Perma-Columns

[jcook5003]

Hey Guys-

I was wondering those of you who have had pole barns built with or have used Perma-Columns in D.I.Y. pole barn build could share how you added shear bracing for the Perma-Column to post connection point.

From my light engineering (no degree) background, obviously a pole barn gets most of it's shear strength from the poles being buried 4' in the ground. When using a Perma-Column you've effectively moved the "moment loading" (I think that's the right term) point to ground level. The pole now acts more like a stud wall where it needs some kind of shear bracing. On a stud wall this is accomplished through the sheathing. Since it is common to add purlins with metal siding with a couple nails between the posts with metal siding screwed to the purlin, you don't get the same shear that you would with common stud framing. As a side note, the wind loading is 90MPH in my building department code book, pole barns dont get inspections though, but I do want to do it right.

My question is, how do you add enough bracing to make the structure sound again? Obviously not looking for an engineering report but some actual builds of things that work. I had though about using "commercial" or "ladder" style purlins in between my posts with the metal screwed through the flats into them. Then the metal acts much more like typical sheathing. Added side benefit of easy insulation and interior wall finishing.



Thoughts? Opinions?

 

[GLyford]

I think with the perma-columns, the bolted connection at the bottom is to transfer that moment to the ground you are talking about. If you just set posts on sonotubes, then yes, it acts more like a hinge. But I think that fancy bracket is supposed to help transfer moment, too.

 

[QRTRHRS]

Some eleven years ago I hauled pole barn materials for a firm that had a "sister" company Perma-Column franchise. First off, I suggest reviewing the Perma-Column website then calling for more info. But, as I recall, the engineering data suggested that the resistance to windload was as good as or better than a pole in the ground. At that time I think the quoted cost would have been 25% more than traditional.

This particular company had several crews in the field so I was privy to a number of installations. The horizontal wall purlins were the same as standard. No special cross bracing as I recall.

The crews ended up to be the only ones negatively affected by them. They ended up setting them by hand with the posts attached due to the expense of crane setting them even with our own crane. In order to have a smooth workload, the P-C plant crew would air nail 2x stock together and bolt them to the column. Heavy is the word.

The columns themselves were well made under controlled conditions. I personally was impressed by them though they did take some "finessing" to haul, wooden columns attached or not.

 

[s219]

Couple data points I can relay to you.

I was required to add diagonal bracing between all poles in my barn to satisfy the building inspector. Those poles were in the ground 3-4' but he wanted diagonals for more shear resistance.

The other thing I had to do was get an engineer to sign-off on the footings. One thing he determined is that the poles should be backfilled with concrete (dry mix). If backfilled with the spoils from digging the hole, there would not have been sufficient wind resistance -- a wind load could have pushed the walls over because the backfill soil would not be able to resist the moment.

 

[Rustyiron]

I've read in "Frame building", a post/frame (pole building) trade magazine that up-lift was the greatest concern in high wind areas. I think that the perma column is tapered and provides better uplift resistance than a normal post. I too would suggest that contact them with your concerns.
But there is always diagonal bracing, enough of that will cure anything:laughing:

 

[s219]

I had always thought uplift was the biggest concern too, but it turns out it was walls blowing over sideways because of posts moving in the soil (in my case with our local clay soil). I suppose once that is eliminated by backfilling with concrete mix, uplift would be next.

 

[sailfast]

Closed cell spray foam is a great way to add rigidity to a buildings walls... And uplift/peel resistance to a roof.

 

[IvanS]

The majority of shear resistance in a typical pole building comes from the metal roof and siding. You can experience this first hand as you apply the metal and screw it down. Before any metal is added the entire structure is pretty flimsy and can move around just by a man walking on the trusses and roof purlins. As more and more metal is added, it becomes very rigid. Take a look at pole barn plans available online to get an idea of wind ratings. Diagonal bracing is a good idea to insure squareness during construction, but it does not contribute as much to shear resistance as it might seem - until the metal ties everything together.

My "pole" building does not have any portion of the poles in the ground, they are all attached to the concrete slab at floor level. They are made of three 2 x 6's nailed together to support the trusses. Wall girts are face nailed to the outside of the posts.

The moment loading of the posts has little to do with the shear resistance of the entire building once it's all constructed, the wall covering provides the majority of it. As described above the uplift resistance is more fulfilled by the post attachment method.

 

[s219]

Ivan, the problem there is that metal siding has limited shear resistance only where it is under tension, and only until the corrugations give. Compared to something like OSB wall sheathing, the shear resistance is nowhere near as uniform. Anywhere the siding is in compression it will want to buckle. And tension load won't ever hit the metal's tensile strength limits before the corrugations give. So with a metal structure, what shear resistance you have is somewhat "accordion" in nature and you will see permanent deformation of the metal if it has to resist a heavy wind load.

The engineer who did the wind load calculations for my barn showed me a few of the plots that come out of their software, which graphically exaggerates the movement of the structure so you can easily visualize how it wants to fail. On a big long wall, the middle bows in, and shear resistance of the end walls won't help a lot there because they are so far away. If the only cross bracing to the opposite wall is the roof trusses, they don't help any either, as they are weak in that direction. But having poles sunk into the ground really helps in the middle of the wall.

 

[EddieWalker]

The majority of shear resistance in a typical pole building comes from the metal roof and siding. You can experience this first hand as you apply the metal and screw it down. Before any metal is added the entire structure is pretty flimsy and can move around just by a man walking on the trusses and roof purlins. As more and more metal is added, it becomes very rigid. Take a look at pole barn plans available online to get an idea of wind ratings. Diagonal bracing is a good idea to insure squareness during construction, but it does not contribute as much to shear resistance as it might seem - until the metal ties everything together.

My "pole" building does not have any portion of the poles in the ground, they are all attached to the concrete slab at floor level. They are made of three 2 x 6's nailed together to support the trusses. Wall purlins are face nailed to the outside of the posts.

The moment loading of the posts has little to do with the shear resistance of the entire building once it's all constructed, the wall covering provides the majority of it. As described above the uplift resistance is more fulfilled by the post attachment method.

Do you have any diagonal bracing at the corners of your barn? The bracing will give you more strength then the metal, which is limited to the thickness of the metal and the ability of the screws to hold it secure. Most screws are just snug so the rubber will keep out water, they do not secure the metal to the wood beneath it enough to actually be structural.