MSC.NASTRAN Sine Vibration Frequency Response Example

Following is an example of a NASTRAN swept sine frequency response analysis run. All the NASTRAN cards necessary to perform a frequency response run are described here. Comments follow the card (or cards) being described. The frequency response-specific cards are in bold. The model is a simple cantilever beam with 48 elements and 49 nodes. It lies along the Y-axis with the first node, 101 at the origin. The downloadable data file can be found at the bottom of this page.

NOTE: This method will only work for MSC's version 2001 or later of NASTRAN because they (finally!) did away with the large mass needed to do frequency response runs. This method is extremely similar to UAI's version of frequency response runs. (See notes below for the differences.)

If you are using MSC/NASTRAN version 70.7 or earlier, then you still need to use the large mass. An example of this method is given on the Random Vibration Seismic Mass Frequency Response page.

Don't know what Double Amplitude (D.A.) means? An explanation is available.

Index:
ACCELERATION | Bulk Data Deck | Case Control | DLOAD | Eigenvalue Output | EIGRL | ENDT | Example Plot | Executive Control | FREQ | FREQ1 | FREQ3 | Frequency Response Output | INCLUDE | METHOD | Output Summary | OUTPUT(XYPLOT) | PHASE | PUNCH | Punch Output | RESVEC | RLOAD2 | SDAMPING | Shaker Point | SOL | SORT2 | SPC1 | SPCD | SPCFORCES| TABDMP1 | TABLED1

$EXECUTIVE CONTROL DECK
ID SINE,TEST
SOL 111
Modal Frequency Response Solution Number
$
TIME 10
CEND
$
$CASE CONTROL DECK
TITLE = SINE VIBRATION EXAMPLE MODEL
SUBTITLE = 48-ELEMENT CANTILEVER BEAM ON THE X-AXIS
LABEL = Y-DIRECTION SINE INPUT
$ Beam is along the X-axis and is 48 inches long

$
ECHO = NONE
$
SPC      = 1
METHOD   = 1
DLOAD    = 10
SDAMPING = 20
FREQ     = 40

METHOD refers to EIGRL card;
DLOAD refers to RLOADx card;
SDAMPING refers to TABDMP1 card;
FREQ refers to all FREQx cards used
$
$SET 98 = 101
$SPCFORCES(PHASE) = 98
For force recovery, these two cards create SET1 for the shaker point, GRID 101,
then request forces (as magnitude/phase) at the shaker point (see SPCD card below)
$
SET 99 = 101, 149
Defines GRIDs for response output
$
ACCELERATION(SORT2,PRINT,PUNCH,PHASE) = 99
Requests frequency response output for SET 99;
Acceleration output given as magnitude/phase angle;
Output is written to the .F06 and punch files;
SORT2 sorts output as GRID1, all frequencies; GRID2, all frequencies; etc.
(SORT1 sorts output as frequency 1, all GRIDs; frequency 2, all GRIDs; etc.)
$
OUTPUT(XYPLOT)
XYPEAK,XYPUNCH,ACCE/ 101(T2),149(T2)
Cards used for plot output.
XYPEAK recovers peak responses in the .prt file;
XYPUNCH creates a .pch file of response vs. frequency;
ACCE requests acceleration output (FORCE requests force output);
GRID point 101 and 149 responses recovered; T1=X, T2=Y, T3=Z, as well as R1, R2, and R3
(NOTE: the punch file will contain both ACCELERATION and OUTPUT data.)
$
BEGIN BULK
$
PARAM,AUTOSPC,YES
PARAM,GRDPNT,0
PARAM,RESVEC,YES
PARAM,RESVEC,YES is REQUIRED for accurate results when NOT using the seismic mass. It computes residual vectors.
$
GRID         101       0      0.      0.      0.       0
Shaker grid point.
$
SPC1           1  123456     101
The Shaker point is constrained in all DOF. Often this is an RBE element attached to many GRID points with the independent GRID being the shaker point.
$
RLOAD2,10,11,,,12,,ACCE
RLOAD defines frequency response dynamic loading;
LOAD = SPCD * TABLED1 [= A * B(f)];
11 IS SPCD card;
12 IS TABLED1 card;
ACCE refers to type of dynamic excitation, enforced acceleration in this case.
$
SPCD,11,101,2,1.0
Y-axis input at GRID 101;
2 refers to input direction (1=X, 2=Y, 3=Z);
1.0 is any scale factor;
$
$SPCD,11,101,2,386.4
Use this card instead when recovering forces or displacements, such as at the shaker point. If you're using the Metric system, for example mm, then use 9807 instead of 386.4.
$
$
TABLED1       12     LOG     LOG                                        +
+            2.0    0.25   500.0    0.25    ENDT
TABLED1 is a tabular function defining the frequency-dependent portion of RLOAD2
This table gives the sine vibration input spec;
The table is given as X1,Y1, X2,Y2, X3,Y3, etc.;
2.0 is the beginning frequency (X1); 0.25 is the magnitude (Y1);
500.0 is the ending frequency (X2); 0.25 is the magnitude (Y2);
If your input is written as double amplitude (D.A.), a conversion is available.
 
All tables MUST end with an ENDT card.
$
TABDMP1       20                                                        +
+            2.0     0.1   500.0     0.1    ENDT
Defines modal damping of Q=10;
The damping is 1/Q over the range of frequencies; in this case Q is constant.
$
FREQ1,40,2.0,2.0,250
FREQ3,40,2.0,500.0
FREQx cards input frequencies where responses are recorded;
FREQ1 gives beginning frequency, freq. increment size and number of increments;
FREQ3 adds each natural frequency up to 500 Hz
$
EIGRL          1      0.    500.               0                    MASS
EIGRL determines normal modes up to 500 Hz
$
INCLUDE 'sinebulk.dat'
Bulk Data can be elsewhere in a different file.
$
ENDDATA


Eignevalue Output

The eignevalue and frequency table from the .F06 file is given below. It gives the frequency range requested in the EIGRL card, 0-500Hz. Note that the frequencies listed in this table under CYCLES correspond with the peaks in the plot at the bottom of the page.

   SINE VIBRATION EXAMPLE MODEL
   48-ELEMENT CANTILEVER BEAM ON THE X-AXIS                                                                                      
   Y-DIRECTION SINE INPUT                                                                                                        

                                           R E A L   E I G E N V A L U E S
MODE    EXTRACTION      EIGENVALUE            RADIANS             CYCLES            GENERALIZED         GENERALIZED
 NO.       ORDER                                                                       MASS              STIFFNESS
  1         1          5.673224E+03        7.532081E+01        1.198768E+01        1.000000E+00        5.673224E+03
  2         2          5.673224E+03        7.532081E+01        1.198768E+01        1.000000E+00        5.673224E+03
  3         3          2.220890E+05        4.712632E+02        7.500386E+01        1.000000E+00        2.220890E+05
  4         4          2.220890E+05        4.712632E+02        7.500386E+01        1.000000E+00        2.220890E+05
  5         5          1.733352E+06        1.316568E+03        2.095384E+02        1.000000E+00        1.733352E+06
  6         6          1.733352E+06        1.316568E+03        2.095384E+02        1.000000E+00        1.733352E+06
  7         7          6.614887E+06        2.571942E+03        4.093373E+02        1.000000E+00        6.614887E+06
  8         8          6.614888E+06        2.571942E+03        4.093373E+02        1.000000E+00        6.614888E+06
  

Example of Frequency Response Output

The first data row gives frequency and magnitude, the second data row gives phase angle in degrees. The POINT-ID refers to the GRID point where the acceleration was recovered. Notice that for GRID 101 the response is the same as the input because GRID 101 is the shaker point.

   SINE VIBRATION EXAMPLE MODEL
   48-ELEMENT CANTILEVER BEAM ON THE X-AXIS
   Y-DIRECTION SINE INPUT
   POINT-ID =       101
                                    C O M P L E X   A C C E L E R A T I O N   V E C T O R
                                                      (MAGNITUDE/PHASE)
 
   FREQUENCY   TYPE          T1             T2             T3             R1             R2             R3
2.000000E+00     G      0.0            2.500000E-01   0.0            0.0            0.0            0.0
                          0.0            0.0            0.0            0.0            0.0            0.0
3.109742E+00     G      0.0            2.500000E-01   0.0            0.0            0.0            0.0
                          0.0            0.0            0.0            0.0            0.0            0.0
4.000000E+00     G      0.0            2.500000E-01   0.0            0.0            0.0            0.0
                          0.0            0.0            0.0            0.0            0.0            0.0
   .
   .
   .
etc.
   SINE VIBRATION EXAMPLE MODEL
   48-ELEMENT CANTILEVER BEAM ON THE X-AXIS
   Y-DIRECTION SINE INPUT
   POINT-ID =       149
                                    C O M P L E X   A C C E L E R A T I O N   V E C T O R
                                                      (MAGNITUDE/PHASE)
 
   FREQUENCY   TYPE          T1             T2             T3             R1             R2             R3
2.000000E+00     G      4.345413E-17   2.221159E-01   1.012504E-12   0.0            3.272628E-15   1.007428E-02
                        181.9821       358.3475       178.8464         0.0          351.0273       180.8535
3.109742E+00     G      3.963935E-17   2.388943E-01   1.406906E-12   0.0            1.447238E-14   9.613853E-03
                        183.6875       357.3896       178.2196         0.0          355.5177       181.5473
4.000000E+00     G      3.502847E-17   2.593850E-01   1.322254E-12   0.0            1.189495E-14   9.052319E-03
                           185.9450       356.5729       177.8896         0.0          354.0833       182.3873
   .
   .
   .
etc.

The following table give the output peak summary from the .F06 file.

   SINE VIBRATION EXAMPLE MODEL
    48-ELEMENT CANTILEVER BEAM ON THE X-AXIS
    Y-DIRECTION SINE INPUT
                            X Y - O U T P U T  S U M M A R Y  ( R E S P O N S E )
SUBCASE  CURVE FRAME    CURVE ID./       XMIN-FRAME/    XMAX-FRAME/    YMIN-FRAME/      X FOR        YMAX-FRAME/      X FOR
   ID     TYPE   NO.  PANEL  : GRID ID     ALL DATA       ALL DATA       ALL DATA       YMIN           ALL DATA       YMAX
      1  ACCE      0      101(  4)      2.000000E+00   5.020000E+02   2.500000E-01   6.438968E+00   2.500000E-01   2.000000E+00
                                        2.000000E+00   5.020000E+02   2.500000E-01   6.438968E+00   2.500000E-01   2.000000E+00
      1  ACCE      0      149(  4)      2.000000E+00   5.020000E+02   1.527553E-01   5.020000E+02   3.915089E+00   1.198768E+01
                                        2.000000E+00   5.020000E+02   1.527553E-01   5.020000E+02   3.915089E+00   1.198768E+01

Example of a Frequency Response punch (.pch) file output

There are two types of output given in the punch file. The first is the same as the output in the .F06 file shown above, but presented in an alternate form. First data row gives frequency and the magnitudes for T1-T3; second data row fives magnitudes for R1-R3; third and fourth data rows give phase angles for T1-R3 in degrees.

etc.

The second type of output in the punch file is simply magnitude vs. frequency. This is useful for plotting the response curve in a spreadsheet.

etc.

Plot of the Example Sine Vibration Data

Below is a plot of the data given in the punch file (sine_test.pch). It shows the input and the response. The input is a straight line at .25 G. The response shows the peaks corresponding to the modes of the beam.

Sine example response plot

Example Files

You can download the three files for this example to run and study:
sine_test.dat, sine_test.f06, and sine_test.pch.

Ryan Simmons
July 2001, updated September 2007

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