Building a temporary foot bridge over a small creek.

[dstig1]

It would depend on the wood grade. If it is common lumber, 2x10s may only be good for 10-12' span. If you had 30 people on a 20' deck that I am guessing was 4' wide, you are running abuot 55 psf live load. Here is a table for 50 psf:
http://www.southernpine.com/downloadpdf.asp?filename=table06.pdf

And that is SYP, so it is towards the stronger end of available structural wood species.

Carrying a half dozen 20' 2x10s plus all that decking does not sound like my idea of fun, but that's going to be true to some degree of all options if you have to portage it in.

 

[Deere Dude]

In the center I would put a couple of stabilizers into the creek bottom or people will think it's a trampoline and probably wreck it for you.

 

[IndyIan]

In the center I would put a couple of stabilizers into the creek bottom or people will think it's a trampoline and probably wreck it for you.

+1 Sounds like a rocky bottom creek so a support in the middle will also let you go much lighter on the shorter two spans.
Maybe have a back up plan too if you get a whole whack of rain the night before...

 

[rd_macgregor]

I've pretty much stopped putting things together with nails. Not only do screws (eg deck screws) hold better then even galvanized nails, they can be backed out if you ever have to replace a part or if you want to disassemble your construction project later.
I crossed forty feet of stream bottom a few years back with a 6' wide bridge/causeway. The actual stream crossing is only about 8' span, supported on driven steel posts on each side. The causeway part across the rest of the floodplain is supported on post spikes with short 4x4 verticals. Because it is a permanent installation, all the wood is PT...and all parts are screwed, not nailed.
The suggestions others have given, using 2x10's sound good to me. If you have an ATV, you could use your building materials to make a stone boat/sled to drag everything down close to the building site...carrying 20' 2x10's any distance wouldn't be a lot of fun.
BOB

 

[Short Game]

We have a few temporary plank bridges over seasonal wet spots. Just 2 2X10s screwed to cross beams. These worked well but were like ice when wet. Someone put me onto a great non-skid idea: 24" chicken wire, put down with a staple tacker.

Anyway, there's a cheap temporary walking bridge.

You would only need to figure out a support for the center of the creek. For your situation, I might go three planks wide and add some workable hand rails.

 

[beppington]

I've pretty much stopped putting things together with nails. Not only do screws (eg deck screws) hold better then even galvanized nails, they can be backed out if you ever have to replace a part or if you want to disassemble your construction project later.
I crossed forty feet of stream bottom a few years back with a 6' wide bridge/causeway. The actual stream crossing is only about 8' span, supported on driven steel posts on each side. The causeway part across the rest of the floodplain is supported on post spikes with short 4x4 verticals. Because it is a permanent installation, all the wood is PT...and all parts are screwed, not nailed.
The suggestions others have given, using 2x10's sound good to me. If you have an ATV, you could use your building materials to make a stone boat/sled to drag everything down close to the building site...carrying 20' 2x10's any distance wouldn't be a lot of fun.
BOB

I have leaned toward doing that, too, but will point out that the deck screws at my Home Depot/ Lowe's (usually) have a narrower shaft than the equivalent length nail *, and I don't know exactly what metal is involved in making the deck screws, but on my new fence several of the screws have snapped in the middle, from the shrinking of the newly-pressure-treated boards as they dry out. I've also had some snap while backing them out. They just don't seem to have nearly the same shear strength as nails.

It could also be that the twist force applied to the deck screw metal is harder on them vs the pounding (almost entirely compression force) a nail experiences.

* I don't know the precise shaft diameters, but if you just hold a 3" galvanized nail up to a 3" deck screw, the deck screw shaft is much smaller.

 

[radioman]

I wouldnt worry about the bridge getting wet if it high enough above water.

I would just go and get 3 20 footers wooden I joists. place it so its 4 ft wide i joist 2 ft apart. screw down the plywood and be done with it. Its temporary right? easy to put down and easy to take apart and put away. If you have the time, install railing on one side as there will be people who will want a rail to hold on when crossing. its lightweight and strong individually.

 

[HarryN]

Don't forget to post a picture of your final project.
Good luck with the wedding!

 

[Deere Dude]

Don't forget to post a picture of your final project.
Good luck with the wedding!

If you fall in the drink we want a video of that.:laughing:

 

[radioman]

If you fall in the drink we want a video of that.:laughing:

ditto!