Complete guide to Exposure B, C, and D terrain classifications
Wind exposure categories are one of the most critical factors in ASCE 7-22 (or ASCE 7-16 for older IBC editions) wind load calculations. The exposure category describes the surface roughness of the terrain surrounding a building and directly affects the velocity pressure coefficients used to calculate wind loads.
ASCE 7-22 Section 26.7 defines three exposure categories—B, C, and D—each representing different terrain conditions that produce distinct wind speed profiles and pressure distributions on structures. Selecting the correct exposure category is essential for accurate and code-compliant wind load determination.
Suburban/Urban Terrain: Numerous closely-spaced obstructions (buildings, trees, houses). Produces the lowest wind pressures due to significant surface friction.
Open Terrain (Default): Open terrain with scattered obstructions. This is the ASCE 7 default exposure and represents typical farmland, grassland, and light development.
Coastal/Flat (Highest Loads): Smooth surfaces with minimal friction—open water, mudflats, salt flats. Produces the highest wind pressures of all exposure categories.
The key concept: as surface roughness decreases (B → C → D), wind speeds near the ground increase, resulting in higher design pressures on structures.
Exposure category affects wind loads through the velocity pressure exposure coefficient (Kh or Kz), which adjusts wind pressure based on height and terrain. For example, at 30 feet height:
This means a building on a coastal site (Exposure D) can experience over 60% higher wind pressures compared to the same building in a suburban area (Exposure B) at the same basic wind speed!
Incorrectly selecting Exposure B when Exposure C or D applies can result in significant under-design of structural components, potentially leading to code violations or structural failure during high-wind events. Always err on the conservative side when uncertain.
| Characteristic | Exposure B | Exposure C | Exposure D |
|---|---|---|---|
| ASCE 7-22 Definition | Urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions | Open terrain with scattered obstructions having heights generally less than 30 ft | Flat, unobstructed areas and water surfaces (excluding shorelines in hurricane-prone regions) |
| Typical Terrain Examples | • Dense residential suburbs • Forested areas • Urban neighborhoods • Downtown districts |
• Open farmland • Agricultural areas • Grasslands • Light industrial parks |
• Open water surfaces • Mudflats and salt flats • Smooth ice • Coastal beaches |
| Minimum Upwind Fetch | 2,600 ft OR 20h (whichever greater) Exception: 1,500 ft for h ≤ 30 ft |
N/A (default exposure) | 5,000 ft OR 20h over water AND within 600 ft OR 60h from shore |
| Kh at h = 30 ft | 0.70 | 0.98 | 1.14 |
| Relative Wind Pressure | Lowest (33% less than C) | Moderate (baseline) | Highest (16% more than C) |
| α (power law exponent) | 7.0 | 9.5 | 11.5 |
| zg (gradient height) | 1,200 ft (366 m) | 900 ft (274 m) | 700 ft (213 m) |
| Typical Applications | Residential homes in suburbs, buildings in wooded developments | Agricultural buildings, warehouses in open areas, most commercial/industrial | Beachfront structures, offshore platforms, coastal condos |
| Selection Difficulty | Moderate (requires verification of fetch distance) | Easy (default when B/D don't apply) | Moderate (coastal proximity calculations required) |
The velocity pressure exposure coefficient (Kh or Kz) varies with height and exposure category per ASCE 7-22 Table 26.10-1. Understanding these values is critical for calculating design pressures.
| Height Above Ground (ft) | Exposure B Kh | Exposure C Kh | Exposure D Kh | D vs B Pressure Ratio |
|---|---|---|---|---|
| 0-15 ft | 0.57 | 0.85 | 1.03 | +81% higher |
| 20 ft | 0.62 | 0.90 | 1.08 | +74% higher |
| 25 ft | 0.66 | 0.94 | 1.12 | +70% higher |
| 30 ft | 0.70 | 0.98 | 1.14 | +63% higher |
| 40 ft | 0.76 | 1.04 | 1.19 | +57% higher |
| 50 ft | 0.81 | 1.09 | 1.23 | +52% higher |
| 60 ft | 0.85 | 1.13 | 1.27 | +49% higher |
| 80 ft | 0.93 | 1.19 | 1.32 | +42% higher |
| 100 ft | 0.99 | 1.24 | 1.37 | +38% higher |
Key Observation: The difference between exposures is most dramatic at lower heights where surface roughness effects dominate. As height increases, all exposure categories converge toward higher velocities because the wind profile extends above surface friction effects.
Location: Wilmington, NC (V = 140 mph, Risk Category II)
Calculation: qh = 0.00256 × Kh × V²
This 70% difference in velocity pressure directly impacts structural member sizing, component ratings, and overall building cost!
Selecting the correct exposure category requires systematic evaluation of upwind terrain for each wind direction. ASCE 7-22 Section 26.7.3 provides specific criteria.
Exposure D applies if BOTH conditions are met:
If YES → Use Exposure D for that wind direction
If NO → Proceed to Step 2
Exposure B applies if:
If YES → Use Exposure B for that wind direction
If NO → Proceed to Step 3
Exposure C is the default exposure when neither B nor D criteria are satisfied.
This includes:
ASCE 7-22 Section 26.7.3 requires evaluation for each wind direction (typically 8 compass directions: N, NE, E, SE, S, SW, W, NW). A single building site can have different exposures from different directions.
Example: A building 1 mile inland from the ocean may be:
Design software must calculate wind loads separately for each exposure category and select controlling values for each building surface.
Exposure category selection errors are among the most frequent mistakes in wind load calculations. Understanding these common pitfalls helps ensure code-compliant designs.
Error: "This is a house, so it's automatically Exposure B."
Reality: Exposure B requires ≥2,600 ft of suburban terrain upwind. A house on the edge of a subdivision surrounded by farmland is likely Exposure C, not B.
Error: "The site is Exposure C, so I'll use C for all directions."
Reality: Many sites have mixed exposures. A coastal site may be Exposure D from ocean directions but C or B from inland directions.
Error: "There are trees and houses nearby, so it's Exposure B."
Reality: Exposure B requires sufficient upwind fetch (2,600 ft or 20h). A few nearby obstructions don't qualify without adequate fetch distance.
Error: "The building is 500 feet from the ocean, so it's Exposure D."
Reality: For tall buildings, the 60h criterion may extend the Exposure D zone well beyond 600 ft. For an 80 ft building, D applies out to 4,800 ft!
When uncertain about exposure category determination:
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