Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Which ones are zero force members?

  1. Feb 25, 2010 #1
    Any help please.
    Picture in attachments

    Attached Files:

    Last edited: Feb 25, 2010
  2. jcsd
  3. Feb 25, 2010 #2
    Are we supposed to do the work for you?
  4. Feb 25, 2010 #3
    Haha. No. I have to find the load in each member. I know some of those members are probably zero force members but which ones?
  5. Feb 25, 2010 #4
    LN is a zero force member?
  6. Feb 25, 2010 #5
    Look again at joint L

    If LN is redundant then so is LC.

    What makes you think this framework is redundant?
    What do you know about the conditions for static indeterminancy?
  7. Feb 25, 2010 #6
    The members that are perpendicular to the slope are zero force members, because the pin joints must be in equilibrium and zero moments
  8. Feb 25, 2010 #7
    Do you mean like LC?

    Look again at joint L
  9. Feb 25, 2010 #8
    No member LC is not a zero force member

    The members that are zero force members are: BM, DN, FO, and HJ; because at pin joints B, D, F, & H these members cannot have load because there would be no way of obtaining force equilibrium at these pin joints.
  10. Feb 25, 2010 #9
    Okay. I will double check with my teacher. Thanks
  11. Feb 25, 2010 #10


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That would be the case only if there were no load applied at B, D, F, & H. Since there is a load applied at these joints, these are not zero force members.
  12. Feb 26, 2010 #11
    You are correct Jay I didn't notice the external loads at Joints B & D; Then, the only zero force members are FO and HJ. Thanks Jay
  13. Feb 26, 2010 #12
    I did pass comment on your last effort, which you seem to have ignored. The same comment applies again.

    If the force in HJ is zero then, by vertical equilibrium of joint J, GJ is also zero.

    I ask again why do you think there are any redundant members.

    and again

    What do you know about the conditions for static determinancy/ indeterminancy?
  14. Feb 26, 2010 #13
    I did asked my teacher and he said that there is no zero forces on this structure
  15. Feb 26, 2010 #14
    Kris I was referring to Gannet.

    I hope your teacher also told you the formula for a perfect frame (statically determinate, no redundant members) - which this is

    m = 2j -3

    Where m = number of members
    j = number of joints.

    You can work out for yourself that this frame conforms.

    I leave the task of calcualting the member forces to you - thre are many methods available.
  16. Feb 26, 2010 #15


    User Avatar
    Science Advisor
    Homework Helper

    It currently appears the zero-force members are EO, FO, GJ, GK, GO, HJ, and KO.
  17. Feb 26, 2010 #16
    This false statement helps the student in precisely what way?
  18. Feb 26, 2010 #17
    Probably he just don't want us to include the zero force members.
  19. Feb 26, 2010 #18
    Because of the question, I was assuming that Kris was just starting "Theory of Structures" and was using Method of Joints for analyzing Statically Determinant Truss Structures. If Kris is analyzing Statically Indeterminant Truss Structures then he would be using Deflection Methods.

    Kris, I am sorry that I mislead you because I missed the two external loads at Joints B and D, the sketch is really light.
  20. Feb 26, 2010 #19
    Certainly an idea of what you are studying and at what level would be useful to pitch responses and explanations to.
  21. Feb 27, 2010 #20


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    So light, in fact, that the loadings on the right side of the frame appear to be missing. If the frame is symmetrically loaded, there are no zero force members; if the frame is loaded only on the side left of center, then there are many zero force members (see nvm's response). It appears there is a 24 unit force reaction at the left support, implying that the frame is symmetrically loaded,and that the applied joint loads on the right side of the frame are missing.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook