Geometric invariance

We discuss changes of variables deriving from changes in the placement of reference entities in the modeling of rigid bodies, beams and shells. We show that standard numerical procedures in general fail to be invariant with respect to these particular changes of variables, and we characterize the nature of the error. Finally, we show how the fixed pole form of the equations of equilibrium automatically guarantees invariant numerical processes, being based on global kinematic and co-kinematic quantities.