We are given this frame and asked to find the forces at C, D, E and F. Distance from D to E is 16 feet. Distance from C to D is 8 feet; distance from E to F is also 8 feet. Distance from A to D is 6 feet; distance from D to G also 6 feet.[/B]
The Attempt at a Solution
We first look at the entire frame, and take the moment about G. This gives us Hy as 150; Gy must be -150 to balance the Y forces. This agrees with the book's answer.
Sum of x forces: Gx plus the 200LB force=0, so Gx = -200. Hx, because it is a roller, must be 0. This agrees with the book too.
OK. So far so good.
Now, why can't we make 2 immediate assumptions:
1) Assume Ax and Ay both are zero, since A is just sitting there with no forces on it.
2) Both C and F should be zero, since they are 2-member elbows with no external forces; aren't CD and EF classic examples of zero-force members?
I see that CG and HF are 2-force members under influence of G and H, respectively, but again this seems to directly contradict what we learned, when we looked at trusses, about how to spot zero-force members.
Thank you in advance for any help!