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Homework Help: Reaction forces on simple 2 strut truss

  1. Apr 8, 2015 #1
    1. The problem statement, all variables and given/known data
    Calculate reaction forces R1x, R1y, R2x, R2y. Load of 100N. Node 1 and 2 are fixed. Distances listed on image.


    2. Relevant equations
    I have:
    ∑Fx = 0 = R1x + R2x
    ∑Fy = 100 = R1y + R2y

    ∑M2 = 0 = (-100 x 0.1) + R1 x 0.06 + R1y x 0.02
    ∑M1 = 0 = (-100 x 0.04) + (-R2x x 0.06) + (-R2y x 0.02)

    3. The attempt at a solution
    Using this equations I have tried to solve using matrices but I get a number error in Excel. Could someone please help clarify if I've made a mistake or why this can't be solved?

    Thank you
  2. jcsd
  3. Apr 8, 2015 #2


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    Perhaps too obvious to be the problem but I assume you mean...

    ∑M2 = 0 = (-100 x 0.1) + R1x x 0.06 + R1y x 0.02
  4. Apr 8, 2015 #3


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    For static equilibrium, you can write one force equation and one moment equation. You should pick either support #1 or support #2 as the reference for you moment equation and solve for the remaining unknowns.

    The OP said the connections at support Nos. 1 and 2 were "fixed", but your equations assume each is a pinned connection, which cannot develop a moment reaction.
  5. Apr 8, 2015 #4
    Thank you for your response SteamKing. You are correct that the supports should be fixed instead of pinned. How could I go about solving the 4 variables in this instance? You're saying I can write one force and one moment but then I have 4 unknowns and 2 equations don't I?
  6. Apr 8, 2015 #5
    Haha yes sorry it should be R1x
  7. Apr 8, 2015 #6


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    If the two supports are indeed fixed, then the struts are statically indeterminate, and the equilibrium equations alone are insufficient to determine the reactions. You have to develop additional equations based on the deflections of the members.
  8. Apr 8, 2015 #7
    I see, thank you for your response!
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