I am trying to write a MATLAb code for determining the natural frequencies of a first-order shear deformation plate using Ritz method. I am using Legendre polynomials as trial functions in Ritz method, and my code works well for static analysis of plate. I have solved few structural problems with my code and compared the results with MSC NASTRAN. The results are accurate with maximum error of 5%.
However, when I try to find eigenvalues, at least one eigenvalue comes out to be negative. Also, the eigenvalues do not match with eigenvalues obtained from MSC NASTRAN (Modal Analysis).
Is it possible for a plate to have negative eigenvalues?