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Homework Help: Truss Analysis

  1. Apr 23, 2013 #1
    Attached is a picture of the truss. I am trying to calculate the member forces.

    I know a moment is = force * perpedicular distance, but in this case, for example, if i am taking moments about A, Would the equation be

    -(1*2KN)+(1*c) = 0

    or using an alternative perpedicular distance

    -(2.4*2KN)+(0.8*c) = 0

    For my preliminary working, assuming i have used correct perp distance i have

    ƩFx = 0
    ƩFy = 0
    ƩMoments about a point = 0

    Ay + By - 2KN = 0

    Momments about A = -(2KN * 2.4) + 1*ReactionC = 0
    -4.8KN = reactionC

    Sub into first eq

    Ay +(-4.8) -2 = 0
    Ay = 6.8

    To find out the reaction in member AB

    I have

    6.8 - Tab Cos(tan-10.8/1) = 0
    Tab = 6.8/Cos(tan-10.8/1) = 8.738

    I have the answer to be 2.56

    Attached Files:

    Last edited: Apr 23, 2013
  2. jcsd
  3. Apr 23, 2013 #2


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    In taking moments, the 2kN force has the same direction as the vertical distance on 1m, so its moment will be 0. If you get confused, you can always calculate M = r x F, where x is the cross product operator.
  4. Apr 23, 2013 #3
    Im dont quite understand sorry, i would understand if the force was acting through the point i am taking momoments about but how can the 2KN force x distance away not cause a moment
  5. Apr 23, 2013 #4


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  6. Apr 23, 2013 #5
    Using the value of of 4.8 but the same working i end up with a final equation of

    2.8/cos(38.66) = 3.59
  7. Apr 23, 2013 #6
    Ok i realise where my confusion was on this problem thanks for your help
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