I recently designed & built a new bridge in one of our town's conservation lands. It was a good project that went pretty smoothly. Here's a pic of the old bridge that was replaced.
First my son and I started off precasting a couple of concrete abutments in my garage. Something to do on a rainy weekend, anyway. Here's the form with rebar and anchor bolts:
We prefabricated a steel skeleton at Importech, a local imported car repair shop. I used 26' steel bar joists for stringers and welded on horizontal and diagonal bracing. Here's the last piece of wood sill being bolted down:
I towed it to the site, then got behind it and wheeled it into place, using the old bridge and pipe rollers for the last bit (after removing the VW axle).