Interactive guide to Risk Categories I, II, III, and IV - Understanding occupancy classifications and importance factors for wind load calculations
Buildings and structures that represent a low hazard to human life in the event of failure
Agricultural storage buildings
Minor storage facilities
Construction trailers, temp storage
Unoccupied agricultural structures
| Aspect | Risk Category I Requirements |
|---|---|
| Importance Factor (Iw) | 0.87 |
| Mean Recurrence Interval | 300 years |
| Design Wind Speed | V × 0.87 (effectively reduced) |
| Typical Occupancy | Minimal or none |
| Economic Impact | 13% savings on wind resistance |
Buildings and structures that represent the standard level of risk - most common building types fall into this category
Single-family and multi-family residential
Standard commercial office space
Shopping centers, strip malls, stores
Hospitality buildings, motels
Standard commercial storage
Dining establishments under 300 occupancy
| Aspect | Risk Category II Requirements |
|---|---|
| Importance Factor (Iw) | 1.00 (baseline) |
| Mean Recurrence Interval | 700 years |
| Design Wind Speed | V (no modification) |
| Typical Occupancy | Standard residential/commercial |
| Percentage of Buildings | ~80-85% of all structures |
| Design Philosophy | Code-minimum baseline protection |
Buildings where more than 300 people congregate in one area, or structures containing hazardous materials
K-12 schools with large assembly areas
Houses of worship with 300+ capacity
Performance venues, cinemas
Assisted living, elderly care facilities
Facilities with hazardous materials
Large-capacity treatment facilities
| Aspect | Risk Category III Requirements |
|---|---|
| Importance Factor (Iw) | 1.15 |
| Mean Recurrence Interval | 1,700 years |
| Design Wind Speed | V × 1.15 (effectively increased) |
| Occupancy Threshold | 300+ people in one area |
| Economic Impact | ~15% increase in structural costs |
| Special Populations | Children, elderly, incarcerated, mobility-impaired |
Buildings designated as essential facilities that must remain operational during and after a disaster event
Emergency medical facilities with surgery
Emergency response facilities
Law enforcement facilities
Electricity generation and distribution
Emergency communication centers
Designated disaster shelters
Critical medical supply facilities
Critical government/emergency data
| Aspect | Risk Category IV Requirements |
|---|---|
| Importance Factor (Iw) | 1.15 (same as Risk III) |
| Mean Recurrence Interval | 3,000 years |
| Design Wind Speed | V × 1.15 (effectively increased) |
| Post-Storm Function | Must remain operational after disaster |
| Design Philosophy | Maximum protection and resilience |
| Additional Requirements | Enhanced durability, backup systems, redundancy |
| Economic Impact | 15%+ increase plus enhanced systems |
While both Risk Category III and IV share the same importance factor (Iw = 1.15), the key difference lies in post-disaster functionality:
Risk IV structures often require backup power, redundant systems, enhanced structural protection, and higher construction standards beyond just the wind importance factor.
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