Tag Archives: design wind speed

Buildings Designed/Built to Code

Designed / Built to Code

Sounds pretty impressive to think you are going to be investing in a new building designed and/or built to “Code”.


Well – maybe not so much. To begin with “Code” happens to be bare minimum requirements to adequately protect public health, safety and welfare. This does not mean a structure built to “Code” will withstand all possible circumstances. As an example, residential structures (R-3) are designed so as there is a 2% probability of their design loads being exceeded in any given calendar year!

So, how does a consumer best protect their interests?


Whether investing in a complete building kit, or having a builder provide materials as well as erection labor – if you receive a proposal stating only “to Code” or not mentioning “Code” at all…..


All proposals and agreements for buildings should mention what Code and Code version is being used. IRC (International Residential Code) and IBC (International Building Code) do have some differences between them. Every three years there is a new Code version published. Each version has latest updated changes due to testing, research and new products being introduced. Your new building should either match your jurisdiction’s adopted Code version or (if no structural permits are required), most recent version.


Unless you are building within prescriptive ‘cook book’ restrictions of a Code, I am a firm believer of buildings being fully engineered. Not just engineered trusses (as an example) but every component and connection being checked and verified by a Registered Professional Engineer specific to your building’s features on your site. This is for everyone’s protection (not just yours, but also your provider and any hired builder).


Beyond applicable Code version, there are other factors you should have included:

Ground Snow Load (Pg) in areas where it snows. Ground snow load is not the same as roof snow load, but is important as it affects drift zones on each side of roof ridges. In these areas, roof purlins often must be closer together, larger dimension or higher graded material to compensate for drifting.

Flat Roof Snow Load (Pf) is usually calculated from Pg and incorporates factors such as Occupancy (low risk buildings get a 20% reduction), wind exposure (an exposed building has snow blow off, a protected site has snow sit) and temperature (heated or unheated and well or poorly insulated). Some jurisdictions mandate a minimum Pf, ignoring applicable laws of physics.

No snow? Then Lr applies, rather than Pf. Lr is a reduced uniformly distributed roof live load ranging from a minimum of 12 to a maximum of 20 psf (pounds per square foot), depending upon the area being carried by a given member.

Design Wind Speed in either V (basic design wind speed, sometimes expressed as Vult) or Vasd, in mph (miles per hour). These values are directly correlated as Vasd equals V multiplied by square root of 0.6.

Wind Exposure – rarely mentioned and extremely important. Most buildings will be on Exposure C sites, meaning they must resist a 20% greater wind force than a fully protected Exposure B site. Become more knowledgeable by reading here: https://www.hansenpolebuildings.com/2012/03/wind-exposure-confusion/

If wind exposure is not delineated on a proposal or agreement, it is not a good sign.

Allowable Foundation Pressure – most people are not interested in having their buildings settle. This value relates to your site’s soil being able to support a given value per square foot of building weight INCLUDING roof and floor live (or snow) and dead (permanent) loads. Keeping it simple, easier to dig equals lower values.  In an ideal world, a geotechnical engineer has tested your site’s soils and can provide an exact measure of soil strength in his or her report. Many providers assume a value of 3000 psf, this would exclude soils including any silts or clays and using this as a value could compromise structural integrity.

Seismic Zone: for single story wood or steel frame structures with low or no snow and more than just bare minimum design wind forces, seismic forces will not dictate structural design. However, they should be checked.

If you are negotiating with a provider or builder who is not clearly stating all of these factors, you are very well paying hard earned money for something you are not getting.

Contact your local jurisdiction so you are aware of what Code minimum requirements are. Ask your provider or builder for any additional investment to upgrade to a greater roof load and/or design wind speed – in most cases it is negligible and it allows you to make informed choices as to risk/reward.

Getting the Best Deal on Your New Post Frame Building

A price quote is merely a number without a complete understanding of exactly what is or is not included in said quote.

You have requested quotes for your new post frame building from a dozen or more providers and actually gotten four back, even after having to hound all of them for pricing! Frustrating when you are ‘knocking at their door’ trying to spend your cash.

One quote stood out above all others with an exceptional price, so you place your order. Only after “everything” arrives and you try to assemble it do you find out what you thought you bought and what you really purchased are not quite equal.


If you prefer to read books by starting with the last chapter, you can skip to there to find a solution.

Here are a few points to be aware of:

Will Your Building Meet Minimum Building Code Standards?

Those quotes you got….few, if any, will specify what loads your building are designed for.

Some of them will just be a list of materials! Are they right? Is there even enough there to construct a building?

Every quote should include (at a minimum): engineer sealed plans specific to your building at your site. Complete Building Code information – including Code version (there is a new one every three years), Ground snow load (Pg), Flat roof snow load (Pf), Design wind speed (Vult or Vasd), Wind Exposure (there is a big difference between Exposure B and C) and assumed soil bearing pressures.

You can easily acquire this information for yourself, so you have a point to check from: https://www.hansenpolebuildings.com/2019/01/building-department-checklist-2019-part-1/

If Code information is not on a quote, chuck it.

Do Roof Trusses Quoted Meet Your Needs?

Here is where investing in an engineered building comes into play, as your Engineer of Record (person who seals your building plans) should be reviewing prefabricated roof truss drawings for their adequacy for his or her building.

Planning on supporting a ceiling, either now or at a later date? If so a ceiling load of no less than five pounds per square foot (psf) should be indicated on engineered plans as well as a BCDL (Bottom Chord Dead Load) to match on sealed truss drawings.

At Hansen Pole Buildings, we ran into this situation so often, we decided to upgrade all trusses up to 40 foot clearspan to support a minimum five psf load.


How is Roof Steel Condensation Being Controlled? Most providers are not even going to mention this. Most of us prefer it not to rain inside of our new buildings. 

I answer questions online every Monday. Problem/question number one is regarding condensation.

From cheapest up – a Radiant Reflective Barrier (aka bubble wrap – if going this route you only need single bubble, six foot wide rolls with an adhesive pull strip); Integral Condensation Control (https://www.hansenpolebuildings.com/2017/03/integral-condensation-control/); Sheathing with 30# felt; Closed cell spray foam.

Planning on insulating and finishing walls? If not using closed cell spray foam you will want to apply a Weather Resistant Barrier between wall framing and steel siding.

What Written Warranty Comes With Your Building?

If it does, how long does it last? What does it include? When it comes to Post Frame Building kits, Hansen Pole Buildings stands alone with a Limited Lifetime Structural Warranty (https://www.hansenpolebuildings.com/2015/11/pole-building-warranty/).

Are Assembly Instructions Included?

If not, there is plenty left to chance. Hansen Pole Buildings provides a fully illustrated, step-by-step 500 page Construction Manual. And, if you get stuck, there is unlimited FREE Technical Support from people who have actually assembled buildings!


How About Your Potential Provider?

How long have they been in business 2 years, 5 years? How about 18 years? How many post frame buildings have they provided? How about roughly 20 thousand buildings located in ALL 50 states!

Here is how to vet any potential provider: https://www.hansenpolebuildings.com/2015/01/pole-building-suppliers/

I promised you a solution (aka Last Chapter of Book)

We are offering to shop for you.  Seriously? Yes! You provide up to three names of competitors to Hansen Buildings, where you can purchase a complete wood framed post frame building kit package, and we will shop them to get quotes for you.

Now we say three, because frankly, some people just are not very prompt or cooperative when it comes to getting back with price quotes.

Why would we do this?  Comparing “apples to apples”, we know our price will beat theirs, every single time. We offer to do this for your peace of mind.   We guarantee all other prices will be higher.  And we will provide you with documentation to prove it!

There is a catch…..before we go shopping you have to place your order for your new Hansen Pole Building kit….. subject to us “proving our point” by going shopping. Your payment to us will not be processed for ten calendar days. Within seven days of order, you’ll have competitive quotes in hand, or our documentation of having hounded them every week day for a week trying to get pricing for you (seriously, if you have to hound someone for a price, what kind of after sale service will you get?). 

After we email you proof, if you seriously want to purchase from one of these competitors, just let us know before ten days pass and we tear everything up and go away friends.

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!

OSB Under Steel Roofing On Pole Buildings

Almost anything can be sold as a benefit…

Most post frame buildings are sold to the unsuspecting or ill-informed client based upon a set of features. These features may or may not have an actual benefit to the long term performance of the building. What is most unique is – the sales person often has no clue as to whether the feature has a benefit or not! They are just selling it as if it actually was beneficial.

Case in point – one of our recent clients in Colorado has hired a building contractor to erect his building. This particular contractor happens to admit to sheathing over 90 percent of all of the post frame buildings he has put up in 7/16” OSB (Oriented Strand Board) beneath the steel roofing and siding. While this may sound great and wonderful (not to mention expensive), does it truly offer a benefit to the client?

Think of roll formed steel roofing and siding as performing like a very strong, very thin plywood and you will have a better grasp of how it performs. The only reason to put OSB on a roof would be in the case of a very narrow, very long, very tall building with high wind loads – ones which would exceed the wind shear carrying capacity of the steel. This case is most certainly the exception to the general rule and happens in far less than 5% of all post frame buildings. Even in those cases, the OSB is only effective in the bays closest to the endwalls, where shear forces are greatest. As for the walls – on endwalls with a significant number of openings, it becomes necessary to reinforce the steel siding with OSB (generally at the corners). Outside of this, there would not be an economically practical reason to sheet the walls with OSB.

Let’s look at some numbers…

From the 2012 IBC (International Building Code) Table 2304.6.1 – over framing spaced 24 inches on center, 7/16” OSB fastened with 8d common nails six inches on center at the edges and in the field will support a maximum design wind speed (Vult) of 110 mph (miles per hour) in Exposure B or 90 mph in Exposure C. Table 2304.7(3) gives the 7/16” OSB the ability to carry a roof live load of 40 psf (pounds per square foot) when spanning the same 24 inches.

The 2009 IBC in Table 2306.2.1(1) gives the shear value for 7/16” OSB with 8d nails six inches on center as 230 pounds per foot for a Case 1 layup (long side of the panel towards the load, and joints staggered by four feet) in Douglas Fir, with an 18% reduction for use with SPF (Spruce-Pine-Fir) or Hem-Fir.

Compare this to 29 gauge steel. From information provided by Fabral (https://www.fabral.com/grandrib-3-load-tables/), spanning 24 inches the wind load is good for 213 psf. Converting psf to wind speed (P = .00256 x mph^2), would equate to 288 mph! For roof loads (gravity) 112 psf.

For shear loads – we actually tested 30 gauge steel (nearly 12% thinner than 29 gauge), over spans of 28 inches (nearly 17% greater than 24) and using SPF lumber. We were looking for a worst case scenario for roof applications. Without the use of sidelap stitch screws we obtained a tested value of 110 pounds per foot and with stitch screws 160 pounds per foot. These values are published in the NFBA Post Frame Building Design Manual.

Doing a little fuzzy math 160 X 1.17 (spacing adjustment) X 1.12 (thickness adjustment) X 1.18 (species adjustment) = 247 or roughly comparable to the OSB value!


So why would we even add OSB? Because in conjunction with the steel, the strength vales of the two products in withstanding wind shear can be added, up to twice the value of the lesser strength product.

Moral of the story – OSB has its place structurally in limited circumstances on post frame buildings, but to try to sell it in overall use as a benefit, just doesn’t hold true.

Fabric Covered Building and Wind

One of the Hansen Buildings designers recently asked me what I knew about fabric covered buildings. He was speaking with a client who was comparing one of our post frame buildings versus a fabric covered structure.

My only up close and personal experience with a fabric structure was with the United States pavilion at the 1974 World’s Fair in my home town of Spokane, Washington. The original covering of the pavilion was a thick vinyl sheeting. It was allowed to remain until it began to deteriorate, become unsightly and was thought a safety hazard.

As I started to do more research, I found article after article about the May 2, 2009 failure of a fabric covered building with steel frame practice facility owned by the National Football League’s Dallas Cowboys. This structure collapsed under wind loads significantly less than those required under applicable design standards, according to a report released October 6, 2009 by the Commerce Department’s National Institute of Standards and Technology (NIST).

Located in Irving, Texas, the facility collapsed, during a severe thunderstorm. Twelve people were injured, one seriously. Based on the national standards for determining loads and for designing structural steel buildings, NIST researchers studying the Cowboys facility found the May 2 wind load demands on the building’s framework—a series of identical, rib-like steel frames supporting a tensioned fabric covering—were greater than the capacity of the frame to resist those loads.

Assumptions and approaches used in the design of the Cowboys facility led to the differences between the values originally calculated for the wind load demand and structural frame capacity compared to those derived by the NIST researchers. For instance, the NIST researchers included internal wind pressure due to the presence of vents and multiple doors in their wind load calculations because they classified the fabric covered building as “partially enclosed” rather than “fully enclosed” as stated in the design documents.

Even more damning, the NIST researchers determined the building’s fabric could not be relied upon to provide lateral bracing (additional perpendicular support) to the frames in contrast to what was stated in the design documents and the expected wind resistance of the structure did not account for bending effects in some members of the frame.

“Our investigation found that the facility collapsed under a wind load that a building of this type would be expected to withstand,” said study leader John Gross. “As a result of our findings, NIST is recommending that fabric-covered steel frame structures be evaluated to ensure the adequate performance of the structural framing system under design wind loads.”

The NIST report recommends such evaluations determine whether or not: (1) the fabric covering provides lateral bracing for structural frames considering its potential for tearing; (2) the building should be considered partially enclosed or fully enclosed based on the openings which may be present around the building’s perimeter; and (3) the failure of one or a few frame members may propagate, leading to a partial or total collapse of the structure.

Shortly after the Cowboys facility’s collapse, NIST sent a reconnaissance team of three structural engineers to assess the failed structure and wind damage in the surrounding area, and collect relevant data such as plans, specifications and design calculations. Using the data acquired during the reconnaissance, the NIST study team developed a computer model of a typical structural frame used in the practice facility and then studied the frame’s ability to resist forces under two wind conditions: the wind loads based on the design standard wind speed of 90 miles per hour (mph) and the actual wind loads based on conditions at the time of the collapse.

NIST worked with the National Oceanic and Atmospheric Administration’s (NOAA) National Severe Storms Laboratory to estimate the wind conditions at the time of collapse. The researchers determined, at the time of collapse, the wind was blowing predominantly from west to east, perpendicular to the long side of the building. Maximum wind speed gusts at the time of collapse were estimated to be in the range of only 55 to 65 mph!

In the conversion of actual wind speeds to pounds of force applied to a building the wind speed is squared. A 65 mph wind speed creates a force of 10.816 pounds per square foot (psf), whereas the required load carrying capacity of 90 mph would be 20.736. The structure failed to carry much more than half of the wind load force it should have carried!

This evidence could lead one to be highly skeptical about the ability of a fabric covered structure to adequately support wind loads.  If one is considering such a fabric covered building, my advice would be to carefully gather evidence (backup data) to clearly substantiate the building supporting wind loads…in all circumstances.

Verifying Design Wind Speed

A client from Florida and I have been discussing wind speeds. The data we show in our system for his county was for the design wind speed to be 120 mph.

Now where do we get our data? In many cases, direct from Building Departments. In other cases, we use the wind speed maps published in Chapter 16 of the International Building Codes, or the maps from ASCE (American Society of Civil Engineers) 7. The Metal Building Manufacturers Association (MBMA) also lists design wind speeds, by county.

This particular client had also gotten a quote on an all steel building, and they used a design wind speed of 138 mph. At these speeds 18 mph can make a significant difference in structural design.

Now I know 18 mph does not sound like much, but in the formula to convert from miles per hour, to pounds per square foot of load, the wind speed is squared! While 138 is only 15% faster, the effective load placed on the building is over 32% more. Huge difference.

I asked the client if he had discussed the design wind load with his Building Department. He had, and his Building Department did have a solution which I was unfamiliar with. I like learning new things. I learn new “stuff” every day.

His Building Official had him go to: https://www.atcouncil.org/windspeed/ which finds the design wind speed for any given latitude and longitude in the country. What if you do not know the latitude and longitude? On the same website is information on how to look it up! Technology is so great when it works.  Many thanks to this Building Official for this new “tool” I can add to my internet reference toolbox.

When Building Departments establish design criteria, those are the “minimum” design loads. When it comes to wind, I would recommend everyone use the link above to check their own actual design wind speeds. In the event the speed shown happens to be greater than your Building Department’s requirements, we would strongly recommend using the higher speed.

Many times it costs very little to increase the wind resisting ability of your new pole building. As more buildings fail due to wind, than any other cause, this is not a place to be penny wise and pound foolish.