gp'', node2='', lab='', stif='', gap='', damp='', **kwargs)

Defines a gap condition for transient analyses.

APDL Command: GP

  • node1 – Node I of gap. If NODE1 = P, graphical picking is enabled and all remaining command fields are ignored (valid only in the GUI).

  • node2 – Node J of gap (must be different from NODE1). Non-grounded gap nodes must be defined as master degrees of freedom or be unconstrained, active DOF in a full analysis type. Grounded gap nodes (those not defined as MDOF) need not appear elsewhere in the model.

  • lab – Direction of gap action in the nodal coordinate system (implied from the following force labels): FX, FY, FZ, MX, MY, MZ.

  • stif – Stiffness (Force/Length) of closed gap (may be positive or negative).

  • gap – Initial size of gap. A zero (or positive) value assumes an initially open gap. A negative value defines an interference condition. For a rotational gap, GAP should be in radians.

  • damp – Damping coefficient (Force*Time/Length) of closed gap using pseudo velocity (Newmark finite difference expansion scheme).


Defines a gap condition for the mode superposition transient analysis (ANTYPE,TRANS with TRNOPT,MSUP). If used in SOLUTION, this command is valid only within the first load step. Gap conditions specified in subsequent load steps are ignored.

Repeat GP command for additional gap conditions. Gaps are numbered sequentially as input.

Note:: : Gaps may be renumbered by the program during the solution (see output listing)

The mode-superposition transient analysis does not allow gap action with the standard gap elements. However, you can define gap conditions which are similar to gap elements; gap conditions can be specified between surfaces that are expected to contact (impact) each other during the transient. The gap condition simulates the basic gap action of the COMBIN40 element.

The gap condition is treated as an explicit force (equal to the interference times contact stiffness) and affects only the load vector calculation and not the stiffness matrix. The interference is calculated from the displacement extrapolated from the previous time points.

Gap conditions can only be defined between two master degree of freedom (DOF) nodes or between master DOF nodes and ground, as shown in the following figure.

Master degrees of freedom are the unconstrained and active degrees of freedom. Gap nodes not defined as active degrees of freedom or attached to an element are assumed to be grounded. Grounded gap nodes do not need a spatial location, nor do they need to be located on an element.

Gap conditions may be defined in parallel (across the same nodes), with varying gap and stiffness values, to simulate a nonlinear (piecewise) force-deflection curve.

The gap direction is determined from the force label input on the GP command; i.e., FX defines a translational gap acting in the UX nodal degree of freedom direction, and MZ defines a rotational gap acting in the nodal ROTZ degree of freedom direction. The actual degree of freedom directions available for a particular node depends upon the degrees of freedom associated with the element types [ET] at that node.

If the coordinate systems of the nodes connecting the gap are rotated relative to each other, the same degree of freedom may be in different directions. The gap, however, assumes only a one-dimensional action. Nodes I and J may be anywhere in space (preferably coincident). No moment effects are included due to noncoincident nodes. That is, if the nodes are offset from the line of action, moment equilibrium may not be satisfied.

The contact stiffness value represents the stiffness of the closed gap. Stiffness values are related to the integration time step size and should be physically reasonable. High stiffness will require a small integration time step; otherwise, due to the displacement extrapolation, the solution may go unstable. Negative stiffness values may be used with gaps in parallel to produce a decreasing force- deflection curve.

The order of specifying the gap nodes is important; i.e., a gap condition connecting two nodes will act differently depending upon which node is specified first on the GP command. For example, for Node 1 at X = 0.0, Node 2 at X = 0.1, and the gap defined from Node 1 to 2, a displacement of Node 1 greater than Node 2 will cause the gap to close. For the gap defined from Node 2 to 1, a displacement of Node 2 greater than Node 1 will cause the gap to close (like a hook action). In general, the gap closes whenever the separation (defined as UJ - UI + GAP) is negative. UJ is the displacement of node J, UI is the displacement of node I, and GAP is the input gap value. The gap force output appears in the printout only for the time steps for which the gap is closed. A negative spring force is always associated with a closed gap (even with the hook option).

Some guidelines to define gap conditions are presented below:

Use enough gap conditions to obtain a smooth contact stress distribution between the contacting surfaces.

Define a reasonable gap stiffness. If the stiffness is too low, the contacting surfaces may overlap too much. If the stiffness is too high, a very small time step will be required during impact. A general recommendation is to specify a gap stiffness that is one or two orders of magnitude higher than the adjacent element stiffness. You can estimate the adjacent element stiffness using AE/L, where A is the contributing area around the gap condition, E is the elastic modulus of the softer material at the interface, and L is the depth of the first layer of elements at the interface.

A mode-superposition transient using the nonlinear gap damping provided through the DAMP field runs faster than a full transient analysis using a gap element (COMBIN40).

Use the GPLIST command to list gap conditions and the GPDELE command to delete gap conditions.

This command is also valid in PREP7.