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Question on Statics -- San Francisco Bay Bridge Bolts

  1. Sep 2, 2015 #1
    1. The problem statement, all variables and given/known data
    http://cisephysics.homestead.com/files/NewSanFranciscoBridge.pdf

    My question is on this problem. For some reason, I have trouble seeing which direction the force is pushing when it comes to statics. In this case, why is the column pushing up? I thought it would be going down because of gravity. There have been similar problems to this in my physics book, where I just don't get it. I'm fine on solving once I have the direction.

    One that I can think of off the top of my head was with a tether ball. It showed the ball pushing against the pole, rather than into it. The diagram would look something like this below:


    o|

    Where the o is the ball
    and the | is the pole

    Then the vector for the x-direction is going to the left, but intuitively I'd imagine it's force pressing into the pole.

    Any help would be appreciated.
     
  2. jcsd
  3. Sep 2, 2015 #2

    SteamKing

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    The diagram is somewhat misleading. It's not so much that the column is pushing up, it's that the cables holding up the bridge span are pushing the column down. Because the column can't move down, an equal and opposite reaction force is created in the footings of the column.

    I think a better way to visualize what is happening is to suppose that the head of the force vector in the column was placed instead where the two force vectors T1 and T2 come together. In other words, the column is in compression.
     
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