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[FAQ]

The order of specifying the gap nodes NODE1and NODE2 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).