# Homework Help: Forces in structure

1. Apr 4, 2014

### MMCS

See attachment for question and diagrams.
This problem is concerned with using the stiffness matrix method to find displacements and force in a structure. My problem is pre-eliminating the members. i have attached 2 diagrams. i would like help in trying to get from the first diagram, to the second diagram ( with eliminated members and forces calculated)

I know that

∑Fx=0
∑Fy=0
∑M=0

However, the UDL and other factors are causing me problems.

Thanks

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2. Apr 4, 2014

### SteamKing

Staff Emeritus
From the diagram attached, it's not clear what the magnitude of the distributed load q is.

If you are trying to eliminate the beam 1-6-7 and the support strut 5-6 from the rest of the structure,
isolate it and draw a FBD for these members. You should be able to write equations of static equilibrium using the FBD and solve those to find the reaction forces, which can be applied to the remaining structure.

3. Apr 7, 2014

### MMCS

ok so my attempt,
I have isolated 1-6-7 and 5-6.
Taking moments about point 1 i have

6metres * 5kn/m * 3metres = Force at 5 * sqrt(2^2 + 2^2)
Force at 5 = 31.8

This value seems correct. I have tried the get the forces at 1 but have had no luck. Where is the best place to take moments from ? intuitively it seems odd that the force in the x direction is greater than P, as P seems like the only force acting this way

Thanks

4. Apr 7, 2014

### MMCS

ok so my attempt,
I have isolated 1-6-7 and 5-6.
Taking moments about point 1 i have

6metres * 5kn/m * 3metres = Force at 5 * sqrt(2^2 + 2^2)
Force at 5 = 31.8

This value seems correct. I have tried the get the forces at 1 but have had no luck. Where is the best place to take moments from ? intuitively it seems odd that the force in the x direction is greater than P, as P seems like the only force acting this way

Thanks

5. Apr 7, 2014

q=5kn/m

6. Apr 7, 2014

### SteamKing

Staff Emeritus
Static equilibrium is more than just taking moments about an arbitrary point. You've got to write both the force and the moment equations to get the complete picture.