# Homework Help: Analysing a 3D truss problem

1. Oct 31, 2011

### La82

1. The problem statement, all variables and given/known data

I am trying to analyse a 3D truss problem (attached). The idea is that when you pull with a force F in the direction of the arrows, the pin in the middle will be pressed downwards, decreasing the length of p2. In general the overall structure will be consistent of a series of these in both directions.

I am having some difficulties getting started with this. As far as I can tell I should be ok to only analyse the section shown in the bottom picture?

2. Relevant equations
I wish to get some sort of solution that relates the length L1 and L2 to p1 and p2.

3. The attempt at a solution
My first attempt at this is to use simple statics in 3D:

Point at top of pin:
Sum(F_y) = 0
0 = - F_(BE) - F*sin(Beta)*2 (symetry)

Sum(M_top) = 0
0 = - F * sin(Beta)*2*p_1 + F*cos(Beta)*A_1-F*cos(Beta)*A_1+F*sin(Beta)*2p_2

So something clearly goes wrong here since I just end up with p1 = p2?

Can anyone help me in the right direction?

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