Closely Spaced Mode Criteria/Time History Time Step (ms) - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
12

(Available for: Spectrum/GROUP and Time History)

Specifies a frequency or time-slice spacing as described below. The usage of this parameter varies with the analysis type.

Spectrum Analysis

For a spectrum analysis with the GROUP Modal Combination Method (as defined by USNRC Regulatory Guide 1.92), this value specifies the frequency spacing defining each modal group, that is, the percentage of the base frequency between the lowest and highest frequency of the group. Regulatory Guide 1.92 specifies the group spacing criteria as 10%, or 0.1. This is the default value in CAESAR II. For more information, see Modal Combination Method (Group/10%/DSRSS/ABS/SRSS).

Time History Analysis

For a time history analysis, this value is the length of the time slice, in milliseconds. The software uses the value during its step-by-step integration of the equations of motion for each of the extracted modes. CAESAR II uses the unconditionally stable Wilson q integration method where any size time step provides a solution. A smaller step provides greater accuracy but more strain on computational resources. The time step should be sufficiently small that it can accurately map the force versus time load profile (that is, the time step should be smaller than typical force ramp times). Additionally, the time step must be small enough that the contribution of the higher order modes is not filtered from the response. For this reason, the time step should be selected so that time step (in seconds) times maximum modal frequency (in Hz) is less than 0.1. For example, if Frequency Cutoff (HZ) is 50 Hz, this value should be set to no more than 2 milliseconds:

0.002 sec x 50 Hz = 0.1