tension and compresions

Hi,

I am working on a small experiment. I have a platform. The frame is made of wood and it is a construction element. this frame is hanging with 4 rods to one point. 

I would like to know how i can put different material properties to my script.

With this script my goal is to see were the tension and compression lines are.

the is just a symple model. mij next step is to change the hanging structure in to branching system. 

Can some one help me or explain to me how it works?