Leo, I used 3 rows 2x12s for my beams. Then set them directly on top of the posts and secured them with metal brackets. The foundation is 20'x40' My beams are made up 3 rows of two 2''x12''x16' and one 2"x12"x8' for each row.
The posts are at 8 ft apart so the ends and the mid-section of all of my boards fall on top of post.
I plan on putting more posts in later in between the 8 ft. posts. Just to make me feel better about the longevity of the foundation.
The entire build up has been nailed, glued, and screwed together, then I placed 5/8" lag bolts and large washers at the top and bottom of the beams every four feet to compress the assembly. This has made one sturdy beam.
It probably would have been less work to buy a full size beam. But, I dont think I could lift it.
[size=13]Disclaimer: This is the way I did it. And it may not be the safest way or the best way, but it has worked for me so far. [/size]