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!

The number of reactions in pinned support

  1. Jan 12, 2017 #1
    1. The problem statement, all variables and given/known data
    why the number of reaction is 3 ? How to know that ? shouldnt the total number of reaction = 8 ?

    2. Relevant equations


    3. The attempt at a solution
    There are 4 pinned support in the structure . Each pinned support can support vertical and horizontal reactions...
     

    Attached Files:

    • 523.jpg
      523.jpg
      File size:
      82.3 KB
      Views:
      25
    • 524.jpg
      524.jpg
      File size:
      41.4 KB
      Views:
      22
  2. jcsd
  3. Jan 12, 2017 #2

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    r is the number of external support reactions. Note that one is a pinned support and the other is a roller support.
     
  4. Jan 12, 2017 #3
    Where are the roller ? Can you point out ? ?? I didnt see any roller in the structure
     
  5. Jan 12, 2017 #4
    Where are the roller ? Can you point out ? ?? I didnt see any roller in the structure
     
  6. Jan 12, 2017 #5
    The roller is labelled 'B' , although I do agree that the difference is not that obvious...
     
  7. Jan 12, 2017 #6

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It's sort of a different way of showing it, but a is a pinned support fixed to the floor, and B is the roller support allowed to slide or roll across the floor. Look carefully at each and compare the difference.
     
  8. Jan 12, 2017 #7
    ok , i accept that the B is roller , then how about D ? Is it a roller or pinned support ? Why we dont have to consider the reactions at part D ?
     
  9. Jan 12, 2017 #8
    A is a pin support, at D there is nothing supporting it (restricting it from moving) so there are no reaction forces.
     
  10. Jan 12, 2017 #9
    How
    How do we know that ? There's something connecting 2 force members together at D , right ? So , it must be pinned or roller or other supporting parts , right ? Why you said there's no support there ?
     
  11. Jan 12, 2017 #10
    By support, I mean fixed to a medium like Earth. Here, the "cap" on point D is just to connect the truss members together. On a free body diagram, there won't be any external support reactions
     
  12. Jan 12, 2017 #11
    What do you mean ?
     
  13. Jan 12, 2017 #12
  14. Jan 12, 2017 #13
    ok , understand . But , how do you know that ? It's not stated in the question . And the author also doesnt stated that the B is a roller , how do we know that ?
     
  15. Jan 12, 2017 #14
    Again , there is nothing externally connected at point D , so no external reaction forces

    Usually, the author should state what is a pin/roller support...but it comes with practice.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted