A method for computing the mass and center of gravity for basic and combined substructures stored in the NASTRAN Substructure Operating File (SOF) is described. The three step method recovers SOF data blocks for the relevant substructure, processes these data blocks using a specially developed FORTRAN routine, and generates the NASTRAN gridpoint weight generator (GPWG) table for the substructure in a PHASE2 SOF execution using a Direct Matrix Abstraction Program (DMAP) sequence. Verification data for the process is also provided

Allen, Tom

English

NASA Technical Reports Server

N91 "20512
NASTRAN GPWG TABLES FOR COMBINED SUBSTRUCTURES
Tom Allen
McDonnell Douglas Space Systems Co.
Huntsville Alabama
ABSTRACT
A method for computing the mass and center of gravity for
basic and combined substructures stored on the NASTRAN
Substructure Operating File (SOF) is described. The three
step method recovers SOF data blocks for the relevant
substructure, processes these data blocks using a specially
developed FORTRAN routine, and generates the NASTRAN
gridpoint weight generator (GPWG) table for the
substructure in a PHASE2 SOF execution using a Direct
Matrix Abstraction Program (DMAP) sequence. Verification
data for the process is also provided in this report.
1.0 INTRODUCTION
Once a basic substructure has been put on the Substructure
Operating File, the ability to obtain the mass and center
of gravity (cg) of the basic substructure or any combined
substructure of which it is part is lost in a normal
NASTRAN execution. The user is unable to verify the mass
and cg of the subsequent combined substructures nor is he
able to attest to the quality of the PHASE1 reductions
performed on his models. The method described here allows
the user to obtain the mass and cg of any substructure that
is stored on the SOF and to recover them in the form of the
customary GPWG tabular format.
The three step method that is used to obtain the mass and
cg of the substructures is described in the next section.
Verification of the process is provided after the method
description.
2.0 METHODOLOGY
The method used to obtain the GPWG table of a substructure
is divided into three steps. The first step is the
recovery of the SOF data blocks BGSS, EQSS, and CSTM for
the substructure of interest. The second step reformats
these data blocks into standard NASTRAN input bulk data.
During the third step, the user executes a DMAP sequence
51
https://ntrs.nasa.gov/search.jsp?R=19910011199 2020-03-19T18:32:33+00:00Z
that uses the bulk data from the second step to define the
geometry, and the stored mass matrix to calculate the GPWG
table. Each of these three steps will be described below.
2.1 Step 1 - Data Block Recovery
Step 1 of the process involves obtaining the SOF data
blocks BGSS, EQSS, and CSTM for the substructure of
interest from the SOF. The PHASE2 substructure control
deck required for this operation is shown in Figure I. The
data is taken from the SOF and written to NASTRAN file
FORT17. Sample records for each of the three data blocks
are provided in Figure 2.
2.2 Step 2 - Defining Input Bulk Data
Step 2 takes the data blocks recovered from Step 1 and
converts them into standard NASTRAN Bulk Data that is used
to define the geometry of the model. The SOF data blocks
will be described below. The bulk data that is created
from the data blocks will also be defined.
2.2.1 BGSS Data Block Description
The BGSS data block contains the location in the basic
coordinate system of each internal point in the
substructure as well as the output coordinate system of the
internal point. If the output coordinate system is -i, the
internal point is a scalar point rather than a physical
gridpoint of the substructure. The BGSS data are converted
to GRID or SCALAR bulk data cards that define the
substructure geometry.
2.2.2 EQSS Data Block Description
The EQSS data block contains data that describe the degrees
of freedom (DOF) that are associated with each internal
point of the substructure. The data is binary coded data
that is stored in an integer variable. The on/off sequence
of the bit sequence tells NASTRAN the DOF that are retained
for that point. For example, the integer value 7 has a bit
sequence of IIi000 which indicates that DOF 123 were
retained and DOF 456 were removed during the PHASE1
execution. The EQSS data are converted to DMI bulk data
cards that are used to merge the reduced mass matrix stored
on the SOF to a full sized mass matrix in the DMAP
sequence.
52
2.2.3 CSTM Data Block Description
The CSTM data block contains the transformation matrix for
each of the output coordinate systems in the substructure.
These data are used to make CORD2R bulk data cards so that
the correct coordinate transformations are performed inside
the GPWG Module.
At the end of Step 2, the user has created a set of NASTRAN
bulk data. A sample set of input bulk data is shown in
Figure 3.
2.3 Step 3 - Calculate Mass and CG
Step 3 of the process uses a DMAP sequence in conjunction
with the bulk data that was created in Step 2 to obtain the
GPWG table. The DMAP sequence that is used to calculate
the GPWG table is shown in Figure 4. Verification of the
method is provided in the next section.
3.0 VERIFICATION
Three test cases were executed to verify the method. The
first case uses a simple beam element model that contains
no MPC, SPC, or OMIT cards. The second case is a complex
combined substructure that comprises 16 basic
substructures. The third case uses a Craig-Bampton modally
reduced model of the second case. The data provided for
these cases demonstrate the validity of the method. Each
of the example problems will be discussed below.
Figure 5 shows the simple beam element model that was used
for the first test case. The GPWG table from the
'straight' execution (non-substructuring) is shown in
Table i. Table 2 is the GPWG table of the substructure
model. The data contained in these tables are identical.
The second case used a combined substructure (pseudo-
structure in NASTRAN parlance) made up of 16 basic
substructures. The mass and cg of the combined
substructure calculated using PHASE1 GPWG data taken from
each of the basic substructures is provided in Table 3.
The GPWG table of the combined pseudostructure is provided
in Table 4. Any discrepancies between these data can be
attributed to changes introduced by the SPC, MPC, and Guyan
reductions performed during the PHASE1 executions of the
individual basic substructures.
53
The final test case used a Craig-Bampton modally reduced
model of the second test case. The GPWG table for this
execution is provided in Table 5. A comparison of the data
in this table with that in Table 4 shows good agreement
between the two models, the only large discrepancy being in
the x direction. The discrepancy in the x direction can be
attributed to the addition of extra x mass to the
Craig-Bampton model. Discrepancies in the y and z
directions can be attributed to the Craig-Bampton modal
reduction.
The three test cases discussed above show good agreement
with expected results. Hence, the method is considered
verified.
4.0 CONCLUSION
A method for obtaining PHASE2 Grid Point Weight Generator
tables of substructures stored on the Substructure
Operating File has been described. Data from several test
executions were provided. These data verify the method.
54
ACRONYMSAND ABBREVIATIONS
cg
DMAP
DOF
GPWG
SOF
NASTRAN
Center of gravity
Direct Matrix Abstraction Program
Degree(s) of freedom
Gridpoint Weight Generator
Substructure Operating File
NASA Structural Analysis program
55
SUBSTRUCTURE PHASE2
PASSWORD = password
SOF(1) = FTxx
SOFOUT(EXTERNAL) FORT17, DISK
POSITION = REWIND
NAMES = model id
ITEMS = BGSS
SOFOUT(EXTERNAL) FORT17, DISK
POSITION = NOREWIND
NAMES = model id
ITEMS = EQSS
SOFOUT(EXTERNAL) FORT17, DISK
POSITION = NOREWIND
NAMES = model id
ITEMS = CSTM
FIGURE I. NASTRAN PHASE2 DECK FOR RECOVERY
OF SOF DATA BLOCKS
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65


NASTRAN GPWG tables for combined substructures

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