6.1.1.1. Single-Point Response Spectrum (SPRS).
In a single-point response spectrum (SPRS) analysis, you specify one response spectrum curve (or a family
of curves) at a set of points in the model, such as at all supports, as shown in Figure 6.1: Single-Point
and Multi-Point Response Spectra (p. 136) (a).
6.1.1.2. Multi-Point Response Spectrum (MPRS).
In a multi-point response spectrum (MPRS) analysis, you specify different spectrum curves at different
sets of points, as shown in Figure 6.1: Single-Point and Multi-Point Response Spectra (p. 136) (b).
Figure 6.1: Single-Point and Multi-Point Response Spectra
s(b)s=spectralvalue
f=frequencysf
(a)fsfsf6.1.2. Dynamic Design Analysis Method (DDAM).
The Dynamic Design Analysis Method (DDAM) is a technique used to evaluate the shock resistance of
shipboard equipment. The technique is essentially a response spectrum analysis in which the spectrum
is obtained from a series of empirical equations and shock design tables provided in the U.S. Naval
Research Laboratory Report NRL-1396.
6.1.3. Power Spectral Density
Power spectral density (PSD) is a statistical measure defined as the limiting root mean-square (rms)
value of a random variable. It is used in random vibration analyses in which the instantaneous magnitudes
of the response can be specified only by probability distribution functions that show the probability of
the magnitude taking a particular value. It is assumed that the dynamic input has a zero mean value
and the range of values takes the form of a Gaussian or normal probability distribution.
A PSD is a graph of the PSD value versus frequency, where the PSD may be a displacement PSD, velocity
PSD, acceleration PSD, or force PSD, that captures both the power or intensity of the input vibration
and its frequency content. The PSD value is in (unit)^2 /Hz, such as g^2 /Hz.. Mathematically, the area under
a PSD-versus-frequency curve is equal to the variance (square of the standard deviation) of the input
vibration. Likewise, the output also takes on a Gaussian distribution and zero mean value. The output
values of a PSD analysis are the response PSDs, with the area under the response PSD curve being the
variance (the square of the standard deviation) of the response.
Similar to response spectrum analysis, a random vibration analysis may be single-point or multi-point.
In a single-point random vibration analysis, you specify one PSD spectrum at a set of points in the
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