Problem data ========================== The data set for a calculation of natural vibration modes is classical and common to most mechanical calculations in *Code_Aster*: * reading the mesh (operator LIRE_MAILLAGE), * assignment of the characteristics of the model: beam or 3D model or...? (AFFE_MODELE), * definition and assignment of materials (DEFI_MATERIAU and AFFE_MATERIAU) and/or assignment of the characteristics of structural elements (AFFE_CARA_ELEM), *possible imposition of boundary conditions (step absent if the structure is completely free). The particularity of the modal calculation in*Code_Aster* currently is that it is generally necessary that the conditions at the displacement limits, if any, be imposed by dualization (AFFE_CHAR_MECA, keyword factor DDL_IMPO — the most common — or FACE_IMPO or ARETE_IMPO) rather than by kinematic loads (AFFE_CHAR_CINE). The schematization of the data of a modal calculation problem. .. image:: images/Object_1.png :width: 5.0252in :height: 5.6736in .. _RefSchema_Object_1.png: **Figure** 2-a **: Data entry of a modal calculation problem.** **Notes:** * *For a modal calculation, no excitation is necessary, unless you want to take into account the effect of geometric stiffness brought by a static loading (advanced study). If applicable, the paragraph :ref:`3.2.2.3 `* indicates the approach to be adopted (advanced study) . * * *For a simple study, the structure is not prestressed: any boundary conditions when moving are generally zero. If we want to take into account the prestress generated by non-zero displacements, we must also adopt the approach indicated in paragraph :ref:`3.2.2.3 `* . *