About Exposure D
Exposure ‘D’ is a multiplier when converting wind velocity to wind pressure that represents coastal areas. It’s used in many formulas in ASCE 7 for wind, a larger topic than covered here. Non coastal areas have Exposure categories B and C. Exposure D is a moving target from the coastline based on several factors, the height of the building in question being a major one. Below is a how to checklist. Be sure to see our graphic attached for an easier visual representation.
1. CONFIRM SURFACE ROUGHNESS DISTANCE AND THE INITIATION POINT OF EXPOSURE D: The Initiation pPoint (Ip) of Exposure D occurs at the point on land where “Surface Roughness D” prevails in the upwind direction for a distance of 5000 feet minimum (Think: the runway for wind approaching the structure shall be at least a mile – 5,280ft to create an exposure D area or start / insertion point).
INITIATION POINT (Ip) = SURFACE ROUGHNESS > 5000FT
2. DETERMINE DISTANCE OF STRUCTURE FROM THE INITIATION POINT (Sdist).
3. CONFIRM PRIMARY EXPOSURE D REGION: The primary Exposure D condition occurs (for all structures) from the Exposure D Initiation Point at the directly exposed coastline area (that mark that was at the end of the runway above), running inland for a distance of 600 feet (Dprimary = 600 FT) (Think: Any structure 600 ft from what we have determined creates exposure D). AND
IF (Sdist) < (Dprimary) THEN EXPOSURE D APPLIES
4. CONFIRM BUILDING-SPECIFIC EXPOSURE D REGION: A secondary Exposure D condition may occur based on the structure itself. To determine the building-specific Exposure D region, multiply the Mean Roof Height (MRH) of the structure by 20 (Dsecondary=MRH*20). Compare Dsecondary to the Sdist. (Think: D can be greater than 600ft from this insertion point if the distance of the structure is less than or equal to the height of the structure * 20).
IF (Sdist) < (Dsecondary) THEN EXPOSURE D APPLIES
Click for larger image of visual explanation of Exposure D
Other Exposure Categories:
(With excerpts from the ASCE 7-05 Commentary to help explain):
Exposure Category A
Exposure A was deleted. Previously, Exposure A was intended for heavily built-up city centers with tall buildings. However, the committee has concluded that in areas in close proximity to tall buildings the variability of the wind is too great, because of local channeling and wake buffeting effects, to allow a special category A to be defined. For projects where schedule and cost permit, in heavily built-up city centers, Method 3 is recommended because this will enable local channeling and wake-buffeting effects to be properly accounted for. For all other projects, Exposure B can be used.
Exposure Category B
Exposure Category C