Find Member Forces of AB and DE: Cut Section Method

[fonseh]

Homework Statement

I am asked to find the member force of AB and DE

Homework Equations

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 ...

 

[PhanthomJay]

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.

 

[CivilSigma]

I agree with Jay,

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