Base Excitation for a Dynamic Time 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 Time Dependence—Specifies the direction of motion of the supports and the base acceleration as a function of time 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 from this dropdown list. Items displayed on the dialog box change based on the selected type of excitation.
◦ Unidirectional translation—Allows the supports to have only translational motion in one direction. Specify the direction in terms of its X, Y and Zcomponents 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 for 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. To use 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 select a coordinate system to specify the translational and rotational components of acceleration.
For Translations at 3 points you define the coordinate system when you define the point constraints for the three point supports.
For a selected excitation type you need to specify the following:
• 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 time dependence function is multiplied by value to give the base acceleration of the support for each component. When the excitation type is Translations & rotations or Translations at 3 points you need to specify individual values for each component.
• Time Dependence—Create or select a function for time dependence, or use the default function. The time dependence function is a function without units. The value of the function varies as a function of time.
• When the excitation type is Translations & rotations or Translations at 3 points, you must select the check box for each direction to activate the Value, and Time Dependence fields.
 For models created in Wildfire 5.0 and earlier releases, excitation type is selected as Unidirectional. Value is specified as 1 and the default function is selected for time dependence. 