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

[bcp]

In Maryland, my former home, and Pennsylvania, requirements for water bridges from both structural and environmental standpoints are strict. Likely treated telephone poles would be considered poison.

I recommend getting a permit. EPA fines are very, very high. $10,000 not unusual. Plus you have to dismantle and may have to pay to dispose as hazardous waste. Once they are on you, they are always on you.

"That isn't a bridge, That's my raft that got stranded there in the last flood. I'm waiting for the next one to float it off."



Bruce

 

[/pine]

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.

Spot on...:thumbsup: :thumbsup:

 

[lilranch2001]

I am not quite sure I would go for this design, but if you do let us know how it goes

 

[bindian]

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.

Like MossRoad said. It's the debris that piles up on the upstream pole that the water pressure works on. Debris pileup can tear off planks. Secure it good..............then if it breaks lose...................."bridge, what bridge, I was building a ferry crossing".

We need photos from start to finish. Remember, you never built it without photos.
hugs, Brandi

 

[MossRoad]

I used to work at a man-made whitewater course and was trained in whitewater rescue. This was 30 years ago, so forgive me if I'm mistaken.... The power behind moving water can be a real living nightmare. The pressure exerted by moving water increases with the square of its velocity. Water weighs about 62# per cubic foot. A 30' telephone pole averaging 6" diameter is the equivalent of about 15 square feet of surface area. Its rounded, so the calculations are beyond me. Anyhow, you got about 1000# of force on that pole at a low flow of just a couple miles per hour.

So if you have 1000# of force on the leading edge of the bridge in 2MPH creek, if the water speed doubles to 4mph, the force goes up to something like 4000#. And if it goes up to 8mph, you're looking at something like 16,000# of force on just the leading edge of that bridge.

 

[4570Man]

I used to work at a man-made whitewater course and was trained in whitewater rescue. This was 30 years ago, so forgive me if I'm mistaken.... The power behind moving water can be a real living nightmare. The pressure exerted by moving water increases with the square of its velocity. Water weighs about 62# per cubic foot. A 30' telephone pole averaging 6" diameter is the equivalent of about 15 square feet of surface area. Its rounded, so the calculations are beyond me. Anyhow, you got about 1000# of force on that pole at a low flow of just a couple miles per hour. So if you have 1000# of force on the leading edge of the bridge in 2MPH creek, if the water speed doubles to 4mph, the force goes up to something like 4000#. And if it goes up to 8mph, you're looking at something like 16,000# of force on just the leading edge of that bridge.

The telephone poles I have seen are more 10-12 inches at the dirt line.

 

[MossRoad]

Mine are about 10" at the butt end and maybe 6" at the tip. It was just an example. So, larger pole = more surface area and even more force.

On that whitewater course, the first year it opened, they let some guys take a metal canoe down it. They turned sideways on a cowcatcher under an old railroad bridge and the water promptly bent the canoe in half around the piling. The guy in the front was facing downstream sitting right next to the guy in back who was now facing upstream. They sat there looking at each other. Lucky they didn't get their feet locked into the bent metal and water didn't start going over their heads. Had to shut off the water (by raising dam gates) and pull the canoe off the piling with ropes and caribiners set up for 6:1 mechanical advantage.

 

[ArlyA]

I've been messing with bridges for something over 15 years now. These were as narrow as 6ft and one 14 wide. In lengths from 16 up to 42ft one we built this past summer which is the longest we have. Log pole structures is a design I can't wait to leave behind and forget, we have just one remaining. There terrible to work on, its a weak design and I'd refuse to use them again. Below I'll list some points I've learned over the years worked on the bridge we have. One was widened three times.....

Build them wider than needed, greater load capacity than imagined and higher out of the water than you'll ever see your creek get. Its just to much work to retrieve the structure from down stream and replant it. Its also to much work to make it higher out of the water the second time around.

And we'd never, ever installed a intermediate creek support. They don't work in high water with the debris, same as other posters have mention.

Its the same ol "do it right the 1st time"

 

[DrRod]

I hear you on the intermediate creek support. I know a guy who used a couple of large plastic barrels to support the middle of the bridge. The idea was to let it rise and fall with the water -- sort of like a floating dock. First good rain in spring and the barrels were gone.

And yes, I appreciate the force of water. I live on a cliff overlooking the old Main Street of Ellicott City, MD. We made the news at the end of July for a flash flood resulting from six inches of rain in an hour. There's lots of exciting video of cars being rapidly shoved down the street and into the river, sometimes in pairs. By last week they had towed 247 cars and had found another 30.

In any case, that's why I thought a flow-through design might mitigate the force of water against the structure. We used to have foot logs made of a single big log with a cable for a hand rail. They would last for at least a decade if placed against a tree on each side [no anchoring at all]. I figure that the water could flow over them and thus take the pressure off. But I'm paying attention to all the experience here and will post pictures of whatever we come up with.

 

[topbuilder]

The power of water. I did a job for a customer who had a culvert in a ravine to access his 40 acre "weekend ranch". He drove up one night to get a chainsaw to clean up storm damage at his regular house. When he got to his gate there was no road and no culvert. His culvert was 8' x 40' long