What size joist for footbridge...?

[Philly20]

Hey guys. I know it might be a bit off topic, but I figure there is some joist knowledge here. This spring I plan on rebuilding and updating an old 16' footbridge on my property that spans a ditch. The current bridge is built of 2X4 that are 16" OC with a duel post/footing in the middle (approx 8') for center span support. Dad and I just toenailed everything years back and the bridge has been fine for foot traffic. However, I want to update the bridge from 48" to 60" in width and beef it up so it can support the weight of a fully loaded ATV (conventional or side-by-side). I was thinking 2X10's @ 15" OC could easily handle the 8' span. What do you think? Also, could I use hangers to attach to the center post header or should I set them on a 2X12 header?

 

[p_mori7]

I would place a 6x8 beam (or a R/R tie) across the mid-point poles, then use 2x8 pressure treated on 12" centers to span either side, resting them atop the 6x8, then use 2x6 pressure treated for the planking. on either end of the span you can rest the ends on concrete piers / ramps, or just bury another section of R/R tie that you can rest the deck on it and have it flush with the ground.