Consider a "beam". In order to perform modal analysis using the finite element method, the beam is meshed with 2nd order hexa elements. The total count of nodes is 1280. The same beam can be either clamped on one side or both sides. Clamped ends are modelled by zero displacements (dx=dy=dz=0) for all nodes on the clamped surface. There are 80 nodes per clamped surface.
The mass and stiffness matrices are generated using code_aster.
The clamped-clamped beam matrices are 4800x4800, meaning that the system has 4800 degrees of freedom (dof).
The simply-clamped beam matrices are 4320x4320, meaning that the system has 4320 degrees of freedom (dof).
I don't understand where the number of dof comes from. I would expect 6 dof per node which gives: 1280 • 6 = 7680
Worst than that, if constraints are already included, I would expect the clamped-clamped beam to have less dof than the simply-clamped.
Can anyone help me?