Hi Peter, 

Thanks you so much for your help.

I got confused because I didn't understand what the purpose of having stiffness is. I would appreciate if you take a look at my model attached here and see if the those modal analysis are true. I'm really grateful if you do this favor for me.

Yes both ways give me the same results.

Also if you look at the plot of force reaction for six parts( which are human teeth), you will see some are positive and the rest are negative which I expect only tensile ones not compression because the connection between teeth and tray are not bonded. What would be the reason?