Agricultural Buildings & Low Hazard Structures with Minimal Occupancy
Risk Category I represents buildings and other structures that pose a low hazard to human life in the event of failure. These structures typically have minimal or no human occupancy, are used for agricultural purposes, or are temporary in nature. The reduced importance factor of 0.87 recognizes that failure of these structures would not endanger public safety.
This classification provides a 13% reduction in design wind loads compared to standard structures, making it the most economical risk category for appropriate applications.
Livestock barns, equipment storage barns, and hay storage buildings with limited occupancy and low economic value to human life safety.
Grain storage silos, bulk material storage, and agricultural processing facilities not occupied during extreme weather events.
Simple storage sheds, pole barns, and accessory structures used exclusively for equipment or material storage.
Open-sided or partially enclosed structures for protecting farm equipment and machinery from weather.
Construction trailers, temporary storage buildings, and portable structures not intended for permanent occupancy.
Minor agricultural processing facilities with automated operations and minimal worker presence.
The importance factor Iw = 0.87 is applied to the velocity pressure calculation, effectively reducing the design wind speed:
VRisk I = V × √Iw = V × √0.87 = V × 0.933
This results in approximately a 6.7% reduction in design wind speed, which translates to a 13% reduction in wind pressures (since pressure is proportional to velocity squared).
Given:
Step 1: Determine velocity pressure coefficient
Kz = 0.85 (at h = 20 ft, Exposure C)
Step 2: Calculate velocity pressure with importance factor
qz = 0.00256 × Kz × Kzt × Kd × Iw × V²
qz = 0.00256 × 0.85 × 1.0 × 0.85 × 0.87 × 115²
qz = 20.9 psf
Step 3: Apply pressure coefficients
p = qz × G × Cp (for MWFRS)
p = qz × (GCp) (for components & cladding)
The following structures cannot be classified as Risk Category I, even if used for agricultural purposes:
Facilities with regular human occupancy such as dairy milking parlors, processing plants with workers, or farm offices.
Buildings housing valuable breeding stock, show animals, or livestock worth more than minimal economic value.
Storage of toxic, highly toxic, or explosive materials that could endanger public safety if released during failure.
School farms, university research facilities, or vocational training centers with student occupancy.
Farm stands, corn mazes, petting zoos, or other agricultural facilities open to public visitors.
Structures whose failure could cause water supply contamination or environmental damage affecting public health.
| Risk Category | Importance Factor (Iw) | Mean Recurrence Interval | Load Comparison | Typical Use |
|---|---|---|---|---|
| I (Agricultural) | 0.87 | 300 years | -13% vs Risk II | Low hazard, minimal occupancy |
| II (Standard) | 1.00 | 700 years | Baseline (100%) | Typical buildings |
| III (Assembly) | 1.15 | 1,700 years | +15% vs Risk II | Substantial public hazard |
| IV (Essential) | 1.15 | 3,000 years | +15% vs Risk II | Essential facilities |
Carefully document that the building meets all Risk Category I criteria before applying the reduced importance factor. Consult with the building official early in the design process.
If there's any possibility of future conversion to occupied use, consider designing to Risk Category II standards to avoid costly retrofits later.
Clearly document in construction documents that the structure is designed for Risk Category I and specify prohibited uses that would require reclassification.
Some jurisdictions restrict or prohibit Risk Category I classifications. Verify that local building codes allow this classification for your specific application.
For qualifying agricultural buildings, Risk Category I classification can provide significant cost savings:
Typical cost savings range from 5-15% of structural costs, depending on wind speed and exposure category.
Clearly label drawings with "Risk Category I" designation. Include a note stating the building is designed for minimal human occupancy and listing prohibited uses.
Submit written justification for Risk Category I classification with permit application. Include owner acknowledgment of use restrictions and limitations.
Ensure CO reflects Risk Category I classification and includes conditions limiting occupancy and use of the structure per the approved plans.
If building use changes to include regular occupancy, a structural evaluation and possible retrofit to Risk Category II standards will be required.
Design professionals should clearly communicate the implications of Risk Category I classification to building owners. Document that the owner understands:
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