Mechanical APDL Structural Analysis Guide

5.2.2. Mode-Superposition Method

The mode-superposition method sums factored mode shapes (eigenvectors) from a modal analysis to
calculate the structure's response. This is the only method available in the ANSYS Professional program.
Its advantages are:

• It is faster and less expensive than the full method for many problems.


• Element loads applied in the preceding modal analysis can be applied in the transient dynamic ana-
  lysis via the LVSCALE command.


• It accepts modal damping (damping ratio as a function of mode number).

The disadvantages of the mode-superposition method are:

• The time step must remain constant throughout the transient, so automatic time stepping is not al-
  lowed.


• The only nonlinearity allowed is simple node-to-node contact (gap condition).


• It does not accept imposed (nonzero) displacements.

For procedural information about using the mode-superposition method, see Performing a Mode-Su-
perposition Transient Dynamic Analysis (p. 120).

5.3. Performing a Full Transient Dynamic Analysis

Note - Before reading this section, you are encouraged to become familiar with the concepts presented
in Structural Static Analysis (p. 9).

We will first describe how to do a transient dynamic analysis using the full method. We will then list
the steps that are different for the mode-superposition method.

The procedure for a full transient dynamic analysis (available in the ANSYS Multiphysics, ANSYS Mech-
anical, and ANSYS Structural products) consists of these steps:

1. Build the Model (p. 110)


2. Establish Initial Conditions (p. 110)


3. Set Solution Controls (p. 113)


4. Set Additional Solution Options (p. 115)


5. Apply the Loads (p. 117)


6. Save the Load Configuration for the Current Load Step (p. 117)


7. Repeat Steps 3-6 for Each Load Step (p. 117)


8. Save a Backup Copy of the Database (p. 118)


9. Start the Transient Solution (p. 118)

10.Exit the Solution Processor (p. 118)

Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information

Performing a Full Transient Dynamic Analysis