For beam-to-beam contact, an important assumption is that of constant circular beam cross section. The contact radius is assumed to be the same for all CONTA177 elements in the contact pair. Likewise, the target radius is assumed to be the same for all TARGE170 elements in the contact pair. You supply the target and contact radii through real constants R1 and R2, respectively.

For a general beam cross section, you can use an equivalent circular beam in the contact definition (see figure below). Use these guideline to define the equivalent circular cross section:

Figure 5.6: Equivalent Circular Cross Section

Use the first real constant, R1, to define the radius on the target side (target radius r_t). Use the second real constant, R2, to define the radius on the contact side (contact radius r_c).

To model external contact between the exterior surfaces of two cylindrical beams, set KEYOPT(9) = 0 (default) for the TARGE170 target elements in the contact pair. To model internal contact (a beam or pipe sliding within another beam or pipe), set KEYOPT(9) = 1 for the TARGE170 elements and input R1 as the inner radius of the outer beam.

In a general contact definition, the equivalent beam radius is specified via SECTYPE and SECDATA commands instead of using real constants. For more information, see the discussion on applying surface geometry in a general contact definition.

For both pair-based contact and general contact, if the contact radius and/or target radius are not defined, the program automatically calculates the equivalent radius for each individual contact/target element based on the associated geometry of underlying elements. In this case, the equivalent radius may vary within a contact pair or within a general contact surface.

In the case of a rigid target defined with TARGE170 elements, the program cannot calculate an equivalent radius because there are no underlying elements. Therefore, you must explicitly input the target radius for all rigid target elements.

For beam-to-beam contact, the thickness effect is accounted for through the contact and target radii. Therefore, KEYOPT(11), which controls the thickness effect for line-to-surface contact, is not used.

For CONTA177, KEYOPT(3) determines the type of 3D beam-to-beam contact as follows:

KEYOPT(9) of the target element (TARGE170) determines whether you are modeling external contact between beams (KEYOPT(9) = 0, default) or internal contact between beams (KEYOPT(9) = 1). Internal contact can be a beam inside a hollow beam or a pipe inside a pipe.

KEYOPT(3) also allows you to choose between a contact force-based model (KEYOPT(3) = 0 or 4), and a contact traction-based model (KEYOPT(3) = 1, 2, or 3). For the contact traction-based model, the program determines the area (based on beam element length and the contact radius, R2) associated with the contact node.

When the traction-based model is specified, certain real constants (FKN, FKT, TNOP) and postprocessing items (PRES, TAUR, TAUS, SFRIC, and so on) have the same units as in the surface-to-surface contact elements (CONTA172 and CONTA174).

When the force-based model is specified, the units of these quantities have a factor of AREA with respect to those used in the traction-based model. For example, contact stiffness FKN has units FORCE/LENGTH for the force-based model, but FORCE/LENGTH³ for the traction-based model. PRES is the contact normal force in the force-based model, but contact pressure in the traction-based model. See the CONTA177 element description for detailed information on the units for these quantities.

For CONTA177, the contact normal is uniquely defined and is perpendicular to both the contact and the target surfaces (two contacting beams). Real constant TOLS is used to add a small tolerance that will internally extend the edge of the target surface. TOLS is useful for problems where contact nodes are likely to lie on the edge of the target (as at symmetry planes or for models generated in a node-to-node contact pattern). In these situations, the contact node may repeatedly slip off the target surface and be completely out of contact, resulting in convergence difficulties from oscillations.

Units for TOLS are percent (1.0 implies a 1.0% increase in the target edge length). A small value of TOLS will usually prevent this situation from occurring. The default value is 2 for both small deflection (NLGEOM,OFF) and large deflection (NLGEOM,ON).

You can use CONTA177 with the multipoint constraint (MPC) approach (KEYOPT(2) = 2) to define surface-based constraints. The KEYOPT(4) setting determines the type of surface-based constraint (rigid surface, force-distributed, or coupling constraint).

Note that in the case of collinear nodes, the moment along the axes parallel to the collinear nodes cannot be transmitted.

For the force-distributed constraint and the rigid surface constraint, you can use the Lagrange multiplier method (KEYOPT(2) = 3) as an alternative to the MPC approach.

See Surface-based Constraints for more information.

KEYOPT(14) allows each contact detection point to interact with multiple target segments simultaneously:

The maximum number of contact interactions allowed at each contact detection point is determined by the KEYOPT(14) and KEYOPT(3) settings, as outlined in the table below.

The following example models show different combinations of KEYOPT(14) and KEYOPT(3).

Figure 5.7: Using KEYOPT(14)

KEYOPT(14) = 0 with Parallel Beams (KEYOPT(3) = 1)	KEYOPT(14) = 1 with Parallel and Crossing Beams (KEYOPT(3) = 2)
KEYOPT(14) = 2 with Parallel Beams (KEYOPT(3) = 1)	KEYOPT(14) = 2 with Crossing Beams (KEYOPT(3) = 2)