An exposure category (B, C, or D)
is one of the main factors in determining design pressures. It is a condition that adequately reflects the location characteristics of ground surface irregularities for the site for the structure. The exposure category is used in calculating the required design wind pressures for a structure with exposure B yielding the lowest wind pressures and exposure D yielding the highest wind pressures.
Exposure B
applies to urban and suburban areas, wooded areas or other terrains with numerous closely spaced obstructions having the size of single-family dwellings or larger. Exposure B is typically associated with site locations in a residential subdivision. Most site locations are assumed to be Exposure B unless the site meets the definition of another type of exposure.
Exposure C
applies to open terrain with scattered obstructions having heights generally less than 30 feet extending more than 1,500 feet from the building site. Exposure C includes flat open country, grasslands, and shorelines in hurricane-prone regions.
Exposure D
applies to flat, unobstructed areas exposed to wind flowing over open water (excluding shorelines in hurricane-prone regions) for a distance of at least 1 mile. Exposure D includes shorelines in inland waterways, the Great Lakes, and coastal areas of California, Oregon, Washington, and Alaska. Exposure D extends inland from the shoreline a distance of 1,500 feet or 10 times the height of the building or structure, whichever is greater.