Such repairs and maintenance work as required to keep it in sound condition may be made to a nonconforming building or structure, provided no structural alterations shall be made except such as are required by law or ordinance or authorized by the Board of Zoning Appeals. Except as otherwise provided in thin Zoning Ordinance, the total structural repairs and alterations that may be made in a nonconforming building or structure shall not, during its life subsequent to the date of its becoming a nonconforming use, exceed fifty percent of its assessed value for tax purposes of such date, unless such building or structure is changed to a conforming use.