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Homework Help: Truss Free Body Diagram & initial conditions

  1. Dec 1, 2015 #1
    1. The problem statement, all variables and given/known data
    Complete a free body diagram for the truss structure with a weight attached to node 2 (this is known as 151N) photo (23).JPG

    2. Relevant equations

    3. The attempt at a solution
    I have attached my Free Body Diagram
    The diagram is to display what is know at the start of the problem before working out the forces, stresses etc..

    Obv at the end of the problem everything is know but this is for initial conditions

    The part of the diagram I'm unsure of is Node 5 & Node 3. This Is what i'm thinking

    I need to indicate the known / unknown for force & displacement for the nodes.

    U1(5)=? F1(5) = 0
    U2(5)=? F2(5)=-151.9

    U1(3) =? F1(3)=0
    U2(3)= ? F2(3)=0

    Bit confused for each node Force (F) & Displacement (U) in the x,y axis
    I know you can only have 1 unknown for each axis

    Attached my drawing
  2. jcsd
  3. Dec 1, 2015 #2


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    You can't know the displacements of the internal nodes of the structure until you've solved for them. Otherwise, analysis would be pointless.

    As far as your free body diagram is concerned, you must show the loads applied to the structure and the reaction forces (and moments, if necessary) at the supports. Just copying a diagrammatic representation of the supports is insufficient. Are nodes 1, 4, and 6 supposed to be fixed, pinned, or what?
  4. Dec 2, 2015 #3
    Thoes nodes are pinned yes

    How can I show reaction forces when it hadn't been analysed yet?
  5. Dec 2, 2015 #4


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    You know that pinned connections can support only forces.

    A fully fixed connection can support a force and a moment. You can't show any magnitudes yet, because these haven't been determined.

    At this stage of the analysis, the free-body diagram is just a sketch to show what you're looking for, in terms of reactions at the supports.
  6. Dec 2, 2015 #5
    So I shouldn't put anything sown for Nodes 5 & 3 because both force & displacements are unknown & stating this is not correct as you can only have one unknown?
  7. Dec 2, 2015 #6


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    A free body diagram needs to show:
    1. The layout of the structure.
    2. The location of any applied forces and moments.
    3. The location of the reacting forces and moments which make the structure a free body.

    All of the internal forces, moments, deflections, etc., and the reactions to the applied loads are determined after the analysis is completed. The applied forces and moments are necessarily known beforehand, so their magnitudes may be shown on the FBD.

    It's not clear what you mean by 'you can have only one unknown'. In this particular structure, you have many unknowns which must be obtained from the analysis.
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