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Wind Exposure and Confusion

WIND EXPOSURE AND 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.
What adds into confusion, for all involved, is (even though written by same group of experts) IBC and IRC definitions do not exactly align!

Choosing a proper wind exposure is crucial to your building’s proper structural performance. Exposure C buildings must withstand a roughly 20% greater wind force than Exposure B and Exposure D, yet another 20%! This can result in one or more of deeper and/or wider column embedments, more concrete required to prevent uplift, larger columns and/or more closely
spaced, larger dimension and/or higher grades for wall girts and roof purlins, changes in truss design (larger and/or higher graded chord lumber, more webs, larger steel connector plates), ‘beefier’ connections, etc. In a nut-shell, it can change nearly every structural member and connection. To ignore proper wind exposure can result in catastrophic failures.
For utter confusion’s sake, I’ll list 2021 code sections (just in case you need some “put me to sleep” late night reading material.)
(HINT: At end, I include a broad generalization providing a close idea for most building sites.)
Picture entering a code book resembling a surrealistic painting by Salvadore Dali.
IBC Section 1609.4‐‐Exposure Category: “For each wind direction considered, an exposure category that adequately reflects the characteristics of ground surface irregularities shall be determined for the site at which the building or structure is to be constructed. Account shall be taken of variations in ground surface roughness that arise from natural topography and vegetation as well as from constructed features.”
IRC Section R301.2.1.4 Exposure Category: “For each wind direction considered, an exposure category that adequately reflects the characteristics of ground surface irregularities shall be determined for the site at which the building or structure is to be constructed. For a site located in the transition zone between categories, the category resulting in the largest wind forces shall apply. Account shall be taken of variations in ground surface roughness that arise from natural topography and vegetation as well as from constructed features. For a site where multiple detached one- and two-family dwellings, townhouses or other structures are to be constructed as part of a subdivision or master-planned community, or are otherwise designated
as a developed area by the authority having jurisdiction, the exposure category for an individual structure shall be based on the site conditions that will exist at the time when all adjacent structures on the site have been constructed, provided that their construction is expected to begin within year of the start of construction for the structure for which the exposure category is determined. For an given wind direction, the exposure in which a specific building or other structure is sited shall be assessed as being one of the following categories:”

Exposure determination is not relegated to a nice, comfortable chart or table. This section’s main part explains ground roughness variations from natural topography and vegetation need to be take into account when determining Exposure Category.
IBC Section 1609.4.1‐‐Wind Directions and Sectors “For each selected wind direction at which the wind loads are to be evaluated, the exposure of the building or structure shall be determined for the two upwind sectors extending 45 degrees either side of the selected wind direction. The exposures in these two sectors shall be determined in accordance with Sections 1609.4.2 and 1609.4.3 and the exposure resulting in the highest wind loads shall be used to represent wind loads from that direction.”
Here we get started in determining Exposure Category, but this process is three‐step from here.
Breaking this babble down to something making 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 chosen wind direction side are markers.
3) Use IBC Section 1609.4.2 and Section 1609.4.3 to determine exposure in those sectors
4) Exposure with 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! Tune in next time for yet
another fascinating installment!

This entry was posted in Pole Barn Design, Building Department, Pole Barn Planning, Pole Barn Structure and tagged wind exposure, Wind Exposure Category, wind load, IRC building code, vegetation, IBC building code, wind direction, topography on June 23, 2022 by admin.

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.

Mike,

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?
  Mike: 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)
Mike: 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!

This entry was posted in Uncategorized and tagged Wind Exposure Category, rat wall, pressure preservative treated columns, Exposure, crawl space, rigid perimeter insulation on October 7, 2020 by admin.

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?