What is Exposure D in ASCE 7?

Exposure D is the terrain category for flat, unobstructed areas where wind sweeps in over open water such as oceans, bays, sounds and large lakes, plus smooth mud or salt flats. Because the water offers almost no friction, wind keeps nearly full speed down to grade, giving Exposure D the highest velocity pressure coefficients of any category per ASCE 7-22 Section 26.7.4.

When do I use Exposure D instead of Exposure C?

Use Exposure D for sites directly exposed to wind coming off open water with a long, smooth, unobstructed fetch: beachfront buildings, marinas, docks, ports and barrier islands. Evaluate each wind direction separately. A coastal site can be Exposure D for winds off the water yet Exposure C or B for winds coming off the land.

How much higher are Exposure D wind loads than Exposure B?

At 30 ft, the Exposure D velocity pressure coefficient Kz is 1.16 versus 0.70 for suburban Exposure B, about 66 percent higher. Near grade, at or below 15 ft, Kz is 1.03 (D) versus 0.57 (B), roughly 81 percent higher. Versus open-terrain Exposure C at 30 ft, D is about 18 percent higher (1.16 vs 0.98).

What are the Exposure D terrain constants?

From ASCE 7-22 Table 26.11-1, Exposure D uses a minimum height zmin of 7 ft, the lowest of any category, a profile exponent alpha of 11.5, and a gradient height zg of 1935 ft. Below 7 ft the velocity pressure coefficient Kz is held at 1.03 because the water surface provides almost no friction to slow the wind.

How is velocity pressure calculated for a coastal Exposure D site?

Velocity pressure follows qz = 0.00256 x Kz x Kzt x Kd x Ke x V squared. Kz comes from the Exposure D column of Table 26.10-1, the directionality factor Kd is 0.85 for buildings, and Kzt is the topographic factor. The basic wind speed V is read from the ASCE 7 map for your site's risk category, with the exact mph varying by location.

ASCE 7-22 · SECTION 26.7.4 · EXPOSURE CATEGORIES

Exposure D — wind straight off the open water

Flat, unobstructed coastlines and water surfaces. Almost no friction to slow the wind, so Exposure D carries the highest velocity pressure coefficients of any category.

1.03Kz AT ≤15 FT (TABLE 26.10-1)

+66%Kz VS EXPOSURE B AT 30 FT

7 ftzmin — LOWEST OF ALL TERRAIN

HOW THE TERRAIN BEHAVES

Smooth fetch means full-speed wind at grade

Over open water the boundary layer barely develops, so the wind keeps its strength all the way down to the structure.

TERRAIN CONSTANTS · TABLE 26.11-1

The three numbers that define Exposure D

A very low minimum height plus a thick profile exponent is what drives the pressures up.

7 ftzmin — Kz HELD AT 1.03 BELOW THIS

11.5α PROFILE EXPONENT

1935 ftzg GRADIENT HEIGHT

zmin = 7 ft α = 11.5 zg = 1935 ft ASCE 7-22 & 7-16 §26.7.4

VELOCITY PRESSURE COEFFICIENT · TABLE 26.10-1

Kz by height — D is always the highest

Exact Exposure D values compared to C and B at the same heights. Every cell is from ASCE 7-22 Table 26.10-1.

Height z (ft)	Exp B Kz	Exp C Kz	Exp 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	0.74	1.04	1.22	+17%	+65%
50	0.79	1.09	1.27	+17%	+61%
60	0.83	1.13	1.31	+16%	+58%
80	0.90	1.21	1.38	+14%	+53%
100	0.95	1.26	1.43	+13%	+51%
120	1.00	1.31	1.48	+13%	+48%
160	1.08	1.39	1.55	+12%	+44%
200	1.14	1.44	1.61	+12%	+41%

Below zmin = 7 ft the coefficient is held at Kz = 1.03 — no reduction near the surface, because the water offers almost no friction.

WHERE IT APPLIES

When to use Exposure D

Flat, unobstructed sites where wind sweeps in over open water (ocean, bays, sounds, large lakes) or smooth mud and salt flats.

Beachfront

Oceanfront homes, condos and hotels directly on the Atlantic, Pacific or Gulf coast.

OPEN OCEAN

Marinas & Docks

Piers, docks and waterfront structures on coastal bays, sounds and harbors.

WATERFRONT

Coastal Industrial

Ports, shipyards and coastal warehouses within reach of open-water wind.

PORTS

Large Lake Shore

Great Lakes and large reservoir shores where wind crosses long stretches of open water.

BIG WATER

Coastal Bridges

Elevated roadways and bridge structures along immediate coastline with clear wind flow.

SPANS

Island Structures

Barrier-island buildings and platforms effectively surrounded by water on all sides.

ISLANDS

SEVERITY CHECK

Exposure D is the most severe terrain

At any given height, swapping a site into Exposure D raises Kz above both C and B. Mis-classifying a coastal site under-designs the structure.

D vs B at 30 ft

Kz 1.16 vs 0.70 — Exposure D is +66% higher than suburban Exposure B.

+66%

D vs C at 30 ft

Kz 1.16 vs 0.98 — Exposure D is +18% higher than open-terrain Exposure C.

+18%

At ≤15 ft

Kz 1.03 vs 0.85 (C) and 0.57 (B) — up to +81% over Exposure B near grade.

+81%

Check each wind direction on its own: a beachfront site can be Exposure D for winds coming off the water and Exposure C or B for winds off the land.

RUN THE NUMBERS

Calculate Exposure D loads in minutes

WindLoadCalc.com applies the correct ASCE 7-22 Exposure D coefficients automatically for coastal structures.

Open the Wind Load Calculators Exposure Selection Guide

RELATED WIND SPEED RESOURCES

Exposure D — wind straight off the open water

Smooth fetch means full-speed wind at grade

The three numbers that define Exposure D

Kz by height — D is always the highest

When to use Exposure D

Beachfront

Marinas & Docks

Coastal Industrial

Large Lake Shore

Coastal Bridges

Island Structures

Exposure D is the most severe terrain

D vs B at 30 ft

D vs C at 30 ft

At ≤15 ft

Calculate Exposure D loads in minutes

Compare the other categories

Exposure B

Exposure C

Selection Guide

Transition Zones

Categories Overview

HVHZ Requirements

Pin down your design wind speed

ASCE 7-22 Wind Speed Maps

Wind Speed by Location

Wind Speed by Zip Code

3-Second Gust Wind Speed

Basic Wind Speed Explained

ASCE 7 Wind Load Calculator

Exposure D — wind straight off the open water

Smooth fetch means full-speed wind at grade

The three numbers that define Exposure D

K​z by height — D is always the highest

When to use Exposure D

Beachfront

Marinas & Docks

Coastal Industrial

Large Lake Shore

Coastal Bridges

Island Structures

Exposure D is the most severe terrain

D vs B at 30 ft

D vs C at 30 ft

At ≤15 ft

Calculate Exposure D loads in minutes

Compare the other categories

Exposure B

Exposure C

Selection Guide

Transition Zones

Categories Overview

HVHZ Requirements

Pin down your design wind speed

ASCE 7-22 Wind Speed Maps

Wind Speed by Location

Wind Speed by Zip Code

3-Second Gust Wind Speed

Basic Wind Speed Explained

ASCE 7 Wind Load Calculator

Kz by height — D is always the highest