**Power-spectral-density (PSD)** analysis is
a type of frequency-domain analysis in which a structure is subjected to a
probabilistic spectrum of harmonic loading to obtain probabilistic
distributions for dynamic response measures. A root-mean-square (RMS)
formulation translates the PSD curve for each response quantity into a single,
most likely value. Because PSD curves represent the continuous probability
density function of each response measure, most of the integrated area will
occur near the resonant frequencies of the structure. For accuracy, it is
important to capture response at frequency steps near the natural modes of
the structure.

Harmonic loading, characterized by sine and cosine functions, may have various components at different phase angles. Either hysteretic or viscous damping may be specified, though hysteretic (displacement-based) is most common for frequency-domain applications. The frequency-dependent behaviors assigned to links and supports are applied during PSD analysis.

**Steady-state
analysis** is another type of frequency-domain analysis in which a structure
is subjected to a given set of one or more harmonic load patterns. The response is
then calculated in a deterministic manner for each frequency of vibration.

__Additional
notes on PSD include:__

- Fatigue analysis is one application for which PSD
analysis is useful.
- Use a constant function to ensure that the
magnitude of the probable load is the same at all frequencies.
- The shape of the power-spectral-density input function is dependent upon the probability of loading for each frequency, and
the variation in likely load magnitude as a function of its frequency.

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