Decks, Patios, Porches and Docks - Foot Bridge: 4x4s or 2x6s?

I am planning a simple foot bridge across a creek. The span will be 15 feet. I know 4x4s (using two, of course) will work but I suspect that using two 2x6s will be better. I'm thinking with the 2x6s there will be less deflection and a load capacity at least as good as 4x4s.

I'd use 2x8s, but weight is a concern (it's going to be a draw bridge).

Opinions, please, on whether my suspicions are correct?

Check your span tables. Will the bridge be in two sections or one long span?

The bridge is one span. Where are my span tables?

I believe you would need at least 2x8 for a 15 foot span if it was only for foot traffic or maybe a bicycle, but If there is a chance a quad or snowmobile will try to cross then for sure this would not be enough.

Is it to be a draw bridge to keep people off of it?
One thing you should seriously consider is the danger of having a 15 foot long bridge of any construction suspended in the air.

Will you have a railing as well?

It's going to be a draw bridge because the channel is subject to high water events and anything left in place (including 400 lb. stones) will be gone.

There will be no motorized vehicles using this bridge.

One end is going to be anchored to the bank (within 36" of a large tree) and when in the open position, it will be leaning against that tree and secured with a rope.

There will be a handrail.

How will you raise and lower the bridge?

How will you raise and lower the bridge?

One end of a rope will be attached to the free end of the span, routed through a crotch 24 feet high in the tree and the span will be raised by a person pulling on the other end of the rope.

http://www.1st-golf-info.com/images/bridge.jpg

For each 100 pounds of weight at the free end of the bridge, you will need approximately 140 pounds of force to pull the rope. Plus a great amount of friction as the rope slides through the crotch of the tree. Since you will be pulling down against the tree crotch, the friction will be increased greatly. You really need to keep the weight as low as possible. Consider an aluminum walk board such as would be used with ladders to make a scaffold.

Thank you for your help, Wirepuller38. I'm going to make a saddle (possibly out of PVC conduit) to go over the tree's crotch to minimize friction.

How do you figure that 140 pounds of force is needed to lift 100 pounds of weight? I'm not at all doubting it, just want to know how it's done.