Select the Split command 
.
Select the element you want to split.
Click a point on the element where you want to split it.
A closed element requires you to define two points to split it.
Select the Split command .
Select the element you want to split.
Click the first point on the element where you want the split to begin.
Click the second point on the element where you want the split to end.
Tip:
The Split command applies appropriate geometric relationships at the split point automatically. In some cases, this deletes an existing relationship.