No building or structure, or part thereof, hereafter erected or altered in any M-3
district, the principal use of which is heavy industrial use, shall be located or
extended in front of a building line established by ordinance, or in the event that
no such building line has been established nearer than 100 feet from the front lot
line or nearer than 60 feet from a side or rear lot line abutting on a street.
