Any city, city and county, or county may assign allowable working stresses to existing materials based on substantiating research data or engineering judgment. Such allowable working stresses shall be limited by a safety factor which is reasonably commensurate with the importance of the element in which the material is used. In the event the local jurisdiction does not have the ability to assign such allowable working stresses, it may employ as a consultant the office of the State Architect. Allowable working stresses prepared by the office of the State Architect for any city, city and county, or county shall be subject to approval by the Seismic Safety Commission.