We've made 3 types of bridges: most are simple ladder frame, one is a deck and another is a suspension bridge.
The ladder frame bridges range from 8 feet long to 15 feet long. They consist of two 2x10 or 2x12 joists joined by 5 "rungs", with diagonal cross bracing to help stabilize them. We've used both 2x6s and 5/4x6 planking with success. I recommend using 2 deck screws on each side of each plank for rigidity. We made 4 cuts on each joist to give the bridges an arched look. The first cut, from the edge of the board, is fairly steep. The second cut is shallower. The center of the board is left uncut, as the flat center of the arch, then 2 cuts are made on the other side. It is much easier making 4 straight cuts with a circular saw than 1 curved cut with a jigsaw, and the results are comparable. We placed a treated 4x4 on the ground on each side for the bridge to rest on. Before putting the planks on, we drilled 2 holes through the first "rung" on each side and through the 4x4, then hammered rebar to join them and anchor into the ground.
The deck bridge is built like a stand-alone deck, with posts at each corner, set in concrete. We wanted a bridge and a place for a bench, so it is 6x12 feet in size.
The suspension bridge is 75 feet long. There are two 8-foot 6x6 posts at each end, set 3 feet in the ground with concrete. A horizontal 6x6 joins them just above ground level. Two 4-foot 4x4 posts anchor guy wires to each 6x6. Four cables are strung across the gulley; two 3/16 plastic-coated cables from the top of the vertical 6x6s, and two 1/4 cables from the horizontal 6x6s. By doing it this way the cables form a trapezoid with the high wires further apart than the lower wires. This gives more room at your shoulders for walking over the bridge while allowing shorter, lighter planks. We used 5/4x6 planking, cut to 2 foot lengths. We cut two 1/8 inch deep grooves in the bottom of each board with a dado blade, then drilled holes so that bolts could be used to hold washers over the cables. Every 5th board has an eyebolt instead of a bolt. We used lengths of chain from the eyebolts to the upper cables and back down, secured with quick links, to join the upper cables to the lower cables. We used 2x2s cut to 2 1/2 inches long with a hole drilled through the middle as spacers. They were strung on the lower cables and go between planks. There are 105 planks - not counting the cable, posts, or hardware attached to the posts, the bridge weighs 400 pounds.
Pictures
Suspension bridge from directly below
Suspension bridge from below and to the side
Suspension bridge while planking
Links
For the suspension bridge, I bought the 1/4" cable and the large eyebolts from Zip Line Hardware
The posts, planks and 3/16 cable came from Lowes
All the other hardware came from McMaster-Carr
