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Homework Help: Cut section method

  1. Mar 30, 2017 #1
    1. The problem statement, all variables and given/known data
    I am asked to find the member force of AB and DE
    2. Relevant equations

    3. The attempt at a solution
    I make an imaginary cut of the question and consider the upper part of the cut . Bur , i didnt get the same ans provided . However , when i consider the lower section of the cut , i get the same ans with the book . Anything wrong with my working ?
    Taking moment about A = 20(4) +30cos45 (3) + DE(4) = 0 , DE = -35.9 upwards

    btw , the vertical reaction at a is 51.3 kN upwards , at E , vertical reaction = 35.9 kN upwards , and 21.2kN to the left ...

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  2. jcsd
  3. Mar 30, 2017 #2


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    Check your vertical reaction at A again.
    Edit: You said that you looked at the section below the cut, but actually, you looked at the entire truss and solved for the reaction force at E, not the force in member DE. When you look at the section below the cut, the applied forces do not show up, because they are applied above the cut. It is best to find member force DE by looking at the section above the cut, and find member force AB by looking at the section below the cut, after first correcting the reaction force at A.
    Last edited: Mar 31, 2017
  4. Mar 31, 2017 #3
    I agree with Jay,

    and when ever dealing with any structure (Determinant or not), it's better to solve for the reaction forces first.
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