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Author Topic: Round pole timberís and basic joinery design  (Read 819 times)

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Offline Rgdsolution

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Round pole timberís and basic joinery design
« on: October 21, 2021, 04:18:19 PM »
Hi Iíve got a few basic questions about a timber frame build (round poles) mainly to build confidence and hopefully gain some new insights. It is a 16x20 with a loft on half of the frame. I am using wood from my property most of which Iím milling with an Alaskan chainsaw mill. Iíve already finished the basement and first floor decking with round log joists and chainsaw milled flooring - I left a gap filled the gaps with 1/4Ē - 1/2Ē (3/8Ē looks best) sisal rope it worked great and looks cool but beware every roll of sisal is different size but itís cheap enough to just buy lots of rolls and bigger is better to jam in gap with white rubber hammer if itíll go. 

So my basic confidence issue is in the joinery on round poles. I want to use mortise and tenon but my poles arenít very oversized (6.5-7.5Ē for 6 posts) but big ole 8x10 plates and tie beams. The posts are a little bigger on the opposite end (around 8Ē) and Iím thinking of flipping them upside down and lagging the joists to the floor with 9Ē 1/2Ē lag bolts at an angle. Iíll have sips all around the outside plate to sill. The posts are made of yellow birch and good grade. 

To put it all frankly Iím intimidated by the outward force on the tie beam pins and since 2Ē tenons seem to make the most sense Iím concerned about the amount of meat removed from my post. I made a drawing and the tenon only goes in halfway and leaves some room on the side but I just wonder about it. Iím using Will Beemers book as my original first plan and he doesnít use a ridge beam and so I was planning to just butt the rafters at a 45 degree angle and bird mouth them at the plate although his book shows yet another cut in the plate as angling the rafter further into the plate I wonder also how necessary that will be. Iím going to add collars to every other rafter except for on the loft and I figured that would be the extra reenforcement but Will only does that on the end walls Iíd love to get some other opinions on outward thrust. When I calculate it the thrust is in the thousands and Iím just saying to myself who wants to put that must faith in a few tie beam pins and sip screws on the corners. I might just add king posts on the two ends with a ridge beam I just wonder is all of that really necessary.


If anyone has input on outward thrust without a ridge beam involved Iíd love to hear it?

Also what are your thoughts on two or three braces coming together almost at one spot removing all the meat from a post?? Iím actually planning to make my brace tenons only 1 1/2Ē deep and putting 1/2Ē threaded rod recessed a little in and trimmed over to alleviate that concern I just wonder how itís normally considered, even with 7Ē or 8Ē Square posts it can be a lot of wood removed? 

Any thoughts on round post building ideas would be greatly appreciated - I have great ideas to square everything but my worries are in what I donít yet know?? I would like to  also reenforce with steel plates or lags but round logs to steel plates seems like itís gonna look crappy or maybe Iím just over worrying. 

Thanks!
Rgdsolution

Offline Don P

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Re: Round pole timberís and basic joinery design
« Reply #1 on: October 21, 2021, 04:50:10 PM »
That's gonna take some sketches to sort out. I'm not going to assume I understand what a loft on half the frame means.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline snobdds

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Re: Round pole timberís and basic joinery design
« Reply #2 on: October 21, 2021, 05:47:19 PM »
That's gonna take some sketches to sort out. I'm not going to assume I understand what a loft on half the frame means.
I think it means a loft will be half way down the length of the cabin.
There is a discussion right now, in this section, about the need for collar and rafter ties on cabins.  

Offline Rgdsolution

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Re: Round pole timberís and basic joinery design
« Reply #3 on: October 22, 2021, 02:01:45 AM »
I tried uploading a few pictures but they wouldnít upload but yes itís basically a loft on half of the length of the cabin and yes if I join the floorboards right I can seemingly reenforce the outward force strength , maybe if Iím worried Iíll just install a small tie beam across the unlofted side just for extra security although the collars should do it. I know it should be fine I just wonder on like a pavilion or something that without a ridge beam or collar ties if the outward pressure becomes an issue or if itís just something to keep in mind. I tried using the outward force calculator but Iím not certain I was seeing it right if I remember it said I had like 3000ib of outward force and Iím thinking thatís s lot of trust in tie beam pins. I know most people add lags and screws and such but in the Old days I wonder if outward force on plates caused common problems.
Rgdsolution

Offline Don P

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Re: Round pole timberís and basic joinery design
« Reply #4 on: October 22, 2021, 10:08:10 AM »
It is not something I would assume will take care of itself, well actually it does take care of itself. The theory of gravity has my vote  :D.

The ends of this plate, were pinned with 1-1/4" pegs, they sheared and it was sliding.

<br
>
 

The half dovetail with packing piece on the tie here was withdrawing, The pin into the log below and alls that shimming and whatall was sheared and sliding. Not the best design going on right there anyway, but no shortage of that. 



 
The future is a foreign country, they will do things differently there - Simon Winchester

Offline mike_belben

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Re: Round pole timberís and basic joinery design
« Reply #5 on: October 25, 2021, 07:11:54 PM »
You need some sort of member that is good at tensile load to prevent the walls from ballooning out as the rafters try to do the splits.  If you want an open interior feel then use a few cables with turnbuckles and hang decorative curtains or something to make them feel cutesy.  


Dont just cross your fingers.  You need a tension cord at the bottom of the rafters.
Isaiah 63:10

Offline bannerd

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Re: Round pole timberís and basic joinery design
« Reply #6 on: October 26, 2021, 07:23:06 AM »
That's what I ended up doing for my house, driving lag screws in all the tie beams because it's hard to know how much pressure will be in a certain area.  In Wills book he talks about dead loads, live loads and it bothered me to think about that.  If you invited 20 people over for a party and they all gather in one spot, think about the passive weight load there.  It really is a gamble, hard to know if it will fail, hard to know what the tipping point would be.  Some of the oak pegs I drove into my house in a off set bore pulled in tie beams.  I watch the tie beam get drawn in but the next day I notice my pencil marks were off.  I drove another dowel in at the same spot... and when the dowel one came out the grain was split, 7/8 inch bore with 1/2" offset so nothing fancy.  I drove two lags in behind it, a floor will be there so you can't see them but it's a little extra insurance.  

I think that is why it's very important to have a backup.  A lot of timber frame barns I look at that were build 1900's era use planking with wood nails midway in the structure to create a loft but to also alleviate the outward pressure.  The floor would transition the pressure across the entire section at that point.  I've also seen double knees like this;



 

In any case with a beam coming out like the above I think some serious ratchet strapping and metal plating is in hand.  Wood tends to rot after 100's of years so it's expected.  The secondary tie beam looks good which is important to have that redundancy.

Offline beenthere

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Re: Round pole timberís and basic joinery design
« Reply #7 on: October 26, 2021, 10:08:04 AM »
Quote
Wood tends to rot after 100's of years so it's expected.

Not expected if kept dry. 
It is not the age, but over those years the chances of not being protected from getting wet can result in rot. 
south central Wisconsin
 It may be that my sole purpose in life is simply to serve as a warning to others

Offline Don P

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Re: Round pole timberís and basic joinery design
« Reply #8 on: October 26, 2021, 08:15:37 PM »
I don't know if any of y'all got to see the King Tut tomb artifacts as they toured around. The part I remember was the masons tools, a wooden cubit stick, a wooden triangle with a cord, plumb bob and alignment notch which makes a perfect level, a wooden speed square. Going back to the geneology discussion, proof that Swanson of speed square fame was a direct descendant of King Tut. After 4000 years the wood was fine, they had DH down back then. Generally back here at home though, century old wood is usually at the end of service. We are wetter and with the decrease in ready Isrealites somebody is gonna drop the ball on upkeep. 
The future is a foreign country, they will do things differently there - Simon Winchester

Offline mike_belben

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Re: Round pole timberís and basic joinery design
« Reply #9 on: October 27, 2021, 02:35:53 PM »
well thats not kosher!   ;)
Isaiah 63:10

Offline Rgdsolution

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Re: Round pole timberís and basic joinery design
« Reply #10 on: November 17, 2021, 03:57:35 PM »





Thanks for the feedback though I hope im not too late in trying to dig a little deeper. To clarify this is for the 16x20 with a half loft very similar to Will Beamers book where the cover shows the 12x16 shed. I used that as my basis due to this being my first build I wanted something I can tackle with a lot of confidence though by raising my plate height and reading up on rafter outward thrust my confidence stopped quick. My plate size is 8x10 and my post size for now is remaining 7" though with a tie beam height of 7'6" to the bottom and me wanting my plate height to go up several feet I have realized a conundrum. Ive tried using the outward thrust calculator and without further explanation Im not certain how to see the end results, with the info it shows 7680ibs. I can assume that means total plate load and so I split it in two to get 3840ibs per plate and then since I have three bents I used 1920ibs for the center bent and the two end bents should be 960ibs?

 From the frames that Ive commonly seen, it seems normal that a 1.5Ē or 2: tenon is meant to resist that force after angled right, though I wonder how many builders are interested in adding strength. Timber lock screws etc add strength but are they enough? If the tie beam is at the height of the plate it seems there are more options but in most cases the plate is higher. So in my case I was hoping to raise the plate considerably but after running the numbers it seems im quite limited. If I use the calculator for beam overhanging one support its saying with a 7x7 I can raise it 14Ē without failing and with an 8x8 I can raise it 24Ē without failing. The general rule is never raise a plate further than 24Ē over a tie beam and even that rule has failed. Im assuming because you would have to account for the mortise from the tie beam and in my case I am concerned all 2000Ibs being held by just a wedged tenon and a few lags and so Im considering an angle bracket above the tie beam with threaded rod through the beam and post. I know with flooring and other supports the tenon is reinforced in most cases but for arguments sake Id like to get others opinion on an angle bracket as a solution. Im using an 8x10 for a plate most likely without a scarf joint and so im not too worried about roof sag Ė what do you guys think?

To speak frankly as well here, Ive only recently become concerned about rafter outward thrust after realizing how much force it can have and I was wondering how come that subject isnít beaten to death more? Im using a 12/12 pitch and im terribly concerned and there is often occurences of far less pitch and I just donít see too much discussions around rafter thrust. I also see ridge beams and truss supports on the same type of structures as rafters with a butt and pass design with no support and thatís like worlds apart? Id like to keep it simple and butt my rafters together like my original plan and reinforce my tie beam tenon Ė do you think if I used 7x7 posts and a 12Ē plate height over beam(calculators allow 14Ē) or better yet 8x8 posts with A 18Ē plate height (allowance 24Ē) I would be in the clear?

Im curious what you guys do if worried about tie beam tenons and what are your thoughts of raising plate height for roof space??
Rgdsolution

Offline Don P

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Re: Round pole timberís and basic joinery design
« Reply #11 on: November 17, 2021, 06:58:59 PM »
I just did a quick read thru and think I spotted an early calculations thinking problem, so lets start there before I read it all again.

The tension at either end of the tie. It looked like you were dividing the tie tension in half at each end.

A thought experiment. Lets say I weigh 200 lbs. I throw a rope over a large oak limb and tie it there. I stand on a stepladder and tie the other end to my toolbelt and step off. There is 200 lbs of tension in the rope. Will a 100 lb connection between me and the rope or the rope and the limb work?

It looked like a lot of further calculation came after that assumption.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline Rgdsolution

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Re: Round pole timberís and basic joinery design
« Reply #12 on: November 19, 2021, 07:06:15 AM »
No it will not unless An awful heavy rope on a very high limb. 

To explain when I split the calculation by the thrust calculator I was under the assumption that it was for total thrust acting on both plates, I then split that in half to show each half on a plate then since their is three beams attaching at the plate but the center one is holding double what the ends are I showed the center tie beam holding half of the plate thrust and the end ones are only covering the one side each. Therefore the center tie beam on each side is holding half of half of the total thrust. Iím most curious if that is how you read the outward thrust calculator and if it all adds by looking at those numbers. Iím not terribly concerned about the post bending because Iíll stay below limits though it does seem like quite an important factor. I do want to reengorce the tie beam connections especially in the center- I know I may be over worrying about it but Iíd feel more comfortable reenforcing whether with brackets, flooring whatever. I can always add collars a ridge beam and king post but I was hoping to keep it simple with rafters butt and pass. I also know a bracket with four holes and lags would be awful strong but I just wanted to get some opinions since it seems like an issue that can be overlooked but still a grave weight especially on lets say a pavilion with full snow load is be scared of all that weight on a few tie beam connections.
Rgdsolution

Offline Don P

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Re: Round pole timberís and basic joinery design
« Reply #13 on: November 19, 2021, 07:45:50 AM »
Let's go back to reply #1, sketch it out with dimensions and I'll try to go through what I know of the thrust calcs with you. Let's make sure we are quantifying the load right, then look for resistance.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline Rgdsolution

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Re: Round pole timberís and basic joinery design
« Reply #14 on: November 20, 2021, 05:39:28 AM »



This is a quick sketch with what I had I tried uploading a copy of the thrust calculator but I was having trouble so Iíll just copy it down quick. - the span is 192Ē the total load is 60ibs psf and Iím pretty certain thatís a good number I beat that horse until it was Wilbur. 12/12 pitch and 7x6 rafters @ 36oc. - the results show 7680ibs so the way Iím seeing it is as a total on both plates then I divy it up to the tie beam connections which makes the center tie beam have 1920ibs of tension on each side. If you happen to know a simple equation for rafter thrust Iíd love to try to play with it. I looked up a few online but everyone seems to have equations for Einstein or for the person who wrote it. Iím good at math when I can solve it and compare not when Iím looking at it crosseyed. Thanks for the help!
Rgdsolution

Offline Don P

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Re: Round pole timberís and basic joinery design
« Reply #15 on: November 20, 2021, 09:09:35 AM »
I looked at the center tie, you are right on, it carries about twice the load as the end bents. This is the tributary area loading that tie 10' x 16', 160 square feet.


 
Im assuming that 60 pound per square foot design load is the combined live snow load of 45 psf + 15 psf dead load... in other words I'm asking for confirmation that you included the dead load  ;).
160 sf x 60 psf =9600 lbs vertical load

The calc here has the equation in the description paragraph just under the picture;
Rafter Thrust (forestryforum.com)

Lets walk through the equation.
Quote
L= Rafter Span (building width)
w= Load on Rafter (roof design load)
s= Rafter Pitch
T= Horizontal Thrust (wL/4 x run/rise x on center spacing/12)
Watch your units... I just drove myself nuts  :D
w= 60 psf
L=16'... !
s=12/12 ...1

T= ((60/16)/4) x (12/12) x 120/12)
  = (240) x (1) x (10)
  = 2400 lbs of horizontal reaction, thrust

The tie connection needs to be better than that. A hidden "bed bolt" is not uncommon there, wedged half dovetail, which will give somewhat. Not a great place to rely on pegs alone but in a perfect world a 1" peg shoul be good for ~1000 lbs allowable. If you don't go enthusiastically drawboring and busting fiber.

Then I think I saw before you were investigating the post as a beam overhanging a support with a concentrated end load. that would be correct. Be sure to use the least remaining net section of the post after the joinery is cut in the post/tie area.

You gave a rafter on center spacing of 3'... run a thrust check on one rafter couple longhand, check it with the calc and holler if any of this didn't make sense.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline Jim_Rogers

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Re: Round pole timberís and basic joinery design
« Reply #16 on: November 20, 2021, 12:05:06 PM »
Years ago, at a joint busting experiment at a guild event, they pull apart a tie post connection that I brought to the event.
It took over 6000 lbs of force before the joint failed. It was a 2" through tenon with two 1" pegs. The joint failed by the pegs bending.
I'm not saying that this type of joint will always fail the same way. But if the pegs are better than mine, which were hand made and did not fit snuggly to the holes then it could be even stronger.
Just my thoughts on this subject.

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Offline Rgdsolution

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Re: Round pole timberís and basic joinery design
« Reply #17 on: November 22, 2021, 11:01:02 AM »
Thanks for the feedback. Every time i was trying to figure out the thrust it just seems like itís 1/2 of the roof load or close to it on a 12/12 pitch so thatís convenient. I feel confident in the strength and Iíll just stay below limits on the post height above the tie beam. I kind of wonder if itís the leverage or the flex of the post that matters more(I know deflection fails first every time). If it were up to me Iíd raise the 8x8 post to 20Ē above the tie beam and then put the plate above that so I can get maximum loft head height but unless I find hard evidence otherwise Iíll probably just keep the top of the plate below the 24Ē limit mitigating any possible inadequacy. Then Iíll probably just add timberlocks and maybe an angle bracket with lags if Iím worried but I know those are awful strong joints. Thanks again for all of the info!
Rgdsolution

Offline Rgdsolution

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Re: Round pole timberís and basic joinery design
« Reply #18 on: November 22, 2021, 11:14:58 AM »
I did include dead load by the way and yes Iím figuring 45 snow and 15 dead load to equal 60ibs. The ground snow load per mass.gov is 65. The rule of thumb Iím using is 70% of ground snow load plus dead load. I used calculators and read up on it a lot and it seems like a fairly conservative number. With a 12/12 pitch and metal roof it lowers even more, plus my dead load is closer to 10ibs but ID rather stay hefty. I do see people being even more conservative and raising their roof load to 65 or 70 in my area but I couldnít see a reason to warrant that. Iíd be curious if anyone felt differently. 
Rgdsolution

Offline Don P

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Re: Round pole timberís and basic joinery design
« Reply #19 on: November 23, 2021, 10:02:32 PM »
Rgdsolution and I have had a couple of pm's back and forth on rafter thrust. I was going to scan and send some notes from one of Dr Woeste's short courses but in case there is anyone else following I think I'll try to paraphrase the notes.



 

T=0.5 x w x S / tan(theta)

T (tension)=0.5 x w (total load in pli) x S (horizontal rafter span, inches)/ tan (theta)(the angle between the rafter and horizontal)

The "0.5" in the equation, half the roof load is acting on the ridge, half is acting on the walls. I think that part is what you were asking about.

You can tan theta if you're calculator has the keys... but what is tan theta?
Remember trig, SOHCAHTOA ... TOA, Tangent = opposite /adjacent. Or, rise/run. We've got a 12/12 roof pitch, 1, well it doesn't get any easier than that.

w=60psf x (120"/12)/12=50pli

Rewrite that equation for man with hammer
T=.5xwxS/(rise/run)

T=.5 x 50pli x 96" / 1

T= 2400 lbs

In Frank's equation rafter span is the span from wall to face of ridge board where in Jordan's equation previously span is building width.

That tensile strength is needed in the tie material and connections need to resist that force. His sketch showed a center splice to show that a ceiling joist lapped over a midwall has the same tension connection design. I included it in my sketch.

Frank calls thrust the axial force down the length of the rafter. If you are doing a typical lightweight metal connected truss I'd be most concerned with that force. I guess what I'm saying is be sure everyone is talking about the same "thrust".

The equation for the axial compressive force down the rafter is;
wS{1-cos2 (theta)/2}

Hopefully I didn't just make mud of it  :)
The future is a foreign country, they will do things differently there - Simon Winchester


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