Mechanical APDL Structural Analysis Guide

3.9.5. Enforced Motion Method for Mode-Superposition Transient and Har-

monic Analyses

In a mode superposition transient or harmonic analysis, the enforced motion method can be used when
acceleration or displacement base excitation is present.

3.9.5.1. Understanding the Enforced Motion Method

In a modal analysis, Mechanical APDL calculates pseudo-static modes and writes them to the Job-
name.MODE file. They can then be used in a subsequent mode-superposition analysis with base excit-
ation.

3.9.5.2. Using the Enforced Motion Method

The procedure for calculating pseudo-static modes in a modal analysis is as follows:

1. Specify modal analysis (ANTYPE, MODAL).


2. Select one of the mode-extraction methods:
   
   
   • Block Lanczos (MODOPT,LANB)
   
   
   • SNODE method (MODOPT,SNODE)
   
   
   
   


3. Turn on the enforced motion calculation key by using the MODCONT command (EnforcedKey=ON).


4. Using the D command, specify the support points where imposed motion will be applied (node and
   degree of freedom label), as well as the enforced base identification number. This number is input as
   VALUE, which must be an integer.


5. Issue a SOLVE command.


6. Issue a FINISH command.

Note

The enforced motion pseudo-static modes calculation can be performed during a modal

analysis restart. Refer to Modal Analysis Restart in the Mechanical APDL Basic Analysis Guide

for more details.

To use the pseudo-static modes in the mode-superposition transient/harmonic analysis, issue the DVAL
command.

3.9.5.3. Sample Input for Enforced Motion Mode-Superposition Analysis

A sample input listing for a mode-superposition transient analysis follows:

! Build the Model

/FILNAM,...! Jobname

/TITLE,...! Title

/PREP7

---

---! Generate model

---

NSEL,...

CM, BASE1,NODE! Define component BASE1 with selected support nodes undergoing the first enforced motion

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Modal Analysis