I am planning to build a bridge across a ravine on my property and rather than buy a bridge kit I want to build it. The span is 40ft so I was going to buy a bunch of 2-in x 12-in x 16-ft pressure treated lumber and make beams.I would like to know the weight capacity of these beams once completed. The plan is to sister them and bolt them to make a beam
This is something that needs a structural engineer. If you want to try to do your own calculations, here is how you do it. First you start with what a beam can hold at all. https://en.wikibooks.org/wiki/Strength_of_Materials/Loading_of_Beams
Then you move to what dimensional lumber is capable of:
But to calculate what sistering would do for you is actually not that simple in that ostensibly if you have two 2x12s sistered, it should be the same as a 4x12. But in reality if they are finished dimensional lumber, the 2" is actually 1-12", so two of them sistered is 3", vs a 4" dimensional beam would be 3-1/2" wide. Then you are complicating it further by staggering sistered beams to try to get a longer span. I'm not saying it can't be done, but this site is not the place for it, you need an experienced a pro to do that.