I have always thought about using an old house trailer frame for a bridge, one could cut the cross member between the main stringer beams and lap them to bring your main beams closer together like 3 or 4' of space between them, then cut the outside off or shorten them to what ever width you wanted the bridge, also a person could take and cut a small v in the bottom of the stringer beams in a couple places and bend the beam down till it closed up and weld them back together and create and arch, then a support steel angle iron or something, going straight from end to end beneath the beams would make it super strong, person should pretty much be able to drive a side by side or lawn mower across it then, and most of the time them old frames are pretty easy and cheap to come by, I have demoed a few old trailers just in the last year that I had to get rid of the frames.
