The City Engineer shall not issue a permit to move any building or structure when the weight of such building or structure, plus the weight of the vehicle or other equipment, exceeds the weight permitted by the California Vehicle Code; except, that if it appears to the City Engineer that the size, shape or physical characteristics of the building or structure or portion thereof, to be moved, or of the street over which such building or structure is to be moved, makes it impossible or impracticable to keep within such weight limits, the City Engineer may issue a permit to move a building or structure on a vehicle every wheel of which is equipped with pneumatic tires.