Base Excitation for a Dynamic Frequency Analyses
Base excitation is available only 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 Frequency Dependence—Specifies the direction of motion of the supports and the amplitude and phase of the base acceleration as functions of frequency when you select Base excitation as the loading type. This area displays the following items:
• Excitation Type—Specifies the type of excitation of the supports. 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 6 translational directions at 3 point supports. For this option, the referenced modal analysis must 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 the coordinate system is used to specify the translational and rotational components of acceleration.
For Translations at 3 points the coordinate system is defined 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 of acceleration or angular acceleration from the adjacent dropdown list. Value acts as a multiplier. The amplitude function is multiplied by value to give the resultant base acceleration of the supports.
When the direction of base acceleration is Translations & rotations or Translations at 3 points you need to specify individual values for each component.
• Amplitude—Specify a function for amplitude or use the default function specified for an analysis. The amplitude function is a function without units. Amplitude varies as a function of frequency.
• Phase—Create or select a function for phase, or use the default function. Phase 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, Amplitude and Phase fields.
 For models created in Wildfire 5.0 and earlier releases direction is selected as Unidirectional. Value is specified as 1 and the default functions are selected for amplitude and phase. 