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Beam with uniformly distributed springs

  1. Oct 3, 2013 #1
    1. The problem statement, all variables and given/known data
    Suppose you have a beam with a transverse load P pointing down in the center, directly over the middle "spring". This beam is supported by 9 uniformly distributed springs of constant k. Compute the loads of the springs.


    2. Relevant equations
    You know that the problem is symmetric, and that the ends of the beam go off to infinity. You, thus, do not know the displacement of the endpoints. You know the spacing of the springs. This beam is assumed to have no mass.


    3. The attempt at a solution

    Basically, I have been racking my brain over how to set up the system of equations.

    Since the problem is symmetric, I know that there are only 5 unknowns (spring loads 1 2 3 4 = springs 9 8 7 6, respectively)

    I know that, since the beam is symmetric, the slope in the middle is 0. If I deconstruct the beam into halves, I can combine the loads of 4 of the springs to an initial shear force where the load is being applied, as well as an initial moment.

    So, equation 1: slope in the middle is 0.

    I need 4 more equations, but I do not know where to retrieve them from. In this case, there are no points where the slope is 0, or where the displacement is 0. I cannot find any points where the shear is 0, or where the moment is 0.

    How do I find further boundary conditions to solve this?
     
  2. jcsd
  3. Oct 3, 2013 #2

    SteamKing

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    You should state the problem is a concise fashion, in accordance with the homework template. It is very hard to follow what is given in the way it is expressed above.

    With this admonition out of the way, it appears that what you have is a special case of an infinite beam on an elastic foundation.

    The following article discusses some approaches for solution:

    http://www.me.ust.hk/~meqpsun/Notes/Chapter4(202).PDF

    See the example on p. 12

    The solution to beams on elastic foundations is modified somewhat from the usual beam analysis. Read over the first part of the article for an illustration of the general procedure.

    You can always google 'beams on elastic foundations' for more articles.
     
  4. Oct 3, 2013 #3

    This is not a case of smearing springs. I already solved that problem. This is specifically a case of solving for the forces and displacements of the individual springs using classical methods that do not involve a continuous elastic foundation.

    Since I am unable to make custom pictures, there is only so much I can do to describe this. I will try again.

    Horizontal beam of length 8c, properties EI. Springs of constant k under the beam distributed at 0, c, 2c, 3c, 4c, 5c, 6c, 7c, 8c. Downwards point load located at 4c. The ends are free. I need to solve this classically without superposition or without an elastic foundation.
     
  5. Oct 3, 2013 #4

    SteamKing

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    Your original post said, "You know that the problem is symmetric, and that the ends of the beam go off to infinity. You, thus, do not know the displacement of the endpoints. You know the spacing of the springs. This beam is assumed to have no mass." That's why I pointed you to the article I did. Beams of infinite length are treated differently from beams with finite length. Discrete springs are a limiting case for continuously distributed support.

    Now, if you have a beam of finite length with a finite number of supports > 2, you've got a continuous beam. There are several different approaches you can take to solve for the reactions. This article illustrates the Three-Moment Theorem with a beam on spring suppports:

    http://www.facweb.iitkgp.ernet.in/~baidurya/CE21004/online_lecture_notes/m2l13.pdf

    In regular continuous beam analysis, the deflection at the supports is taken to be zero. With spring supports, the reaction force is proportional to the deflection of the spring.
     
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