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Base Excitation...

The base excitation option in Pro/MECHANICA assumes that the input load will excite any part of the structure that has a constraint or is attached by a spring to ground. The excitation is always defined as a linear acceleration. The load may be a time, frequency or PSD function. It can be scaled and applied in any of the three orthogonal directions using a single input curve.

The following example is a time function load. The load function curve would be defined in units of G's while the scale factor in the Y direction converts the curve to in/sec^2 acceleration units.

With results defined to be relative to ground, the displacement, velocity and accelerations of any point on the structure will include the base motion. If a measure were defined at a support location the result would be the same as the input motion.

If the results are defined relative to supports then displacement, velocity and accelerations for any point on the structure will be the difference between base motion and the point on the structure. If a measure were defined at a support location the result would be zero.

The base motion option in Pro/MECHANICA Vibration is limited to linear acceleration and a single function curve for all three orthogonal directions. Another method called the 'Big Mass' method may be used to circumvent these limitations. The Big Mass method allows the application of rotational as well as linear accelerations. This method also allows a different input function for each load direction.

To use this method a mass equal to 1000 times the mass of the structure (include rotary inertia terms) is modeled at the CG of the structure. This mass point is attached to the support locations of the structure using rigid connections and at least one beam element. One thousand unit forces and/or moments are applied to the mass point using different 'member of set' names for each. These forces are scaled by the load functions on the Analysis Definition form such that the proper acceleration will be achieved based on the basic equation F=MA.