APPENDIX C2
Definition of Weight and
Inertia Loading
Objectives:
■ Apply boundary condition and gravity load to the model.
■ Utilize the WTMASS option to obtain the correct result.
■ Compare results.
MSC.Nastran 105 Exercise Workbook
C2-1
C2-2
MSC.Nastran 105 Exercise Workbook
APPENDIX C2
Definition of Weight and Inertia Loading
Model Description:
This exercise is designed to study the effect of gravity on a model. When
modeling a gravity load in English units, users may sometimes forget to
convert the mass density to weight density. Failure to do so will yield
results that are incorrect.
Figure C2.1-Model Geometry
ID
B
h
OD
Table C2.1
Outer Diameter of Cylinder (OD)
4 in
Inner Diameter of Cylinder (ID)
3.25 in
Height (B)
16 in
Hole Diameter (h)
1.5 in
Weight Density
0.101 lbs/in3
Weight/Mass Factor
2.59E-3 sec2/in
Elastic Modulus
10.0E6 lbs/in2
Poisson’s Ratio
0.33
MSC.Nastran 105 Exercise Workbook
C2-3
Figure C2.2-Loads and Boundary Conditions
Gravity = 7 g
123
C2-4
123
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MSC.Nastran 105 Exercise Workbook
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123
123
APPENDIX C2
Definition of Weight and Inertia Loading
Suggested Exercise Steps
■ Open the previous workshop.
■ Define material (MAT1) and element (PSOLID) properties.
■ Apply the fixed boundary constraints (SPC1).
First Job (Solid)
■ Apply gravity load of 7 with grav constant of 1. (GRAV).
■ Prepare the model for a static analysis (SOL 101).
■ Generate an input file and submit it to the MSC.Nastran solver
for static analysis.
■ Review the results.
Second Job (Solid2)
■ Apply 7g gravity with gravity constant of 386.4. (GRAV).
■ Prepare the model for a static analysis (SOL 101).
■ Generate an input file and submit it to the MSC.Nastran solver
for static analysis.
■ Review and compare the results.
Third Job (Solid3)
■ Apply 7g gravity with gravity constant of 386.4. (GRAV).
■ Prepare the model for a static analysis (SOL 101).
■ Specify the weight-mass conversion value.
■ PARAM, WTMASS, 0.00259
■ Generate an input file and submit it to the MSC.Nastran solver
for static analysis.
■ Review and compare the results.
MSC.Nastran 105 Exercise Workbook
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APPENDIX C2
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Definition of Weight and Inertia Loading
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MSC.Nastran 105 Exercise Workbook
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APPENDIX C2
Definition of Weight and Inertia Loading
Exercise Procedure:
1. Users who are not utilizing MSC.Patran for generating an input file should go
to Step 8, otherwise, proceed to step 2.
2. Open the database named Solid.db.
File/Open...
Change to the directory where the database is located.
Existing Database Name
Solid.db
OK
3. Create a set of material properties for the plate.
◆ Materials
Action:
Create
Object:
Isotropic
Method:
Manual Input
Material Name
alum
Input Properties...
Elastic Modulus =
10.0E6
Poisson Ratio =
.33
Density =
.101
Apply
Cancel
4. Define the plate thickness.
◆ Properties
Action:
Create
Dimension:
3D
Type:
Solid
Property Set Name
bar
MSC.Nastran 105 Exercise Workbook
C2-9
Input Properties...
m:alum
Material Name
(Select from Material Property Sets box.)
OK
<Select Solid Element Icon in
select menu and select All the
elements (Elm 1:4480)>
Select Members
Add
Apply
5. Apply constraints to the model.
Constrain the bottom surface from displacement in all directions.
Rotate to Left side view.
Left side view
◆ Load/BCs
Action:
Create
Object:
Displacement
Type:
Nodal
New Set Name
pinned
Input Data...
Translations <T1 T2 T3>
<0, 0, 0>
OK
Select Application Region...
● FEM
Select Nodes
(see Figure C2.3)
C2-10
MSC.Nastran 105 Exercise Workbook
<Select all nodes at
the bottom surface of
the model (Left edge
in the display)>
Definition of Weight and Inertia Loading
APPENDIX C2
Add
OK
Apply
Figure C2.3-Model Constraint
Select these nodes.
MSC.Nastran 105 Exercise Workbook
C2-11
6. Apply gravity load to the model.
Constrain the bottom surface from displacement in all direction.
Return to the Iso 3 view.
Iso 3 view
◆ Load/BCs
Action:
Create
Object:
Inertial Load
Type:
Element Uniform
New Set Name
gravity
Input Data...
Load/BC Set Scale Factor:
1.0
Trans Accel <A1 A2 A3>
<0, 0, 7>
Note: It is not necessary to specify that gravity force is negative because
Trans Accel <A3> is defaulted to point down.
OK
Apply
7. Now, run the analysis.
Be sure to activate the Node i.d. for Weight Generator so MSC.Nastran
will output the weight table in the .f06 file.
◆ Analysis
Action:
Analyze
Object:
Entire Model
Method
Analysis Deck
Job Name
solid
Solution Type...
● LINEAR STATIC
Solution Parameters...
C2-12
MSC.Nastran 105 Exercise Workbook
APPENDIX C2
Definition of Weight and Inertia Loading
Node i.d. for Wt.Gener. =
0
OK
OK
Apply
An MSC.Nastran input file called solid.bdf will be generated. The process
of translating your model into an input file is called Forward Translation.
The Forward Translation is complete when the Heartbeat turns green.
MSC.Patran Users should proceed to step 9.
MSC.Nastran 105 Exercise Workbook
C2-13
Submitting the input file for analysis:
8. Submit the input file to MSC.Nastran for analysis.
8a. To submit the MSC.Patran .bdf file for analysis, find an available
UNIX shell window. At the command prompt enter: nastran
solid.bdf scr=yes. Monitor the run using the UNIX ps command.
8b. To submit the MSC.Nastran .dat file for analysis, find an available
UNIX shell window. At the command prompt enter: nastran solid
scr=yes. Monitor the run using the UNIX ps command.
9. When the run is completed, edit the solid.f06 file and search for the
word FATAL. If no matches exist, search for the word WARNING.
Determine whether existing WARNING messages indicate modeling
errors.
C2-14
MSC.Nastran 105 Exercise Workbook
APPENDIX C2
Definition of Weight and Inertia Loading
Comparison of Results
10. Compare the results obtained in the .f06 file with the results on
the following page:
MSC.Nastran 105 Exercise Workbook
C2-15
C2-16
MSC.Nastran 105 Exercise Workbook
DIRECTION
MASS AXIS SYSTEM (S)
MASS
X
6.723973E+00
Y
6.723973E+00
Z
6.723973E+00
0
0
0
1
X-C.G.
0.000000E+00
-6.540650E-10
-6.540650E-10
Y-C.G.
1.548170E-10
0.000000E+00
1.548170E-10
SPCFORCE RESULTANT
T1
T2
T3
R1
R2
-7.3691053E-14 -2.8186134E-13 -4.7067810E+01 -7.2845907E-09 -3.0786040E-08
Verify the SPCFORCE Resultant:
Mass * gravity = Resultant
6.724 * 7 = 47.068
Z-C.G.
8.000008E+00
8.000008E+00
0.000000E+00
R3
7.6456202E-14
APPENDIX C2
Definition of Weight and Inertia Loading
11. MSC.Nastran Users have finished this exercise.
MSC.Patran Users should proceed to the next step.
12.
Proceed with the Reverse Translation process, that is importing the
solid.op2 the Analysis form and proceed as follows:
◆ Analysis
Action:
Read Output2
Object:
Result Entities
Method
Translate
Select Results File...
Select Results File
solid.op2
OK
Apply
To simplify the view, turn off the entity labels and reset graphics using the
toolbar.
Hide Labels
Reset graphics
13.
Plot the results of the analysis.
To plot the results to posted FEM use the Results Application radio
button.
◆ Results
Action:
Create
Object:
Quick Plot
Select Result Case
Default, Static Subcase
Select Fringe Results
Stress Tensor,
Quantity:
von MIses
MSC.Nastran 105 Exercise Workbook
C2-17
Select the Fringe Attributes icon.
Display:
Element Edges
Label Style...
Label Format:
Fixed
Significant figures
4 <use slider bar>
OK
Apply
The results should resemble Figure C2.4.
Figure C2.4
Although the solution is correct, it was just a coincidence that this
occurred. The weight density of aluminum is 0.101 lbs/in3. This value was
entered in the density input field for material. However, MSC.Nastran
C2-18
MSC.Nastran 105 Exercise Workbook
Definition of Weight and Inertia Loading
APPENDIX C2
only recognizes mass density and not weight density. Therefore, the value
of 0.101 was taken as mass density. This will yield an incorrect solution.
Remember, mass desity = (weight density / gravity). Therefore, the result
has to be divided by gravity in order to yield the correct solution.
Another mistake was the value entered for gravity (acceleration). The
value 7 was entered. However, the actual value is 7g. Therefore, the
solution will be off by the factor of 386.4 in/sec2.
Although both the entered mass density and gravity values were incorrect,
the solution happened to be correct. The reason is because the weight
density needs to be divided by value of gravity (386.4 in/sec2) while the
gravity (acceleration) needs to be multiplied by 386.4 in/sec2. Thus, the
two 386.4 in/sec2 canceled out each other.
Next, we will make the mistake more obvious to see. Most users, who are
not familiar with how MSC.Patran defines its density, will often make this
mistake.
To reset the graphics, click on this icon:
Reset Graphics
14. Apply the correct gravity load to the model.
Apply the 7 g gravity load, where g is 386.4 in/sec2.
◆ Load/BCs
Action:
Modify
Object:
Inertial Load
Type:
Element Uniform
Select Set to Modify
gravity
Modify Data...
Trans Accel <A1 A2 A3>
<0, 0, 2704.8>
OK
Apply
15.
Now, run the analysis.
◆ Analysis
Action:
Analyze
MSC.Nastran 105 Exercise Workbook
C2-19
Object:
Entire Model
Method
Analysis Deck
Job Name
solid2
Solution Type...
● LINEAR STATIC
Solution Parameters...
Node i.d. for Wt.Gener. =
0
OK
OK
Apply
An MSC.Nastran input file called solid2.bdf will be generated. The
process of translating your model into an input file is called Forward
Translation. The Forward Translation is complete when the Heartbeat
turns green.
C2-20
MSC.Nastran 105 Exercise Workbook
APPENDIX C2
Definition of Weight and Inertia Loading
Submitting the input file for analysis:
16.
Submit the input file to MSC.Nastran for analysis.
16a. To submit the MSC.Patran .bdf file for analysis, find an
available UNIX shell window. At the command prompt
enter: nastran solid2.bdf scr=yes. Monitor the run using
the UNIX ps command.
16b. To submit the MSC.Nastran .dat file for analysis, find an
available UNIX shell window. At the command prompt
enter: nastran solid2 scr=yes. Monitor the run using the
UNIX ps command.
17. When the run is completed, edit the solid2.f06 file and search
for the word FATAL. If no matches exist, search for the word
WARNING. Determine whether existing WARNING
messages indicate modeling errors.
MSC.Nastran 105 Exercise Workbook
C2-21
Comparison of Results
18. Compare the results obtained in the .f06 file with the results on
the following page:
C2-22
MSC.Nastran 105 Exercise Workbook
1
Y-C.G.
1.548170E-10
0.000000E+00
1.548170E-10
SPCFORCE RESULTANT
T1
T2
T3
R1
R2
-3.0187408E-11 -1.0866241E-10 -1.8187002E+04 -2.8147510E-06 -1.1895723E-05
Z-C.G.
8.000008E+00
8.000008E+00
0.000000E+00
R3
2.3724134E-11
MSC.Nastran 105 Exercise Workbook
Verify the SPCFORCE Resultant:
Mass * gravity = Resultant
6.724 * (386.4 * 7) = 18187.0 <= Notice that this is different from previous solution.
C2-23
Definition of Weight and Inertia Loading
0
0
0
X-C.G.
0.000000E+00
-6.540650E-10
-6.540650E-10
APPENDIX C2
DIRECTION
MASS AXIS SYSTEM (S)
MASS
X
6.723973E+00
Y
6.723973E+00
Z
6.723973E+00
19.
Proceed with the Reverse Translation process, that is importing the
solid2.op2 the Analysis form and proceed as follows:
◆ Analysis
Action:
Read Output2
Object:
Result Entities
Method
Translate
Select Results File...
Select Results File
solid2.op2
OK
Apply
20.
Plot the results of the analysis.
To plot the results to posted FEM use the Results Application radio
button.
◆ Results
Action:
Create
Object:
Quick Plot
Select the Fringe Attributes icon.
Select Result Case
Default, Static Subcase_2
Select Fringe Results
Stress Tensor,
Apply
The results should resemble Figure C2.5.
C2-24
MSC.Nastran 105 Exercise Workbook
APPENDIX C2
Definition of Weight and Inertia Loading
Figure C2.5
Most users input the correct value for gravity (acceleration). However,
they may not realize the density value is incorrect. After all, most density
values found in references are given as weight density. Thus, it is
important to remember that mass density is needed for calculating forces
and stresses for unit consistency.
The solution will be off by a factor of 386.4, which is the value of gravity.
Now, we will utilize the weight-mass conversion option. This option will
allow the users to input weight density as density; at the same time, it will
also signal MSC.Nastran to treat that value was weight density and make
the conversion.
To reset the graphics, click on this icon:
Reset Graphics
MSC.Nastran 105 Exercise Workbook
C2-25
21. Apply the correct gravity load and also include weight-mass conversion
to the model.
Note: F = CMa. Where C = 1/g = 0.00259 (English weight system).
◆ Analysis
Action:
Analyze
Object:
Entire Model
Method
Analysis Deck
Job Name
solid3
Solution Type...
● LINEAR STATIC
Solution Parameters...
Wt.-Mass Conversion =
0.00259
Node i.d. for Wt.Gener. =
0
OK
OK
Apply
An MSC.Nastran input file called solid3.bdf will be generated. The
process of translating your model into an input file is called Forward
Translation. The Forward Translation is complete when the Heartbeat
turns green.
C2-26
MSC.Nastran 105 Exercise Workbook
APPENDIX C2
Definition of Weight and Inertia Loading
Submitting the input file for analysis:
22.
Submit the input file to MSC.Nastran for analysis.
22a. To submit the MSC.Patran .bdf file for analysis, find an
available UNIX shell window. At the command prompt
enter: nastran solid3.bdf scr=yes. Monitor the run using
the UNIX ps command.
22b. To submit the MSC.Nastran .dat file for analysis, find an
available UNIX shell window. At the command prompt
enter: nastran solid3 scr=yes. Monitor the run using the
UNIX ps command.
23. When the run is completed, edit the solid3.f06 file and search
for the word FATAL. If no matches exist, search for the word
WARNING. Determine whether existing WARNING
messages indicate modeling errors.
MSC.Nastran 105 Exercise Workbook
C2-27
Comparison of Results
24. Compare the results obtained in the .f06 file with the results on
the following page:
C2-28
MSC.Nastran 105 Exercise Workbook
1
Y-C.G.
1.548168E-10
0.000000E+00
1.548168E-10
SPCFORCE RESULTANT
T1
T2
T3
R1
R2
-7.8159701E-14 -2.8299585E-13 -4.7104336E+01 -7.2902302E-09 -3.0809911E-08
MSC.Nastran 105 Exercise Workbook
Verify the SPCFORCE Resultant:
C(weight/mass conversion) * Mass * gravity = Resultant
(1/386.4) * 6.724 * (7 * 386.4) = 47.068
Z-C.G.
8.000008E+00
8.000008E+00
0.000000E+00
R3
6.3584554E-14
C2-29
Definition of Weight and Inertia Loading
0
0
0
X-C.G.
0.000000E+00
-6.540666E-10
-6.540666E-10
APPENDIX C2
DIRECTION
MASS AXIS SYSTEM (S)
MASS
X
6.723973E+00
Y
6.723973E+00
Z
6.723973E+00
25.
Proceed with the Reverse Translation process, that is importing the
solid2.op2 the Analysis form and proceed as follows:
◆ Analysis
Action:
Read Output2
Object:
Result Entities
Method
Translate
Select Results File...
Select Results File
solid3.op3
OK
Apply
26.
Plot the results of the analysis.
To plot the results of the posted FEM, use the Results Application
radio button.
◆ Results
Action:
Create
Object:
Quick Plot
Select Result Case
Default, Static Subcase_3
Select Fringe Results
Stress Tensor,
Apply
The results should resemble Figure C2.6.
C2-30
MSC.Nastran 105 Exercise Workbook
APPENDIX C2
Definition of Weight and Inertia Loading
Figure C2.6
As shown above, the solution is the same as that of our first job (solid).
This time, the correct solution is not by a coincidence.
This concludes our workshop.
Quit MSC.Patran when you are finished with this exercise.
MSC.Nastran 105 Exercise Workbook
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