Did you find the answer with a google search? I didn't find any.
I posted this subject in Structural engineering forum and someone wrote: Probably not the best question for the structural engineering forum. You might want to delete this thread and create a new one in the Finite Element Engineering Analysis forum... which tends to be more technical related to numerical methods.
Hopefully I started posting the subject here. If you like, I can post my step-by-step calculation, I got the tridiagonal matric. Now I'm stuck with the eigenvalue and eigenvector calculation.