Load Case Data - Power Spectral Density Form

The  Load Case Data - Power Spectral Density form is used to view and change the definition of a power-spectral-density (PSD) load case. Click the Analysis > Type > Power Spectral Density > New command to display the Load Case Data - Power Spectral Density form.

A PSD load case solves for the response of the structure resulting from cyclic (harmonic, sinusoidal) loading over a range of frequencies, and then integrates the resulting spectrum weighted by a probabilistic power-spectral-density function to get a root-mean-square (RMS) expected response. The structure may be damped or undamped. Frequency-dependent stiffness and damping (complex impedance) properties may be included for the Link elements.

Important: In a single PSD load case you may apply one or more loads at the same or different phase angles. All loads applied in the same load case are assumed to be fully correlated, i.e., they are algebraically added.

To do this, each applied load is multiplied by its specified scale factor and the square-root of the specified power-spectral-density function, applied to the structure at its specified phase angle, and summed with all the other loads, similarly applied. The response to this combined load is squared to generate the power-spectral-density response, which can be integrated to get the RMS response. You may also plot the power-spectral density function for any response quantity.

To combine uncorrelated loads, use SRSS type Combinations of PSD load cases. The result will be a single RMS value for each response quantity.

For example, consider two uncorrelated machines vibrating on the same platform. Each machine may generate a horizontal X and a vertical Z force, 90 degrees out of phase with each other. The two forces for a given machine are fully correlated since they are both generated by the same eccentric, spinning mass. This problem requires the following:

Use the various options available on the Load Case Data - Power Spectral Density form to solve this and other similar problems.

See Also

Load Case Data

Define Load Cases

Define Power-Spectral-Density Functions