Analysis Case Data - Modal Form

Access the Analysis Case Data - Modal form as follows:

Click the Define menu > Analysis Cases command to display the Analysis Cases form.
Click the Add New Case button or highlight a previously defined analysis case and click the Add Copy of Case or the Modify/Show Case button to display the Analysis Case Data form.
Select Modal from the Analysis Case Type drop-down list to display the Analysis Case Data - Modal form.

The Analysis Case Data - Modal form is used to view and change the definition of an modal analysis case. A modal analysis is always linear. Choose between Eigenvector or Ritz vector modal analysis types. Eigenvector analysis determines the undamped free-vibration mode shapes and frequencies of the system. These natural modes provide an excellent insight into the behavior of the structure. Ritz vector analysis seeks to find modes that are excited by a particular loading. Ritz vectors can provide a better basis than do eigenvectors when used for response-spectrum or time-history analyses that are based on modal superposition.

Tip: The program automatically creates an eigenvector modal analysis case when a new model is started. Calculating and reviewing the first few natural modes (eigenvectors) of a structure is an excellent way to check the model for errors and to gain insight into its behavior.

Note: An infinite number of modal analysis cases can be created. However, for most purposes, one modal analysis case is enough.

Stiffness to Use. Choose to solve for the vibration modes using the stiffness of the unstressed structure (i.e., the Zero Initial Conditions-Unstressed State option), or at the end of a nonlinear static or nonlinear direct-integration time-history analysis case (i.e., Stiffness at End of Nonlinear Case option). See Stiffness to Use. If you don't know what to do, choose the Zero Initial Conditions - Unstressed State option.
Type of Modes. Choose to calculate eigenvectors or Ritz vectors. The remainder of the data on the form depends on the type of mode chosen.
Maximum Number of Modes. The program will not calculate more than the specified maximum number of modes. This number includes any static correction modes requested (eigen only). The program may compute fewer modes if there are fewer mass degrees of freedom, all dynamic participation targets have been met, all modes within the cutoff frequency range have been found (eigen only), or the maximum number of cycles has been reached for all loads (Ritz only.)
Minimum Number of Modes. The program will not calculate fewer than the specified minimum number of modes, unless there are fewer mass degrees of freedom in the model.
Load Applied and Show Advanced Load Parameters check box. When this check box is checked, additional load related input is required on the form. Loads are not really applied in a modal analysis case, but they may be used in the calculation of the modes:

For Ritz vectors: Specify the loads to be used as starting vectors for the Ritz-vector algorithm. These should include all loads that will be used in all response-spectrum and modal time-history analysis cases that will be based on this modal analysis case.

Important: If any nonlinear modal time-history analysis case will be based on this modal analysis case, be sure to include the Link vector loads in the set of applied loads.

Load Type drop-down list. Choose whether the load to be applied is a load case, a built-in acceleration load, or the set of all Link loads. Link loads are special built-in loads corresponding to the nonlinear degrees of freedom in the Link elements. These are needed only if a nonlinear modal time-history analysis will be performed based on this modal analysis case.
Load Name drop-down list. Choose the load case name, or the global direction of ground acceleration, depending on the type of load. For Link loads, the only choice is "All."
Maximum Cycles edit box. Specify the maximum number of Ritz vectors to be generated for each load by specifying the maximum number of generation cycles allowed for that load. Specifying zero (the default) is the same as infinity, i.e., no limit. These limits will not be used until the minimum number of modes has been found.
Target Dynamic Participation Ratio edit box. After the minimum number of modes has been found, the program will continue to seek modes until all specified participation ratios have been achieved, or some other criterion governs, whichever comes first. If a participation ratio for a load is not specified, or if zero is used, the participation ratio will not be used.

Important: The Target Dynamic Participation Ratio feature is not yet available in this release of the program. Any values set will be ignored, i.e., they are set to zero.

Note: The dynamic participation ratio for an acceleration load is exactly the same as the mass participation ratio.

Tip: For Ritz vectors, static correction modes for the starting load vectors are always program calculated.

For eigenvectors: Normally it is not necessary to specify loads because the modes are properties of the structure, not the loading. However, to request static correction modes or participation targets, check the Show Advanced Load Parameters check box and specify a load(s) using the following options:

Load Type drop-down list. Choose whether the load to be applied is a load case, a built-in acceleration load, or the set of all Link loads. Link loads are special built-in loads corresponding to the nonlinear degrees of freedom in the Link elements. These are needed only if a nonlinear modal time-history analysis will be performed based on this modal analysis case.
Load Name drop-down list. Choose the load case name, or the global direction of ground acceleration, depending on the type of load. See Applying Acceleration Loads for more information. For Link loads, the only choice is "All."
Target Mass Participation Ratios. After the minimum number of modes has been found, the program will continue to seek modes until all specified participation ratios have been achieved, or some other criterion governs, whichever comes first. If a participation ratio for a load is not specified, or if zero is used, the participation ratio will not be used.

Important: The Target Mass Participation Ratio feature is not yet available in this release of the program. Any values set will be ignored, i.e., they are set to zero.

Note: The dynamic participation ratio for an acceleration load is exactly the same as the mass participation ratio.

Static Correction drop-down list. Use the Static Correction option to request that a static correction mode be calculated for one or more mode loads. If "Yes" is chosen for one or more loads, the number of eigenvectors sought will be reduced as necessary and static correction modes will be calculated for any requested load whose dynamic (mass) participation ratio is less than 100%.

Note: For acceleration loads, a static correction mode is exactly the same as a "residual-mass" or "missing-mass" mode. Calculating a static correction mode does not guarantee that the mass participation ratio is 100%, only that the missing high-frequency modes have been captured statically.

Other Parameters - For Eigenvectors Only

Frequency Shift (Center) edit box. The shift sets the center of the cyclic frequency range for which modes will be sought. Modes are found in order of increasing distance from the shift. Specify in Hz.
Cutoff Frequency (Radius) edit box. The cutoff frequency sets the radius of the cyclic frequency range for which modes will be sought. Modes are found in order of increasing distance from the shift, until the distance exceeds the cutoff frequency. The default value of zero indicates infinity, i.e., there is no limit. Specify in Hz.
Convergence Tolerance edit box. Enter a relative tolerance to be used to determine when the eigenvalues have converged. The default value is usually adequate.
Allow Automatic Frequency Shifting check box. Check this check box to allow automatic frequency shifting for the Eigen vector solution.

Add button. To add a load to the set of applied loads, enter the load type, load name, and other parameters at the top of the table, then click the Add button.
Modify button. To modify a load in the set of applied loads, click on the load in the table to select it, make any changes to the load type, load name, and other parameters at the top of the table, then click the Modify button.
Delete button. To remove a load from the set of applied loads, click on the load in the table to select it, then click the Delete button.