span rating for a 6x6 beam?

Question

I have a 12 foot long 6"x6" beam made out of pressure treated lumber (pine I think). It is supported by concrete on each end. Is there a formula to figure out how much weight it is rated to support if the weight is distributed evenly?

Also, if I added an addional concrete support in the center, would that double the beam's support rating? Or would it increase it by a factor greater than two?

Thanks.

Answer 1

Seems to be not quite as straight-forward as simply 6 X 6 inches.
Try to digest this:
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch06.pdf