I built a bridge over the creek in the far back of my yard and it was fairly easy except for the three inches of rain we received in one day this past spring. Originally I built it using timbers cut from the yard with 2×6 treated boards crossing it. That turned out to be super strong, but it was far too low and when we got the rains I basically built up the embankments on either side and fortified them more and started fresh with a new bridge. On the second bridge I used 2×10's screwed together on either side and the 2×10's at 12" on center for the center joists, then came back with the 2×6's for planking. It is 54 inches wide and the span is 6 feet, so it overhangs the embankments on either side about a foot. I then filled the voids in the overhang with concrete and made the ramps on either side with dirt. I drive my 900 lb John Deere with me (200 lb) over it and it doesn't budge. I think what I learned on the first one was the sides on either side must be built so they absolutely won't wash away. One side on my original one did and it destroyed the whole bridge. When I re-did it, both sides are now solely built with rock stacked and they have seemed to hold up immensely.
