1. Module 7
Modal Analysis

2. 7. Modal Analysis
Modal analysis is used to determine a structure’s vibration characteristics — natural frequencies and mode shapes.
It is the most fundamental of all dynamic analysis types and is generally the starting point for other, more detailed dynamic analyses.
In this chapter, we will describe how to do a basic modal analysis via the following topics:
A. Procedure
B. Workshop
For details, refer to your Structural Analysis Guide or Dynamics Training Manual.
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3. Modal Analysis A. Procedure
The steps to do a modal analysis are as follows:
Preprocessing
Geometry
Meshing
Solution
Analysis Type and Options
Loading
Solve
Postprocessing
Review results
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4. Modal Analysis ...Procedure
Geometry & Meshing
Same considerations as a static analysis:
Include as many details as necessary to sufficiently represent model geometry.
A fine mesh will be needed to resolve complex mode shapes.
Both Young’s modulus and density are required.
Specify mass density instead of weight density when using British units.
weight density (lb/in3)  g (in/sec2) = mass density (lb-sec2/in4)
For example, density of steel = 0.283/386 = 7.3E-04 lb-sec2/in4
Linear elements and material properties only. Nonlinearities are ignored.
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5. Modal Analysis ...Procedure
Analysis Type
The analysis type is modal.
Solution > -Analysis Type- New Analysis
Or issue ANTYPE,MODAL
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6. Modal Analysis ...Procedure
Analysis Options
Mode extraction options
MODOPT command or Solution > Analysis Options...
Method: Block Lanczos is generally recommended.
Number of modes: Specify the number of modes you want to extract.
Mode expansion options
MXPAND command or Solution > Analysis Options...
Expanding modes allows you to view them in the postprocessor.
Number of modes is usually the same as the number extracted.
Prestress effects
PSTRES command or Solution > Analysis Options...
calculate mode shapes of a prestressed structure, such as a spinning turbine blade.
Default settings are usually sufficient
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7. Modal Analysis ...Procedure
Loading & Solve
The only “loads” valid in a modal analysis are displacement constraints:
Solution > -Loads- Apply > Displacement >
Or use the D family of commands: D, DK, DL, DA, etc.
If you don’t specify constraints, ANSYS will calculate rigid body modes (zero frequency).
After all constraints have been specified, obtain the solution:
First save the database (SAVE command or Toolbar > SAVE_DB).
Then issue SOLVE (or Solution > -Solve- Current LS).
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8. Modal Analysis ...Procedure
Review Results
POST1 (the general postprocessor) is used to review modal analysis results.
Typically, the first step is to list the natural frequencies:
General Postproc > Results Summary
Or issue SET,LIST
Note that each mode is stored in a separate substep.
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9. Modal Analysis ...Procedure
To plot a mode shape:
First read results for that substep
General Postproc > Results Summary...
Or use the SET command
Then plot the deformed shape
General Postproc > Plot Results > Deformed Shape
Or use the PLDISP command
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10. Modal Analysis ...Procedure
You can also animate mode shapes:
Utility Menu > PlotCtrls > Animate > Mode Shape
Or use the ANMODE command
Another option is to plot contours stresses and strains. Note, however, that displacement and stress values are simply relative numbers and have no real meaning.
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11. Modal Analysis B. Workshop
This workshop consists of the following problem:
W6. U-Bracket
Please refer to your Workshop Supplement for instructions.
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