Public, semipublic buildings, public utilities, public service buildings, civic buildings, hospitals, schools, churches, temples, grain elevators, scenery lofts, elevator bulkheads, and silos, where permitted, may be erected to a height not to exceed 100 feet, provided that the side and rear yards are increased by one foot for each foot of additional building height above the height regulations for which the building is located; and also, providing that the main shadow area shall be 2 1/2 times the height of the buildings. (See definitions for "shadow area.")