Eigenvalue analysis provides dynamic properties of a structure by solving the characteristic equation composed of mass matrix and stiffness matrix. The dynamic properties include natural modes (or mode shapes), natural periods (or frequencies) and modal participation factors.
1. Natural mode: A natural mode pertains to free vibration in an undamped system. 1st mode, 2nd mode… and nth mode represent the order in which least energy is required to deform the structure.
2. Natural Period: A natural period is the time that it takes to freely vibrate the structure into the corresponding natural mode one full cycle.
3. Modal Participation Factor: The ratio of the influence of a specific mode to the total modes.
Enter the control data for the eigenvalue analysis.
midas Civil follows the procedure noted below for an eigenvalue analysis.
1. The self weight of the elements in the structure is converted into mass in Structure Type. Masses, which are not included in the model, can be entered by using the Loads to Masses conversion or by directly specifying Nodal Masses. Total mass can be checked in Mass Summary Table. Enter the mass data of the model using the various mass defining functions in the Load> Masses menu.
2. Invoke the Analysis > Eigenvalue Analysis Control menu and specify all the required data for performing eigenvalue analysis including the number of modes.
3. Select the Analysis > Perform Analysis menu or click 
4. Once the structural analysis is successfully completed, use either the Results > Vibration Mode Shapes function or the Results > Result Tables > Vibration Mode Shape function to review various vibration mode shapes and eigenvalue mode frequencies or periods.
Note1
Tension-only/Compression-only element will be changed to Truss element in eigenvalue Analysis.
Note2
Eigenvalue analyses must precede dynamic analyses such as Time history analysis or Response spectrum analysis.
The response spectrum analysis uses the natural periods from the eigenvalue analysis. Thus, the entered spectral data must include the expected range of natural periods.
Note
Use the Load > Static Loads > Loads to Masses function to convert the desired static load data to nodal mass data. The conversion function may become quite useful when we try to apply the dead load already entered to the mass data required for a seismic analysis. In case DOF of the nodes to which nodal masses have been assigned are constrained, the corresponding masses are included in the total mass, but excluded from the effective mass. Therefore, the user must not constrain the DOF of the nodes assigned with masses when checking mass modal participation factors.
Details
| Title | Eigen Value and Response Spectrum |
| Duration | 48 Mins |
| Language | English |
| Format | MP4 |
| Size | 92 MB |
| Download Method | Direct Download |
| Download Links | BECOME A MEMBER VIEW DOWNLOAD LINKS |
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