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Modal Case Data Form

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Modal Case Data Form Use this form to view and change the definition of a modal load case. A modal analysis is always linear.

General: 

Modal Case Name. Accept the default or enter a name for the load case being defined. It should be unique among all load cases of all types.

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Modal Case Subtype can be one of the following: 

Eigen vector 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.

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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 eigen vector modal load case when a new model is started. Calculating and reviewing the first few natural modes (eigen vectors) 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.

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Notes - Modify/Show button. Click this button to access the Load Case Notes form. Use that form to add Notes about the load case to the model file.

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Design button. Click this button to access the Design Load Type form. Choose Program Determined, or User Specified. If user-defined is chosen, select a load pattern from the drop-down list; the selected load pattern will be used in the load case. The specified load case design types are used in creating automatic design load combinations.

P-Delta/Nonlinear Stiffness: 

Use Preset P-Delta Settings option. Click the Modify/Show button to access the Preset P-Delta Options form. Use that form to set the P-Delta analysis parameters.

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Use Nonlinear Case (Loads at End of Case NOT Included) option. Choose to solve the modal case starting at the end of a nonlinear load case. Use the drop-down list to select the load case.

Loads Applied: When the Advanced check box is checked, additional load related input is required on the form. Loads are not really applied in a modal load case, but they may be used in the calculation of the modes. Begin using this area of the form by clicking the Add button to add a data row to the spreadsheet area. 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 load cases that will be based on this modal analysis case.

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Modal Case Data Form

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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.

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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."

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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.

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Target Dyn, Par. Ratio, % (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 Eigen Vectors: 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 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.

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

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Target Mass Par. Ratio, % (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

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Modal Case Data Form

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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 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.

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Add button. Click this button to display the Load Type drop-down list, and the Load Name and Scale Factor edit boxes. Use these items to specify the load pattern(s) to be included in the load case.

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Delete button. 1. Highlight the load pattern to be deleted in the set of applied loads. 2. Click the Delete button.

Other Parameters: 

Maximum Number of Modes. The program will not calculate more than the specified maximum number of modes. Note: 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.)

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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.

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Entries only available when Eigen Vectors are selected: 

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.

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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

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Modal Case Data Form

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is no limit. Specify in Hz. 

Convergence Tolerance edit box. Enter a relative tolerance to be used to determine when the eigen values have converged. The default value is usually adequate.

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Allow Automatic Frequency Shifting check box. Check this check box to allow automatic frequency shifting for the Eigen vector solution.

See Also Load Case Data Define Load Cases Load Combination Data Form

Access the Load Case Data form for a Modal case as follows: 1. Click the Define menu > Modal Cases command to display the Modal Cases form. 2. Click the Add New Case button or highlight a previously defined load case and click the Add Copy of Case or the Modify/Show Case button to display the Load Case Data form.

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