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
