The word superstructure as used in sections 39-812 to 39-815 and 39-820, is hereby defined to mean all that part of a bridge or an approach thereto directly above and resting upon the stone or concrete abutments or piers, iron tubings, pile caps or other similar supports. The word substructure, as used in said sections, is hereby defined to mean all that portion of a bridge or approach thereto directly under and upon which rests the superstructure. The term unit quantity in place, as used in said sections, shall mean cubic feet of stone, cement, brick or concrete; lineal feet of piling; feet of lumber, board measure; lineal feet of tubing; and pounds of all other metal when properly placed in the bridge, approach or culverts according to the terms of the contract.