Here's a link to how to calculate loads. Good information all in one place; http://www.engineersedge.com/
A friend and I are building an I-beam bridge that needs to be 14ft wide and span 25ft. He was thinking of using 4 W14x38 beams. How would you go about calculating the weight that the bridge could support? This is a bridge he's using to get trucks across so it has to be fairly tough.
What are the dimensions of that beam? What is the tensile strength of the steel being used? Are there going to be any supports (piling in middle, gussets). What do you want the factof of safety to be (typically 2.5 or higher for bridges). What is the footing going to be (concrete?), thickness, etc. What about the material under the concrete? There are a lot of variables need to be specified. What I would do is build the bridge based on the load needed, then use a factor of safety of 3 (build 3 times stronger than what would barely hold up).