Tag Archives: design wind speed

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.