Generally, that exception only applies to decks and platforms less than 55 sq.m in area. That may or may not help you in this instance...

Any chance you would want to save me some time and point me to the Part 9 section? That would be just what I need IF I could find it in the US residential codes.

almost every Alberta ski hill

Yeah... back side of Lake Louise is pretty much all that I do now because it's not so windy there. I like Sunshine on the right day but often feel as though I'm about to be swept from the face of the earth.

The Swiss SLF Institute is generally the reference that I use in estimating these.

I'd never even heard of that, thanks.

 

[Craig_H]

Sure thing! 9.12.2.2 deals with foundation depth, 9.12.2.2.(7) gives exceptions for decks with less than 600mm clear space underneath. My interpretation is that this is allowed because:
- Kids likely don't find a space less than 2' high to be prime fort-building territory, meaning they are unlikely to be underneath if the whole thing falls.
- The fall for deck occupants is not likely to cause life-threatening injuries in the event of a failure of the deck structure.

9.17.2.2.(3) allows you to ignore lateral support for the deck if it is also the same low height

My AHJ doesn't require a building permit for decks that are less than 600 mm above grade. Most decks that meet that 600 mm criterion in my neck of the woods are supported on grade, often with the hokey deck-blocks sold at your favourite big-box "home center". You might be able to just omit the deck from your scope.

 

[KootK]

...often with the hokey deck-blocks sold at your favourite big-box "home center

Yes... my 5' above grade deck is built that way unfortunately.

I have to say, the NBCC stuff strikes me as eminently reasonable. Any of our US compatriots know if similar exemptions might be found down there?

 

[phamENG]

Not that I've seen. Our codes are really quiet when it comes to decks. The last couple code cycles have progressively increased the codification of deck structures (finally), so I figure we have a good 10 years before anyone gets around to deciding there should be exceptions.

 

[KootK]

So ASCE 7-16 and Rourke's guide to snow do cover this somewhat, as shown below. This confirms a lot of our own conclusions but, unfortunately, provides no simple answers. I feel like there should be some maximum ground fetch based on the width of the building but I don't know that that should be.

 

[KootK]

Update: here's what the AHJ had to say about this:

1) They trust the stamp. Whatever I say goes. Interestingly, this is the only jurisdiction in which I'm licensed under the "ridiculous amount of experience" clause. They didn't want to bother evaluating my foreign education.

2) They do not make any explicit relaxations for decks, at any elevation, other than the trusting the stamp business.

I'm planning for drift to the level of the second story and no higher this go 'round.

 

[dauwerda]

I have to say, the NBCC stuff strikes me as eminently reasonable. Any of our US compatriots know if similar exemptions might be found down there?

The IRC specifically excludes decks not exceeding 200SF, not more than 30" above grade, not attached to dwelling, AND not serving a required egress door from requiring a permit in section R105.2. It also states that even though a permit it not required it does not mean any work shall be performed that is in violation of the code. That is as close as you are going to get to the NBCC Stuff.

That said, section 507.1 specifically states that, "Decks shall be designed for the live load required in Section R301.5 or the ground snow load indicated in Table R301.2. whichever is greater."

Table R301.2 is simply the table the local jurisdiction is supposed to fill out/modify with the local requirements. Table R301.5 requires 40psf live load for decks and balconies.

To me the specific statement of the load requirement for ground snow loads is essentially excluding the requirement to check drift loads for snow (When building/design per the IRC).