One of the more sickening events for a post-frame building contractor is being on the receiving end of a call advising that some or all of one of your buildings has collapsed.
Déjà vu all over again
I was one of those contractors in the mid-1990s in Spokane, Wash. Under jurisdiction of the Uniform Building Code, we hired a registered design engineer to do all of our designs and plans for us. At the time, the design wind speed in our area was 70 miles per hour.
On a late summer afternoon, the wind decided to blow, and blow hard. Recorded speeds approached 80 mph. Just west of Hayden, Idaho, we had constructed a steel roofed and sided 40-foot wide post-frame building with a 10-foot eave height. The building also had overhangs.
This was typical Northwest style post-frame construction, with columns spaced every 12 feet, two-ply prefabricated metal plated wood roof trusses aligned with the columns, and 2×6 msr (machine stress rated) roof purlins every 24 inches. The purlins were stood on edge, joist hung into the side of the first pair of trusses and extended across the top of the end truss to support the overhang. At the end truss, the purlins were toenailed to the top, and also nailed to blocking placed between the purlins.
The call we did not want to hear was the one telling us the first bay of the roof had been caught by the wind and folded over onto the roof behind it! After investigating, it was determined the attachment of the purlins to the end truss met code and engineering requirements yet had failed due to winds in excess of the design parameters.
The important lesson from any failure is to learn from it. On future projects with end overhangs, we upgraded this connection to use a Simpson hanger and felt more than comfortable with the added safety of this connection. The side benefit – hangers look impressive to new building owners.
I recently attended the 2014 Frame Building Expo in Nashville, Tenn., sponsored by the National Frame Building Association. The NFBA (www.nfba.org) is the only national trade association which represents post-frame industry professionals. The association is the country’s primary source of post-frame building resources, research, networking, news and education.
While the Frame Building Expo highlights for me are typically being able to interact with the hundreds of vendors who are displaying the latest post-frame innovations and products on the trade show floor, this year the National Frame Building Association had a surprise in store for me.
The daily Expo “breakout” sessions included the fields of sales and marketing, management and technical knowledge.
My surprise session was: “Avoiding Common Building Failures in the Post-Frame Industry”. The presenter was Ryan Michalek, a registered professional engineer, who is one of several engineers employed by Nationwide Insurance to help its policy holders avoid catastrophic structural building failures.
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, of the buildings which failed were designed by a Registered Design Professional (registered engineer or architect), as they are nearly exclusively “agricultural” structures, which are exempted from the building permit process in many states.
In my humble opinion every building should be designed by an RDP, as well as being subjected to structural review by a building official. Knowing the size of the insurance industry, I questioned why it was that Nationwide and other insurance companies were not lobbying for stricter rules for these now permit-exempt buildings. Mr. Michalek minced no words in stating that the agricultural lobby in the United States is far more powerful than the insurance industry.
What was surprising to me was the actual most prevalent failures. Although column size and embedment always seem to be big concerns from purchasers, it wasn’t a contributor to the three major causes of failures: lateral bracing of trusses; purlin to truss connections; and unbalanced and snow drift loads on trusses.
In the Midwest, where Nationwide Insurance’s engineer is based, the purlin/truss scenario is generally from one of two schools. What most would consider an “Eastern” U.S. style post-frame building, roof trusses are placed (usually every four feet) across a “truss carrier” at each end. Purlins are most typically 2x4s flat across the top of the trusses, and connected only by nails driven through the purlin into the top of each truss.
Roof sheathing and purlins peeled off during a wind event on this building, the result of using smooth shank nails to connect the purlin to the top chord of the truss. – Nationwide Insurance Co. photo
The other scenario has widely spaced single trusses, usually anywhere from 7 foot 6 inches to 10 foot on center, and aligned with the sidewall truss bearing columns. Here, the roof purlins are 2×4 on edge, over the tops of the trusses, with a long thin nail driven through the purlin into the top of the truss.
A truss bottom chord incorrectly laterally-restrained resulted in this roof framing failure during a high wind event. – Nationwide Insurance Co. photo
For sake of discussion, I’ll use a 50 foot wide building with Occupancy Category I (a building which represents a low hazard to human life in the event of failure). Under the 2012 International Building Code (IBC), for most of the country the lowest design wind speed for this Occupancy would be 105 mph. With a 14 foot eave height and a 4/12 roof slope, with a B wind exposure, for an enclosed structure the uplift force on the truss 4 feet in from the end of the building is about 31 pounds per square foot (psf) or roughly 46 psf within 5 feet of the eave and ridge. This assumes a fully enclosed building with openings closed by wind-rated windows and doors (sliding barn doors are not wind rated).
With purlins 24 inches on center, and trusses every four feet, each connection has to resist 8 square feet of uplift force, or 368 pounds in the most critical areas. For safety sake, we will assume the lowest Specific Gravity (G) of the most commonly used materials for trusses, G = 0.42 for Spruce-Pine-Fir. Using a 10d common nail (3 inch length x .0148 inch diameter) the withdrawal value is 23 pounds per inch of nail into the top of the truss (since the nail penetrates 1-1/2 inches, the value per nail would be 34.5 pounds). Adjusting for duration of load for wind, it would take SEVEN of these nails to hold the purlin to the truss. Of course there is no way to get seven nails into this connection. Even the less critical areas of the roof (more than 10 percent of the building width away from the end, ridge or eaves) would take three 10d commons, and there is only so much area to be nailing through!
What happens when the purlins go over the top of a single truss at say 9 foot on center?
The truss is now going to be out of the “end zone” for wind, however some of the purlins are going to lie completely in the high force areas near the eave and ridge. Now we are talking about 18 square feet of roof with a total uplift force of over 558 pounds! The most common method for resisting uplift in this scenario is to use a 60d threaded hardened nail. These nails have a withdrawal value of 35 pounds per inch of nail into the truss, so adjusted for duration of load, the nail would need to penetrate nearly 10 inches into the top chord! As these nails are only six inches long, and 3-1/2 inches is consumed by the purlin, it would take FOUR of these to make an adequate connection.
This roofing failure, caused by heavy snow, was the result of a truss top chord incorrectly laterally-restrained. – Nationwide Insurance Co. photo
After watching virtually every AETN History Channel show about building failures, more often than not they have been caused by inadequate connections. Even in the case of my “roof flop”, while the connections were adequate by code, they did not perform in real life.
Now, this Expo breakout session did not help my roof two decades ago, but perhaps it gave pause to some of the other attendees to give added thought to how they design or construct post-frame buildings.
Mike Momb is Technical Director for Hansen Pole Buildings, LLC of Browns Valley, Minn. His daily post-frame blog, as well as his weekly “Ask the Pole Barn Guru” column can be followed at www.hansenpolebuildings.com/blog/.