Contact element CONTA176 follows the
contact pair concept used by the surface-to-surface elements (CONTA171 through CONTA174).
You must pair CONTA176 with 3-D target line
segments (TARGE170) to model 3-D beam-to-beam
contact. See *Identifying Contact Pairs* for more information. CONTA176 uses most of the same element KEYOPTs and real
constants as the surface-to-surface contact elements. Any differences
are described in the next section.

The basic steps for performing a 3-D beam-to-beam contact analysis
are the same as those used for a typical surface-to-surface analysis
using CONTA171 through CONTA174. See *Steps in a Contact Analysis* for details.

Use the **ESURF** command to generate CONTA176 elements between corresponding contact pairs.
(In the GUI, pick .)
This procedure is similar to that used for the surface-to-surface
contact elements. If the underlying elements of target element TARGE170 are part of shell edges, use **ESURF**,,,LINE to generate 3-D line or parabola target segments.

When using the line segments to form the target surface or using line-to-line contact elements to form the contact surface, the nodes must be entered in a sequence that defines a continuous line, as shown in the figure below.

The line can be made up of linear or parabolic segments, depending
on whether the attached beam is made up of first order or second order
elements. If the nodal ordering of the underlying beam elements is
not consistent, you must either change them by reversing the node
number order of the selected elements (**ESURF**,,REVERSE
command) or make a consistent element ordering (**ENORM** command).

You can list the CONTA176 results using
the **PRESOL**,CONT or **PRETAB** commands.
Since CONTA176 is a 3-D line element, you
can also use the **PLLS** command to display element
table items.

CONTA176 uses most of the same KEYOPTs
that are used by the surface-to-surface contact elements, CONTA171 through CONTA174.
KEYOPT(3) and KEYOPT(4) differ from the other contact elements, and
KEYOPT(11) is not used by CONTA176. See *Element KEYOPTS* for a listing of the remaining KEYOPTs.

CONTA176 uses the same real constants
used by the surface-to-surface contact elements (CONTA171 through CONTA174), except for the units
of FKN and FKT. See a listing of the real constants in *Real Constants*.

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 CONTA176 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:

Determine the smallest cross section along the beam axis.

Determine the largest circle embedded in that 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}).

The contact radius R2 is always a positive number. The target radius R1 can be entered as either a negative or positive value. Use a negative value when modeling internal contact (a beam or pipe sliding within another beam or pipe), with the input value equal to the inner radius of the outer beam. Use a positive value when modeling external contact between the exterior surfaces of two cylindrical beams.

For beam-to-beam contact, the thickness effect is accounted for through the contact and target radii. Therefore, KEYOPT(11) (which is used for the surface-to-surface contact elements to include or exclude thickness effects) is not used for CONTA176.

For CONTA176, KEYOPT(3) determines the type of beam-to-beam contact as follows:

Use KEYOPT(3) = 0 or 2 to model external contact between parallel beams or internal contact between beams (a beam inside a hollow beam or a pipe inside a pipe). For this case, the sign of the target radius, R1, will determine whether you are modeling parallel beams (R1 > 0) or a beam inside a beam (R1 < 0).

Use KEYOPT(3) = 1 or 3 to model beams that cross. The target radius must be a positive value.

KEYOPT(3) also allows you to choose between a contact force-based model (KEYOPT(3) = 0 or 1), and a contact traction-based model (KEYOPT(3) = 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 (CONTA171 through 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/LENGTH^{3} 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 CONTA176 element
description for detailed information on the units for these quantities.

For CONTA176, 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 10 for small deflection
and 2 for large deflection (**NLGEOM**,ON).

You can use CONTA176 with the multipoint constraint (MPC) approach (KEYOPT(2) = 2) to define surface-based constraints. The KEYOPT(4) setting will determine the type of surface-based constraint. Set KEYOPT(4) = 1 for a force-distributed constraint, or set KEYOPT(4) = 0 for a rigid surface constraint. See Surface-based Constraints for more information.