# Simple truss question I can't seem to get right

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1. Jul 2, 2016

### nothingsus

1. The problem statement, all variables and given/known data

ignore all the other writing!

3. The attempt at a solution
I can't tell where I'm making a mistake. Probably a massive oversight somewhere but I don't know where!

First find the reaction forces at the pin and roller by treating everything as a rigid body. (I am now aware this isn't necessary since I can start at joint D since one of the equations has 1 unknown, but this method is still bugging me)

sum F_x = 0
R_Dx + R_Ax - 5000sin(15) = 0 (equation 1)

sum F_y = 0
-R_Ay - 5000cos(15) = 0
so R_Ay = -4829 N

sum Moments about point A = 0
(2*R_Dx) - (5*5000sin(15)) = 0
so R_Dx = 3235 N

so plugging R_Dx into equation 1 gives
R_Ax = -1941 N

starting at joint D

sum F_x = 0

R_Dx + BD = 0
so BD = -R_Dx = -3235 N

but the answer says it is 12.89 kN compression!
I can't continue because everything else will be wrong.

What am I screwing up on?

2. Jul 2, 2016

### SteamKing

Staff Emeritus
It appears you have accounted for the moments about A due to the horizontal component of the load at C.

However, additional moment at A will be created due to the vertical component of the load at C, since this component is offset horizontally from the line A-D.

3. Jul 2, 2016

### nothingsus

Dang, can't believe I missed that.