  Non-intrusive methods have been used since two decades to derive
reduced-order models for geometrically nonlinear structures, with a particular
emphasis on the so-called STiffness Evaluation Procedure (STEP), relying on the
static application of prescribed displacements in a finite-element context. We
show that a particularly slow convergence of the modal expansion is observed
when applying the method with 3D elements, because of nonlinear couplings
occurring with very high frequency modes involving 3D thickness deformations.
Focusing on the case of flat structures, we first show by computing all the
modes of the structure that a converged solution can be exhibited by using
either static condensation or normal form theory. We then show that static
modal derivatives provide the same solution with fewer calculations. Finally,
we propose a modified STEP, where the prescribed displacements are imposed
solely on specific degrees of freedom of the structure, and show that this
adjustment also provides efficiently a converged solution.
