This section illustrates the *BASELINE CORRECTION and *BASE MOTION options by two examples.

The first example (pmodbase.inp, pmodbas2.inp, and pmodbas2a.inp) is a modal dynamic, time history analysis that is performed on a one-element cantilever structure using a B23 element. As the base motion record, a simple sine-shaped accelerogram is assumed for the time of one sine period. The record is corrected for the total time of the record duration. The choice of the base motion record as a sine function allows the analytical calculation of the parabolic correction to the record using the formulæ from “Baseline correction of accelerograms,” Section 6.1.2 of the ABAQUS Theory Manual. The values of the three constants for the parabolic correction are = –0.8308, = 0.4207, and = 2.1717; and the corrected accelerogram is

The second example (pmodbas3.inp and pmodbas4.inp) illustrates the application of multiple base motions in a time history modal dynamic analysis in which part of the structure is fixed while another part of it is subjected to excitation. The structure analyzed is a quarter-symmetry axisymmetric model of a cylinder made of rubberlike material. An 8 × 8 mesh with CAX4H elements is employed for the analysis. The structure is first preloaded in compression statically in the axial direction by a rigid platen, which is modeled as a rigid surface in pmodbas3.inp and as a rigid body in pmodbas4.inp; perfect bonding between the platen and the top surface of the cylinder is assumed. The response to applied axial (acceleration) excitation at the rigid surface reference node is sought. The acceleration records are the same as those used in the first problem. Since both fixed boundary conditions and applied acceleration boundary conditions occur in the same global (axial) direction in different parts of the structure, we use two *BASE MOTION options to specify these boundary conditions, treating the fixed boundary conditions as a primary base motion and the applied accelerations as a secondary base motion.