Load Case Data - Modal Form

The  Load Case Data - Modal form is used to view and change the definition of an modal load case. Click the Analysis > Type > Modal > New command to display the form.

A modal analysis is always linear. Choose between Eigenvector or Ritz vector modal 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 load 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 load cases can be created. However, for most purposes, one modal load case is enough.

Important: If any nonlinear modal time-history load case will be based on this modal load 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 pattern, 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 load case.

      • Load Name drop-down list.  Choose the load 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.

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 eigen vectors 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.

See Also

Load Case Data