Frequency Cutoff (HZ) - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
13

(Available for: Modal, Spectrum, and Time History)

Specifies a frequency cutoff point in Hertz as described below.

When extracting modes to be used in dynamic analysis, you can specify a value for either Max. No. of Eigenvalues Calculated or a frequency cutoff. Modal extraction ceases when the Eigensolver extracts either the number of modes requested or extracts a mode with a frequency above the cutoff, whichever comes first.

You can select a frequency cutoff point for modes up to, but not far beyond, a recognized "rigid" frequency, and then include the missing mass correction. For more information, see Include Missing Mass Components. Choosing a cutoff frequency to the left of the resonant peak of the response spectrum provides a non-conservative result, because resonant responses may be missed. During spectrum analysis, using a cutoff frequency to the right of the peak, but still in the resonant range, yields either over- or under-conservative results, depending upon the method used to extract the ZPA from the response spectrum. For time history analysis, selecting a cutoff frequency to the right of the peak, but still in the resonant range, usually yields non-conservative results. The missing mass force is applied with a dynamic load factor of 1.0. Extracting many rigid modes for calculation of the dynamic response may be conservative in the case of spectrum analysis, because all spectral modal combination methods (such as SRSS, GROUP, and ABS) give conservative results versus the algebraic combination method used during time history analysis. This gives a more realistic representation of the net response of the rigid modes. Based upon the response spectrum shown below, an appropriate cutoff point for the modal extraction is about 33 Hz.

  1. Non-conservative cutoff (Misses amplification of any modes in resonant range)

  2. Conservative cutoff (Multiplies missing mass contribution by excessive DLF—1.6)

  3. Optimal cutoff (Includes all modes in resonant range, uses low DLF—1.05—for missing mass contribution, minimizes combination of rigid modes)

  4. Conservative Cutoff (Too many rigid modes combined using non-conservative summation methods)

When the analysis type is SPECTRUM, MODES, or TIMEHIST, either this parameter or Max. No. of Eigenvalues Calculated must have a value.