[vash]

if you take some dimensions of the beam,someone can go thru the steel manual and probably estimate the exact beam. from there, the calcs are pretty vanilla. the trick will be to pour some kind of slab at the ends..(we call them abutments) to anchor and support the ends. you typically anchor only one end, and allow the other to slide.

10" is nothing. i wouldnt support from mid span. running water puts crazy loads on columns. the added math wouldnt be worth spanning such a short distance. cool thing about the dually and trailer, you wont be able to put them both on the bridge at one time, so the loading is relatively light.

talking out of my butt here, i bet a simple 12x12 wood corbel bridge would hold up your dually. what does the trailer weigh? a camping trailer? less than the dually?