Building primitve bridge -- weight vs. flow-through for flood resistance

[DrRod]

I'm about to launch my long-stalled foot bridge project. I have some telephone poles and good heavy planks ready to span a 30-foot creek. My plan is to shove the poles across at a high spot and nail the planks across with a 1-foot gap between each. That way, when we get our annual flood [not more than a foot over the banks] the water will flow through the gaps rather than push on the bridge. I should mention that I will also anchor the bridge with cables to a large tree on each side.

My neighbor noted that I have three telephone poles and so should use them all. He thinks the weight will help prevent the bridge from washing away in high water. Plus he thinks the extra beam will help support the cross planks better [this I disagree with since the planks are plenty hardy for the width of the bridge and a third beam will inevitably cause the planks to rock in place].

So my questions are:

1. While the poles are really heavy, I wonder if they would just float regardless of their weight.
2. Won't the extra beam running across the creek help to trap the flow of water and make it less porous to the flow of water?

Thanks

 

[RedNeckRacin]

1. Wood floats. It doesn't matter how many beams you have. 2. You are gonna lose the battle with the floods if you are going to knowingly put a wooden bridge in a flood zone. Get above the flood zone with abutments and concrete, or look at some sort of large enough culverts, or a low water crossing.


Telephone poles are really not that strong for spanning a true 30' span. How much weight are you planning on putting on it? We might be able to talk you out of a mistake.

 

[DrRod]

I've put a number of foot logs over the creek in this spot and found the high water just washes past. I know a bridge will catch more water but that's why I plan to leave lots of open space in the planking. I have a good pile of large rocks to raise it some and also deflect water on the upstream side. Also, I've seen the cable anchors work on some of my neighbors farms. Nothing will beat a flood like Hurricane Hugo but we seem to do alright with the spring runoff type of flood.

As for load weight, there is the weight of the planks plus maybe two people walking at a time. But feel free to talk me out of a mistake.

 

[RedNeckRacin]

Well, Moving water is very powerful. Even engineers underestimate it. If you have direct experience with it and have neighbors who have dealt with it. I'd give it a shot. If its just foot traffic, you should be ok. I would say if you have a third pole, put it in. I was assuming you were going to take the 4110 over it. Oh btw, we like pictures!

 

[bcp]

...and nail the planks across with a 1-foot gap between each. That way, when we get our annual flood [not more than a foot over the banks] the water will flow through the gaps rather than push on the bridge.

Until floating debris comes along and plugs your gaps.

I should mention that I will also anchor the bridge with cables to a large tree on each side.

I would anchor it to a large tree on one side, and let the other end swing downstream in flood conditions. Or maybe one strong anchor and one weak anchor.

Bruce

 

[CobyRupert]

Until floating debris comes along and plugs your gaps.



I would anchor it to a large tree on one side, and let the other end swing downstream in flood conditions. Or maybe one strong anchor and one weak anchor.

Bruce

:thumbsup:
Heck, if yer gettin' cables involved, make it a draw bridge.

 

[hslogger]

Gotta admit I like the cable one end idea and let it swing.
But my reason for replying is I would definitely pour pads on either side and get the ends OFF the ground and OUT of the soil. i helped build a bridge just like this once, it is still in place, although sagging. We rigged block and tackle and pulled the phone poles across the creek, quickly with a team of horses. It was a cinch. Once on the other side, rolled em a bit to get the position on the pads and secured them with galvanized bolts.

And just to wreck your fun, yours might not pass gov't muster.

 

[Deere Dude]

I once made a 16' bridge from barn timbers to cross a small creek. It worked great until the Mississippi overflowed and backed up my creek. Then I had to go into the farmers field and float it back about 1/2 mile. So, at the very least tether it.

 

[MossRoad]

1' gaps + 1 misstep = 1 broken leg. Why do that? It isn't going to help much at all. The water is going to push against the leading edge of the upstream pole and blow over and under it. That's where all the force will be to get it moving in the first place. Once it starts slipping off its piers, its gone regardless of the decking.

 

[Luke'sScreenName]

Why not put the poles right next to each other? Less stuff to catch debris and more likely to survive a flood in one piece. With a small overhang of planks could easily be 3' wide and easier to construct. Just a thought.

I love projects like this.