The IRS (Improved Reduced System) reduction method was developed by O’Callahan [2]. The IRS method improves upon Guyan reduction by including an estimation of omitted mass effects in the development of the transformation matrix. The first step in the IRS method is identical to the Guyan reduction. The next step uses the static transformation matrix to estimate the omitted mass effects. The final transformation matrix for the IRS method is:
(2.3.3-1)
where
(2.3.3-2)
The stiffness and mass matrices are reduced to the TAM DOF using the transformation matrix (2.3.3-1) and the transformation equation (2.3-1).