The height limitations of this chapter shall not apply to the following structures: church spires, chimneys, elevator bulk heads and other mechanical equipment that is part of the principal structure, conveyors, flagpoles, silos, standpipes, elevated water tanks, derricks, gas, oil or mineral extraction drilling rigs and supporting facilities, public utility structures, barns and other structures not intended for human habitation which do not exceed the height limitations of the zoning district by more than 15 feet.