1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

[ME statics] Two force members

  1. Oct 21, 2014 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    Was trying to solve for the horizontal reaction at B. Why is BC a two-force member but not AE?
  2. jcsd
  3. Oct 21, 2014 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Who says AE is not a two-force member?
  4. Oct 21, 2014 #3
    Well, nobody but the solution says: CB is a two force member so the reaction at B only in the horizontal direction. Then the sum of the moments about A bla bla bla.
    How can they know that there is no reaction in the Y-direction and why isn't the same true for A?
  5. Oct 21, 2014 #4
    For a two force member, the forces are necessarily colinear and along the axis of the link. The link ADE supports shear and bending as well as the axial load, which is why it is not a two force member.
  6. Oct 21, 2014 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    I think that's why you have to work out the reactions. Remember, the two bars appear to be pin-connected at D, so what kind of forces and moments can be applied here?
  7. Oct 22, 2014 #6
    SteamKing: I agree that the members are pinned at D (that is the way I read the figure), and, by definition, a pin only transmits forces, no moments. Without working through the details, if we think about moments wrt A, it is evident that the load at E puts bending in the member ADE.

    On the other hand, since there is no applied load at C, and there are only pins at B and D, there cannot be any bending in the member BDC. No bending implies no shear, and BDC must be a two force member.
  8. Oct 22, 2014 #7
    I don't get that at all. It doesn't seem to make any intuitive sense to me. The force has a negative Y component so there should be a reaction force in the Y direction at D. That should create some moment on beam BC and also some reaction in it.... I don't see the two force member.
  9. Oct 22, 2014 #8
    @ Feodalherren: You have noted the negative component of the force applied at E and the fact that it must be reacted somewhere. But there is no reason it has to be reacted at D; it can be equally well reacted at A. If there were a vertical force component at D, then there would be a moment about B on the link CDB. There is nothing to react such a moment, so that moment cannot exist.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: [ME statics] Two force members