(08-23-2009, 11:11 AM)fashion Wrote: essenza, I think what your r saying applies more to surface f.e. rather than to linear ones. While, for a simple frame structure, you (or the software) form the stifness matrix, it is easy to and straightforward to compute the eigenvalues of the matrix by simple algebra, and find the eigenmodes, for a structure modeled with shear walls (i.e. surface f.e.) I agree that eigenmode analysis is aproximate. I will try to find some papers about the issue
there where you got it wrong. Stiffness matrix in finite element (frame, surface,solid) that we know is static stiffness matrix. Try to find out about dynamic stiffness matrix, you will not find that 12EI/L3, EA/L, 6EI/L2 etc :D, because now it is frequency dependent or in mathematical form it is trancendental.