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Homework Help: Simple statics question

  1. Jan 17, 2015 #1
    1. The problem statement, all variables and given/known data
    There isn't really a problem statement. This is for something I am myself designing but I'm struggling conceptually with the basic statics analysis. It's a simple square truss. If I were to write a problem statement it would be "What are the reaction forces at A and B?"

    My problem is, I feel like there should be a moment reaction at B. Am I simplifying this too much? Thank you in advance for any help

    2. Relevant equations
    2nd law:
    ∑F=0, ∑M = 0

    3. The attempt at a solution

  2. jcsd
  3. Jan 17, 2015 #2


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    Please clarify. Is it a rigid weightless square, attached at (not merely sitting upon) supports A and B? And the only applied force is F1?
    If so, your equations are OK, but you need to take moments too. What point you take them about is not important - just make it somewhere convenient.
  4. Jan 17, 2015 #3
    The square is not rigid, I think that is the fact that is confusing me.

    The points A,B,C,D are not pinned, they are fixed via some kind of joint (to represent a more complex joint). So A - D can fail given some moment or max force, and I want to analyze the system to ensure that those values will not reach the point of failure.
  5. Jan 17, 2015 #4


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    I guess you mean the joints do not freely rotate but they can flex in response to load, yes? So there is some stiffness factor, which needs to be known?
    What about the rods themselves? Are they to be rigid or flexible to some degree?
    If the lower corners are not pinned to their supports, the only force preventing the frame from rotating is its own weight, but you do not show that on the diagram.
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