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Author Topic: Timber sizes for king post pole barn  (Read 704 times)

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Offline JMoirGA

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Timber sizes for king post pole barn
« on: December 18, 2017, 12:27:29 PM »
I'm new to the world of timber framing and looking for a bit of advice. I'm new to the world of online forums as well so bear with me. My project is a pole barn that I'll likely close in later. The barn is for lumber storage. I'd like to be able to drive my tractor into the barn with a load of 20' or shorter lumber on the forks. I'm thinking my basic footprint will be 25x40. I'm planning on spanning the 25' with king post trusses. The problem is that my mill is only 21' long. I have an Alaskan that I can use to cut longer but my woodmizer lt40 has gotten me spoiled. Most of the logs I get are usually 20' or less anyhow. My idea was to split the tie beams and extend the king posts down and join them on the king posts. Adding in some steel hardware at this connection seems like a good idea since this joint will be in tension. I'd like everything in the roof to be at 12' or slightly higher so there will be no chance of me hitting anything with my tractor. The back plate on my forks is 11'8" at full extension. I'm thinking 10x10 posts spaced 10' apart down the long sides. I'd like to use girders between the posts instead of trying to make long plates. I want to set the trusses directly on the posts. I'm going to raise the bottom of the posts on some block piers 16x16" and 16 or 24" high. The roof will be a 6/12 pitch. I haven't been able to calculate the size of all the timbers for the trusses. I'm also wondering if the split tie beam is a good idea or should I quit being lazy and cut some 25' long tie beams. I'm sure I'm leaving out a bunch of details. Any help pointing me in the right direction would be greatly appreciated. I'm in southeastern Georgia so there's no snow load but there is the occasional hurricane. All my timbers will be Syp. It'll be a mix of long needle, loblolly, and slash pine. There's hardwoods available too but I haven't put much thought into using them as timbers.

Offline starmac

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Re: Timber sizes for king post pole barn
« Reply #1 on: December 18, 2017, 03:27:42 PM »
I can't answer your question, but am eager to see the answers.

Myself if I was going to go 12 foot tall, I would go on up to at least 14, so that a truck could fit under. I see folks build shops here all the time at 12 foot, then when they eventually go to sell it they find their property would be worth considerable more and easier to sell if they had of built it big enough to get a truck in.
Old LT40HD, old log truck, old MM forklift, and several huskies.

Offline Don P

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Re: Timber sizes for king post pole barn
« Reply #2 on: December 18, 2017, 06:32:08 PM »
This is a 24' span with  discontinuous bottom chord tabled into the kingpost. There is a black gum spline (interlocked grain) passing through the kingposts and buried in the ties. IIRC there are 7 pegs on each side to take care of the tension. I have not run a string across the bottom chord but I believe it is now down a bit. If I were to do it again I would start with a bit of camber in the bottom chord.

Eastern white pine, 8x8 posts @12' tall, 8x10 top and bottom chords and yup I can only get the cab of a truck in da door.

In my gallery under 24' kingpost truss there are more pics. The second bent was tabled into the kingpost the same way but instead of a spline I inset a 2x10x14' on each side of the tie across the kingpost and used 3@1"dia bolts on each side of the kingpost to handle the tension. This was considerably stronger. If I had had an Alaskan mill at the time I would have one pieced the bottom chord, it is faster in the long run and stronger.

Offline JMoirGA

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Re: Timber sizes for king post pole barn
« Reply #3 on: December 19, 2017, 08:59:16 PM »
That's a good suggestion about the height. I'll probably never park a truck under there but you never know. I was wondering how much the fibers of the wood crushing around the pegs would come into play with that kind of connection. I was also pondering lapping a 2x10 or even a 3x10 tabled onto the tie beams. I'll probably just bite the bullet and sharpen up the old chainsaw. Cutting a couple beams with the Alaskan would probably save a lot of headache later in the project. Any thoughts on using a bridle joint at the post to tie beam joint instead of a regular mortise and tenon? I was thinking since the posts will be 10x10s and the tie beam would be an 8x10 it might work out nice. I'm bothered by the idea of the tie beam twisting a bit as it dries and causing the post to split. My idea was to neck the tie beam down to say 4 inches and shouldered of course, then leave a fork on the top of the posts with 3 inches on each side. Then use 2 bolts to clamp the whole thing together.

Offline Don P

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Re: Timber sizes for king post pole barn
« Reply #4 on: December 19, 2017, 10:26:01 PM »
I don't see a problem, anyone else?
I've been sketching something along those lines. I think Viking frames often had a forked post with a tie that went back up to a full sized head outside the post as a restraint. The forks of the post extended above the tie. The plates then ran across the ties and tucked up against the extended forks as a thrust restraint. These roofs had common rafters on those plates. With the same thought in mind though, capturing the back of the post and extending the forks up might be incorporated into a heel joint for the top chord.



Here's where I was going with that roof;

Offline Dave Shepard

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Re: Timber sizes for king post pole barn
« Reply #5 on: December 20, 2017, 01:13:39 PM »
Don, that is very close to the medieval cruck frame we built at Hrartwood School.


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Offline Don P

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Re: Timber sizes for king post pole barn
« Reply #6 on: December 20, 2017, 10:41:19 PM »
A cross between that and a Viking boathouse I had seen a picture of is where I wandered off to with that sketch above. The post/tie is similar to what J is talking about I think.

Offline JMoirGA

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Re: Timber sizes for king post pole barn
« Reply #7 on: December 22, 2017, 12:08:58 PM »
That post and tie is pretty much what I had in mind. That's a good suggestion about extending the post up to incorporate into the top chord. That would surely help it resist any outward thrust. That's an awesome cruck frame.

Offline JMoirGA

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Re: Timber sizes for king post pole barn
« Reply #8 on: January 05, 2018, 06:22:17 PM »
Any recommendations on the purlin size? I'm planning on going with 8x10s for the tie beams and principal rafters. The bents will be 10'10" on center. I'm planning on one for a plate at the bottom of the trusses, one in the middle of the run with a strut coming from the king posts and one at the ridge. Pretty much a standard setup. Then use some 2x4 common rafters stripped with 1xs for metal roofing. I'm also considering putting the purlins on 2' centers and installing the roofing directly to the purlins but I'm not sure which way would be better. It seems like the extra purlins would save a lot of work over using the common rafters. I was thinking something like 5x7 purlins with the common rafter system or trying to get by with something like some good stiff 4x6s if I go with the purlins on 2' centers. I'm not sure which way to go with it.


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