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Thoughts on this King Post and Beam design?

Started by Mestak, January 19, 2023, 04:10:03 PM

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Mestak

 

 

Hi folks. I need to build a barn/shed to store my tractor and other equipment beneath and I've been searching around for designs that meet the following criteria.

1. Can be feasibly built by one guy w/modest skills.
2. Takes advantage of my saw mill and access to lots of mature pine trees (lobLolly).
3. Also meaning built with green lumber.
4. Has some of the aesthetics of a classical timber framing but with simplicity of post and beam.

Pole barns are probably out because I don't want to sink untreated lumber into the ground. I'd rather not do traditional stick framing because of the amount of individual sticks of lumber/milling and complexity of DIY trusses involved. Site will be professionally graded, but jury is out on whether I want to spring for a concrete slab. Building will be in S. Central VA.

I happened upon this 20' x 30' post and beam design by the guy that goes by "BarnGeek" and I'm wondering what people think of this. As far as I can tell all the main beams and posts are 6x6. He uses structural screws to assemble the king post trusses and also sells (& I assume fabs) the steel u-brackets for the connections between posts and footers. I have an 8" auger to dig the footers and would likely use homemade forms for the post bases as seen in the picture.

I will eventually side this, likely with board & batten, or potentially lap siding if cutting it on the mill proves feasible. Initially I figured I would use half this building to store the tractor and half for a small shop, but now I'm wondering if I can put a lean-to off one side, use that to store the tractor, and make better use of the main interior space.

Thoughts? I plan to show this to a structural engineer friend, but probably won't hire an architect.

Don P

Hmm, lost that brilliant post  :D
The kingpost is just for pretty on that thing, take it or leave it as desired.
The highest force is the connection between ties ends and rafters. Calc here;
Raised Rafter Ties (forestryforum.com)

That also puts an equal bending load on the rafter at that connection point as well as the snow /wind/self weight of the roof on the rafter. And the connection reduces the net section of the rafter depending on how much you hack at it to make a joint.

...  see what you come up with for connection force first and see how it'll hook up.

And make a scale drawing.

Mestak

Thanks much for the reply, Don. At risk of exposing my ignorance, I don't know how to find or determine RDL so I can't complete the calculator.





In BarnGeek's design the joint connections are all done with 8" structural screws, so no true timberframing.

Don P

It looks like the area is 10'x10' or 100 square feet. 
I'd call dead load 10lbs per square foot so 1000 lbs dead load.
For live load go here if you do not know it for your location;
ASCE 7 Hazard Tool

Does that get you there?

Hilltop366


Don P

What Hilltop is worried about is the lateral push from wind and it would blow what is pictured above right over. It does need a bracing plan. An equipment room at each end with a shed between them is a good setup. Where I grew up it would be 2 tobacco barns about 16' square x 2 stories with a machine shed/ work area in between.

But, let's work the gravity problem first.
Rereading the op slower, sounds like you are just east of me.. call it 25 psf(snow) +10psf DL=35psf x 100 sf = 3500 lbs on each side of the rafter couple under high wind or snow. At 6/12 that is giving 5666 lbs of tension in that raised bottom chord. It is putting an equal bending point load on the rafter right there, you'll be upsizing for that. You need to shackle each end of the tie to the rafters with that strength connection, or better.

I suspect under full load the geeks tie will mercifully swing down and render you unconscious before the rest of the roof snaps through and maims you. I dunno, I think you already have an architect on the job, that's thoughtful  :D.

Let's check out dropping the tie to the plate .. 3500 lbs tension, and easier to brace to.  This is probably going to be easier to build .. is there a reason for raising the tie?

Either can be built, one is more challenging. Talk to your friend before buying that kit.

Mestak

Thanks much for the detailed reply, Don. Given my ignorance, I'm not sure I deserve your expert attention, but I sure do appreciate it. There's no specific reason for the raised tie from my perspective (beyond shortening the length of the tie to something I can cut on my 16'6" mill), that's just how BarnGeek designed it. I was fishing around for a design that used large, builder-milled posts and beams and relatively few total # of pieces instead of stick framing, saw this, and I guess just assumed a guy who sells his designs knew what he was doing (my fault for assuming). Glad I haven't paid for the plans yet ;-)

Your restrained but dubious reaction to this design sent me back to square one, and I thank you for it. Putting the tractor under a lean-to off one side would solve some of the reasons I was attracted to this big clear span and would mean a more traditional build with standard rafters and girts.

In the interim I've read a few threads and understand there are ways to do pole barns with rough milled, untreated lumber. What I'm not clear on is whether a concrete pier + steel bracket + untreated post will have the same strength as a post sunk some ways into the ground. Off to do some more reading before I trouble you and anyone else with bad ideas. 

Thanks again.
-Mike








Don P

You're just getting the opinion of a noisy carpenter, its all good  :D.
Basically if you aren't using the time tested caveman approach of earthfast construction which attempts to brace the structure using the depth of embedment, diameter of the pole and quality of the mud to resist lateral forces.(no bias here  :))

Or that bracing must be somehow provided by the structure itself, as it should be, mud is unpredictable. People hope for some moment resistiance from those post base connectors. With the exception of one that I know of they shoulsd be thought of as a hinge pin type connection, The post is fixed the the ground but is free to rotate, tip over. The one that has some document moment capacity lists it in inch pounds. That is not going to stop a several thousand pound roof on 8' long lever arms from breaking the posts if the connector does hold. Basically I look at buried footings and posts or posts on piers as being hopefully designed for gravity but incapable of resisting the lateral loads of the building.

For a house it is easy, we make big rigid wall floor and roof planes and attach them to one another, each bracing the building in its' plane.

You'll need bracing in the longitudinal and lateral axes of the frame. While you can brace up to the area of the raised tie, that is all getting pretty sketchy to me. I think it will be lower stress and easier to frame and brace with a plate level tie if possible... but there are many ways to do just about anything.  

This is a sketch I did recently, it shows sheathed bracing along the length of the building. Think about the number of nails I can get around the perimeter of the bracing triangle. That is far more rigid than a bolted or mortise and tenon connection.



 

And a discontinuous bottom chord on a kingpost truss;


 

 
That notch in the rafter heel and tie is what is resisting the tension.

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