ASCE-7 is the governing referenced standard for design loads on buildings & other structures for building codes throughout the US. ASCE 7 (26.7 of ASCE 7-16) describes the process to which wind speed is converted into wind pressure to design such structures. The formula is based on many variables. One of which is called ‘Exposure Category’ and reflects the characteristics of ground surface irregularities at a site which the building or structure is to be constructed. The rougher the surface, the lower the multiplier that converts wind speed to pressure.
Learn more about the conversion of wind speed to pressure by clicking here.
About Exposure D & ASCE 7
Exposure ‘D’ is the largest multiplier when converting wind velocity to wind pressure which represents coastal areas and the ‘smoothness’ of water relative to wind. Non coastal areas have Exposure categories B and C with greater roughness coefficients.
The exact dimension of where Exposure D lies relative to the coast is a moving target and based on the height of the building in question. Below is a how to checklist in determining Exposure D. Be sure to see our graphic below for an easier visual representation. The example & illustration have been verified for use with ASCE 7-16 as well.
1. CONFIRM SURFACE ROUGHNESS DISTANCE AND THE INITIATION POINT OF EXPOSURE D: The Initiation Point (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
Visual Example of Exposure D Per ASCE 7:
Visual Representation Of Exposure Roughness from ASCE 7 Commentary:
Update Regarding ASCE 7-16
Something to note, ASCE 7-16 has expanded on the application of exposure categories by clarifying a directional requirement for (8) sectors of wind directions. An Exposure of one condition could exist in one sector but not another. This was in previous ASCE standards but was expanded upon in the commentary
From 26.7.1 (exists in earlier ASCE 7 codes): Wind Directions and Sectors. For each selected wind direction at which the wind loads are to be determined, the exposure of the building or structure shall be determined for the two upwind sectors extending 45° on either side of the selected wind direction. The exposure in these two sectors shall be determined in accordance with Sections 26.7.2 and 26.7.3, and the exposure the use of which would result in the highest wind loads shall be used to represent the winds from that direction.
From the ASCE 7-16 Commentary this illustration has been expanded:
We recommend consulting with the ASCE 7-16 design manual for more information before use in designing a project for wind & using directional exposures with caution. Worst case is always recommended.
Other Exposure Categories:
(With excerpts from ASCE 7 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
Last Update: June 11, 2021