# # Tied Interfaces

Tied interfaces allow two sets of elements that do not have connectivity to be joined without the need for complex mesh refinement. Instead the elements on one side are taken as primary and the other side as constrained. The nodes on the boundary of the constrained side are tied to the nodes on the boundary of the primary side through a set of constraint equations.

As tied interfaces do not give proper continuity conditions across the boundary they should not be used in area where accurate determination of stresses is required.

## # Definition

Name

The name is only used as a convenient way of identifying a tied interface.

Stage List

This specifies a stage list using one any of the forms detailed in “Lists”. If this is set to “all” the tied interface applies to the whole model, irrespective of stage. Otherwise it applies only to the specified stage(s).

Constrained Type

The constrained side of the tied interface can be defined either as nodes or elements. When elements are selected GSA will find the nodes on the constrained side that need to be connected.

Constrained List and Primary List

The nodes in the constrained list are tied to elements in the primary list to form a continuous entity. The primary retains all its degrees of freedom. The primary elements must be 2D elements, but the constrained elements can be of any element type (although link elements should be avoided as these will give rise to conflicting constraints). So for example a beam element can be connected to the face of a 2D elements.

The primary list should contain the larger elements.

There are two options for the type of linkage if the constrained type is “nodes”:

• Auto – the tied interface behaves as tied if the constrained type is element and rigid if the constrained type is node.
• Tied – the tied interface connects the node on the constrained side to the element faces on the primary side using constraint equations.
• Rigid – the tied interface take the node on the constrained side as a primary and creates a rigid constraint making the element on the primary side rigidly connected with the node.

The third of these options is intended for situations where the node represents a line entity such as a column that is to be attached to a slab. In this case, if the beam element cross section is a similar size to the 2D element on the other face the effect of the connection will be to make the 2D element behave in a manner close to being rigid. This option will not work where there are nodes to connect to the same element face or adjacent element faces.

The rigid linkage cannot be used when the constrained type is element as this would constraint conflict.

Surface Tolerance

This is used to limit the nodes that are connected to only those along the edge. Nodes outside this tolerance are excluded from the tied interface.

## # Tied Interface

A tied interface works by connecting the nodes associated with the constrained elements to the elements on the primary side to form a continuous entity.

A tied interface is a way of joining two meshes with dissimilar mesh densities by establishing a set of constraint equations to “stitch” the nodes on the constrained side to those on the primary side. The tied interface can be used to attached elements along two edges or elements on an edge to a face, or to connect a beam element to the face of a 2D element.

The primary elements must be 2D elements, but the constrained elements can be of any element type (although link elements should be avoided as these will give rise to conflicting constraints). The primary retains all its degrees of freedom.

The elements on the primary side should be of the same size as or larger than those on the constrained side for the tied interface to work well. The accuracy of the tied interface will be controlled by the mesh size on the primary side.