Public, semi-public 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 the zone in which the building is located, and also provided that the main shadow area shall be 2 1/2 times the height of the buildings. (See § 980-6 for definition of "shadow area.")