Water bridge (engineering question?)

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DanielCopelin
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Water bridge (engineering question?)

Postby DanielCopelin » Thu Oct 30, 2008 1:32 pm UTC

I did a bit of a search and couldn't find any answers to this question here or elsewhere. Also, I know I'm technically not allowed to post links this early, but it's a photo relevant to my question and my post wouldn't make sense without it. I hope that in the interests of science my link is permitted.

A while back I saw this photo:
http://ewordpress.files.wordpress.com/2008/05/water_bridge_in_germany.jpg

The website had a bit of a description of the bridge and, among other things, listed a weight limit for boats wishing to cross the bridge.

My first thoughts were:

1. Whoa! There is a river, and there is another river on top of it!
2. Awesome.
and most importantly, 3. Hold on, why is there a weight limit?

As I see it, the bridge would be designed to cater for the dead load of its own structure, the dead load of the water (a function of its depth), and some live loads (pedestrians, wind, ice, earthquakes, that sort of thing). However, I can't see where the weight of a boat crossing the bridge would come into it.

The weight of the boat is "carried" by the displacement of the water surrounding it. In a real-world river scenario, the displaced water is allowed to spread over a large length of the river and will result in a negligible change in water depth of the bridge. Hence, the actual load carried by the bridge does not change.

Is there a fatal flaw in my logic somewhere, or can the weight limit only be described by some other (non-load related) operational constraint?

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jaap
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Re: Water bridge (engineering question?)

Postby jaap » Thu Oct 30, 2008 2:47 pm UTC

DanielCopelin wrote:The website had a bit of a description of the bridge and, among other things, listed a weight limit for boats wishing to cross the bridge.


I don't see any weight limits on the page. The only weight listed is the amount of steel used to build the tub that contains the water.

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MotorToad
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Re: Water bridge (engineering question?)

Postby MotorToad » Thu Oct 30, 2008 3:08 pm UTC

That is really neat. If nothing else just because it's something I never really expected to see.

You're right about the weight displacement, and I can't think of a good reason to have a weight limit if there is one. Size, speed, and depth are all obvious since it has to fit, but there might possibly maybe be a weight limit set for how much the sides can take in an accidental bump at max speed for the canal. That's the only reason I could think for it, anyway.
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alterant
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Re: Water bridge (engineering question?)

Postby alterant » Thu Oct 30, 2008 4:31 pm UTC

My guess is that the bridge is not like a river, in the sense that the bridge has a finite length & finite volume of water. I would imagine a system of locks would move boats on and off of it. If it's a finite length, then there should be a weight limit (because the water level will go up with more boats, not stay constant).

And yes... very cool.

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Red Hal
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Re: Water bridge (engineering question?)

Postby Red Hal » Thu Oct 30, 2008 4:45 pm UTC

To the OP, you are correct, there should be no weight limit, except perhaps the weight of water "on" the bridge at any one time, to go above that, you would need a vessel longer than the bridge, which could lead to some interesting flows of water.

To the rest of you, it's just an aqueduct! If you want to see cool, try the Falkirk wheel. Now there's a good example of the Archimedes principle being put to good use.

1200 tonnes of steel and 600 tonnes of water, moved with 1.5kw of motor!

http://www.thefalkirkwheel.co.uk/about/engineering.html
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Mr_Rose
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Re: Water bridge (engineering question?)

Postby Mr_Rose » Thu Oct 30, 2008 4:53 pm UTC

Red Hal wrote:To the OP, you are correct, there should be no weight limit, except perhaps the weight of water "on" the bridge at any one time, to go above that, you would need a vessel longer than the bridge, which could lead to some interesting flows of water.

To the rest of you, it's just an aqueduct! If you want to see cool, try the Falkirk wheel. Now there's a good example of the Archimedes principle being put to good use.

1200 tonnes of steel and 600 tonnes of water, moved with 1.5kw of motor!

http://www.thefalkirkwheel.co.uk/about/engineering.html

Dammit! I was just going to post that!

Well, here's the Wiki article about it anyway...
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alterant
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Re: Water bridge (engineering question?)

Postby alterant » Thu Oct 30, 2008 5:59 pm UTC

Oh, I see. In order to keep the water level constant, the excess water displaced by the boats drains out. That makes sense, except of course that water has to be pumped back in...
Neat.

DanielCopelin
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Re: Water bridge (engineering question?)

Postby DanielCopelin » Thu Oct 30, 2008 11:33 pm UTC

jaap wrote:
DanielCopelin wrote:The website had a bit of a description of the bridge and, among other things, listed a weight limit for boats wishing to cross the bridge.


I don't see any weight limits on the page. The only weight listed is the amount of steel used to build the tub that contains the water.

It wasn't that exact page that I first read a while ago. I couldn't find the same one again.

alterant wrote:My guess is that the bridge is not like a river, in the sense that the bridge has a finite length & finite volume of water. I would imagine a system of locks would move boats on and off of it. If it's a finite length, then there should be a weight limit (because the water level will go up with more boats, not stay constant).

And yes... very cool.

I was thinking about this also. Say the finite length of the river was equal to the length of the bridge. It seems obvious that the full weight of the boat will be carried by the bridge, but can someone explain to me what is the mechanism for this load transfer? I.e. what is the load path?

Is it explained by the increase in depth leading to an increase in the hydrostatic pressure at the bottom of the water?

Red Hal wrote:To the OP, you are correct, there should be no weight limit, except perhaps the weight of water "on" the bridge at any one time, to go above that, you would need a vessel longer than the bridge, which could lead to some interesting flows of water.

To the rest of you, it's just an aqueduct! If you want to see cool, try the Falkirk wheel. Now there's a good example of the Archimedes principle being put to good use.

1200 tonnes of steel and 600 tonnes of water, moved with 1.5kw of motor!

http://www.thefalkirkwheel.co.uk/about/engineering.html

Yeah, that thing is sweet.

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smw543
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Re: Water bridge (engineering question?)

Postby smw543 » Sat Nov 01, 2008 7:51 pm UTC

According to the Wikipedia page, there are indeed locks at both ends, so that answers the initial question.

Concerning the more theoretical follow-up, I think a full answer would require application of fluid dynamics, which is an area I'm weak in (I tend more towards the "less practical" fields like magnetism and atomic physics), but I would say that it has a lot to do with that your thinking involves an essentially labratory-like situation. In reality, there are issues like surface tension, friction, and (I can't think of the technical terms, but there are a couple other more specific ones.) Try posting this in the logic puzzles thread, although some purists may ask that you rephrase it in a more "puzzling" form.

Also, in the realm of real life factors, there is the fact that a heavier ship will create more wake/stronger forces on the sides of the bridge. This sort of ties in with what I clumsily said about surface tension; water doesn't flow and conform perfectly and instantly. To give an example, some sea mines used in WWII were placed at the ocean floor (or floated in really deep areas) and were pressure-sensitive - they didn't require a ship to physically touch them to be set off. They could actually detect the pressure difference if there was a ship directly above them, even if it wasn't moving. In other words, the influence was enough that a mine quite a ways down could detect a ship in the ocean, where displacement is, of course, much easier and faster than in a more constricted body such as a canal.
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Re: Water bridge (engineering question?)

Postby Dobblesworth » Sun Nov 02, 2008 3:07 am UTC

Viaducts have carried canals in bridges over gaps in topography since the Victorian era, and as mentioned, aqueducts did the same in Roman times. All we're seeing here is a much wider canal, and probably deeper. The supports at the base of the bridge look more significant so it all looks fairly rational to me. It's too late/early/3am for me to pull out my first year engineering free body diagram routines for structural mechanics.


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