Wood member design for residential decks

[gtsinc]

It’s been years since I studied beam design in college so I could use some assistance.
I am designing my 20x14 deck and need help with sizing joists.
I have a 20’ Ledger board and 12’ out from the house I have 3 Piers spaced 8’ apart with a 2’ overhang on each side. I want a 14’ Joist so I have 2’ overhang over my beam edge. I am using 10 PSI DL and 40 PSI LL for a total of 50 psi. – Deck is 20’*14’ with Joists 12” on center.

Joist:
Max Bending Moment = 850.7 Ft-#
Max Shear = 408.3 #

Using a 2X8 (1.5”x7.5”) I calculated:
Max Shear Stress Fv = 54 psi
Max Bending Stress S = 726 psi

I found that the AWC de-rated Southern Pine Lumber. And a lot of span tables don’t include a cantilevered option – just simple span. However I found this reference:

http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf

This table only allows 10’1” span with 2x8 on 12” centers with overhang using No 2 grade Southern Pine. It does allow me to use 2x10 HOWEVER I read where my Ledger Board should not be deeper than my Band Joist. So this presents a problem if my existing band is 2x8 and I need 2x10 framing.

Can I assume the following for Wet Service SP #2 treated lumber? Not sure if these are the de-rated values:
Size Fb Single Repetitive Fv E
2x4 1,500 1,720 90 1,600,000
2x6 1,250 1,440 90 1,600,000
2x8 1,200 1,380 90 1,600,000
2x10 1,050 1,210 90 1,600,000
2x12 975 1,120 90 1,600,000

If these are valid then I should be ok with 1200/726 = FS= 15.8 for Fb and 90/54 = 1.67 FS for Fv

Next is the Beam: The tributary area would be 8’ x 20’ * 50 psf on 12” Centers or 400 lbs/ft.
I plan on 3 20’ 2x8’s
I calculated:
Max Moment = 12000 ft-#
Max Shear = 4000 #

Using 3 2x8 (4.5” x 7.5”) I calculated:
Max Shear Stress Fv = 3413 psi total /3 = 1138 each Close to 1200
Max Bending Stress S = 726 psi total / 3 = 54 each FS ~2
Is this analysis valid? Could I use 2x8’s as my members?

 

[LorenAWC]

First, why aren't you using the latest version of DCA6?
Second, DCA6 can help you size your beam too.
Are you having doubts about DCA6's answers? Or do you just not like them?

 

[Archie264]

If you use this you probably won't go too far wrong:

http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf

It's interesting to see that they updated it for the newly reduced values of Southern Yellow Pine.

(As an editorial note it's frustrating to me to see that our industry continually complicates design parameters and yet has to sheepishly admit that published design values are wrong, years after the fact, well after millions of structures have been built using the prior values. If the industry gurus can't get such fundamental things correct why should they feel entitled to impose continual complications on us? Sigh...)

 

[Archie264]

OP, whoops, sorry, I didn't fully read your post. That'll learn me...or ought to.

 

[gtsinc]

I am using that as a reference - It states I need a 2x10 but my band joist is a 2x8.
It also states that I should not have a ledger deeper than my band.

 

[RWW0002]

I am not sure you are going to find anyone who is willing to design your deck on here, but here are a couple of things worth noting:

1. 50 psi is a heck of a load for a deck - 7200 psf! (Im assuming you mean 50 psf)

2.Section properties for a 2x8 should be calculated using 1.5x7.25 (which will give you a section modulus of 13.14 for a 2x8).

3.I would not consider live load on the cantilever when looking at positive bending moment for the 12' span. This will result in a 900ft-lb moment as opposed to the 851 you calculated.

4. The allowable design values you note are the values prior to the 2013 NDS design supplement and do not include the adjustment for wet service mentioned in the DCA document. For a #2 southern pine 2x8 adjusted for wet use the allowable bending stress should be 925 psf (considering Cm of 1.0 for fb<1150).

5. The design values noted are "allowable" stresses, which indicates that factors of safety are included (although adjustments are made to these values based on use).

6. I am not following your girder calculations at all. For a simple span (8') beam carrying 12' or deck and 2' of cantilever the moment in the beam should be around 3.3k'

7. If you are not familiar with the NDS and wood design, I would recommend either following the prescriptive design guide or hiring someone who can design the deck for you. In some areas, the code officials look closer at deck construction than the actual house, and they enforce the DCA guide to a T.

 

[gtsinc]

1) sorry, PSF
6) I considered the tributary area and the ledger to support the first 6' of the deck. So used the last 8' @ 50 psf or 400lbs / linear foot along the beam length.

Thanks for the help