In seismic design, the importance factor I is used to increase the margin of safety against collapse. For example, I = 1.50 for essential facilities, I = 1.25 for hazardous facilities, and I = 1.15 for special occupancy structures. Essential structures are those that must remain operative immediately following an earthquake such as emergency treatment areas and fire stations. Hazardous facilities include those housing toxic or explosive substances. Examples of special occupancy structures are those not classified as special or hazardous, and required for continuous operation. Standard occupancy structures such as office buildings, hotels, and residences are designed for I = 1.0. The above values of I apply to facilities designed under the regulations of the 2001 California Building Code (2001 CBC). For buildings designed under the 1997 UBC, the values of I are as follows (Table 16-K, UBC 1997):
• Special occupancy structures I = 1.0
• Standard occupancy structures I = 1.0
• Miscellaneous structures I = 1.0
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