The Basic Design Wind Speed, V (mph), corresponds to a 3-second gust speed at 33\' above ground in Exposure Category "C" and is associated with an annual probability of 0.02 of being equalled or exceeded (50-year mean recurrence interval).

Surface Roughness Categories for the purpose of assigning Exposure Category are defined as follows:

Surface Roughness "B":
Urban and suburban areas, wooded areas or other terrain with numerous closely spaced obstructions having the size of single family dwellings or larger.

Surface Roughness "C":
Open terrain with scattered obstructions having heights generally < 30 ft. This category includes flat open country, grass lands, and all water surfaces in hurricane prone regions.

Surface Roughness "D":
Flat, unobstructed areas and water surfaces outside hurricane prone regions. This category includes smooth mud flats, salt flats, and unbroken ice.

Exposure Categories are defined as follows:

Exposure "B":
Exposure B shall apply where the ground surface roughness condition, as defined by Surface Roughness B, prevails in the upwind direction for a distance of at least 2600 ft. or 20 times the building height, whichever is greater.

Exception: For buildings whose mean roof height <= 30 ft., the upwind distance may be reduced to 1500 ft.

Exposure "C":
Exposure C shall apply for all cases where exposures B and D do not apply.

Exposure "D":
Exposure D shall apply where the ground surface roughness, as defined by Surface Roughness D, prevails in the upwind diection for a distance >= 5,000 ft. or 20 times the building height, whichever is greater. Exposure D shall extend into downwind areas of Surface Roughness B or C for a distance of 600 ft. or 20 times the height of the building, whichever is greater.

The eave height, 'he', is the distance from the ground surface adjacent to the building to the roof eave line at a particular wall.

If the height of the eave varies along the wall, the average height shall be used.

This program assumes that a Gable roof is symmetrical, as the ridge line is assumed in the center of the building width, L.

For flat roofs (roof angle = 0 degrees), either Gable (G) or Monoslope (M) may be used.

The Topographic Factor, Kzt, accounts for effect of wind speed-up over isolated hills and escarpments (Sect. 6.5.7 and Fig. 6-4).

The effect of wind speed-up shall not be required to be considered (Kzt = 1.0) when H/Lh < 0.2, or H < 15' for Exposures 'C' and 'D', or H < 60' for Exposure 'B'.

Note: this factor shall only be applied when used in conjunction with load combinations specified in Sect. 2.3 and 2.4. Otherwise, use Kd = 1.0.

1. An enclosed building is a building that does not comply with the requirements for open or partially enclosed buildings.
2. An open building is a structure having all walls at least 80% open.
3. A partially enclosed building complies with both of the following conditions:
   1. the total area of openings in a wall that receives positive external pressure exceeds the sum of the areas of the openings in the balance of the building envelope (walls and roof) by more than 10%; and
   2. the total area of openings in a wall that receives positive external pressure exceeds 4 sq ft or 1% of the area of that wall, whichever is smaller, and the % of openings in balance of the building envelope does not exceed 20%.