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Force in members of truss; section method

  1. Jan 1, 2012 #1
    1. The problem statement, all variables and given/known data
    295eq8w.jpg

    2. Relevant equations
    torque = force x distance


    3. The attempt at a solution
    35mihxs.jpg

    words: hopefully this one is clearer than the last one and someone can follow what I did and figure out where I went wrong. Thanks.

    Answer is Ffg=5kn T, Ffh = 20 kn T (only these two members are being questioned)
     
    Last edited: Jan 1, 2012
  2. jcsd
  3. Jan 2, 2012 #2

    vela

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    About what point are you calculating the moments when solving for FH? Your scan is kind of fuzzy.

    By the way, I don't get answers that match what you say they're supposed to be.

    Also, you might get more responses if you post this type of problem in the engineering section.
     
  4. Jan 2, 2012 #3
    I see. I calculated for the moment around E. (also double checked the answer in back of book and 5 and 20 are what it says)
     
    Last edited: Jan 2, 2012
  5. Jan 2, 2012 #4

    vela

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    You left out the moment due to GH.

    Try calculating the moments about G instead of E.
     
  6. Jan 3, 2012 #5
    that sounds like 2 equations with 2 unknowns, I'll work it
     
  7. Jan 3, 2012 #6
    Me = -28.8 = 1.8FH - 2.4GH

    = -16 = FH - 1.333GH

    So two unknowns there in calculating for Me. For Mg:

    Mg = -(1.8m)(Fh) + (2.4m)(10kN)
    = -1.8FH + 24
    FH = 13

    of course not knowing whether to assume tension or compression, in any case the Mg I calculated is obviously wrong
     
  8. Jan 3, 2012 #7

    vela

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    Why is it obviously wrong (other than you didn't specify the units)?
     
  9. Jan 3, 2012 #8
    Ffh = 20kN (tension) according to the back of the book... (Ffh = Force FH)
     
  10. Jan 3, 2012 #9

    vela

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    This is a straightforward problem, so I'm certain the book is wrong. I'll see what the other helpers say.
     
  11. Jan 3, 2012 #10
    an additional question; when using section method you aren't supposed to take into account forces operating on the other side of the section except for the initial calculation of forces holding up the entire truss?
     
  12. Jan 4, 2012 #11

    vela

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    Right. You start by looking at the structure as a whole to find the external reactions, but once you look at a section, you're only interested in the forces acting on that section.

    Another helper got the same results I found. Your result of FFH=13 kN is correct, though the member is under tension, and you should find FFG=3.3 kN, also under tension. The book's answers are wrong.
     
    Last edited: Jan 4, 2012
  13. Jan 4, 2012 #12
    I just wanted to say thanks I'm getting fairly good at these questions now.
     
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