So do I have the layout right.

Stagger the joints in the 2x12's by 4'. The plywood is just to flush out the beam to standard bracket dimensions

All 4'. That is what I have here. So we should be good to go.

No you have your.plywood.joints stagered.4'. Your 2x12's have all the joints in the same place. Its your 2X's that matter, the plywood can be anywhere

example:
My posts are going to be 8' apart. So i will use 16' 2x12s.

First row will.be 16'-16'

Second.row i cut one in half so i go 8'- 16'- 8'

Third row is back at 16'-16'

That way the joints are stagered and all the joints sit on top of a post
yours may not work out as evenly as mine depending on your pier width but i like to try n keep all my joints on the pier, or post.

There is no try, do 

(make all splices in the 2x's over supports)

aplogies to Yoda 

Will need to do more! Not try!

So the plywood is just a filler? Seams don't mater?

The piers are a every 6'.

Looks like it cant be done with 6' spacing. And end over piers.
Will need to try 18' 2x12? Do they make 18' 2x12???
Nop, just wont work.

They do make 18' 2x12 but at 6' pier spacing 12' material works. 1st ply of 2x starts with 12', next ply of 2x starts w/ 6' third ply starts w/ 12'. This knits the plies over the posts. Plywood is just filler but does help to knit the joints.

12-12-6

6-12-12

12-12-6 or 12-6-12, either will work.

You can have two of the three joints in the same place as long.as its on a pier and they.are not right next to each other.

DonP and CjAl are giving good advice. If you can get all the joints in the built-up beam to land on supports then any ability of the nails and adhesives to bond the beam into a working unit is pure gravy. If you are building a generalized beam of uncertain support spacing the 4' overlap in 2x spacings gives you a good working distance for the nails to do this bonding.

Stager.Ok that will work. 

Quote from: CjAl on October 06, 2011, 09:55:43 AM
12-12-6

6-12-12

12-12-6 or 12-6-12, either will work.

You can have two of the three joints in the same place as long.as its on a pier and they.are not right next to each other.

Now that that is resolved let me ask. I am good to go with the tripple 2x12's extending out 9.5" beyond the edge of my outer most piers right?  or does the rule about being able to extend the same as the width of the board not apply to the main beams?

something like this?

If one ply of the plywood was treated 1/2", extended down out of the girder and had a frame of 2x4's it would be a well tied brace in the direction of the girder.

Great idea Don!

I will be using a system like this. Seam Don's way would not work?