Seek a harmonic solution of the form

The vector is the generalized coordinate vector. The undamped, homogeneous equation is transformed through substitution and algebraic manipulation into the generalized eigenvalue problem.

TThe eigenvalues can be found by setting the determinant equal to zero.

There is an eigenvalue for each degree-of-freedom. Each angular natural frequency is then calculated from the square root of the respective eigenvalue. The corresponding eigenvectors represent orthogonal mode shapes. The eigenvector for each mode is found via

An eigenvector matrix can be formed. The eigenvectors are inserted in column format.

The coefficient represents the modal displacement of mass i for mode j.

Each eigenvector can be multiplied by an arbitrary scale factor. A mass-normalized eigenvector matrix can be calculated such that the following orthogonality relations are obtained.

The superscript T represents matrix transpose. The identity matrix is I, a diagonal matrix of ones. The diagonal matrix of eigenvalues Ω.

Now define a modal coordinate in terms of the normalized eigenvector matrix such that the displacement vector is

Substitute equation (3.1) into equation (1.2).

Premultiply by the transpose of the normalized eigenvector matrix.

The orthogonality relationships yield

Furthermore, the following simplifying assumption is made for the damping matrix.

The equation of motion can now be written as

The two equations are now uncoupled in terms of the modal coordinates. The modal displacement for free vibration response to initial conditions is found via Laplace transforms as shown in Reference [3]. The modal displacement for dof i is

The damped angular natural frequency is

The physical displacement can then be found by substituting equation (3.7) into (3.1). The physical displacements can be expressed in terms of the eigenvector coefficients and modal coordinates as

The modal displacement initial conditions are required for the complete solution. These can be found from the following transformations.

The participation factors and effective modal masses are indicators of how excitable the modes are given uniform base excitation to the given system. Some modes are more readily excited than others, and some cannot be excited at all for this excitation type.

The system’s generalized mass matrix is given by

Again, the generalized mass will be the identity matrix if the eigenvectors are mass normalized. Let be the influence vector which represents the displacements of the masses resulting from static application of a unit ground displacement. Define a coefficient vector as

The modal participation factor matrix for mode i is

The effective modal mass for mode i is

The system in Figure 5.1 has the following parameters.

Assume 5% damping for each mode. The initial velocity is zero for each mass.

Apply the mass and stiffness values from Table 1.1 into equation (1.1) to form the following matrices.

The 1/386 factor is needed to convert lbm to lbf sec^2/in.

The eigenvalues and vectors are found by inserting these matrices into equations (2.5) and (2.6). The results are shown in the following table.

The mass-normalized mode shapes in matrix format are

The response time histories for the initial conditions are shown in Figure 5.2.