Importance factor is a numerical value used in engineering design to account for the relative importance of a structure or component based on its function, occupancy, and potential risk to human life in the event of failure. It serves as a multiplier applied to design loads such as wind, seismic, snow, or other environmental forces to ensure that critical or essential facilities such as hospitals, emergency centers, and power plants are designed to higher safety standards than ordinary buildings. Structures with higher importance factors must withstand greater forces because their continued operation during or after a disaster is vital for public safety. Conversely, buildings with low human occupancy or minimal risk, like storage sheds, may have a lower importance factor. This approach ensures that the level of structural reliability is proportional to the potential consequences of failure.
Each building and structure shall be assigned a risk category in accordance with Table 1604.5 of this code. Where a referenced standard specifies an occupancy category, the risk category shall not be taken as lower than the occupancy category specified therein. Where a referenced standard specifies that the assignment of a risk category be in accordance with ASCE 7, Table 1.5-1, Table 1604.5 of this code shall be used in lieu of ASCE 7, Table 1.5-1.
Importance Factor (Table 1604.5.2) | |||||
|---|---|---|---|---|---|
| Risk Category | Hazard to Human Life | Snow \(\;I_s\) | Ice \(\;I_i\) | Wind \(\;I_w\) | Seismic \(\;I_e\) |
| \(I\) | Low - Failure Poses Minimal Risk to Life | \(0.80\) | \(0.80\) | \(1.00\) | \(1.00\) |
| \(I \;I\) | Normal - Buildings not Classified as I, III, or IV | \(1.00\) | \(1.00\) | \(1.00\) | \(1.00\) |
| \(I \;I \;I\) | Substantial - Failure Poses a Substantial Risk to Life | \(1.10\) | \(1.15\) | \(1.00\) | \(1.25\) |
| \(I \; V\) | Essential Facilities - Structures Critical for Post-disaster Response or Containing Hazardous Materials | \(1.20\) | \(1.25\) | \(1.00\) | \(1.50\) |
