Flat unobstructed areas and water surfaces with maximum wind exposure
Exposure D represents flat, unobstructed coastal areas and smooth surfaces directly exposed to wind flowing over open water. Defined in ASCE 7-22 (and ASCE 7-16) Section 26.7.4, it produces the highest design wind pressures of all exposure categories.
Exposure D applies to sites within 600 feet of the shoreline or within a distance of 60 times the building height from the shoreline, whichever is greater, for sites with wind flowing over open water.
Flat, unobstructed areas exposed to wind flowing over open water (ocean, bays, sounds).
Within 600 ft of shoreline OR within 60× building height from water, whichever is greater.
17-21% higher wind pressures than Exposure C due to smooth water surface.
Defined in Section 26.7.4 of both ASCE 7-22 (latest) and ASCE 7-16.
Exposure D represents flat, unobstructed areas exposed to wind flowing over open water or smooth mud flats for a distance of at least 5,000 ft or 20 times the building height, whichever is greater.
Exposure D: Flat, unobstructed areas and water surfaces. This exposure applies to sites with surface roughness D as defined in Section 26.7 prevailing in the upwind direction for a distance greater than or equal to 5,000 ft (1,524 m) or 20 times the building height, whichever is greater.
Critical Distance Requirements:
Exposure D produces the highest wind pressures because water surfaces provide minimal resistance to wind flow, allowing higher wind speeds to reach structures near the shoreline.
Understanding when Exposure D applies requires careful measurement from the shoreline. ASCE 7 provides two distance criteria that vary based on whether you're measuring inland or over water.
Exposure D extends inland a distance of:
600 feet OR 60 × building height, whichever is GREATER
Exposure D requires open water for:
5,000 feet OR 20 × building height, whichever is GREATER
| Building Height | 60× Height (Inland) | 600 ft Minimum | Exposure D Distance Inland |
|---|---|---|---|
| 15 ft (residential) | 900 ft | 600 ft | 900 ft (60× governs) |
| 25 ft (small commercial) | 1,500 ft | 600 ft | 1,500 ft (60× governs) |
| 8 ft (low storage) | 480 ft | 600 ft | 600 ft (minimum governs) |
| 40 ft (hotel) | 2,400 ft | 600 ft | 2,400 ft (60× governs) |
Key Point: For most buildings over 10 ft tall, the 60× building height criterion controls the inland Exposure D distance, not the 600 ft minimum!
Oceanfront homes, beach condos, hotels directly on Atlantic/Pacific/Gulf coasts within Exposure D zone.
Waterfront structures, piers, docks, marinas on coastal bays, sounds, or large harbors with open water exposure.
Ports, shipyards, coastal warehouses, offshore support facilities within 600 ft (or 60× height) of open water.
Great Lakes shoreline structures, large reservoir shores where wind flows over ≥5,000 ft of open water.
Bridge structures, elevated roadways along immediate coastline with unobstructed wind flow from ocean.
Barrier island buildings, offshore platform structures completely surrounded by water.
Exposure D uses the highest velocity pressure exposure coefficient (Kh) values, resulting in maximum design wind pressures.
| Height (ft) | Exposure B Kz | Exposure C Kz | Exposure D Kz | D vs C | D vs B |
|---|---|---|---|---|---|
| 0-15 | 0.57 | 0.85 | 1.03 | +21% | +81% |
| 20 | 0.62 | 0.90 | 1.08 | +20% | +74% |
| 25 | 0.66 | 0.94 | 1.12 | +19% | +70% |
| 30 | 0.70 | 0.98 | 1.16 | +18% | +66% |
| 40 | 0.76 | 1.04 | 1.22 | +17% | +61% |
| 60 | 0.86 | 1.13 | 1.31 | +16% | +52% |
Massive Impact: At 15 ft height, Exposure D produces 21% higher wind pressures than Exposure C, and 81% higher than Exposure B! Coastal structures face significantly higher wind loads.
Oceanfront hotel
200 ft from Atlantic Ocean
Mean roof height: 40 ft
Risk Category III
V = 150 mph (ASCE 7-22)
Exposure D (coastal)
Kzt = 1.0, Kd = 0.85
Window - Zone 5
Effective area = 12 ft²
GCp = -1.00
Enclosed: GCpi = ±0.18
p = -68.9 psf
Required: DP-70
(vs DP-60 for Exposure C)
1. Kh for Exposure D, h=40 ft:
Kh = 1.22 (from ASCE 7 Table 26.10-1)
2. Velocity pressure qh:
qh = 0.00256 × 1.22 × 1.0 × 0.85 × 1.0 × 150²
qh = 58.4 psf
3. Wind pressure (governing case):
p = qh[(GCp) - (GCpi)]
p = 58.4[(-1.00) - (+0.18)] = -68.9 psf
4. Component selection (LRFD with 1.0W):
LRFD pressure = 1.0 × 68.9 = 68.9 psf
Select DP-70 (70 psf ≥ 68.9 psf ✓)
Comparison - Same building in Exposure C:
Kh = 1.04 → qh = 49.8 psf → p = -58.8 psf
Would only need DP-60 (vs DP-70 for Exposure D)
WRONG: "My building is 700 ft from the ocean, so Exposure C applies" (but building is 20 ft tall)
RIGHT: 60 × 20 ft = 1,200 ft. Exposure D extends 1,200 ft inland for this building. Use Exposure D!
Impact: 17-21% under-design of wind loads
WRONG: "It's near the coast, must be Exposure D"
RIGHT: If beyond the Exposure D distance (600 ft or 60× height from water) and terrain has obstructions, use Exposure C or B.
Impact: Over-design, wasted material costs
WRONG: Site is 100 ft from small pond → "Use Exposure D"
RIGHT: Exposure D requires ≥5,000 ft (or 20× height) of open water UPWIND. Small ponds don't create Exposure D.
Impact: Over-design for sites not truly exposed to long water fetch
WRONG: Beachfront building → "Use Exposure D for all wind directions"
RIGHT: Exposure D only applies for wind directions flowing OVER the open water. Landward wind directions may be Exposure C or B.
Impact: Over-design for non-water wind directions (though often conservative default is to use D for all)
Measure distance from site to nearest open water (ocean, bay, sound, large lake)
Calculate 60 × building height - compare to 600 ft minimum
If site is within greater of (600 ft or 60× height), check for Exposure D
Verify open water extends ≥5,000 ft (or 20× height) in upwind direction
Check each wind direction separately - may be D for ocean winds, C for land winds
Document with aerial imagery showing water distance and building height
When design is conservative, using Exposure D for all directions simplifies analysis
WindLoadCalc.com automatically applies Exposure D coefficients for coastal structures
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