Tag Archives: wind exposure C

Questionably Designed Steel Truss Pole Barns

Being a member of numerous social media discussion groups, I see a plethora of photos of people’s new (or under construction) steel trussed post frame buildings. Most of these buildings are from Southeastern states where it appears structural building permits and plan checks are minimal or non-existent. This results in my receiving emails like this one from KEVIN in CULPEPER:

Hello! I am in the midst of building a pole barn, 40 wide x 90 long x 12 high posts.  We are using 6x6x16s and steel trusses. All is going well thus far my question is, would it be advisable to close in the long side walls and leave the ends open? Or leave all sides open? How does the wind load change in relation to this? I prefer not to fully enclose for now.  Also, what wind bracing would YOU utilize? Y braces, knee braces, X braces down the side walls, steel cables and turn-buckles spanning from gable peak to second post back? I’m in a windy spot.

THANK YOU in advance for any advice!” 


Mike the Pole Barn Guru comments:

While I appreciate your reaching out to me, these are questions best directed to your building plan’s engineer. He or she has structural design responsibility for your building. If somehow you do not have fully engineered plans, you desperately need one’s (an engineer’s) services. My best guess is 6×6 columns are overstressed in bending either with or without sides and will require an engineered repair. On typical post frame construction covering long sides and leaving endwalls open is pretty much a recipe for a failure as wind loads going into the top half of your sidewalls and roof are transferred through endwall sheeting to ground. No endwall sheeting means a tremendous load is being placed on those corner columns. Required bracing will be called out for on your engineer sealed building plans. Your building site happens to be an Exposure C – it is open to wind coming from one or more directions. Effectively this requires your building to be designed to resist roughly 20% more load from wind, than would a protected (Exposure B) site. This wind condition should be specified on those engineered plans.

I realize it is a huge temptation to throw hard earned money at questionably designed, bargain priced buildings. There is a reason their prices are so cheap – and if you are willing to sacrifice structural integrity for low price, there is nothing I can do to save you.

For everyone else – unless you are investing in a stick frame building following prescriptive requirements laid out within Building Codes (not just a handout from a Building Department) you should demand your provider supply fully engineered plans for your building – specific to your building’s features and your site. Anything short of this is an invitation to a future disaster site.

Building Department Checklist Part I

BUILDING DEPARTMENT CHECKLIST 2020 PART I

I Can Build, I Can Build!

Whoa there Nellie…..before getting all carried away, there are 14 essential questions to have on your Building Department Checklist, in order to ensure structural portions of your new building process goes off without a hitch.  I will cover the first seven today, finishing up tomorrow, so you have a chance to take notes, start your own home file folder of “what to do before I build”.  Careful preparation will be key to having a successful building outcome (whether post frame or some other structural building system).

Provide answers to these questions to your potential building providers!

IMPORTANT SIDE NOTE: Building Departments’ required snow and wind loads are absolute minimums in an attempt to prevent loss of life during extreme events. They are not established to prevent your building from being destroyed. Consider asking your providers for added investment required to increase wind and/or snow loads beyond these minimums.

#1 What are required setbacks from streets, property lines, existing structures, septic systems, etc.?

Seemingly every jurisdiction has its own set of rules when it comes to setbacks. Want to build closer to a property line or existing structure than distance given? Ask about firewalls. If your building includes a firewall, you can often build closer to a property line. Creating an unusable space between your new building and a property line isn’t very practical. Being able to minimize this space could easily offset a small firewall investment. As far as my experience, you cannot dump weather (rain or snow) off a roof onto any neighbor’s lot, or into an alleyway – so keep those factors in mind.

#2 What Building Code will be applicable to this building?

Code is Code, right? Except when it has a “residential” and also has a “building” version and they do not entirely agree with each other.

Also, every three years Building Codes get a rewrite. One might not think there should be many changes. Surprise! With new research even things seemingly as simple as how snow loads are applied to roofs…changes. Obviously important to know what Code version (e.g. 2012, 2015, 2018, 2021) will be used.

 

#3 If building will be in snow country, what is GROUND snow load (abbreviated as Pg)?

Make sure you are clear in asking this question specific to “ground”. When you get to #4, you will see why.  Too many times we’ve had clients who asked their building official what their “snow load” will be, and B.O. (Building Official) replied using whichever value they are used to quoting.  Lost in communication was being specific about “ground” or “roof” snow load.

As well, what snow exposure factor (Ce) applies where a building will be located? Put simply, will the roof be fully exposed to wind from all directions, partially exposed to wind, or sheltered by being located tight in among conifer trees qualifying as obstructions? Right now will be a good time to stand at your proposed building site and take pictures in all four directions, and then getting your B.O. to give their determination of snow exposure factor, based upon these photos.

#4 What is Flat Roof Snow Load (Pf)?

Since 2000, Building Codes are written with flat roof snow load being calculated from ground snow load. Design snow load has become quite a science, taking into account a myriad of variables to arrive with a specific roof load for any given set of circumstances.

Unfortunately, some Building Departments have yet to come to grips with this, so they mandate use of a specified flat roof snow load, ignoring laws of physics.

Make certain to clearly understand information provided by your Building Department in regards to snow loads. Failure to do so could result in an expensive lesson.

#5 What is “Ultimate Design” or Vult wind speed in miles per hour?

Lowest possible Vult wind speed (100 miles per hour) only applies in three possible states – California, Oregon and Washington for Risk Category I structures. Everywhere else has a minimum of 105 mph.  Highest United States requirement of 200 mph for Risk Category III and IV buildings comes along portions of Florida’s coastline (although there are scattered areas nationally defined as “Special Wind Regions).  Don’t assume a friend of yours who lives in your same city has your same wind speed.  City of Tacoma, WA has six different wind speeds within city limits!

Vult and nominal design wind speed (Vasd) are different and an errant choice could result in significant under design (or failure). Make certain to always get Vult values.

#6 What is wind exposure (B, C or D)?

Please Take a few minutes to understand their differences:

(https://www.hansenpolebuildings.com/2012/03/wind-exposure-confusion/).

A Building Department can add hundreds, or even thousands, of dollars to your project cost, by trying to mandate an excessive wind exposure.  Once again, a good place for photographs in all four directions from your building site being shared with your Building Department.  Some jurisdictions “assume” worst case scenarios.  Meaning, your property could very well have all four sides protected and easily “fit” category B wind exposure requirements.  However, your jurisdiction may have their own requirement for every site in their jurisdiction to be wind exposure C, no matter what.  It’s their call.

#7 Are “wind rated” overhead doors required?

Usually this requirements enforcement occurs in hurricane regions. My personal opinion – if buying an overhead door, invest a few extra dollars to get one rated for design wind speeds where your building will be constructed. Truly a “better safe, than sorry” type situation.

I’ve covered seven most important questions for your Building Department Checklist, and they really weren’t so difficult, were they?  Come back tomorrow to find out the last seven!

Self-Designed Pole Buildings

Spring, When a Young Man’s Heart Turns to Self-Designing Pole Buildings

For some obscure reason a plethora of otherwise intelligent people have an idea. This idea being they can structurally design a building to be adequate to resist applied climactic loads, without any actually engineering background. Given an under designed building can lead to failure, injury and even death of occupants and/or bystanders, one might think it would be best left to professionals.

Reader NORM in SILVERTON writes:

“I’m considering building an open pavilion style pole building, with outside (the posts) dimensions of 20’ x 16’ x no more than 9’ to 10’ high posts, secured to cement pad with Simpson CC66 caps.  There would be 3 posts on both the left and right sides, that would be 8’ from middle, of middle post, to outside edge of front and back post.  The alignment of 3 posts on each side, would be 20’ apart with 6/12 gable roof, supported with roof trusses (50 PSI Snow Load).  On each side, the roof overhang would be 3’, which I don’t think matters when considering my question.  The posts are more than sufficient size and strength for the gabled metal roof …..  I’ve been told.  

Question: What “wind gust” strength would I need to be concerned about from side to side, for the “sway” factor ?  Would that “wind strength” be less if directly behind this “pavilion”, was a slightly larger and taller building, AND directly behind that building, was standing forest with trees that were 60’ to 100’ tall ?  We obviously are NOT in tornado country like the Midwest and South.

Thank You.”

About Hansen BuildingsThank you for your interest in a new Hansen Pole Building. We should be able to take care of all of your needs with a third-party engineer sealed set of blueprints specifically for your building. Face it – this eliminates any guesswork, as anything you do without a Registered Design Professional involved is nothing but a W.A.G. (Wild Ass Guess), probably an errant one. Given height of your roof (it takes full brunt of wind coming from a side) it is unlikely a 6×6 column will work in bending (it is plenty strong enough to support downward forces from building weight and roof snow load acting alone).

Even without being an engineer I can tell you a proposed Simpson CB66 is totally inadequate. Frankly your ideal design solution is to embed your six columns into the ground and concrete them in to avoid uplift and overturning challenges. If you feel you must have columns above ground, then we can design using a proper wet set anchor capable of carrying imposed loads.

If your building is wind unprotected on even one side or end chances are it is Exposure C for wind design. You do not get credit for a building being protected on one side (or even two or three) by a larger taller building or a forest – only if it were to be entirely surrounded. (read more here about Wind Exposure: https://www.hansenpolebuildings.com/2012/03/wind-exposure-confusion/).

A Hansen Pole Buildings’ Designer will be reaching out to you to further discuss your proposed project, or dial 1 (866) 200-9657 and talk with one now!