ASCE 7-22 · SECTION 26.7.3 · EXPOSURE
Where terrain roughness changes, exposure changes with direction
Upwind of every site, the ground gets rougher or smoother. ASCE 7-22 makes you read that fetch for each wind direction — so one building can be Exposure D, C, and B at the same time.
PLAN + SECTION · UPWIND FETCH
A site that reads water to suburban upwind
Looking down on the site (plan) and across it (section): the exposure for a direction is set by the surface roughness over the required upwind fetch in that direction.
Until the new boundary layer fully develops over the required fetch, the site behaves like the smoother upwind terrain — which is why a short stretch of suburb does not yet earn Exposure B.
METHOD · PER WIND DIRECTION
How to read exposure direction by direction
ASCE 7-22 26.7.3: for a given direction, exposure follows the ground roughness that prevails over the upwind fetch. Walk all 8 principal directions.
1 · Map each direction
Look upwind along all 8 principal directions and identify the surface roughness from aerial and survey data.
8 SECTORS2 · Measure the fetch
How far does that uniform roughness extend upwind? Compare it against the category's required fetch.
DISTANCE CHECK3 · Apply the criteria
Suburban with 2,600 ft (or 20h) upwind → B. Open coast meeting the shore criteria → D. Otherwise → C.
B / C / D4 · Set the governing case
Each direction can carry its own exposure. The wind analysis then uses the controlling exposure per direction.
DIRECTIONALTABLE 26.11-1 · zmin
The height where each profile starts to count
Below zmin the velocity pressure coefficient is held at its zmin value — rougher terrain has a taller floor.
TRANSITION SCENARIOS
Common roughness changes and what governs
Each pairing flips one direction's exposure. Read the fetch — the smoother terrain wins until the rougher one earns its distance.
Water → suburban
Exposure D from the water side, B from the land side — if the suburban fetch is sufficient.
D ON SEAWARDOpen → suburb
Farmland is C; a building near the suburb's edge may lack the upwind B fetch, so C still governs.
C UNTIL FETCHForest → clearing
Treed B opens to a C field; a structure in the clearing can see higher, more-open pressures.
OPENS UPUrban → industrial
Dense B gives way to open industrial; low buildings feel a transitioning, rougher-to-opener profile.
SELECTION GUIDETABLE 26.10-1 · Kz AT 30 FT
Choosing the more-open exposure raises the pressure
Velocity pressure scales with Kz. At 30 ft, stepping from B toward D is a large jump — so a transition direction that earns D, not B, drives the design.
| Exposure | Kz at 30 ft | Relative to B at 30 ft | Character |
|---|---|---|---|
| B (suburban) | 0.70 | baseline | Rougher, slowest profile low to the ground |
| C (open) | 0.98 | +40% | Open terrain, scattered obstructions |
| D (open water / coastal) | 1.16 | +66% | Smoothest, fastest profile low to the ground |
When a transition leaves a direction borderline, defaulting to the more-open exposure is conservative: at 30 ft, D's Kz of 1.16 is 66% higher than B's 0.70, and C's 0.98 is 40% higher. The governing direction sets the load.
EXPOSURE LIBRARY
Keep going on exposure categories
Selection Guide
Step-by-step process for choosing the right exposure.
START HEREExposure B
Suburban and urban terrain with numerous obstructions.
zmin 30 FTExposure C
Open terrain with scattered obstructions — the default.
zmin 15 FTExposure D
Coastal areas and smooth water surfaces — highest pressures.
zmin 7 FTRUN IT PER DIRECTION
Let the software resolve every directional exposure
Directional analysis across B, C, and D — the governing case found for you, fully ASCE 7-22 compliant.