No ordinance, resolution, or franchise shall become operative until 30 days after its passage by the Council and approval by the Mayor, except measures necessary for the immediate preservation of the peace, health, or safety of the city, but such an emergency measure shall not become immediately operative unless it states in a separate section the reason why it is necessary that it should become immediately operative, and unless it is approved by the affirmative vote of three-fourths of all the Council Members, taken by ayes and nays.