# Base Excitation

Transient/Frequency Response
1. Two types of base excitations are possible. The excitation can be specified in terms of displacements or accelerations. Both are applied in the global X, Y or Z coordinate frames only.
2. You can select the excitation type (displacement or acceleration), time function, scaling amplitude and direction.
3. In addition, displacement-based excitations can be applied to individual supports which can be important for long ground-based structures like pipe systems, bridges, and transportation systems.
Random Response
1. The excitation can be specified in terms of accelerations. They are applied in the global X, Y or Z coordinate frames only.
2. You can select the excitation type (acceleration), frequency/PSD function, scaling amplitude and direction.
3. The Power Spectral Density (PSD) of the response, ${S}_{xo}\left(f\right)$ , is related to the power spectral density of the source, ${S}_{a}\left(f\right)$ , by:(1)
${S}_{xo}\left(f\right)=\left|\left({H}_{xa}\left(f\right)\right|{}^{2}S{}_{a}\left(f\right)$
Where, ${H}_{xa}\left(f\right)$ is the frequency response function.

Let us take an example of base excitation as an input to a random response analysis.

The base excitation with amplitude is used to define the input for frequency response analysis, so the units of the acceleration excitation type would be any of the below highlighted units
• m/sec2
• mm/sec2
• cm/sec2
• G
• in/sec2
Units for PSD function depend on the boundary condition. In this example, as base excitation is given as acceleration, the unit for PSD is (mm/s2)2/Hz.
Note: Only immovable and general constraints can be used as the source location. Hinges, springs and sliders are not supported at this time.