wood connection capacity - settle a bet

[HouseBoy]

Help settle a bet about the capacity of a wood connection:
We have a beam 2) 2x12 #1 SYP that are fastened together with nails. One of the 2x12's extends about 2 1/4" farther than the other and that one 2x12 is supported by 6) 20d nails into a 2)2x6 wood column (#2 SYP) and there is a wood "cleat" underneath the one 2x12 that is nailed. The wood cleat has 2) 20d nails into the 2)2x6 column.
We are trying to ascertin the capacity of the connection.
I think the nial capacity is approximately 185 pounds per nail so roughly the capacity is 8 x 185 = 1480 pounds. The other side of the bet is taking the position that 1) the nail capacity is greater than 185 and 2) the shear capacity of the beam is equal to the shear capacity of BOTH 2x12's together. Remember, only one of the 2x12's is actually connected to the column and it is also supported by (bearing on) the "cleat" with 2 nails in it. The other 2x12 stops just short of any support. I see the shear capacity as equal to 2/3 x 1.5 x 11.25 x 90 =1012.5#. Conceivably this could be increased with a Ch factor equal to 2 for a beam with no splits or checks but I am leery of that with so many nails in it at the end connection. The nailed 2x12 is only lapped over the face of the column by 2 1/4".
I can not find any justification in the 1997 NDS (the one recognized by our building code) for either a greater nail capacity OR the greater shear capacity.
Is anyone familiar with the NDS justification for either position?
BTW the beam is supporting a storage load.
Your thoughts are appreciated.

 

[JAE]

I'm away from my office at the moment so I can't verify the 185 lbs. - but your assertion that the shear capacity should be based upon only one 2x12 is correct.

Follow the load path and ALL the shear eventually must go through the single 2x12 (the one nailed to and bearing on the column). There is zero shear at the end of the other 2x12 since it ends in space.

Is the "cleat" you speak of a single 2x6 x length? that is nailed to the side of the 2-2x6 column? If so, then its likely that the initial shear reaction from the extended 2x12 would ALL go into the cleat until some slippage occurs. After that, then the load would probably be divided fairly evenly between the 8 nails.

 

[HouseBoy]

I agree that it seems that way but are you aware of ANY provisions in NDS that would possibly permit significantly higher assignment of capacity? (either nail capacity or the assignment of the shear for both pieces into one)
Thanks

 

[RARWOOD]

Per the 1997, I believe 185 pounds would be you maximum capacity per nail.

There is one way you might be able to increase the load. If you actually calculate the nail equations based on the length of the nail in the post and the other conditions. The first thing I would look at is the dowel bearing strength of #2 grade SYP. The NDS table is based on mixed SYP.

However I don't think you would get much of an increase. What ever you do, make sure you both are using the 97 NDS.

For example the shear design values have been increased in the 2001 NDS and the increase for no splits has been eliminated.

 

[JAE]

houseguy - as far as using the two 2x12's for the end shear instead of the single 2x12 (which is the only one connected); this isn't a code issue at all. This is simply pure analysis.

The elements of any structure must be checked along the entire load path. So your friend is right in that you can check the shear near the end where the two 2x12's exist. But going a little closer to the column, you must check the shear at the point where ALL the shear load is transferred into the single 2x12. It is a condition that simply exists (all shear on one 2x12) and you must check it.

 

[woodengineer]

With both main and side members consisting of SYP, 1.5" side member and either 3" or 5.5" main member (depending on (2)-2x6 post orientation yields 185# (Z Model IV governs) C(D)=1.0.

AF&PA has an online nail/bolt/lag capacity calculator:

http://www.awc.org/calculators/connections/ccstyle.asp

Since the second ply of the beam does not extend to bearing, I would only account for the shear capacity of a single ply.

Shear is not a usual failure in wood even though we design for it.

Jordan