Tag Archives: wood roof truss

An Avoidable Building Failure

I had already begun working on this article when I saw on Facebook a great post frame prefabricated wood roof truss setting video (https://www.facebook.com/ruralrenovators/videos/2443278165738995/) posted by Kyle Stumpenhorst of Rural Renovators, LLC (https://rrbuildings.com/).

This is not a paid endorsement for Kyle – however I do believe Kyle really cares about doing a job right. If I personally lived in his immediate service area of Franklin Grove, IL and needed a post frame building erected, I would call Kyle – and wouldn’t ask for bids from anyone else. I am willing to pay for someone who truly takes pride in what they do.

Photo above and excerpts in italics are from a July 29 updated posting at www.fox9.com (for full article: http://www.fox9.com/news/widespread-damage-east-of-twin-cities-after-tornado-reports).

Areas east of the Twin Cities were among the hardest hits spots after storms ripped through Minnesota and Wisconsin on Sunday.

There were at least four reports of tornadoes created by the storms across Minnesota — including one near the area of Scandia — but none have been officially confirmed by the National Weather Service as of Sunday night.

Daniel Kaiser said, “In probably 15, 20, 25 seconds, it was kind of in and out of here so that wind, it didn’t really last too long. I was just kind of amazed to see all of the trees down from the wind we had here.”

Several decades of old trees lay across Daniel Kaiser’s lawn in Scandia. He’s also dealing with some unusual debris.

“That’s one of the solar panels from across the street,” he explains. “It’s amazing how much force that must have been coming through here carrying these things because they aren’t light.”
One solar panel ended up stuck several feet off the ground in a tree. Onlookers were surprised by the damage.

“I’ve never seen that,” said Rob Thompson. “Almost 52 years old and I’ve never.”
Down the road, the damage was even worse.

“The siren went off and Terry said, ‘Go downstairs’ and so we all went downstairs,” recalls Mark Johnson.
The Johnson’s roof was ripped off their pole barn.
“It just got underneath the roof and ripped the whole roof off and sucked all of the insulation out.”

I can tell you right now what happened, and then will show why. This entire roof – steel roofing and wood roof purlins was lifted off from roof truss system because of a poor connection. Long time readers will recall me mentioning how most engineering failures are due to poorly designed or improperly installed connections.

Many Midwest pole building suppliers and contractors provide buildings with sidewall columns anywhere from seven to 10 foot on center. A single pole barn roof truss is placed at each column. 2×4 roof purlins are installed (on edge) across purlin tops. One popular supplier uses a nine foot spacing with 20 foot long purlins to span two ‘bays’ (a bay being space between truss columns).

Design wind speed is 115 mph from 2015 IBC (International Building Code) Figure 1609.3(3). This is based upon IBC Risk Category II for buildings like your home. For this purpose, we will assume an Exposure B for wind site (building in the photo is Exposure C, roughly 20% greater loads). Wind exposure is explained here: https://www.hansenpolebuildings.com/2012/03/wind-exposure-confusion/.

Using appropriate calculations wind load (uplift) for components and cladding in this area of roof is 33.547 psf (pounds per square foot). Weight of roof purlins and steel roofing can be used to resist this uplift (roughly 1.105 psf). This makes our net uplift 32.442 psf.

For the sake of this discussion we will assume purlins are spanning eight feet between truss centers and spaced every two feet. This means each purlin end has 16 square feet of surface to possibly uplift x 32.442 psf or a total of 519.072 pounds.

We are going to attach purlin to top of truss using a 60d pole barn nail (roughly 2/10 inch in diameter). From the 2015 National Design Specifications for Wood Construction (NDS) Table 12.2D with a Specific Gravity of 0.55 (assuming roof truss top chords are Southern Pine – other species may be less) and a nail diameter of 0.200 inches, these nails are good for 109 pounds of resistance per inch of depth of penetration (lbs/in) into truss top chord.

109 lbs/in multiplied by 2-1/2 inches = 272.5 pounds. Because this connection is not controlled by metal strength a load adjustment factor of 1.6 may be applied giving total resistance to uplift of 436 pounds or 19% overstressed.

Even worse would be if a purlin is used to span across two adjacent bays of roof. Using the previous example, our uplift loads at each end would be reduced to 389.304 pounds per end (and working), however at our truss at center uplift would be nearly 1300 pounds!

There does exist some solutions, most economical of width is probably to use engineered joist hangers and place purlins between trusses.

“My Guy Says… Materials… Design…”

“My Guy Says….. Materials… Design…”

Title inspired by our Wizardress of all things materials – Justine!

I’ve now been in the post frame industry for nearly 38 years, the majority of them spent providing complete building kit packages, most often to do-it-yourselfers. Some new building owners happen to hire contractors to erect some or all of their building packages for them. It is from this last group where I get the infamous report of…..

“My Guy Says….”

This is actually construction short speak for, “We have really screwed something up here, not quite sure what it is, but it must be someone’s fault other than ours even though we have not truly looked at the engineered building plans, or bothered to open your Construction Manual”.

Here is an actual example.
The building in question happens to be 36 feet in width, and 28 feet deep. It has a single prefabricated metal connector plated wood roof truss on each endwall, and a double truss nine feet from each endwall. Roof purlins for the building are on edge joist hung to the top chords of the double trusses and running over the top of the end trusses in order to support a 12 inch endwall overhang.
Sound simple?
Please read on….

Client: “We have an issue with the purlin/joist hangers.  We appear to be 24 short of the LU-26, but we have 28 of the ESR2523 (H1) hangers.  My guys say the H1 hangers don’t work for mounting to the trusses. I’ve included a picture of the two hangers; the one with the square mounting plate is the H1.”

Hansen Pole Buildings Technical Support: “The purlins go OVER the end trusses and use the H-1 hanger there. Please refer to Detail 9/S-4 of plans.”

Client: “Yes, thanks. We did find that detail.  I went ahead and purchased the missing hangers at Menards.”

Tech Support: “Doing a manual count, we come up with 60 LU26 and 28 H-1 needed, which actually provides 2 extra LU26. If you were not short shipped, the concern is your guys may have done something which is going to cause future challenges. Other than at the double trusses, there should be no other LU26 used on the building.”

Client: “We found them… They were installed on the end trusses. OOPS”

Overhead Crane

OK, I was web surfing again! My wife thinks I spend all of my time on the ‘net Facebooking….well, I actually do some real research.

I found this post recently on www.garagejournal.com, in reference to a comparison between all steel and post frame (pole) buildings. The poster cited this as why he regrets not having constructed an all steel building for himself:

The down side for me was not being able to spec the building with an overhead crane supported by the trusses. If I went with steel I’d have a five ton overhead in my shop right now. I really regret that”.

Hoist Mounted To Wood TrussesShaking the dust off from my prefabricated wood roof truss designer’s hat (I have owned my own truss plants from another life), I can assure the writer he was sadly misinformed. There were numerous times I designed a roof truss system to support suspended crane loads.

Take this example, a 50 foot span pole building, with doubled (two ply) trusses spaced every 12 feet. Adding a five ton load to these trusses would be the equivalent of under 17 psf (pounds per square foot) of load being added. Highly doable. The truss designer would need to know where on the trusses the load would be supported, and then they can be designed accordingly. With very wide spans, or high loads, increasing to a three ply truss, from a double, may be required.

More often than not, overhead crane systems in pole buildings are supported from the wall columns. The columns are very strong in compression (downward load) and the connection of a crane system to the columns can be done relatively easily. The cautions would be to make sure the system (both building and crane attachments) are designed by a RDP (Registered Design Professional – an architect or engineer) and the column footing diameters are increased appropriately to account for the added weight of the system.

With truly informed experts on the design side of the equation, almost anything can be done with a pole building, and usually at a more reasonable cost than other alternatives.