Understanding terrain changes and surface roughness transitions
When wind flows from one type of terrain into another—such as from open farmland into a suburban neighborhood, or from a coastal area inland—the surface roughness changes. These locations are called exposure transition zones, and they require special consideration in wind load calculations per ASCE 7-22 Section 26.7.3 (or ASCE 7-16 for older IBC editions).
Exposure transition zones present a unique challenge: the wind profile doesn't instantly adjust to the new terrain. Instead, the velocity pressure and wind speed profile gradually transition from the upwind exposure category to the downwind exposure category over a certain distance called the fetch length.
The upwind distance of uniform surface roughness that affects wind flow at a building site. Different exposure categories require different minimum fetch lengths to be applicable.
Wind doesn't instantly "change exposure" at a terrain boundary. The velocity profile adjusts gradually over distance as the wind interacts with the new surface roughness.
Exposure must be evaluated for each wind direction. A site may qualify as Exposure B from the north but Exposure C from the west, depending on upwind terrain.
When in doubt, use the exposure category that produces higher wind pressures. For uncertain transition zones, Exposure C is typically the safe default.
ASCE 7-22 Section 26.7.3 provides specific criteria for determining when a site is in a transition zone and how to handle it. The key principle: sufficient upwind fetch is required before an exposure category can be claimed.
| Exposure Category | Minimum Upwind Fetch Required | Alternative Criterion | Notes |
|---|---|---|---|
| Exposure B | 2,600 feet (792 m) | 20× building height (h), whichever is greater | For buildings ≤30 ft tall, minimum fetch reduced to 1,500 ft |
| Exposure C | N/A (default exposure) | Use when B or D don't apply | No minimum fetch requirement; assumed if conditions not met for B or D |
| Exposure D | 5,000 feet (1,524 m) over water OR 600 ft from shore | 60× building height from shore (whichever is GREATER) | Applies when site is at waterfront or on unobstructed ground within distance criteria |
Transition zones occur wherever there's a change in surface roughness. Understanding the physics of these zones helps engineers make better exposure category decisions.
Upwind: Exposure C (open farmland)
Downwind: Exposure B (suburban)
Challenge: Building near edge of suburb may not have 2,600 ft of Exposure B terrain upwind.
Upwind: Exposure D (over water)
Downwind: Exposure C or B (inland)
Challenge: Buildings near shore see high D exposure from ocean direction, lower exposure from land directions.
Upwind: Exposure B (forested area)
Downwind: Exposure C (clearing/field)
Challenge: Structure in clearing may experience transitional pressures higher than Exposure B.
Upwind: Exposure B (dense urban)
Downwind: Exposure C (open industrial park)
Challenge: Low buildings in industrial park see transitioning wind profile.
When wind flows from one surface roughness to another, the internal boundary layer (IBL) develops above the new terrain. This boundary layer grows with distance downwind:
This is why ASCE 7-22 requires 2,600 ft minimum for Exposure B and 5,000 ft for Exposure D over water—the wind profile needs sufficient distance to fully develop to the lower roughness characteristics.
A building located 500 feet into an Exposure B suburban area (coming from Exposure C farmland) experiences wind pressures closer to Exposure C than Exposure B because the wind profile hasn't had sufficient fetch to slow down and develop the lower Exposure B velocity profile. Using Exposure C in this scenario is the correct (and more conservative) approach.
ASCE 7-22 Section 26.7.3 requires engineers to evaluate exposure for each wind direction (typically 8 principal compass directions: N, NE, E, SE, S, SW, W, NW). The process is systematic:
For each of the 8 wind directions, examine the terrain upwind from the building site. Use aerial imagery, site surveys, and topographic maps to identify surface roughness.
Measure how far uniform terrain extends in each direction. For Exposure B, measure the extent of suburban/urban terrain. For Exposure D, measure distance to waterfront and distance of water upwind.
Exposure D Check: Is site within 600 ft OR 60h of shore, with ≥5,000 ft of open water or unobstructed ground upwind? → Exposure D
Exposure B Check: Is there ≥2,600 ft OR 20h of suburban/urban terrain upwind (or 1,500 ft for h≤30 ft)? → Exposure B
Default: If neither D nor B criteria are met → Exposure C
ASCE 7-22 Section 26.7.3 states: "For a given direction, exposure shall be based on ground surface roughness that prevails in the upwind direction." Each wind direction may have a different exposure category.
Project Site: A 35 ft tall warehouse located 1,000 feet from the Atlantic Ocean in coastal South Carolina. The site is in a light industrial area with sparse obstructions.
Terrain Analysis by Direction:
| Wind Direction | Upwind Terrain Description | Fetch Distance | Exposure Determination |
|---|---|---|---|
| East (from ocean) | Open water, then 1,000 ft of low coastal vegetation | 1,000 ft to shore, >20 miles of ocean | Exposure D (within 60h = 2,100 ft, with >5,000 ft water upwind) |
| Southeast | Open water and coastal marshland | ~5,000 ft water/marsh upwind | Exposure D |
| South | Light industrial, then farmland | ~3 miles of open terrain | Exposure C (no qualifying B or D terrain) |
| Southwest | Open farmland and fields | >2 miles | Exposure C |
| West | Developed suburban residential area | ~4,000 ft of suburban | Exposure B (>2,600 ft of qualifying terrain) |
| Northwest | Suburban transitioning to farmland | ~1,800 ft suburban | Exposure C (insufficient B fetch: 1,800 ft < 2,600 ft required) |
| North | Light industrial/commercial | ~1 mile | Exposure C |
| Northeast | Mixed commercial/residential near water | ~2,000 ft to water | Exposure C (beyond 60h = 2,100 ft, so D doesn't apply; insufficient B terrain) |
Result: This site requires directional analysis using Exposure D for easterly winds, Exposure B for westerly winds, and Exposure C for all other directions. Design software must evaluate velocity pressures separately for each exposure category.
Many engineers simplify by using the most conservative (highest pressure) exposure for all directions when directional differences are modest. For the example above, using Exposure D for all directions would be conservative but potentially over-designed for west/northwest winds. Professional wind load software like WindLoadCalc.com can evaluate each direction separately for optimized design.
Coastal areas present the most dramatic exposure transitions, where Exposure D (smooth water surface) meets Exposure C or B (land terrain). These transitions significantly affect wind pressures, particularly for buildings in the first few thousand feet inland.
Exposure D applies when BOTH of the following conditions are met:
Location: 300 feet from ocean shoreline, Outer Banks, NC
Proximity Check: 600 ft OR 60h = 60×25 = 1,500 ft → ✓ Within 1,500 ft (use greater value)
Water Fetch: >20 miles of Atlantic Ocean upwind → ✓ Exceeds 5,000 ft requirement
Result: Exposure D applies for winds from ocean directions
Location: 400 feet from Gulf of Mexico, Florida panhandle
Proximity Check: 600 ft OR 60h = 60×80 = 4,800 ft → ✗ 400 ft < 4,800 ft (not within required distance)
Result: Exposure D does NOT apply (fails proximity test). Use Exposure C for coastal winds (unless Exposure B terrain exists).
Location: 1,200 feet from Chesapeake Bay shoreline
Proximity Check: 600 ft OR 60h = 60×35 = 2,100 ft → ✓ 1,200 ft < 2,100 ft (within required distance)
Water Fetch: Chesapeake Bay is ~8 miles wide at this location → ✓ Exceeds 5,000 ft
Result: Exposure D applies for winds from bay direction
Buildings located just beyond the Exposure D distance criteria are in a transition zone where the wind profile is adjusting from the smooth water surface to the rougher inland terrain. The ASCE 7 approach is to use:
This creates a sharp transition in the code, even though physically the wind profile transitions gradually. For buildings very close to the cutoff distance, some engineers perform sensitivity analyses using both Exposure D and C to evaluate the impact on design pressures.
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