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Truss calculation

  1. May 19, 2012 #1
    I have solved a number of example questions referring to Trusses. I have one where the forces do not balance correctly when I check through.

    A difference with this truss is that there is a couple applied to the truss.

    I have been using the method of Joints to do the working out.

    Does the presence of a net couple mean that I have to do something different?
    Can anyone point me to an example where the calculations are done on a Truss with a couple applied to it?
     
  2. jcsd
  3. May 20, 2012 #2

    PhanthomJay

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    A pure truss consists of 2-force members which carry axial compression or tensile forces and stresses, no bending stresses or shear forces. When a couple is applied, the truss is no longer a pure truss, and it becomes a frame or frame-truss combo, where at least one of the members carries shear load and moment and associated stresses. The frame has to be analyzed differently, often still statically determinate using all 3 equilibrium equations.
     
  4. May 21, 2012 #3
    Mmm, I thought that might be to the problem. The three equilibrium equations being sum Fx, Fy and Moments?
    The issue I have is that I do not know how to include the couple. Can anyone point me to an example?
     
  5. May 21, 2012 #4

    PhanthomJay

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    Can you attach a sketch of your particular question showing where the couple is applied and what the frame looks like and how it is supported?
     
  6. May 21, 2012 #5
    Here is a truss that I have sketched. The question is to resolve all forces and the reactions at the supports, one of which is pinned.
    I have had to export the svg as a non-vector version for this forum.

    The three lengths at the bottom are all 3m, i.e. 9m in all along the bottom with all internal angles as you would expect from the two given, the couple being formed by the horizontal component of the reaction at the pinned support and the 2K sideways force.
     

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    Last edited: May 21, 2012
  7. May 21, 2012 #6
    BTW, that last post was me as I set up two accounts after Chrome related issues with registering on this site.
     
  8. May 21, 2012 #7

    PhanthomJay

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    Oh, when you said a couple was applied, I was envisioning a pure couple and not a force couple consisting of equal and opposite forces a perpendicular distance 'd' apart. Since this is the case here, and all external forces are at joints, this problem is no different than other truss problems, the truss is 'pure' with axial member forces only. So you might want to show your work so we can see what the problem is that you are having.
     
  9. May 21, 2012 #8
    Ah, Thanks, you have given me more confidenec and looking at the diagram, the middle distance can't be 3. I wonder if the question has an error, I will work it through with three and 2.3 (calculated middle brace length) and see if the results come out correctly.
    I will post some working.
     
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