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Thread: Pressure on Bridge

  1. #1 Pressure on Bridge 
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    Recently, driviong under a canal the following question raised its head!
    The downwards pressure on a bridge under a canal - does it increase when a ship passes above the bridge (Forget ships vibrations)?


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  3. #2  
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    Quote Originally Posted by boblalux View Post
    Recently, driviong under a canal the following question raised its head!
    The downwards pressure on a bridge under a canal - does it increase when a ship passes above the bridge (Forget ships vibrations)?
    No. A ship displaces its own weight in water. So all that happens as the ship comes along is that a certain weight of water is replaced by an identical weight of ship and then reverts to an identical weight of water again.


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  4. #3  
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    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by boblalux View Post
    Recently, driviong under a canal the following question raised its head!
    The downwards pressure on a bridge under a canal - does it increase when a ship passes above the bridge (Forget ships vibra(eg)tions)?
    No. A ship displaces its own weight in water. So all that happens as the ship comes along is that a certain weight of water is replaced by an identical weight of ship and then reverts to an identical weight of water again.
    Thanks. Presumably should the ship be between locks, the water-level may rise by (eg) 1mm, which would increase the pressure by a small increment?
    Bob
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  5. #4  
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    Quote Originally Posted by boblalux View Post
    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by boblalux View Post
    Recently, driviong under a canal the following question raised its head!
    The downwards pressure on a bridge under a canal - does it increase when a ship passes above the bridge (Forget ships vibra(eg)tions)?
    No. A ship displaces its own weight in water. So all that happens as the ship comes along is that a certain weight of water is replaced by an identical weight of ship and then reverts to an identical weight of water again.
    Thanks. Presumably should the ship be between locks, the water-level may rise by (eg) 1mm, which would increase the pressure by a small increment?
    Bob
    The pressure would only increase if the water displacement had nowhere else to go other than rise, in other words contained. An example of this would be a bath tub as a person gets in the level rises as the water is displaced, the overall mass increases within the bath tub.
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  6. #5  
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    Quote Originally Posted by boblalux View Post
    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by boblalux View Post
    Recently, driviong under a canal the following question raised its head!
    The downwards pressure on a bridge under a canal - does it increase when a ship passes above the bridge (Forget ships vibra(eg)tions)?
    No. A ship displaces its own weight in water. So all that happens as the ship comes along is that a certain weight of water is replaced by an identical weight of ship and then reverts to an identical weight of water again.


    Thanks. Presumably should the ship be between locks, the water-level may rise by (eg) 1mm, which would increase the pressure by a small increment?
    Bob
    No I don't see why the water would rise in that case either. The displacement occurs as the ship enters the lock. As it leaves, it pushes water past itself to maintain the displacement and this continues as it makes its journey along the canal. You can in fact see this if you watch a vessel move through a waterway that is quite small compared to its size, such as narrow canal.
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  7. #6  
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    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by boblalux View Post
    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by boblalux View Post
    Recently, driviong under a canal the following question raised its head!
    The downwards pressure on a bridge under a canal - does it increase when a ship passes above the bridge (Forget ships vibra(eg)tions)?
    No. A ship displaces its own weight in water. So all that happens as the ship comes along is that a certain weight of water is replaced by an identical weight of ship and then reverts to an identical weight of water again.


    Thanks. Presumably should the ship be between locks, the water-level may rise by (eg) 1mm, which would increase the pressure by a small increment?
    Bob
    No I don't see why the water would rise in that case either. The displacement occurs as the ship enters the lock. As it leaves, it pushes water past itself to maintain the displacement and this continues as it makes its journey along the canal. You can in fact see this if you watch a vessel move through a waterway that is quite small compared to its size, such as narrow canal.
    Exactly! You would have to fill the lock before the boat entered, lift the boat out of the canal and straight into the lock, ergo the bath tub analogy.
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