Mechanical APDL Structural Analysis Guide

6.6.3.1.2. Display the Results

Use the same options available for the SPRS analysis.

Note

Nodal averaging performed by the PLNSOL command may not be appropriate in a random

vibration analysis because the result values are not actual values but standard deviations.

Instead, consider using the PLESOL command to display unaveraged element results.

Note

Displacements, stresses, and strains are always in the solution nodal or element coordinate

system (RSYS,SOLU).

6.6.3.2. Calculating Response PSDs in POST26.

You can calculate and display response PSDs for any results quantity available on the results file (dis-
placements, velocities, and/or accelerations) if the Jobname.RST and Jobname.PSD files are available.
If you are postprocessing in a new session, the Jobname.DB file corresponding to the PSD analysis
solve must be available for resume.

The procedure to calculate the response PSD is as follows:

1. Enter POST26, the time-history postprocessor.
   Command(s):/POST26
   GUI: Main Menu> TimeHist PostPro


2. Store the frequency vector.NPTS is the number of frequency points to be added on either side of
   natural frequencies in order to "smooth" the frequency vector (defaults to 5).The frequency vector
   is stored as variable 1.
   Command(s):STORE,PSD,NPTS
   GUI: Main Menu> TimeHist Postpro> Store Data


3. Define the variables in which the result items of interest (displacements, stresses, reaction forces,
   etc.) are to be stored.
   Command(s):NSOL,ESOL, and/or RFORCE
   GUI: Main Menu> TimeHist Postpro> Define Variables


4. Calculate the response PSD and store it in the desired variable. The PLVAR command can then be
   used to plot the response PSD.
   Command(s):RPSD
   GUI: Main Menu> TimeHist Postpro> Calc Resp PSD


5. You can integrate the response PSD to obtain the variance and take its square root to obtain its 1
   σ value. For example:
   RPSD,4,3,,3,2! variable 4 is the relative accel RPSD of var 3
   INT1,5,4,1! variable 5 is the integral of the RPSD

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Spectrum Analysis