# How is rigid bodies implemented in finite element codes

I am writing a finite element code for structural analysis, and I want to implement rigid bodies. How is this usually done? Say that I have a square mesh, with one half of the mesh being defined rigid and the other deformable. How are the forces on the "boundary-nodes" (the nodes shared between the deformable and rigid part of the mesh) transfered to the 6 (3 in 2D) dofs of the rigid body?

I read somewhere that you could loop through all the nodes that are shared between the deformable and rigid parts, and sum all the forces (or express them as generalized forces) to get the total force acting on the rigid body. I can see how this would work for explicit finite element, but not for implicit/static since this does not contribute to the stiffness matrix.

Any insight in how to implement rigid bodies in my code is appreciated.

• Why not just take the stiffness of the rigid part as very large via the material properties? Jun 14 '18 at 20:14
• @DanielRch it is more efficent to make it rigid because you reduce the number of dofs, and you can not make it to stiff in explicit simulations due to the timestep becoming to small. Jun 15 '18 at 15:43