Mechanical APDL Structural Analysis Guide

3. Obtain the mode-superposition transient solution.


4. Expand the mode-superposition solution.


5. Review the results.

5.4.1. Build the Model

Building the model for a mode-superposition transient dynamic analysis is the same as that described
for the full method. See Build the Model (p. 110) for more information.

5.4.2. Obtain the Modal Solution

Modal Analysis (p. 39) describes how to obtain a modal solution. Following are some additional hints:

• The mode-extraction method should be Block Lanczos, PCG Lanczos, Supernode, Subspace, or QR
  damped. ( The other methods, unsymmetric and damped, do not apply to mode-superposition.) If
  your model has damping and/or an unsymmetric stiffness matrix, use the QR Damp mode-extraction
  method (MODOPT,QRDAMP).


• Be sure to extract all modes that may contribute to the dynamic response.


• If you use the QR damped mode-extraction method, you must specify any damping (ALPHAD,BETAD,
  MP,ALPD,MP,BETD,MP,DMPR or element damping including gyroscopic) that you want to include
  in the modal analysis. During the transient analysis, you can define additional damping, including a
  constant modal damping ratio (DMPRAT) or the modal damping ratio as a function of mode (MDAMP).
  For more details about damping definitions, see Damping (p. 2)


• Specify displacement constraints, if any. Note that additional constraints may not be applied in the
  subsequent mode superposition transient analysis.


• If you need to apply element loads (pressures, temperatures, accelerations, etc.) in a transient dynamic
  analysis, you must specify them in the modal analysis. The loads are ignored for the modal solution,
  but a load vector will be calculated and written to the mode shape file (Jobname.MODE), and the
  element load information will be written to Jobname.MLV. You can generat e multiple load vectors
  (see the MODCONT command in Generating and Using Multiple Loads in Mode-Superposition Ana-
  lyses (p. 68)). You can then scale and use these load vectors in the transient solution.


• To include the contribution of higher frequency modes in the subsequent transient analysis, you can
  calculate the residual vectors in the modal analysis (see the RESVEC command in Using the Residual-
  Vector Method to Improve Accuracy (p. 66)).


• If the transient excitation is coming from the support, you can generat e pseudo-static modes needed
  in the subsequent transient analysis (see the MODCONT command in Enforced Motion Method for
  Mode-Superposition Transient and Harmonic Analyses (p. 70)).


• You should expand the modes and calculate the element results to save computation time in the
  subsequent expansion of the transient results (MXPAND,ALL,,,YES,,YES). Do not use this option if you
  are applying thermal loads, or if you want to postprocess energies. The model data (for example,
  nodal rotations) should not be changed between the modal and transient analyses.


• You can select the modes of interest for the expansion. Because the expansion is performed on selected
  modes only, you will save computational time in the subsequent mode superposition transient ana-
  lysis. See Using Mode Selection (p. 71) for information about this procedure.

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Performing a Mode-Superposition Transient Dynamic Analysis