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Axial force at X

  1. Nov 14, 2017 #1
    1. The problem statement, all variables and given/known data
    I have to find axial force at X for the following structure:

    https://i.imgur.com/8o01fkC.jpg

    3. The attempt at a solution
    These are my workings
    https://imgur.com/JfSfk9Z
    https://imgur.com/3HuDN4d

    I didn't work out moments because the question only asks for axial force, is that alright? I think I have gone wrong somewhere though, because I haven't used x=1.54m anywhere. Thanks!
     
  2. jcsd
  3. Nov 14, 2017 #2

    PhanthomJay

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    You went through more work than necessary, you could have isolated a section of the right hand part with a cut at x to find the axial force. Your answer looks numerically correct, but is the axial force compression or tension in the member?
     
  4. Nov 14, 2017 #3
    Which right hand part do you mean I could have isolated, from point X to C? It would be compression I think, so should be -5.08?
     
  5. Nov 14, 2017 #4

    PhanthomJay

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    Isolate from point x to D.
    Compression is correct, but why do you say that?
     
  6. Nov 14, 2017 #5
    I actually don't know, I've confused myself. At point B the axial force along BC is downwards, so axial force in X is opposite which means it should be tension doesn't it?

    I put the answer 5.08 into the online test that I had to do and I got it wrong...
     
  7. Nov 14, 2017 #6

    PhanthomJay

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    when you isolated the section AB, the vertical force at B acts downward on AB. (Don't rotate the members as you have done, this confuses the issue). Anyway, if the vertical force at B acts downward on AB, then what does Newton's third law tell you about the direction of the vertical force at B on BC?
    the problem statement tells you that a positive value for the axial force in BC indicates tension, implying that compression is entered as a negative value.
     
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