Use the Add Breaklines dialog box to create standard breaklines.
You can use 3D lines, grading feature lines, splines, and 3D polylines as breaklines. The X, Y, and Z coordinates of each vertex on the polyline that you select are converted into TIN vertices. For 3D lines, each line that you select is defined as a two-point breakline.
Important: If you select a 2D polyline with a zero elevation, then it is saved with that elevation. Use proximity breaklines if you want the elevations calculated automatically.