Mechanical APDL Structural Analysis Guide

The procedure for obtaining the modal solution is described in Modal Analysis (p. 39), but the following
additional recommendations apply:

• Use the Block Lanczos, PCG Lanczos, Supernode, or Subspace method to extract the modes. (Other
  methods are not valid for subsequent spectrum analysis.)


• Extract a sufficient number of modes to characterize the structure's response in the frequency range of
  interest. The ratio of the effective mass (used in the subsequent spectrum analysis) to the total mass is
  printed out along with the participation factors at the end of the modal analysis. A ratio greater than 0.9
  (more than 90% of the mass is included) is generally considered acceptable.


• To include the contribution of higher modes, you can either request the calculation of the residual vectors
  in the modal analysis (RESVEC) and use them in the spectrum analysis, or you can include the effect of
  the missing mass directly in the spectrum analysis (MMASS command).


• Expand all the modes (MXPAND,ALL). This is required if you eventually want to compute summed forces
  and moments in postprocessing (for example, obtaining the forces across a cut in the model using FSUM).
  You can also expand the modes after the spectrum solution if the size of the results file Jobname.RST
  is an issue (see the use of the SIGNIF argument in the MXPAND command documentation). In this case,
  use MXPAND,-1 to suppress the expansion during the modal analysis. Expanding modes after a spectrum
  solution will only expand significant modes based on the excitation direction specified in spectrum ana-
  lysis. If you included missing-mass effect (MMASS), then the calculation will be done only for these expan-
  ded modes. Note that you can limit the expanded mode set using the procedure in Using Mode Selec-
  tion (p. 71).


• To include material-dependent damping in the spectrum analysis, specify it in the modal analysis.


• Constrain those degrees of freedom where you want to apply a base excitation spectrum.


• At the end of the solution, exit the Solution processor.

If you intend to perform multiple independent spectrum analyses, keep a copy of the Jobname.MODE
file from the modal analysis. For more information, see Running Multiple Spectrum Analyses (p. 145).

6.2.3. Step 3: Obtain the Spectrum Solution

In this step, the program uses mode shapes extracted by the modal solution to calculate the SPRS
solution. The following requirements and limitations apply:

• The mode shape file (Jobname.MODE) must be available.


• The database must contain the same model from which the modal solution was obtained.


• The Jobname.FULL file must be available for the participation factors calculation.


• If the missing mass calculation is activated (MMASS command), the element matrices file (Job-
  name.EMAT) and the results file (Jobname.RST) from the modal analysis must also be available.


• Spectrum reaction force with missing mass is not supported.

1. Enter SOLUTION.
   Command(s):/SOLU
   GUI: Main Menu> Solution

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

Spectrum Analysis