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Homework Help: Statics problem(simple beam/three supports/one moment)

  1. Apr 20, 2016 #1
    1. The problem statement, all variables and given/known data
    I have to solve for Ay, By, and Cy reaction forces. The constant C is a couple acting at A in a clockwise direction

    2. Relevant equations
    Sum of all forces = 0
    Sum of Moments about any point = 0

    3. The attempt at a solution
    I solved for all four reaction equations(one force sum and three moment equations at a,b, and c) These are the four equations I calculated:

    Ay + By + Cy = 0
    36(By) + 3(36)(Cy) = C
    2(36)(Cy) - 36(Ay) = C
    -3(36)(Ay) -2(36)(By) = C

    But no matter how I try to solve the system of equations I always get 0 = 0 or a singular matrix. I don't know if the problem is that I messed up with one of the moment equations or if there is something else I need to do. Any help would be very much appreciated, thanks!!!
  2. jcsd
  3. Apr 20, 2016 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    This beam is statically indeterminate. You can write only two equations of static equilibrium, but you have 3 unknown reactions to solve for.

    You'll have to find a third equation to use which doesn't involve the sum of the forces or the sum of the moments.

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