Simple truss question I can't seem to get right

  • #1
16
0

Homework Statement


p6v4LnW.png


bRYWUNP.jpg


ignore all the other writing!

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?
 

Answers and Replies

  • #2
SteamKing
Staff Emeritus
Science Advisor
Homework Helper
12,796
1,668

Homework Statement


p6v4LnW.png


bRYWUNP.jpg


ignore all the other writing!

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?
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
16
0
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.
Dang, can't believe I missed that.

Thanks for the reply!
 

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