There are three different scenarios for 3D beam-to-beam contact:
Internal contact, where one beam slides inside another hollow beam (Figure 5.2: Internal Contact (One Beam Sliding Inside Another)).
External contact, where two beams are roughly parallel and contact each other along their outer surfaces (Figure 5.3: External Contact (Two Beams Roughly Parallel)).
External contact, where contact between the exterior surfaces of two beams is pointwise (Figure 5.4: External Contact (Two Beams Cross Each Other)).
For all three scenarios shown above, use 3D line segments (straight line or parabola) to define the target surface (TARGE170). You can attach the contact elements and target elements to 3D beams or pipes, which can be first or second order elements. Both flexible-flexible and rigid-flexible contact between two beams can be considered.
When modeling internal contact, the inner beam (or pipe) should usually be considered the contact surface and the outer beam (or pipe) should be the target surface. The inner beam can be considered as the target surface only when the inner beam is much stiffer than the outer beam. When modeling external contact, the target surface should be associated with the stiffer beam or with the beam having the coarser mesh.