Performing an Inertia Relief Analysis
Introduction
Inertia relief analysis is a type of stationary analysis, where the external loads are balanced by (unknown) inertial forces. Thus, even though rigid body motion constraints must be present to avoid a singular stiffness matrix, there will be no reaction forces.
The basic principle is that different load cases are studied. Arbitrary constraints, sufficient to suppress all rigid body motions (in 3D: six constraints), are added to the model. Then, six load cases (in 3D) are run for the six possible translational and rotational unit accelerations. Each such load case will give a set of reaction forces. A seventh load case, and corresponding reaction forces, are given by the external loads on the structure. It is now possible to scale the unit accelerations and sum them to the external loads so that the reactions are all zero.
For more information, see
Inertia Relief
in the
Structural Mechanics Theory
chapter.
Since the procedure is based on linear superposition of precomputed load cases, the inertia relief analysis can produce incorrect results in case of geometric nonlinearity.
Setting up this procedure manually is possible, but tedious and error prone. For this reason there is a built-in functionality for this.
