Roger, Your best bet is to leave the idea of a suspension bridge somewhere else unless you have a huge amount of time on your hands. To spen 15' with decking and hand rail, (not including suspension) you have a few choices. 1) 4"x6"x16' Pressure Treated 2) (2) 2x6x16 (laminated) PT 3) 2x10x16 PT
Your best bet would be the 2x10 it is the most economical and much easier to handle. If you want to go heavy duty, use 2x8 and double it up.