The
SEISMIC
menu is shown in the figure (right). T/HIS can be
used to handle response spectra information. In particular, displacement,
velocity or acceleration spectra can be read and converted to another
format.
DV
|
Displacement spectrum is converted
to a velocity spectrum
|
DA
|
Displacement spectrum is converted
to an acceleration spectrum
|
VD
|
Velocity spectrum is converted
to a displacement spectrum
|
VA
|
Velocity spectrum is converted to an acceleration
spectrum
|
AD
|
Acceleration spectrum is converted to a displacement
spectrum
|
AV
|
Acceleration spectrum is converted to a velocity
spectrum.
|
DS
|
Produce a design spectrum from a response
spectrum through the specification of a broadening factor..
|
RS
|
Produce a response spectrum from
input accelerations. This gives the response of a damped single degree
of freedom system, given its damping factor and period, to the input acceleration
time-history.
|
FFT
|
Perform a fast Fourier transform. Convert an input signal from the time
to the frequency domain.
There are three options for output;
magnitude only
magnitude and phase
real and imaginary components of the time signal.
The frequency is calculated in Hz NOT radians/s if the time axis is in
seconds.
T/HIS automatically adds points with zero y-value to the end of the curve
to pad the curve out so that the number of points is increased to the
next power of 2.
There are two options for scaling the curves output:
Scaling Option 1 - Consistent with other signal processing software
giving a magnitude independent of any padding. This is the default and
recommended for most purposes. Performing an inverse FFT on the resulting
curves will NOT get back exactly to the original curve if it did not have
a number of points equal to a power of 2.
Scaling Option 2 - With this option, applying an inverse FFT to the
resulting curves will generate a curve the same as the original even if
the original curve did not have a number of points equal to a power of
2. This is useful if users wish to create their own filters, where the
filter characteristic is defined in the frequency domain.
An option to regularise the curve before performing the function is on
by default. The spacing between points on the frequency axis of the resulting
curve is determined by the time duration of the padded input curve; dx
= 1.0/(time).
The highest frequency in the output curve is determined by the time
interval of the input curve; F
(max).
= (#points)/dt
|
IFFT
|
Performs an inverse fast Fourier transform. Converts two input signals
from the frequency to the time domain. The two input signals can be the
magnitude and phase or real and imaginary components of the time signal.
NOTE: If an FFT using scaling option 1 is performed on a curve that does
not have a number of points equal to a power of 2 and then an IFFT is
performed on the resulting curves you will NOT get back exactly to the
original curve. This is because the FFT and IFFT both scale their output
curves by the number of points in the curve, which in this case will be
different. For the FFT the number of points used to scale the curves is
the original number of points before padding. For the IFFT the number of
points used is the original number of points plus the points needed to make
it a power of 2.
If the number of points in the original curve is a power of 2 and no
padding is required, the IFFT of the resulting curves will get back to
the original curve.
|
NCP
|
By default beam element plastic
rotations are always written out by Ansys LS-DYNA as being increasing +ve (i.e.
cumulative). This option allows a non-cumulative plastic rotation to be
calculated by taking two input curves: the moment/time and the cumulative
rotation/time histories for the beam in question.
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BLC
|
Baseline correction.
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