If you are a registered design professional, or a building official, then you are trying to make sense out of this subject on a daily basis. Most people who are selling buildings (either constructed or kit packages), tend to ignore wind exposure, or pretend it somehow doesn’t exist.
For sake of utter confusion, I’ll list the sections of the code (just in case you need some “put me to sleep” late night reading material.)
(HINT: At the end, I include a broad generalization which should give a close idea for most building sites.)
Picture entering a portion of the code book, which resembles a surrealistic painting by Salvadore Dali.
Section 1609.4‐‐Exposure Category: includes three subsections but determination of exposure is not relegated to a nice, comfortable chart or table. The main part of this section explains variations of the roughness of the ground from the natural topography and vegetation need to be take into account when determining Exposure Category.
Section 1609.4.1‐‐Wind Directions and Sectors is the first item for determining Exposure Category, but the process is three‐step from this point. Breaking the babble down to something which makes sense isn’t easy, but a list helps:
1) Select wind direction for wind loads to be evaluated
2) Two upwind sectors extending 45° from either side of the chosen wind direction are the markers
3) Use Section 1609.4.2 and Section 1609.4.3 to determine the exposure in those sectors
4) The exposure with the highest wind loads is chosen for this wind direction
Got all this? If not, you aren’t the only one. But wait, there’s more!
To get the information needed, glance over Section 1609.4.2‐‐Surface Roughness Categories. In this section, roughness is broken down into three categories: B, C, or D. Summarized as follows:
1) Surface roughness B: Urban, suburban, wooded, closely spaced obstructions
2) Surface roughness C: Open terrain with few obstructions (nothing greater than 30 feet), flat open country, grasslands, water surfaces in hurricane‐prone regions
3) Surface roughness D: Flat areas outside of hurricane prone regions, smooth mud flats, salt flats, unbroken ice
Hold on, we still haven’t determined what the Exposure Category is.
In Section 1609.4.3‐‐Exposure Categories, we get the information we want. Exposure is based on the roughness determined earlier and once again broken into Categories B, C, or D:
1) Exposure B: Surface roughness B = Exposure B with these restrictions:
a) Roughness B prevails upwind for at least 2,600 feet or 20 times the building height (choose greater)
b) If the roof height is 30 feet or less, upwind distance is reduced to 1,500 feet
2) Exposure C: Exposure C shall apply for all cases where Exposures B or D do not apply
3) Exposure D: Surface roughness D = Exposure D with these restrictions:
a) Roughness D prevails upwind for at least 5,000 feet or 20 times the building height (choose greater)
b) Exposure D extends inland from a shoreline 600 feet or 20 times the building height (choose greater)
Is choosing an Exposure Category now clear? If not, as a generality, roughness = exposure. Memorizing all the details isn’t necessary, but being able to recognize the letters is probably a good idea.
OK – so here is the “Cliff Notes” version – in generalized, simple terms:
Wind Exposure B is a site protected from the wind in all four directions, within ¼ mile, by trees, hills or other buildings. This would include building sites in residential neighborhoods and wooded areas.
Wind Exposure C is open to the wind in one or more directions, for ¼ mile, with only scattered obstructions generally less than 30 feet tall in the “open” direction. This would include building sites in flat open country, grasslands and ocean exposed shorelines in hurricane-prone regions.
Very few people actually have Wind Exposure D, which is the most severe exposure. It would be in areas with terrain which is flat and unobstructed facing large bodies of water over a mile or more in width. An example is the non-hurricane prone ocean shoreline or the Great Lakes. I am always amazed when I get a request for a quote from someone claiming Exposure D…and they are in the middle of Kansas with not even a river, much less a lake within 100 miles!
I also have those folks who insist “the prevailing wind” comes from the one direction they have “protected”, so they want to claim Wind Exposure B – when the other 3 sides are basically totally exposed. Which means they are really “out in the country”. Exposure determination doesn’t care which side IS protected –just that all 4 sides ARE protected. And, although it doesn’t hurt to claim a higher exposure “just to be safe”, it will cost more in many (but not all) cases. When in doubt, stand on your building site and take photos in all 4 directions, and then take them to your building department for a wind exposure determination. It’s always best to have your local Building Officials working with you from the beginning of your project.