Heavy Snow Deck Design 1

[buzzford]

I am designing a deck for the Donner Summit area of Ca.
The area has very heavy snow loads. At any time there can be 15-20 feet on the ground at a time.
If I use 6 in by 14 in joists and the deck is 8 ft by 20 feet with the 20 foot side attached to the house and the opposite side supported by a 5 1/8 by 14 1/2 glu lam. Can I use 2 in by 6 in redwooding material or do I have to use 4 in by 6 in redwood decking material. How much weight can a square foot of 2 in by 6 in redwood common wood support? Thanks.

 

[DaveVikingPE]

The advantage of working in Donner Summit is that no one will fault you for eating your coworkers...

A 2x6 redwood deck member, supported every foot can support 20 ft of snow above it.

20 ft of snow weighs, say, 60 pcf (conservative', water weighs 62.4 pcf and now is water and air...). 60 pcf X 1 ft (that's two flat 6" wide planks) X 20 ft high = 1200 pounds per foot and with supports at 1 ft, that's a 1200 lb weight the plank is supporting. So, wl^2/8 = 1200X1X1/8 = 150 #' X12 = 1800 #".

S = bh^2/6 = 5.5x1.5x1.5/6 = 2.0625 in^3 (assuming exact dimensions . So, M/S = 1800/2.0625 = 873 psi which is less than the maximum allowable unfactored (fancy engineering term) bending stress for redwood of something a bit above 1,000 psi.

Note, what's important is the spacing. a one-foot spacing is pretty tight; I've seen a wood floor build of 2x8's spaced at 8" o.c. - the purpose of which was to support a bank vault (strange adventure it was).

 

[buzzford]

Great news for the redwood 2 in by 6 in decking!
One more question pertaining to the same deck. It is 8 ft by 20 ft with the 20 against the house.
What about the 6 in by 14 in joists? Are they necessary?
Thanks for the input. Great info. Buzz

 

[923806]

same thing goes with it. if span = 8ft coz u don't go with the larger span, assume joist wt = (6"x14&quot/144 x 45pcf) =26.25 #/ft + (2"x6&quot/144 *45pcf =3.75 #/ft + 1200#/ft.
total wt =26.25+3.75+1200=1230#/ft, M = .125*1230*8²=9840 ft-# , S = 5.5*13.5²/6 =167 in³, so M/S = 58.92psi <1000 psi
ur section is quite over, so u can use a lesser size but u need to check them both for shear and deflection

 

[boo1]

923806 is the 45 pcf typical snow loads?

 

[RockEngineer]

Before we get too far into sizing members it is important to look at the real snowload Buzzford has to design for. I am from Washington state and we have some of the heaviest design snow loads in the United States in the Cascade Mountains due to our wet snow. Even our wet snow doesn't come close to the 60 pcf estimated by Dave Viking.

What buzzford should be designing for is the design snow load in his area. I couldn't find the SEAOC design snow loads for that area of California but the Truckee California has their building requirements on the web.

http://www.townoftruckee.com/pdfforms/Handbook.pdf

Table 1 page 10 shows the highest snow load for Tahoe/Donner at 340 psf. Buzzford should contact the local building officials in the city or county he is in and they will give him the design snow load based on his address. It is much better than guessing and will save headaches. In these high snow areas decks are usually required to be engineered so the engineer of record would specify the member sizes. The town of Truckee California site above states &quot;The entire Town of Truckee is declared a snow area. Therefore, all structures require engineered plans, therby eliminating reference to conventional construction.&quot; Buzzford may not be in the town of Truckee, but the entire Donner Pass area probably has similar requirements.

Boo: The 45 pcf number referred to by 923806 is an approximation of the density of the lumber. Depending on the type of wood it can vary from 30 pcf to 45 pcf. The density of snow varies from area to area. The following web page article gives some ranges on the density of snow.

http://www.avalanche.org/~moonstone/snowpack/snow layer densification.htm

Good Luck on your deck

 

[buzzford]

The design snow load for Donner Summit is 430 pounds per square foot. Truckee has snow areas ranging from 150 pounds per square foot to 360 pounds per square foot, depending on what part of Truckee you are in.

 

[RockEngineer]

Thanks Buzzford.

I had a couple other thoughts after I wrote my response. I would have more concerns about the glulam than the joists and decking. Not the glulam's capacity but it's makeup. The glulam would have to be an exterior grade glulam and should be pressure treated or manufactured of a weather resistant wood like Alaskan Yellow Cedar (beautiful and naturally resistant but expensive). Glulams out in the weather can be a problem. You may even want to design something to keep the water from ponding and soaking in the top of the glulam (Torch down roofing on top the glulam works). Your deck footings are also critital due to the large support area and frost depth. If your deck is under the eve where snow will slide off the roof you need to increase the joist sizes to cover up to 200% of snow load for impact.

P.S. With 450 psf ground snow load your deck beams and redwood decking are ok at 4x12 joists on 16&quot; centers and nominal 2x decking (no impact). Your joist hangers (if you hang them)need about 2500 lb capacity. Your glulam needs to have a capacity of about 1850 lb/ft in wet conditions.