MSC.NASTRAN Frequency Response Example Using A Seismic Mass

Following is an example of a general MSC.NASTRAN frequency response analysis run using a large (seismic) mass. This information is to be used in conjunction with the examples for random and sine vibration analysis runs. All the additional NASTRAN cards necessary to perform a frequency response run with large mass are described here. Comments follow the card (or cards) being described. The frequency response-specific cards are in bold.

NOTE: This method will work for ALL MSC NASTRAN versions, including v. 2001

    SOL 111
Modal Frequency Response Solution Number
    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.
These are the same for both random and sine.
    GRID         101       0      0.      0.      0.       0
Shaker grid point.
    SPC            1     101   13456      0.
Instead of constraining the shaker point using an SPC1 card, when using the large mass you need to use an SPC card with all directions constrained except the forcing direction.
    SUPORT	     101       2
A SUPORT card determines the reaction direction, i.e., the direction being forced. This direction must also be shown on the DAREA card below. Notice this direction is NOT constrained in the SPC card above.
RLOAD defines frequency response dynamic loading;
LOAD = DAREA * TABLED1 [= A * B(f)];
11 IS DAREA card;
12 IS TABLED1 card;
ACCE refers to type of dynamic excitation, enforced acceleration in this case.
    DAREA         11     101       2 386.4E8
DAREA defines the scale (area) factors for the dynamic loading.
Use this card instead of the SPCD card in the random and sine examples.
This example shows a Y-axis input at GRID 101;
2 refers to input direction (1=X, 2=Y, 3=Z);
386.4E8 is a force scale factor. In this example, the scale factor "cancels out" the CONM2 mass and returns an acceleration of 1.0.
    TABLED1       12     LOG     LOG                                        +
    +           20.0     1.0  2000.0     1.0    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.;
20.0 is the beginning frequency (X1); 1.0 is the magnitude (Y1);
2000.0 is the ending frequency (X2); 1.0 is the magnitude (Y2);
All tables MUST end with an ENDT card.
    TABDMP1       20                                                        +
    +            2.0     0.1   500.0     0.1    ENDT
TABDMP1 Defines modal damping of Q = 10;
The damping is 1/Q over the range of frequencies; in this case Q is constant.
    FREQ1         40    20.0     2.0     140
    FREQ1         40   300.0     5.0     140
    FREQ1         40  1000.0    20.0      50
    FREQ3         40    20.0  2000.0
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. (Going out to 2000Hz is usually a waste of time and computing power.)
    CONM2        100     101       0 386.4E8
The seismic mass, the card you've been waiting for. The mass here is 386.4E8 units. It can be whatever you want so long as it is several orders of magnitude larger than the mass you are shaking. Since we still use inches and pounds, this number is convienent for us. (See the DAREA card above.)

Ryan Simmons
August 2001 (updated Jan. 2002)