Where plain solid masonry bearing walls are braced at distances not greater than twelve feet (12 ft.) apart by cross walls, or by internal or external offsets or returns at least two feet (2 ft.) deep, or by reinforced concrete floors, they may be of twelve-inch (12 in.) nominal thickness for the uppermost seventy feet (70 ft.), measured downward from the top of the wall, and shall be increased four inches (4 in.) in thickness for each successive seventy feet (70 ft.) or fraction thereof. 
(Ord. 1799-57.  Passed 5-20-57)