Translations at Three Points

For a dynamic time, frequency, or random analysis, to specify the motion of the base in terms of translations at three point supports you need to constrain your model in a specific way. In the referenced modal analysis you must define point constraints for 3 non-collinear points in your model.

The point constraints must satisfy the criteria illustrated in the following example:

• Consider three points as shown in the figure. The first point, must have all translational degrees of freedom (DOFs) fixed, the second point must be constrained along 2 axes and the third point must be constrained along only one axis. With these three point constraints only the rigid body motion of the model is constrained.

• The constraint set for the three point constraints must be in terms of the WCS or a common Cartesian UCS.

• The first point must have all translational DOFs fixed. In the example Point 1 must have translations constrained along the X-, Y- and Z- directions.

• The line joining Point 1 to Point 2 must be parallel to one of the axes of the coordinate system. Point 2 must be free along this axis. It should be constrained along the other two axes. In the example the line joining Point 1 and Point 2 is parallel to Y-axis. Point 2 should have translations in the X- and Z- directions constrained.

• Point 3 must be in the plane through line joining Point1 and Point2 and parallel to one of the coordinate planes. Point 3 must be constrained in the direction normal to the plane defined by the three points. In the example the three points define the X-Y plane. Point 3 must have translation in the Z- direction constrained.