Exposure Categories
The exposure category defines how wind speed increases with height, based on the terrain surrounding the tower site.
Categories
| Category | Terrain Description | Examples |
|---|---|---|
| B | Urban, suburban, wooded areas | Cities, dense suburbs, forests |
| C | Open terrain with scattered obstructions | Farmland, open fields, grasslands |
| D | Flat, unobstructed areas and water surfaces | Coastal areas, deserts, airports |
Impact on Design
Exposure category determines the Kz profile — the velocity pressure exposure coefficient at each height.
- Exposure B — wind speed increases slowly with height (turbulent ground layer)
- Exposure C — moderate wind speed increase (standard open terrain)
- Exposure D — rapid wind speed increase (smooth terrain, highest loads)
caution
Selecting the wrong exposure category is one of the most common errors in tower analysis. Exposure D produces significantly higher loads than Exposure B at the same wind speed. Always verify the actual site conditions.
Selection Guidance
Per TIA-222-H Section 2.6.4:
- Use the most conservative category if terrain varies by direction
- Consider terrain within 1.6 km upwind of the tower site
- Urban/suburban areas surrounded by open terrain may still qualify as Exposure C