Abstract: In structural dynamics, generally we can not determinate accurately the natural frequencies and eigenvalues due to various approximations. But we may obtain an eigenvalue interval, where the lower bound eigenvalue can be determinated by, say, Dunkerly's or Temple's method; the upper bound eigenvalue can be computed by multi-term Rayleigh-Ritz or Galerkin method. Alternatively, the upper and lower bounds and can be obtained simultaneously by means of Hu Haichang's "Mass Inclusion Theorem and Rigidity Inc...