Mechanical APDL Structural Analysis Guide

*GET,VARIANCE,VARI,5,EXTREME,VLAST! get the integral value

STDDEV=SQRT(VARIANCE)! convert to standard deviation (1-sigma)

Note

This value will correspond to the POST1 1 σ values. POST26 sums all the modes by

default for the response PSD (see SIGNIF on the RPSD command), whereas POST1

only sums the significant modes (SIGNIF equals 0.0001 by default for the PSDCOM

command). You can use this comparison to verify that the significance factor on the

PSDCOM command is small enough and that the curve fitting for the input PSD

curve was adequate.

6.6.3.3. Calculating Covariance in POST26.

You can compute the covariance between two quantities available on the results file (displacements,
velocities, and/or accelerations), if the Jobname.RST and Jobname.PSD files are available.

The procedure to calculate the covariance between two quantities is as follows:

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


2. 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


3. Calculate the contributions of each response component (relative or absolute response) and store
   them in the desired variable. The PLVAR command can then be used to plot the modal contributions
   (relative response) followed by the contributions of pseudo-static and mixed part responses to the
   total covariance.
   Command(s):CVAR
   GUI: Main Menu> TimeHist Postpro> Calc Covariance


4. Obtain the covariance.
   Command(s):*GET,NameVARI,n,EXTREM,CVAR
   GUI: Utility Menu> Parameters> Get Scalar Data

6.6.4. Sample Input.

A sample input listing for a random vibration (PSD) analysis is shown below:

! Build the Model

/FILNAM,! Jobname

/TITLE,! Title

/PREP7! Enter PREP7

...

...! Generate model

...

FINISH

!

! Obtain the Modal Solution

/SOLU! Enter SOLUTION

ANTYPE,MODAL! Modal analysis

MODOPT,LANB! Block Lanczos method

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

Performing a Random Vibration (PSD) Analysis