To perform a mechanical simulation of a component, its geometrical model usually needs to be simplified in accordance with the hypotheses on its mechanical behaviour. Here, the preparation and the simplification of the model for the structural analysis act upon an intermediate polyhedral representation and take into account the mechanical hypotheses specified on the product shape. An a posteriori mechanical criterion has been incorporated into this process to bring an objective estimation of the model simplification. The a posteriori criterion can be applied to FE problems of linear static analysis or thermal problems for stationary linear conduction and is able to estimate the influence of shape transformations over the global analysis results. If a shape detail removed during the shape simplification process proves to be influent on the mechanical behaviour, it can be re-inserted on the simplified model, so readapting the initial simulation model. In this article, the focus is on the description of the geometric operators supporting the automatic process of FE model preparation.