Tag Archives: lateral truss bracing

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!

Buying a Used Pole Building

40’ x 60’ Used Pole Building – $14000 (Silverton)  SERIOUSLY?

The following ad appeared in the Salem, Oregon Craigslist December 6, 2016 in for sale > farm & garden – by owner:

“I have a nice fully enclosed 14 foot tall pole building. It is fully disassembled and ready for transport. All the metal is fully galvanized. The building has a clear span with 4 double trusses and framed ends.”

Now, the ten top reasons why buying this used pole building would be so wrong:

#10 It is all galvanized steel – generally most folks do not find this to be aesthetically pleasing!

#9 You are going to have to pick it up and transport it – plan on a semi pulling at least a 40 foot long trailer, because those trusses are 40 feet long! Might be handy to have either a boom truck or a forklift there to hoist everything onto the trailer;

#8 And unload it when it gets to your site – some offloading equipment could be handy here;

#7 It isn’t designed to current Building Codes – so you cannot get a permit to erect it – Oregon DOES have an agricultural exemption which you might qualify for. Don’t even consider putting it up without a permit unless you are 100% certain it is exempt;

#6 The wall girts flat to the wind on the outside of the columns – they will overly deflect (again not meeting the Building Code);

#5 Plywood gussets on the trusses – even if your seller has the engineered drawings for them, they are not going to meet the current Building Code;

#4 There is no lateral truss bracing – as the trusses are on each side of the columns are acting as single trusses. At a bare minimum, they will need a row of 2×4 “T” bracing no more than 10 feet on center;

#3 The wood framed sliding door is going to be heavy – and it probably has square barn door tracks. You may want to replace it with a steel framed sliding door and a round track so it is light enough and easy enough to roll open and closed;

#2 Sure hope you can get all of the steel back in the exact same places – because if you are unable to, there is a good chance you will experience roof leaks;

And the #1 reason – For about $3,000 more, you could get a brand new post frame building designed to meet the building code, with all new materials, delivered to your site, with all colored steel roofing and siding PLUS engineered plans!

Prefabricated Roof Trusses Part Two

Prefabricated Roof Trusses – They can Make You or Break You

In yesterday’s blog, Mike the Pole Barn Guru started to share some secrets which should both increase your bottom line as well as allow you to sleep soundly at night.

A short recap here, for the full account, read Part One posted yesterday, May 25th.

A case in point, not too many years ago, we provided the post frame building kit package for the Nature Center at the Cheyenne Mountain Zoo in Colorado Springs, Colorado. The Building Department gave the ground snow load as 27 psf (pounds per square foot), yet wanted 40 psf as the roof snow load. When our engineer called the Building Department to discuss this, the explanation given was, “The snow is just different here!”

Moving on to “the rest of the story” on prefabricated roof trusses.

Truss design programs calculate the roof snow load using Pg as the basis and multiplying it by several factors. This is the formula for the relationship between the ground snow load and the roof snow load adjusted for slope or Ps: Ps = 0.7 X Cs X Ce X Ct X I X Pg.

All of these factors should be clearly outlined on any set of plans being submitted for a structural plan check to a building department. If not, or there is some doubt, the engineer of record for the building should be consulted to make a written determination.

Cs is the sloped roof factor. Metal roofs are assumed to be slippery surfaces unless the presence of snow guards or other obstructions prevent snow from sliding. It is calculated based upon whether the roof is warm or cold, the nature of the roofing material, and the slope of the roof. With heated, steeply sloped and/or slippery roofs, these reductions can be significant.

The Exposure Factor (Ce), is most often 1.0, but can vary from 0.7 to 1.2 and can make a huge difference in both truss design and price.

The Thermal Factor (Ct) for heated buildings will be 1.0 or 1.1. However, most post frame buildings are not heated year round, and if so the factor should be 1.2.

Most truss manufacturers assume a Risk Category II (shown as “Is” or “Importance” factors on truss design drawings). Many post frame buildings will not result in the loss of human life in the event of a failure, given they are used for agriculture or storage and are Risk Category I. This alone can reduce the roof snow load by 20 percent as compared to residential, office or manufacturing type structures!

Will the building be in an area with little or no snowfall? Don’t forget area reductions. If the truss span times the truss spacing is over 200 square feet, a reduction is in order. This calculated value can reduce the roof live load (Lr) to as little as 12 psf.

Further, if Lr is greater than Ps (sloped roof snow load), the Duration of Load for roof loads is now 1.25, instead of the 1.15 typical default value.

What can you do? Make sure the values of Pg, Cs, Ce, Ct and I as shown on the plans, are provided to the truss manufacturer and are reflected on the drawings you will be asked to sign off on.

One other important thing to look for – the “fine print” on the truss drawings should state the trusses are designed for an unbalanced snow load (think drifting). Some truss manufacturers will ignore this crucial component of design, in order to reduce their truss price.

Now, let’s save you some money.

Truss bracing is important, and when neglected can result in catastrophic failures. When overdone, it can kill a building budget in materials and labor.

The truss company will produce preliminary drawings and ask you to “sign off” on them, prior to production. Look at the interior members (webs) which require lateral bracing. These are most typically longer webs which are in compression. Often bracing to web members can be reduced or eliminated by asking the truss designer to switch the direction these long webs run, which will place them in tension. Increasing the size and/or grade of the web can also help. These changes are generally less expensive than the cost of the added bracing.

Lateral bracing needs can also be reduced by using trusses which are physically doubled – installed face-to-face without blocking in between. If your standard building design is for single trusses every four feet placed on a truss carrier spanning eight feet between columns, consider going to a double truss every eight feet, which eliminates the need for truss carriers. Why would this reduce bracing? Because the double truss is now a three inch wide member, instead of 1-1/2 inches, for the most part cutting bracing needs in one-half.

Using trusses spaced over ten feet apart? The truss drawings probably show lateral bracing as a single 2x spanning from truss to truss. Any truss braces spanning over ten feet should be done as “T” or “L” braces, otherwise they can fail in weak axis bending (the skinny direction) under a load.

Learning to read and understand the information on truss drawings is crucial to the success of your business. Don’t make the assumption the truss company is going to be right. If there is something on a truss drawing which is unclear, ask the truss provider to explain it – in layperson’s terms. And, if you still have any doubts, ask your RDP.