Building or structure designed or intended for a nonconforming use. Ordinary repairs and alterations may be made to a nonconforming building or structure, all or substantially all of which is designed or intended for a use not permitted in the district in which it is located, provided that no structural alterations shall be made in or to such building or structure except those required by law or except to make the building or structure and use thereof conform to the regulations of the district in which it is located. For the purpose of this subsection, "ordinary repairs" shall include normal maintenance of a building or structure, and the replacement of storage tanks where safety of operation of the installation requires such replacement, and other replacements of, or substitutions for, machinery or equipment not involving structural alterations to the buildings or structure, except as hereinabove provided.