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Homework Help: Need to solve a mechanical problem that's due on Wednesday

  1. Jun 3, 2013 #1
    1. The problem statement, all variables and given/known data
    The system is composed of 3 competing ( one after another) beams, like in the drawing. They are all from the same material, with the same admissible strain σ and the same elasticity module E.
    It is required to calculate the optimum configuration for the system, considering the main objective function to be the material's weight.

    I put the drawing in the attachment.

    2. Relevant equations
    Need to find:
    σ=? ( admissible strain)

    I don't need the scalar values.

    3. The attempt at a solution

    Nobody in my class knows how to solve this, our professor didn't want to help at all, even if he was asked several times. Everybody just gave up and nobody even knows where to start.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    Attached Files:

  2. jcsd
  3. Jun 3, 2013 #2


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    Gold Member

    Not sure i can solve it but can you write equations for the length of each member in terms of some angle, then sum those together to give a total length. That should be representative of the total material mass. Then find the minima for that equation. Then check that in that configuration the load on each member is less than some limit.

    Problem is the max load in one of the members might be exceeded in that configuration. My maths isn't good enough but I suspect that instead of solving for the minima of a curve you need to do something similar in multiple dimensions.
  4. Jun 3, 2013 #3
    The length isn't really an issue, it can be formulated using the angles. Or at least that's just an idea, but length is also dependent on the admissible tension, which I can't figure out. I don't think I even know what I'm saying anymore, this whole problem, when I think about it all at once, just confuses the hell out of me.

    At any rate, I got no idea what to do.
  5. Jun 3, 2013 #4
    If it's all in one plane (the plane of the paper) then you should be able to draw a range of scale force diagrams for the loaded node. In 2-d it is statically indeterminate, but in 3-d it is determinate. The diagrams could imply a way of minimising the weight (length).
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