The primary design standard used for wind load calculations is ASCE 7-02. Exposure classifications are described as:
The magnitude and sense of internal pressure is dependent upon the magnitude and location of openings around the building envelope with respect to a given wind direction. Accordingly, the standard requires that a determination be made of the amount of openings in the envelope to assess enclosure classification (enclosed, partially enclosed, or open). “Openings” are specifically defined in this version of the standard as “apertures or holes in the building envelope and which are designed as “open” during design winds.” Examples include (unprotected and non impact resistant) doors, operable windows,…
• A building having each wall at least 80% open.
• Partially Enclosed
• A design that takes into account the fact or potential for wind to build up pressure on the inside of the building. A building is designed as partially enclosed and is designed for “Internal Pressure” that is created due to the “Partial Enclosure.” This was an option to be used in lieu of opening protection in the wind-bourne debris region in Florida prior to July 2007. The option has been eliminated from the 2006 IRC.
• A building that does not comply with the requirements for open or partially enclosed buildings.
There are four exposure categories defined in ASCE-7 and further defined by the surface roughness categories:
Exposure A, Exposure B, Exposure C , and Exposure D
Surface Roughness Categories. A ground surface roughness within each 45-degree sector shall be determined for a distance upwind of the site as defined in Section 184.108.40.206 from the categories defined below, for the purpose of assigning an exposure category as defined in Section 220.127.116.11
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 less than 30 ft (9.1 m). This category includes flat open country, grasslands, 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 A: This exposure category does not exist in the hurricane prone region of the country.
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 2630 ft. (800 m) or 10 times the height of the building, whichever is greater. Exception: For buildings whose mean roof height is less than or equal to 30 ft (9.1m), the upwind distance may be reduced to 1500 ft (457 m)
Exposure C: Exposure C shall apply for all cases where exposures B or D do not apply
Exposure D: In ASCE-7, this exposure category does not exist in the hurricane prone region of the country. However, the removal was based on the roughness of the ocean surface, particularly near shore where breaking and shoaling waves occur. Data is not available to demonstrate this effect for winds over inland bodies of water or inland waterways. Consequently, exposure D has been re-introduced in the Florida Building Code for bodies of water away from the coast. See the note on post storm investigations below.