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Access the Additional Frequencies form as follows:
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Use the Additional Frequencies form to specify additional frequencies at which the response is to be calculated for a Steady-State or Power-Spectral-Density Analysis case. Only frequencies between the first and last frequency specified for the analysis case will actually be used. Additional frequencies outside that range will be ignored.
Modal Case drop-down list. Parameters specified on the form can be used to request that the response be calculated at or near modal frequencies computed for the structure. Begin by selecting the name of a previously defined modal analysis case from which the frequencies are to be obtained.
Add Modal Frequencies check box. When this check box is checked, the analysis will be performed at the modal frequencies of the structure. The use of this option is recommended, but only if the steady-state or power-spectral-density analysis case being defined has damping. Otherwise, large and meaningless responses will be calculated.
Add Deviations from Modal Frequencies check box. Check this box if the analysis is to be performed at frequencies near the modal frequencies of the structure. After checking this box, create, modify, or delete a list of signed fractional deviations from the modal frequencies. Examples include 0.01, -0.01, 0.02, -0.02, 0.05, -0.05, 0.10. For a mode with natural frequency of 20, this would correspond to frequencies 20.2, 19.8, 20.4, -19.6, 21.0, 19.0, and 22.0. Positive and negative values are independent. Values must be greater than -1.
Add button. To add a value, type a frequency into the edit box and click the Add button.
Modify button. To change a value in the list, click on the value in the list, make changes in the edit box, and click the Modify button.
Delete button. To delete a value in the list, click on the value in the list, and click the Delete button.
The use of this option is strongly recommended, even for models without damping.
Add Specified Frequencies check box. Check this box if the analysis is to be performed at specific frequencies of interest, such as at the operating frequency of machinery in the structure, or at the natural frequencies of sensitive equipment in the structure. After checking this box, create, modify, or delete a list of specific frequencies. Values are given in Hz (cycles per second).
Add button. To add a value, type a frequency into the edit box and click the Add button.
Modify button. To change a value in the list, click on the value in the list, make changes in the edit box, and click the Modify button.
Delete button. To delete a value in the list, click on the value in the list, and click the Delete button.