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Wednesday, 14 May 2008 13:30
Output from the Grid Point Weight Generator

Output from the Grid Point Weight Generator


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The grid point weight generator (GPWG) is activated with the parameter GRDPNT.
The data entry format is:


PARAM,GRDPNT,i


where i is an integer value defining a reference point. The value of "i" can
be any grid point in the model, or if it is set to zero, the reference point
is the origin of the basic coordinate system.


The output from the GPWG includes a rigid body mass matrix, various coordinate
transformations, and the location of the center of mass as shown below.











O U T P U T F R O M G R I D P O I N T W E I G H T G E N E R A T O R
REFERENCE POINT = 0
M O
* 1.480000E-03 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 -1.480000E-03 *
* 0.000000E+00 1.480000E-03 0.000000E+00 0.000000E+00 0.000000E+00 7.400000E-03 *
* 0.000000E+00 0.000000E+00 1.480000E-03 1.480000E-03 -7.400000E-03 0.000000E+00 *
* 0.000000E+00 0.000000E+00 1.480000E-03 2.220000E-03 -7.400000E-03 0.000000E+00 *
* 0.000000E+00 0.000000E+00 -7.400000E-03 -7.400000E-03 5.032000E-02 0.000000E+00 *
* -1.480000E-03 7.400000E-03 0.000000E+00 0.000000E+00 0.000000E+00 5.254000E-02 *
S

* 1.000000E+00 0.000000E+00 0.000000E+00 *
* 0.000000E+00 1.000000E+00 0.000000E+00 *
* 0.000000E+00 0.000000E+00 1.000000E+00 *

DIRECTION
MASS AXIS SYSTEM (S) MASS X-C.G. Y-C.G. Z-C.G.
X 1.480000E-03 0.000000E+00 1.000000E+00 0.000000E+00
Y 1.480000E-03 5.000000E+00 0.000000E+00 0.000000E+00
Z 1.480000E-03 5.000000E+00 1.000000E+00 0.000000E+00
I(S)
* 7.399999E-04 0.000000E+00 0.000000E+00 *
* 0.000000E+00 1.332000E-02 0.000000E+00 *
* 0.000000E+00 0.000000E+00 1.406000E-02 *
I(Q)
* 7.399999E-04 *
* 1.332000E-02 *
* 1.406000E-02 *
Q
* 1.000000E+00 0.000000E+00 0.000000E+00 *
* 0.000000E+00 1.000000E+00 0.000000E+00 *
* 0.000000E+00 0.000000E+00 1.000000E+00 *






The output from the GPWG is generally more than most users need. The mass
and center of gravity (CG) location is typically all that is used. Note the
following items:


o For most models, the mass in all three directions should be the same. If they
are not, the cause may be due to a CONM1, CMASSi, or DMIG input.


o Because the mass may be different in the three translational directions, the
mass may have directional properties. It is for this reason that CG location
is given for each of the three translational masses. If the mass is the same
in all directions, a unique CG exists and is located at the X component of
the Y (or Z) mass, the Y component of the X (or Z) mass and Z component of
the X (or Y) mass. In the above example, the CG of the structure is located
at (5.0,1.0,0.0) in the principal mass coordinates.


o The scale factor entered in the parameter WTMASS is applied to the assembled
element mass before GPWG. The GPWG module, however, converts the mass back
to the original input units that existed prior to the scaling effect of the
parameter WTMASS. (Note: The parameter WTMASS does not get applied to M2GG
or M2PP input, but the M2GG mass does get assembled into the mass matrix prior
to GPWG. Therefore, for GPWG output only, the mass input using M2GG is scaled
to the same units as the element mass input. M2GG input may be scaled independently
using the CM2 parameter.)


o GPWG is performed on the G-size mass matrix which is the mass matrix prior
to the processing of the rigid elements, SPCs, and MPCs.


o Mass at scalar points is not included in the GPWG calculation.


o GPWG for a superelement does not include the mass from upstream superelements.
Therefore, the GPWG for the residual structure includes only the mass on the
residual points. The CG location is also based on the residual mass only.


o If a large mass is used for enforced motion, the large mass dwarfs the structural
mass. For model checkout, remove the large mass and constrain the driving
point. A static run is a convenient way to generate a mass matrix and obtain
output from GPWG.


o The output from GPWG is for informational purposes only and is not used in
the analysis.


o Additional information can be found in Section 4 of the MSC/NASTRAN Programmer's
Manual.


See also MSC/NASTRAN Common Questions and Answers.

 




The Grid Point Weight Generator (GPWG) module computes the rigid body
mass properties of an entire structure with respect to a user specified point and with
respect to the center of mass. Output from the module is requested by a PARAM
GRDPNT
card in the Bulk Data Deck which specifies from which grid point mass
computations are to be referenced. Optionally, the absence of a specific grid point
(i.e. PARAM, GRDPNT, 0) automatically causes the origin of the basic
coordinate system to be utilized as a reference. The mass properties are initially
defined in the basic coordinate system. Subsequently, the mass properties are
transformed to principal mass axes and to principal inertia axes. The actual printout
is composed of several elements. These are:


M0 RIGID BODY MASS MATRIX IN BASIC COORDINATE SYSTEM


This is the rigid body mass matrix of the entire structure in the basic coordinate
system with respect to a reference point chosen by the analyst.


S TRANSFORMATION MATRIX FOR SCALAR MASS PARTITION


S is the transformation from the basic coordinate system to the set of principal axes
for the 3 x 3 scalar mass partition of the 6 x 6 mass matrix. The principal axes for
just the scalar partition are known as the principal mass axes.


XC.G. YC.G. ZC.G.


It is possible in NASTRAN to assemble a structural model having different values of
mass in each coordinate direction at a grid point. This can arise, for example, by
assembling scalar mass components or from omitting some components by means of bar
element pin flags. Consequently three distinct mass systems are assembled one in each of
the three directions of the principal mass axes (the S system). This third tabulation
has five columns. The first column lists the axis direction in the S coordinates. The
second column lists the mass associated with the appropriate axis direction. The final
three columns list the x, y, and z coordinate distances from the reference point to the
center of mass for each of the three mass systems.


I(S) INERTIAS RELATIVE TO C.G.


This is the 3 x 3 mass moment of inertia partition with respect to the center of
gravity referred to the principal mass axes (the S system). This is not necessarily a
diagonal matrix because the determination of the S system does not involve second
moments. The values of inertias at the center of gravity are found from the values at
the reference point employing the parallel axes rule.


I(Q) PRINCIPAL INERTIAS


The principal moments of inertia at the center of gravity are displayed in matrix
form with reference to the Q system of axes. The Q system is obtained from an eigenvalue
analysis of the I(S) matrix.


Q TRANSFORMATION MATRIX I(Q) = QT*IBAR(S)*Q


Q is the coordinate transformation between the S axes and the Q axes. IBAR(S) is the
same as I(s) except that the signs of the offdiagonal terms are reversed.

 

Last Updated ( Wednesday, 14 May 2008 14:46 )
 
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