Ground Snow Load vs. Deck Snow Load 2

[designres]

Hi, I’m working on a deck renovation and the snow load is extremely high. Ground snow load is 372 psf. Crazy, right? There’s a deck on the second floor, above the garage and the client would like to remove the center post supporting the deck so the snow plow can get in closer to the garage, in the middle of the driveway. So, in order to size the new beam and posts, do I need to use the full ground snow load? Or is there a reduction since the deck is about 8’ off the ground? Thank you for any advice.

 

[driftLimiter]

Treat it like a lower roof. It may be subject to INCREASED loads from drift.

 

[designres]

Thank you, driftLimiter. So, 0.7 factor applies and then add drift to that?

 

[JLNJ]

The load factors depend on your jurisdiction and what code is enforced. Where is your project?

 

[driftLimiter]

Like JLNJ says you need to check with the AHJ. They often prohibit some reductions allowed by the ASCE in these high snow areas. Watch out for the snow load duration factor as well, some AHJ's don't allow it to be over 1.0.

 

[designres]

The project is in Truckee, California. I’ll look through their posted materials and if I can’t find anything on the load duration factor and the ground snow load reduction, I’ll try to get a hold of somebody at the building department or someone in the engineering or architecture field. Thank you for the tips, JLNJ and driftLimiter!

 

[JLNJ]

372 is pretty crazy, and oddly precise.

Something has to give, at least a little. That's six feet of water weight or twelve feet of 30 pcf (which is the max) drift. Then again, it's a cold roof, so no reduction there.

I have never dealt with ground snow loads of this magnitude. Let us know what you find out.

 

[CrabbyT]

I did some googling and found this paper. These snow loads are insane.

https://www.townoftruckee.com/home/showpublisheddocument/8714/635326508598730000

 

[driftLimiter]

The snow loads are very real I've lived in this area my entire life. Just a within the last few years there was a multiple snow storm event that collapsed many roof and deck structures.

 

[le99]

I am puzzled. If the platform can support a moving snow plow but not the snow? Also, where in CA has that much of snow accumulation?

 

[driftLimiter]

The sierra nevada mountains especially the eastern side of Placer county. The snow load requirements can be a little hard to decipher I made this map that shows the approximate location of the demarcation line for Placer county. The eastern side of this line can see VERY large snow accumulation.
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1657302811/tips/placer_country_snow_load_map_uhtvy0.pdf[/url]

Here are the snow load tables that correspond with this map.
[URL unfurl="true"]https://library.qcode.us/lib/placer_county_ca/pub/county_code/item/chapter_15-article_15_04-part_ii-15_04_290[/url]

The storm I was talking about in 2017:
[URL unfurl="true"]https://sacramento.cbslocal.com/2017/01/23/storms-dump-23-feet-of-snow-on-tahoe-resorts-in-first-weeks-of-2017/[/url]

23' snow accumulation in less than 3 weeks in the same area.

 

[ProgrammingPE]

You need to use a balanced snow load, ps, of Is * pg.

The importance factor times the ground snow load Is * pg shall be used as the balanced snow load for snow accumulation surfaces, such as decks, balconies, and other near-ground level surfaces or roofs of subterranean spaces, whose height above the ground surface is less than the depth of the ground snow, hg (hg = pg / γ).

Snow Density, γ = MIN(0.13 pg + 14, 30) = MIN(0.13 * 372 + 14, 30) = MIN(62.4, 30) = 30 pcf
Depth of Ground Snow, hg = pg / γ = (372 psf) / (30 pcf) = 12.4 ft

Since the depth of the ground snow exceeds your 8 foot high deck, you no longer get to use 0.7, Ce, or Ct since the snow has nowhere to blow away to, and the ground will not melt the snow. As others have mentioned, you also need to apply drift loads.

Structural Engineering Software:

http://www.structuralcentral.com

Structural Engineering Videos:

http://www.youtube.com/structuralcentral

 

[le99]

I think you can make a decision from the angle of the "importance" of the deck under such extreme snow accumulation - will the collapse jeopardize the occupants or severely damage the building/garage.

 

[designres]

ProgrammingPE that is very helpful. Would the snow load factor 0.7, for flat roofs, then be interpolated? If the deck is at 8’, expected snow level above that is 4.4’. So, 0.7+(4.4/12.4)*0.3 = 0.81?

 

[ProgrammingPE]

There's nothing in the code or commentary that suggests that interpolation should be done here. If the code writers thought interpolation was applicable, then they could have just written the alternate equation with interpolation built into it. (They could have including an additional coefficient similar to what you suggested.)

Also, you likely have a Thermal Factor, Ct, of 1.2 so you're actually comparing a factor of 0.84 to 1.0 (pf = 0.7*Ce*Ct*Is*pg = 0.7*Ce*1.2*Is*pg = 0.84*Ce*Is*pg vs pf = Is*pg), so interpolating would only decrease the load by 10%.