Tag Archives: Wind Exposure Category

Avoid Being Driven Crazy With Barndominium Questions Part I

Avoiding Being Driven Crazy With Barndominium Questions Part I

Loyal reader and client GREG in KENTWOOD is planning his new post frame barndominium home and has questions no one else will answer. Mike’s answers are in italics.


Good morning, I hope all is well with you.  

 I have some questions that I would like to understand and it is driving me crazy, because no one other than Hansen Pole Building, it seems will answer my questions and return my calls.  Most the crazy part is about the slab. I find cement workers are not great communicators.  I am 56, a mechanical engineer, have renovated several houses from the studs up, I can do plumbing without butt crack showing, I raced stock cars for 10 years, I tell you this, because I know how to build stuff and am not afraid of hard work. My job requires a detailed list of Bill of Materials, precision drills and reamers and very detailed processes to make fuel injection parts, ABS brake parts and other.  I hope you can help out, as I am ignorant about some simple facts that are driving me crazy.

Mike: We believe good communication is essential to successful completion of most any building. You will find we strive towards written communications in order to minimize (or eliminate) possible miscommunication of important facts and details).

I also would like to say Brenner is doing a fine job.

Mike: Brenner has a passion for post frame buildings and he is not at all afraid to reach out to higher authorities for answers to complex structural questions.

Due to your great level of communication, you so far are my #1 choice to partner with on my new house.  I also would be willing to visit you in MN if you have/think I could see examples of my questions first hand.  

Statements from details I learned at Code Meeting:

  • In Michigan, in my county I need to have a 2’ foam, below grade, with R10 barrier, around the full foundation or slab perimeter. 
  • So my building code staff recommends a Rat Wall 2’ below grade, or a crawl space with footers, or a slab with a wooden wall built to hold the 2’ foam below grade.
  • I don’t really have a problem with the Rat Wall, but it certainly will add more cost to cement.

My questions on this topic are:

    • If using a Rat Wall, it seems like the 6” * 6” poles will then be encapsulated in cement for about 2.5’ at minimum.  I thought the poles were not supposed to be in cement as it causes probably more decay than dirt.
      Mike: Concrete does not cause premature decay of properly pressure
      preservative treated columns.
    • What are your thoughts?
      Personally I would build over a crawl space because my knees are far happier living on wood than concrete.
    • What would you recommend for the 2’ below grade issue regarding the slab foundation?  Mike: With a slab on grade, you can use rigid perimeter insulation without a need for a ‘rat wall’ or wood foundation wall. It can be held in place by backfill on each side. It can be placed running from below base trim on exterior of splash plank, or on inside of splash plank. If on inside of splash plank, it eliminates having to protect it from possible UV degradation.

On attachment pictures in the middle pages show where the 6” X 6” poles go and spacing, listed are my questions:

I am concerned with spacing of 12’ and 14’ of the 6” X 6” posts.  (I’m sure the loading is OK, within code, but see below)
Actually with your 21 foot eave height and Exposure C for wind 6×6 columns will not engineer out. Your building will have glulaminated columns manufactured out of high strength 2×6 or 2×8 depending upon location.

    • One concern is getting wood in today’s supply chain that are straight enough for the girt boards and purlins at 12’ and 14’, should I be concerned? ( I can’t get straight 2” X 4” at 8’ for the walls I have built in last year alone.) Mike: Lumber is obviously organic and we can no longer cut down those old growth trees where one might be able to get straight grained, narrow growth ringed lumber with few or no defects and very little warp, twist or cup. One beauty of steel roofing and siding is it hides a plethora of framing imperfections (like warp), due to high ribs of steel siding.

When using double trusses, at all locations, why not just go to 8’ centers on the poles?   I know it is more 6” X 6” poles, but really is not anymore trusses and may help get straighter girt and purlin boards. 
Mike: Wider spaced columns allow for more flexibility in location of doors and windows and an added advantage of not having to dig as many holes. If you were thinking of using a single truss every eight feet, rather than a double truss every 12, I would discourage it. Double trusses allow for true load sharing and eliminate any possibility of a single truss having a weak point, where under extreme loads (beyond design loads) it may fail and bring down your entire roof.

Come back tomorrow for Part II in Barndominium Questions!

Airplane Hangar Exposure C

Why Your Airplane Hangar is Probably Exposure C

I had the joy of growing up “hanging out” (pun intended) at airplane hangars and doing a lot of flying including having my hands on the controls of a Cessna 182 for many hours before I became a teen. One thing for certain about airplane hangars – they are always built with the idea of being able to take off and land the airplanes which are housed inside, somewhere in the general vicinity of the hangar!

Yes, I know this reads like a mission for Captain Obvious.

After all, what would be the use of a hangar if not to be able to fly the plane?

Airplanes do require a certain amount of space to be able to land and the runway better be fairly flat, as well as not obstructed by things like other buildings and trees. Those tall things generally tend to make the life of a pilot miserable.

A Hansen Pole Buildings client recently ordered a new post frame hangar with an Exposure B for wind. This is the short version of the definition of a B exposure:

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.

Whereas, Wind Exposure C is a site where there is open terrain with scattered obstructions having heights generally less than 30 feet high. (Commonly associated with flat open country and grasslands).

If you are curious and want to know all there is to know about Wind Exposure here is some good late night reading: https://www.hansenpolebuildings.com/2012/03/wind-exposure-confusion/.

Being a fairly simple guy, I am scratching my head at this wondering how the plane is going to takeoff through all of this protection.

Hansen Pole Buildings’ Managing Partner Eric did a quick Google search of the site and let me know it is in the middle of a field!

In the event you are in need of a new airplane hangar and you are getting quotes from providers which do not specifically indicate on the Exposure C for wind, chances are good you are being quoted for Exposure B. The difference in design strength for resistance to wind loads is roughly 20%.

Think about it…..

Do you actually want your several hundred thousand dollar airplane to be parked in a building which is under designed for the actual wind conditions which could be applied to the building?

Wind Exposure & Confusion

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.