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[PLEASE HELP ] Forces in members of a truss

  1. Jul 2, 2008 #1
    [PLEASE HELP!!] Forces in members of a truss

    1. The problem statement, all variables and given/known data

    A barrel vault truss is loaded as shown. Knowing that the length of the bottom chords DF, FH, ..., and QS is 3 ft, determine the force is members IK, JL, and JM.


    2. Relevant equations

    Sum of y-components
    [tex]\sum F_{x} = 0[/tex]

    [tex]\sum F_{y} = 0[/tex]

    [tex]\sum M = 0[/tex]

    3. The attempt at a solution

    First I did a free body diagram of the entire truss, which gave me three unknowns tosolve for first.


    [tex]\sum F_{x} = 0:[/tex]
    [tex]A_{x} = 0[/tex]

    [tex]\sum F_{y} = 0:[/tex]
    [tex]A_{x} = (1+1.5+1.5+1+1.5+1.5+1) = A_{y} + V_{y}[/tex]
    [tex] 9 kips = A_{y} + V_{y} [/tex] ---- (1)

    Solving for Moment about A
    [tex]\sum M_{a} = 0:[/tex]
    [tex]V_{y} (18) = (1.5)(3) + (1.5)(6) + (1)(9) + (1.5)(12) + (1.5)(15) + (1)(18) =[/tex]
    [tex]V_{y} = 4.5 kips [/tex] ----- (2)

    Then plugging equation (2) into equation (1) I get
    [tex]A_{y} = 4.5[/tex]

    Then I get stuck. I know I need to pass a section through the truss so I can get two portions, with each including the desired members I am solving for. Then I just draw a FBD and solve for no more than three unknowns.
    Last edited: Jul 2, 2008
  2. jcsd
  3. Jul 3, 2008 #2


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    Re: [PLEASE HELP!!] Forces in members of a truss

    You can get IK pretty quick using the method of joints. Beyond that, I don't like that roller support at the bottom corner. Nonetheless, try then cutting a section from IK through JM.
    Last edited: Jul 3, 2008
  4. Jul 3, 2008 #3
    Re: [PLEASE HELP!!] Forces in members of a truss

    have you considered this.. do you know the reference angle?(i know it, i think you know it too)
    Once you figure out IK, and you cut through as PhanthomJay says, you now have 3 unknowns and 3 equations..in other words, solvable.

    Attached Files:

    • IK.JPG
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  5. Mar 29, 2010 #4
    Re: [PLEASE HELP!!] Forces in members of a truss

    help... anyone can help to settle this problem????
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