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Section 5
5-171
5.30.1 Cut-Off Frequency, Mode Shapes or
Time
Purpose
These commands are used in conjunction with dynamic analysis.
They may be used to specify the highest frequency or the number
of mode shapes that need to be considered.
General Format:
See Section
1.18.3
⎧FREQUENCY f
1
⎫
CUT
(OFF) ⎨
MODE SHAPE i
1
⎬
⎩
TIME t
1
⎭
Where,
f
1
= Highest frequency (cycle/sec) to be considered for dynamic
analysis.
i
1
= Number of mode shapes to be considered for dynamic
analysis. If the cut off frequency command is not provided,
the cut off frequency will default to 108 cps. If the cut off
mode shape command is not provided, the first six modes will
be calculated. These commands should be provided prior to
the loading specifications.
t
1
= Ending time for a time history analysis. If zero (default), the
time history will end when the last forcing function ends.
A maximum of i
1
mode shapes will be computed regardless of f
1
.
If during convergence testing, the 0 through f
1
frequencies are
converged, then the modal calculation will be completed before i
1
mode shapes are calculated.
If the CUT OFF FREQ f1 and CUT OFF MODE i1 commands are
both entered, then after completing each iteration step in the
Subspace iteration, convergence testing is performed. If every
frequency from 0.0 to f1 meets the convergence tolerance, then the
Subspace iteration is done. Similarly, if every mode from 0 to i1