Tag Archives: roof diaphragm

Steps to Minimize Snow Load Failures

The following article will appear in April 2019’s Component Manufacturing Advertiser magazine (www.componentadvertiser.com).

Early every year NFBA (National Frame Building Association) holds its annual Frame Building Expo – where thousands of post-frame builders, design professionals and vendors meet for three days filled with break-out sessions, guest presenters and of course a trade show.

In 2019’s Expo, one breakout session was, “Avoiding Common Building Failures in the Post-Frame Industry” presented by Ryan Michalek, P.E. of Nationwide Insurance.

The “trailer” for this session was, “Would you find it surprising that Nationwide Insurance’s loss experience with post-frame buildings is disproportionately represented by newly constructed facilities? The company’s loss history is full of buildings that are less than 5 years old and that fail when subjected to their first moderate wind or snow loading event or to a modest commodity-loading cycle. This presentation discusses the common oversights in post-frame building design and construction which lead to building loss and offers strategies to eliminate these oversights.”

I quizzed Mr. Michalek myself as to how many of these failures were subjected to a structural plan review by a Building Official. His opinion was few, if any, failed buildings were designed by a registered design professional RDP (registered engineer or architect), as they are nearly exclusively “agricultural” structures, exempted from Building Permit processes in many states.

My personal belief – every building should be designed by a RDP, as well as being subjected to structural review by a Building Official. Knowing Insurance Industry size, I questioned why it was Nationwide® and other insurance companies were not lobbying for stricter rules for these now permit exempt buildings. Mr. Michalek minced no words in stating United States agricultural lobby having far more power than insurance industry lobby.

I am just not grokking thought processes of those who would invest in buildings which will underperform or fail structurally, all for saving a few dollars. Considering many failures come from poultry industry buildings, it seems costs and cleanup of a million dead chickens or turkeys would trump a few dollars saved on construction.

What was surprising to me, was an analysis of actual most prevalent failures – although column size and embedment always seem to be big concerns from informed purchasers, it wasn’t a contributor to three major causes of failures: lateral bracing of trusses; purlin to truss connections and unbalanced and snow drift loads on trusses.

Typically builders, when they do install bracing, will just run it laterally from building end to building end. This results in all trusses bending together, as loads being placed upon bracing are not being transferred to a very stiff surface – like a roof diaphragm. By utilization of properly designed “X” braces, lateral loads can be transferred into roof plane, and keeping trusses where they are happiest – upright.

Many post-frame buildings, especially those designed with widely spaced single trusses rely upon nailed purlin-to-truss connections woefully inadequate to resist uplift forces. This becomes even more crucial in critical areas such as “end zones” and close to eaves or ridge. A solution would be to use appropriate engineered hangers to attach purlins.

A third common area of failure occurs from designs where drifting snow causing unbalanced loads has not been accounted for. Roof truss designers have an ability to turn off a “switch” in their engineering design programs accounting for drift loading. This results in a less expensive, although under designed truss. With no structural design review, no plan’s examiner will catch trusses being inadequately or inappropriately designed.

Not only do trusses need to support unbalanced snow loads, so do roof purlins. It’s not unusual for an engineered building to have purlins either spaced closer together, or of higher grade or larger size in roof drift areas.

Fast forwarding to 2019’s NFBA Expo – where a frequently heard topic of discussion revolved around a plethora of snow related building collapses in Wisconsin, Minnesota, North and South Dakota. A great majority of these roof failures came from buildings exempted from Building Codes.

While it is impractical, unaffordable or unfeasible to retrofit existing buildings to meet Code loading requirements, we can make changes to minimize or eliminate future failures due to snow. Every time a post-frame building roof collapses, it reflects poorly upon our industry. When prefabricated wood roof trusses fail, truss manufacturers get blamed.

Together these two industries can lobby for changes ultimately making for better and safer buildings.

In my humble opinion, these would include:

Require all buildings over 200 square feet to be constructed from plans sealed by a RDP, as well as to be subject to a structural plan review and field inspections.

Eliminate “agricultural exemptions” from permits.

Eliminate Risk Category I as an IBC option. Risk Category I reduces design loads for snow and wind. This doubles annual probability of failures as compared to Risk Category II.

Eliminate use of Cs to downward adjust top chord live loads due to roof slope and “slipperiness” of roof surfaces. Too many buildings have snow retention systems added to roofs, after completion. This would be all well and good if RDPs and truss technicians were made aware when computing their designs.

Create minimum design dead loads of five psf (pounds per square foot) for both top and bottom chords of roof trusses. Some post-frame buildings have sheathing installed between purlins and roof steel, overloading top chords. Personally, I receive numerous inquiries every week from building owners who now want to install ceilings in their existing post-frame buildings and find their building’s trusses are not designed to support this weight.

Have Building Departments re-evaluate values used for snow (and wind) loads in their jurisdictions. Allstate® Insurance has a TV commercial featuring actor Dennis Haysbert. Haysbert sits in an open field and questions why there have been 26 “once in 500 years storms” in last decade, when term alone implies they should only happen every 500 years.  An increase of design snow load of only 15% cuts annual probability of a failure in half!

Hopefully we will learn from this past winter’s collapses and become proactive together to make for better and safer buildings!

Open Endwalls in a Hay Barn

There are plenty of bad structural things which can be done to post frame buildings. While roof only buildings can have their own set of challenges, these can be exponentially made worse by putting walls on the eave sides and leaving the building endwalls entirely open.

Here is a good explanation of why this occurs, in lay person’s terms: https://www.hansenpolebuildings.com/2011/12/lateral-wind-loads/

In my last article, we discussed our engineer friend’s proposed hay barn, and the challenge of fire. Here is the next concern (as posed by Hansen Pole Buildings’ Designer Rick Carr):

“Next, he wants the gable end walls completely open.  The hay will be loaded into and out of the building with equipment moving straight in and straight out of the gable ends.  He understands the wind shear loading issue and I suggested at least six feet walls on each corner explaining there would be steel on the outside  and osb on the inside, he suggested Struc 1 on the inside instead of OSB.

He thought  we could use two additional rows of posts on the gable end walls and running through the interior, running up to the bottom cords of the trusses, on the end walls and through the length of the building,”

Assuming even a minimal wind speed and an Exposure C site (learn about Wind Exposure here: https://www.hansenpolebuildings.com/2012/03/wind-exposure-confusion/), the building endwalls must be able to resist 17,632 pounds of shear each! If this sounds like a big number, it’s because it is. Even using double layers of 7/16” OSB with lots and lots of nails, it would take just over eight feet of shearwall at each corner.

Structural I, also referred to as “STRUC I,” may be OSB or plywood and is, essentially, a subcategory of APA Rated Sheathing. Panels designated Structural I must meet all of the manufacturing and performance standards of Rated Sheathing, as well as certain additional requirements. These additional requirements relate to increased racking performance related to shear wall and diaphragm values and increased cross panel strength and stiffness properties, which are important when panels are applied with the strength axis parallel to support, as is typical in panelized roofs.

The advantages of Structural I Rated Sheathing over standard Rated Sheathing panels of the same Performance Category are realized in certain specialized engineered applications such as engineered shear walls and engineered horizontal diaphragms. Specialized construction situations requiring installation of roof sheathing panels with the strength axis parallel to supporting structure also benefit from the use of Structural I Sheathing. Structural I is of no additional benefit for other traditional construction applications.

The difference in shear strength between standard rated sheathing and Struc 1 is about 10%. This would allow the width of each shearwall to shrink by about 10 inches – still using two plies and the same thickness. Struc 1 is also going to prove to be significantly more expensive.

Adding extra rows of columns is not going to solve the shear problem – those loads are still trying to transfer through the roof diaphragm to the endwalls.

How would I solve this? My recommendation would be to enclose both endwalls to the ground and leave one or both long sidewalls open. Column spacing along the sidewalls could be adjusted to optimize the ability to load.

Not All Pole Barns Are Designed to Code

Not All Pole Barns Are Designed to Code Mandated Wind Loads

Photo by Lewis Geyer, Longmont Times-Call

Debris are all which remains of a pole barn formerly belonging to Tom Bennett of Berthoud, Colorado. According to Berthoud Fire Chief, Rob Stumpf, their office received a call about 9:15 on Monday morning, January 9, 2017 to notify them of a building collapse.

Winds were predicted to approach 90 mph (miles per hour) and they literally ripped the 30 foot wide by 120 foot long building apart. The pole building was used for animal housing, including llamas, however amazingly none of the animals were trapped or injured in the collapse.

Llamas abound in Colorado, where they are often used as pack animals for novice backpackers or serious outfitters. Llamas can be enjoyed by everyone, as they are good pets and companion animals, are easily cared for, and don’t require a lot of space. Llamas are great around kids and are wonderful 4H projects. Llamas are good family fun, and llama shows offer opportunities for all ages to get involved.

Berthoud proper happens to straddle two Colorado counties – Weld and Larimer. Both counties provide handouts of prescriptive requirements for certain dimension post frame (pole) buildings. I have come down pretty hard on Larimer county’s version in a past article: https://www.hansenpolebuildings.com/2015/04/prescriptive-requirements-2/, which a case in point might be the debris field shown in the photo.

Some things about the destroyed building which give me cause for concern would be the aspect ratio of the building. Its footprint has a length to width ratio of 4:1, which leads me to believe the roof steel, even if installed to the specifics of sound diaphragm design, would probably have been unable to withstand 90 mph wind shear forces. I don’t see any signs of OSB (Oriented Strand Board) or plywood in the photo and there should have been portions of the roof and building endwalls with significant quantities to have withstood nature’s forces.

In the end, a properly engineer designed post frame building, constructed in accordance with the plans, would very likely still be standing – without any damage. This again makes my point for all buildings be designed by a Registered Design Professional (engineer or architect) and structural plan reviews and inspections being required.

For this building, people and llamas were lucky, not one was injured. I hate to think it is going to take fatalities for jurisdictions to wake up.