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Offline sangyo soichiro

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Water bridge thought problem
« on: July 17, 2009, 11:51:03 AM »
I was emailed this and thought I'd share it.  It's a nice little physics problem.

This was the question in the email:

Question:
Did this bridge have to be designed to withstand the additional weight of ship and barge traffic,
or just the weight of the water?




Something to think about....    ;)  :)
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Offline Bob Wessner

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Re: Water bridge thought problem
« Reply #1 on: July 17, 2009, 12:01:18 PM »
Hmm, reminds me of a plane taking off on a conveyor belt.  ;D

P.S. I say both.
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Offline mlinder

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Re: Water bridge thought problem
« Reply #2 on: July 17, 2009, 12:06:24 PM »
depends on whether the boats were already in the body of water that is used in the water bridge. If so, the boats displaced the water by the amount of weight already.
Adding the boats later adds weight overall to the entire body of water.
So the bridge has to be designed for the overall weight of both water and ships. Not to mention the weight of the bridge itself :)
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Offline my78k

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Re: Water bridge thought problem
« Reply #3 on: July 17, 2009, 12:08:33 PM »
and all those people just wandering along...damn don't they have jobs?

Dennis
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Offline mlinder

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Re: Water bridge thought problem
« Reply #4 on: July 17, 2009, 12:09:24 PM »
I guess what I should say, is that the ship in the middle of the bridge ads no more weight than if it wasn't there.
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Offline Bob Wessner

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Re: Water bridge thought problem
« Reply #5 on: July 17, 2009, 12:17:31 PM »
Anyone know if the bridge is 'locked' at both ends? It would make a difference in the answer I think. If locked, it would have to be at both ends, duh.
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Offline mlinder

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Re: Water bridge thought problem
« Reply #6 on: July 17, 2009, 12:21:49 PM »
Quote from: Bob Wessner on July 17, 2009, 12:17:31 PM
Anyone know if the bridge is 'locked' at both ends? It would make a difference in the answer I think. If locked, it would have to be at both ends, duh.
If the boat was already between the locked ends, no matter how far apart the locked ends were, the weight would be the same. Even if the 'locked ends' were the ocean on one side, and a giant lake on the other.
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Offline UnCrash

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Re: Water bridge thought problem
« Reply #7 on: July 17, 2009, 12:49:25 PM »
There is no need to calculate the weight of the boats because they in fact weigh as much as the water they displaced.

All the engineers need to do is calculate mean water height for the "bridge" to derive the total load that the bridge needs to support regardless of how much boat traffic is on it.

Now if a boat sinks and no longer displaces water equal to its weight there's a problem.
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Offline Bob Wessner

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Re: Water bridge thought problem
« Reply #8 on: July 17, 2009, 12:52:04 PM »
Quote from: mlinder on July 17, 2009, 12:21:49 PM
Quote from: Bob Wessner on July 17, 2009, 12:17:31 PM
Anyone know if the bridge is 'locked' at both ends? It would make a difference in the answer I think. If locked, it would have to be at both ends, duh.
If the boat was already between the locked ends, no matter how far apart the locked ends were, the weight would be the same. Even if the 'locked ends' were the ocean on one side, and a giant lake on the other.

What if the lock positions were roughly the ends of the bridge? Wouldn't the displaced water still have to be accommodated by the structure?
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Offline Duke McDukiedook

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Re: Water bridge thought problem
« Reply #9 on: July 17, 2009, 12:56:28 PM »
I think the boats would be considered live loads just like traffic on a regular bridge.

But like you said the boats would just displace their equal buoyant weight in water.

This is why I'm not a structural engineer.

Good thought question.
« Last Edit: July 17, 2009, 01:12:02 PM by Dukiedook »
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Offline mlinder

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Re: Water bridge thought problem
« Reply #10 on: July 17, 2009, 01:05:01 PM »
Quote from: Bob Wessner on July 17, 2009, 12:52:04 PM
Quote from: mlinder on July 17, 2009, 12:21:49 PM
Quote from: Bob Wessner on July 17, 2009, 12:17:31 PM
Anyone know if the bridge is 'locked' at both ends? It would make a difference in the answer I think. If locked, it would have to be at both ends, duh.
If the boat was already between the locked ends, no matter how far apart the locked ends were, the weight would be the same. Even if the 'locked ends' were the ocean on one side, and a giant lake on the other.

What if the lock positions were roughly the ends of the bridge? Wouldn't the displaced water still have to be accommodated by the structure?

Well, I figure, the lock would have to open to allow the boat in. Means the lock is open. As the boat moves in, the difference between the volume of water that the boat replaces would move outisde the lock as the boat moved in, so no, no difference.
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Offline Bob Wessner

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Re: Water bridge thought problem
« Reply #11 on: July 17, 2009, 01:25:46 PM »
Good point.
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Offline medic09

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Re: Water bridge thought problem
« Reply #12 on: July 17, 2009, 02:03:17 PM »
Of course, I wanted to know if it really exists.
http://www.snopes.com/photos/architecture/waterbridge.asp

Pretty cool!
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Offline tramp

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Re: Water bridge thought problem
« Reply #13 on: July 17, 2009, 04:08:51 PM »
boats are weighed by the amount of water they displaced
so i would say the bridge would just have to support the water
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Offline mlinder

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Re: Water bridge thought problem
« Reply #14 on: July 17, 2009, 04:19:28 PM »
Quote from: tramp on July 17, 2009, 04:08:51 PM
boats are weighed by the amount of water they displaced
so i would say the bridge would just have to support the water

That's true, but the overall weight of the water in the 'container' increases as much as the boat weighs. It's a matter of percentages. A boat that weighs 600 tons in 6 million tons of water still makes the whole bit weigh 6,000,600 tons. It's just that the amount of weight is spread out all over the entire contact area of water to container, not just directly below it. I don't know the speed of that equalization, and obviously, it's not instantaneous, so a boat moving through that waterway will cause more weight at that moment, where it's located, but it does eventually equalize out.
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Offline lynchj

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Re: Water bridge thought problem
« Reply #15 on: July 17, 2009, 04:23:17 PM »
Quote from: tramp on July 17, 2009, 04:08:51 PM
boats are weighed by the amount of water they displaced
so i would say the bridge would just have to support the water

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