### Two-node unidirectional gap element (GAPUNI)

This is a standard gap element defined between two nodes. The clearance d of the gap and its direction \boldsymbol{n} are defined by using the *GAP card. Let the displacement vector of the first node of the GAPUNI element be \boldsymbol{u_1} and the displacement vector of the second node \boldsymbol{u_2}. Then, the gap condition is defined by (Figure ):

 d+\boldsymbol{n} \cdot (\boldsymbol{u_2} -\boldsymbol{u_1}) \ge 0. (14)