Bracket (Modal Anslysis)

Transcription

1 (Modal Anslysis) Chapter Overview The sample session described in this chapter demonstrates a simple linear static and dynamic analysis on a steel bracket. The bracket restrains a vertical pipe. The bracket also supports some mechanical equipment. First, the bracket will be subjected to a static load. A dynamic analysis will predict the normal frequencies and mode shapes of vibration to determine if there is any interaction with the bracket and surrounding excitation frequencies. Finally, the bracket will be subjected to a time dependent pressure and the dynamic response will be determined. MSC.Marc User s Guide 14-1

2 Dynamic Modal Shape Analysis Dynamic Modal Shape Analysis Overview of Steps Step 1 Step 2 Step 3 Restore the database from the static analysis. Create a modal dynamic loadcase and submit it. Postprocess the results. MSC.Marc User s Guide 14-29

3 Step 1 For the dynamic analysis, the same geometry will be used. The first step is to restore the database. Closing the post file will automatically restore the database. In addition, the commands are shown for restoring the database and resetting the program. (These commands are not necessary here). Finally, the plotting of points, curves, and surfaces will be switched off. RESULTS CLOSE FILES RESTORE RESET PROGRAM VIEW show 4 PLOT draw POINTS draw CURVES draw SURFACES REGENERATE FILL MSC.Marc User s Guide

4 Step 2 The next step is to create a modal dynamic loadcase. The default is that 10 modes are determined which is enough for this structure. (Note that the determination of higher order modes in general required higher mesh densities). The following button sequence will create the loadcase. LOADCASE NEW DYNAMIC MODAL The next step is to create and execute the modal analysis job. The following button sequence will create and submit the job. JOBS NEW MECHANICAL available lcase2 ANALYSIS OPTIONS RUN SAVE SUBMIT 1 MONITOR (Verify that the LANCZOS method is used) MSC.Marc User s Guide 14-31

5 Step 3 The next step is to postprocess the results. For modal analyses, not only the values of the eigenfrequencies but also the shape of the deflections or modal shapes are of interest. For the deformed shape, the automatically scaled deformations should be viewed. For ease of understanding, it is best to show all four views. The following button sequence will do the postprocessing. RESULTS OPEN DEFAULT DEF & ORIG PLOT draw NODES MORE edges OUTLINE SCALAR Displacement z CONTOUR BANDS deformed shape SETTINGS deformation scaling AUTOMATIC NEXT INC (Repeat until all modes have been viewed) Finally, generate an animation sequence of one modal shape: RESULTS MORE animate MODE 9 (Number of frames for animation) ANIMATION VIEW show view 4 FILL PLAY SHOW MODEL MSC.Marc User s Guide

6 Figure The Second Eigenmode Observe that the calculated eigenfrequencies and corresponding eigenvectors are stored as so-called subincrements on the post file. The eigenfrequency value (in cycles/time unit) corresponding to a specific mode is printed on the top left of the screen. MSC.Marc User s Guide 14-33

7 Table 14-1 Eigenfrequencies increment mode value (cycles/time) 0: : : : : : : : : : Observe that the eigen period of mode 1 will be seconds MSC.Marc User s Guide