### Failure of Method of Joints

Well, this is an example as
good as any which cannot be solved "manually" by the classical
method of joints (although program TRUSS does
not have any problems with it if I gave the exact geometry and the
value of the force P).

When looking at a truss in general, the important questions you should
ask yourself are :

**Question :** Is it possible to solve for the unknown forces
within the confines of statics ?

**Anwser :** Yes. We have 7 joints, giving rise to 14 equations.
We have 10 members and 4 components of support forces giving
us a total of 14 unknowns.

**Question :** Is there a joint with only two unknown forces acting
on it ?

**Anwser :** No. Joints 1,4, and 7 have each 4 unknown forces,
joints 2,3,5, and 6 have each 3 unknown forces.

**Question :** Is there a zero-force member present ?
**Anwser :** No.

**Question :** Can you solve any of the overall equilibrium
equations ?
**Anwser :** No. We have four external forces, but only
3 equations AND one cannot find an overall equilibrium equation with
only one unknown force. (At least I cannot).
If you think you have found a way to solve for the forces other then using
program TRUSS,
**
**

**Send a Note to Zig**

**
**

Last revised: 12/07/00

Zig Herzog