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).
For Basic Wind Speed Map (Fig. 6-1) see 'Wind Map' application of this workbook.

TABLE 1-1
Classification of Buildings and Other Structures for Flood, Wind, Snow, and Earthquake Loads
Nature of Occupancy Category
Buildings and structures that represent a low hazard to human life in the event of failure including, but not limited to: I
- Agriculture Facilities
- Certain Temporary Facilities
- Minor Storage Facilities
All buildings and other structures except those listed in Categories I, III & IV II
Buildings and other structures that represent a substantial hazard to human life in the event of failure including, but not limited to: III
- Buildings and other structures where more than 300 people congregate in one area
- Buildings and other structures with day-care facilities with a capacity greater than 150
- Buildings and other structures with elementary school or secondary school facilities with a capacity greater than 250
- Colleges & adult education facilities with a capacity greater than 500
- Health care facilities with a capacity greater than 50 resident patients but not having surgery or emergency treatment facilities
- Jails and detention facilities
- Power generating stations and other public utility facilities not included in Category IV
- Buildings and structures containing sufficient quantities of toxic, explosive or other hazardous substances known to be
dangerous to the public if released but not limited to:
- Petrochemical and fuel storage facilities
- Manufacturing or storage facilities for hazardous chemicals or explosives
Buildings and other structures that are equipped with secondary containment of toxic, explosive or other hazardous substances
(including, but not limited to dbl wall tank, dike of sufficient size to contain a spill, or other means to contain a spill or a blast
within the property boundary of the facility and prevent release of harmful quantities of contaminants to the air, soil, ground,
or surface water) or atmosphere (where appropriate) shall be eligible for classification as a Category II structure.
This reduced classification shall not be permitted for seismic loads.
In hurricane prone regions, buildings and other structures that contain toxic, explosive, or other hazardous substances and do
not qualify as Category IV structures shall be eligible for classification as Category II structures for wind loads if these
structures are operated in accordance with mandatory procedures that are acceptable to the authority having jurisdiction
and which effectively diminish the effects of wind on critical structural elements or which alternatively protect against harmful
releases during and after hurricanes.
Buildings and other structures designated as essential facilities including, but not limited to: IV
- Hospitals and health care facilities having surgery or emergency treatment facilities
- Fire, rescue and police stations and emergency vehicle garages
- Designated earthquake, hurricane or other emergency shelters
- Communication centers and other facilities required for emergency response
- Power-generating stations and other public utility facilities required in an emergency
- Ancillary structures (including but not limited to communication towers, fuel storage tanks, cooling towers, electrical
substation structures, fire water storage tanks or other structures housing or supporting water or other fire suppression
material or equipment) required for operation of Category IV structures during an emergency.
- Aviation control towers, air traffic control centers and emergency aircraft hangars
- Water storage facilities and pump structures required to maintain water pressure for fire suppression.
- Buildings and other structures having critical national defense functions

Exposure Categories adequately reflect the characteristics of ground surface irregularities at the building site. Exposure Categories are as follows:
Exposure "A":
Large city centers with at least 50% of the buildings having a height in excess of 70'. Use of this exposure category is limited to those areas for which terrain representative of Exposure "A" prevails in the upwind direction for a distance >= 0.5 miles or 10 times the height of the building or other structure, whichever is greater.
Exposure "B":
Urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger. Use of this exposure category is limited to those areas for which terrain representative of Exposure "B" prevails in the upwind direction for a distance >= 1,500' or 10 times the height of the building or other structure, whichever is greater.
Exposure "C":
Open terrain with scattered obstructions having heights generally less than 30'. This category includes flat open country, grasslands and shorelines in hurricane prone regions.
Exposure "D":
Flat, unobstructed areas exposed to wind flowing over open water (excluding shorelines in hurricane prone regions) for a distance of at least 1 mile. Shorelines in Exposure D include inland waterways, the Great Lakes and coastal areas of California, Oregon, Washington and Alaska. This exposure applies only to those buildings exposed to the wind coming from over the water. Exposure "D" extends inland from the shoreline a distance >= 1,500' or 10 times the height of the building or other structure, whichever is greater.

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.

This application assumes either Enclosed or Partially Enclosed buildings, and does not consider open buildings.
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:
a. 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
b. 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%.

The Tributary Area pertaining to the Main Wind-Force Resisting System (MWFRS).

The Effective Area, for a component or cladding panel equals the span length times the effective width that need not be less than 1/3 of the span length; however, for a fastener it is the area tributary to an individual fastener.
Note: Major structural components supporting tributary areas > 700
sq ft shall be permitted to be designed using the provisions for
main wind-force resisting systems (MWFRS).

The Effective Area, for a component or cladding panel equals the span length times the effective width that need not be less than 1/3 of the span length; however, for a fastener it is the area tributary to an individual fastener.
Note: Major structural components supporting tributary areas > 700
sq ft shall be permitted to be designed using the provisions for
main wind-force resisting systems (MWFRS).

For buildings with roof angle <= 10 degrees:
h = he (per Sect. 6-3).

For calculating the sum of the wind load on the windward and leeward walls parallel to the building ridge, the average roof height = (hr+he)/2 is used.

Exposure Factor
B 1.00
C 1.40
D 1.66

Area Reduction Factor
(ft.^2) (Interpolation Permitted)
<= 100 1.0
250 0.9
>= 1000 0.8
Note: above factors are applicable to
MWFRS pressures for roof only.

Importance Factor, I (Table 6-1):

Non-Hurricane Prone Regions Hurricane Prone Regions
Category and Hurricane Prone Regions with V > 100 mph
with V = 85-100 mph and Alaska
I 0.87 0.77
II 1.00 1.00
III 1.15 1.15
IV 1.15 1.15

Width 'a' is equal to 10% of least horizontal dimension or 0.4*h, whichever is smaller, but not less than either 4% of least horizontal dimension or 3'.