Structures are categorized into three Importance classes depending on the consequences of their loss of function. The importance classes are characterized by the Importance factor, I. Importance factor of 1.0 is associated with 475 year return period earthquake. Higher values of I represent return periods longer than 475 years. Importance Factor is assigned to a structure based on two factors: (1) Post-earthquake function of the structure: Critical facilities which are required to remain functional during and after the design earthquake. (2) Consequences of failure: Structures which house a large number of occupants, whose failure could lead to large casualties; loss of function detrimental to society. The highest value of importance factor equal to 1.5 is assigned to structures and facilities which are required to remain functional during and after the design earthquake. . Importance factor equal to 1.25 are assigned to structures having high occupancy.