Why are members CD and DE zero force in this truss?

[twj2944]

In the attached problem, I don't understand why members CD and DE are equal to 0. I was under the impression that the combined vertical components of BC and CD should equal 6+downward components of BC and EF, as to counteract the force straight down, but this is not the case.

My questions:

Why are the forces in members CD and DE equal to zero?

If they are equal to 0, what stops the downward force from moving the structure?

Thank you

Note: G and Ay are 6 and 6, not 8 and 6 as shown. I used the right values for my calculations, but for some reason wrote 8 down

 

[SteamKing]

It's not clear from your post: Who is saying that members CD and DE are equal to 0? Are you talking about the forces in these members?

 

[twj2944]

Sorry yes - that's what I meant.

 

[SteamKing]

You still haven't answered the main question: who is saying that the forces in members CD and DE are equal to 0?

 

[twj2944]

The answers in the back of the textbook

 

[pongo38]

IF THE STRUCTURE AND LOADING ARE BOTH SYMMETRICAL THEN THE EQUILIBRIUM OF JOINT d REQUIRES SYMMETRY OF FORCES IN cd AND de. tHAT IS THE CLUE...

 

[SteamKing]

Hey, you don't have to shout.