# Structural Element Nodes

## Mechanical Behavior

Six degrees of freedom, composed of three translational components and three rotational components, are associated with each node. All mass associated with the structural elements is lumped at the nodes; each node inherits mass from the structural elements that use it.

There are two coordinate systems (the global system and the node-local system) associated with each node:

1. The global system can be used to specify generalized velocity and displacement boundary conditions, nodal positions, and applied loads. The global system does not change throughout the course of a simulation.

2. The node-local system is used to specify attachment conditions that control how the node interacts with the grid. Also, the equations of motion are solved in these local directions. Therefore, one may fix or free velocities in these directions only.

The orientation of the node-local system is set automatically at the start of a set of cycles (or when the model cycle 0 command is executed), based on the type of elements that use the node. For beams and shells (3D ONLY), the local system is aligned with the global system. For cables and piles, the local system is oriented such that (1) the $$x$$-axis is aligned with the average axial direction of all cable or pile elements using the node, and (2) the $$yz$$-axes ($$y$$-axis in 2D) are arbitrarily oriented in the cable or pile cross-sectional plane. For geogrids and liners (3D ONLY), the local system is oriented such that (1) the $$z$$-axis is aligned with the average normal direction of all geogrid or liner elements using the node, and (2) the $$xy$$-axes are arbitrarily oriented in the geogrid or liner tangent plane. When running in large-strain mode, the orientation of the node-local system is updated automatically, based on the motion of the elements that use the node (unless this behavior is inhibited by the structure node system-local command).

In general, geometry creation is carried out as described in Geometry Creation. In addition, individual nodes can be repositioned by using the structure node initialize position command. Any elements that are moved as a result of moving nodes will retain their default attachment conditions.

Accumulated displacement and rotation at nodes can be assigned initial values with the structure node initialize command. For example, these components can be initialized to zero at any stage of analysis. Note that displacement values are not used as part of the computation cycle. Instead, incremental displacements are computed by multiplying the nodal velocity by the timestep.

When selecting with the group range element, nodes are considered to be a member of a group by default if either they or any element the node is connected to is a member of that group. The by keyword may be used to restrict this to a specific type of object.

## Response Quantities

Nodal responses include displacement, velocity, position, and out-of-balance force. These quantities can be expressed in either the global or node-local coordinate system. The nodal responses can be accessed via FISH and