This “Code Simple” will attempt to clarify the determination of Exposure Category D. Section 1609.4.3 of the International Code Council’s 2006 International Building Code (IBC) defines Exposure D as follows:
Exposure D shall apply where the ground surface roughness, as defined by Surface Roughness D, prevails in the upwind direction for a distance of at least 5,000 feet (1524 m) or 20 times the height of the building, whichever is greater. Exposure D shall extend inland from the shoreline for a distance of 600 feet (183 m) or 20 times the height of the building, whichever is greater.
Section 1609.4.1 indicates that for each wind direction, the exposure of the building needs to be determined for the two upwind sectors extending 45 degrees from either side of the selected wind direction (see Figure 1). This section also states that the exposure resulting in the highest wind loads needs to be used to represent winds from that direction. Section 1609.4.2 indicates that a ground surface roughness within each 45-degree sector needs to be determined for a distance upwind of the site so that the Exposure Category can be determined.
Section 1609.4.2 defines Surface Roughness D as follows:
Surface Roughness D. Flat, unobstructed areas and water surfaces outside hurricane-prone regions. This category includes smooth mud flats, salt flats and unbroken ice.
Answers to FAQs
Q: I am reading the definition for Exposure Category D in ASCE 7-05 Section 18.104.22.168, and I don’t understand the second sentence which reads as follows: “Exposure D shall extend into downwind areas of Surface Roughness B or C for a distance of 600 feet (200 m) or 20 times the height of the building, whichever is greater.” What downwind area is being referred to in this statement?
A: You have a good question, and for this particular definition, the 2006 IBC has made that sentence more easily understandable. The 2006 IBC does not use the term “downwind areas” and does not make reference to “Surface Roughness B or C” but simply reads: “Exposure D shall extend inland from the shoreline for a distance of 600 feet (183 m) or 20 times the height of the building, whichever is greater.” See Figure 2.
There is a good reason why Exposure D extends so far inland and is dependent on the height of the structure. After a change in surface roughness, wind has to travel a sufficient distance over the new terrain for the velocity or pressure distribution to attain a new, changed equilibrium state over a certain height above ground level; the required travel distance is a function of that height.
S.K. Ghosh Associates Inc., is a structural, seismic, and code consulting firm located in Palatine, Ill., and Laguna Niguel, Calif. President S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at firstname.lastname@example.org and email@example.com, respectively, or at www.skghoshassociates.com.