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Truss analysis using method of joints

  1. Apr 8, 2012 #1
    The question is to calculate the forces ina ll members of the trussnby method of joints.

    First i have calculated the reaction forces for A and B by taking the moment from both these points.
    These came too: Ay=25N and By=25N

    I do not know where to go from here, what point should i analyse first, i tried analysing point H however i am not sure how i should go about this, is F_hj equal to zero. and would CH equal to the force of AC.

    Please help i am lost. i just need to know how to start, so maybe could you just help me solve two points and i can probably finish the rest. thank you for your time.
     

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    Last edited: Apr 8, 2012
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  3. Apr 8, 2012 #2

    PhanthomJay

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    This can't be right; the total applied forces add up to 120 N down, so the sum of the reaction forces must sum to 120 N up. You did your moment equation incorrectly.Try summing moments first about A and then about B
    yes
    If F_hj is zero, then what must F_ch be equal to?
    After correctly solving the reactions, then identify the zero force members if you can. Then proceed to the easiest joints first. The sum of forces at any joint must always be zero in the x and y directions.
     
  4. Apr 8, 2012 #3
    thanks for your quick response.
    so F_ch must equal zero as well.
    i redid my calculations for the reaction forces and i got Ay=65N and By=55N.
    Also could somebody give me an example on how to analyse one of the joints because i don't really know how to go about it.
     
    Last edited: Apr 8, 2012
  5. Apr 8, 2012 #4
    ok if F_hj does equal to zero, i think i have figured out how to continue, could someone please analyse joint c. would the sum of the forces in the x direction be F_cd + F_cj cos45 and the sum in the y direction be F_ch -F_cj sin45 - 65N ????
     
  6. Apr 8, 2012 #5
    thanks for your quick response.
    so F_ch must equal zero as well.
    i redid my calculations for the reaction forces and i got Ay=65N and By=55N.
    Also could somebody give me an example on how to analyse one of the joints because i don't really know how to go about it.
     
  7. Apr 9, 2012 #6
    ok if F_hj does equal to zero, i think i have figured out how to continue, could someone please analyse joint c. would the sum of the forces in the x direction be F_cd + F_cj cos45 and the sum in the y direction be F_ch -F_cj sin45 - 65N ????
     
  8. Apr 9, 2012 #7
    oopps sorry for the double posts, how do i delete them
     
  9. Apr 9, 2012 #8

    PhanthomJay

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    Yes, and those sums are equal to zero. Be sure to watch plus and minus signs and directions of forces. Remember that fch is 0.
     
  10. Apr 10, 2012 #9
    Thank you, I have figured it all out. However i am having trouble with the next part of the question which says to check EK, KL and KF using method of sections. When doing this do i only use the external forces?
     
  11. Apr 10, 2012 #10

    PhanthomJay

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    When using the method of sections, you must use all external forces and reactions and internal forces at the cut section as identified in your free body diagram of the left or right truss section . (The force in EK should be quickly evident).
     
    Last edited: Apr 10, 2012
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