Base Excitation for a Dynamic Random Analysis
Base excitation is only available if your dynamic analysis references one constrained modal analysis.
If no loads are defined for your model, you must define the dynamic analysis using base excitation. In this case the structure is excited by a prescribed shaking of the supports that hold it.
Base Acceleration PSD—Specifies the direction of motion of the supports and power spectral density of the base acceleration when you select Base excitation as the loading type. The following items are displayed in this area:
• Excitation type—Select the type of excitation of the supports from this dropdown list. Items displayed on the dialog box change based on the selected excitation type.
◦ Unidirectional translation—Allows the supports to have only translational motion in one direction. Specify the direction in terms of its X, Y and Z components in the WCS or a selected Cartesian coordinate system and the base acceleration along that direction.

For Unidirectional translation, the magnitude of the direction vector is ignored, and only its directional information affects the acceleration.

◦ Translations & rotations—Allows the supports to have both translational and rotational motion. Specify X,Y, and Z components of both translational and rotational acceleration of the supports along each axis of the WCS or a selected coordinate system. Rotations are about the origin of the referenced coordinate system.
◦ Translations at 3 points—Specifies the motion based on six translational directions at 3 point supports. For this option, the referenced modal analysis needs to be constrained with 3 point constraints. All translations must be constrained at the first point, 2 translations constrained at the second point and 1 translation constrained at the third point in such a way so that only rigid body motion is prohibited.

2D plane stress and 2D plane strain models can have Unidirectional translation or Translations & rotations as the excitation type. For 2D axisymmetric models you can have only Unidirectional translation as the excitation type.

• Coordinate system—For Unidirectional translation select a coordinate system to specify the direction of movement of the supports. WCS is the default. You can also select a Cartesian user coordinate system.
For Translations & rotations select a coordinate system to specify the translational and rotational components of acceleration.
For Translations at 3 points you have to define the coordinate system when you define the constraints for the supports.
For a selected excitation type you need to specify the following items:
• Value—Specify a value for base acceleration. Select units from the adjacent dropdown list. The units are acceleration 2 /Hz for a dynamic random analysis. Value acts as a multiplier. The PSD function is multiplied by value. When the direction of base acceleration is Translations & rotations or Translations at 3 points you need to specify individual values for each component.
• PSD—Specify a function for PSD or use the default functions specified for an analysis. The PSD function is a function without units that is multiplied by value to give the resultant base acceleration of the supports. For a dynamic random analysis PSD varies as a function of frequency.
When the direction is Translations & rotations or Translations at 3 points, you need to select the check box for each direction to activate the Value, and PSD fields.
 For models created in Wildfire 5.0 and earlier releases, direction is selected as Unidirectional. Value is specified as 1 and the default function is selected for PSD. 