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Author Topic: tenon design  (Read 383 times)

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Offline rkb5916

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tenon design
« on: January 04, 2017, 02:08:42 pm »
I've been searching the forum for a few days to find information on tenon design and so far I've found the following:
1) Tenon thickness = ¼ thickness of beam (i.e. 6x6 beam requires an 1 ½” tenon)
2) Peg diameter = ½ the thickness of the tenon (1 ½” tenon = ¾” peg)
3) Peg “relish” = 4 Diameters of the peg beyond the hole (i.e. ¾” Peg requires 3” beyond the peg hole)
4) Peg hole should be 1 ½” off the shoulder and 2” off the bearing end of the tenon

Question:  How do I determine the proper length of a tenon?  Example: Let's say I have a 6x10 tie beam meeting a 6x8 Post.   

 
Using the above information I'm seeing the tenon as 1 1/2" thick, minimum length is 5 1/4"

If more information is needed just let me know

Thanks,
Raymond

Offline Brad_bb

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Re: tenon design
« Reply #1 on: January 04, 2017, 03:02:33 pm »
Raymond,
Where are you getting your information?
While there are good general rules of thumb, each joint must be analyzed to determine if the proposed design is adequate.  Joints under tension will need much more scrutiny.   Even many compression joints may become tension joints under a significant racking condition.  I don't know what you are building.  If anything more than a small shed, or sawmill shed, I'd highly advise working with an engineer experienced in timber framing, that will do a proper force and stress analysis. The rules of thumb are a good starting point, but no one on here can really determine if a joint is sufficient without knowing all of the loads and conditions.

I will say that in your tie beam to post example, I prefer a shouldered Mortise and tenon, which increases resistance to vertical shear, compression of tenon, and reduce twisting. Mortise can be full depth of post, or just shy of that.  If you haven't read Steve Chappell's book "A timber framer's workshop", I recommend reading it.  Despite the thoroughness of his explanations of the engineering, I still recommend an engineer on anything but the most straight forward small structure.  It's very little cost compared to that of the building and the potential cost of a failure. 
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Offline Jim_Rogers

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Re: tenon design
« Reply #2 on: January 04, 2017, 03:24:31 pm »
1 1/2" off the shoulder of the tenon piece to the center of the peg hole not the edge as drawn.
The longer the tenon the stronger.
I'm assuming your going with 6" wide and 8" deep on the posts. If you go with square rule layout then the housing for the tie beam to the post should be 1/2" under or at 7 1/2" over from the reference edge.
If you do that then the through tenon on the tie beam would be 7 1/2" long and should provide plenty of relish beyond the peg hole. If you need more support for the tie beam, depending on the load it will be carrying, and do a 1" housing then the tenon will be 7" long. Still enough relish for the peg hole.
You'll find that at through mortise is easier to cut then a blind mortise.

Jim Rogers
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Offline rkb5916

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Re: tenon design
« Reply #3 on: January 04, 2017, 03:46:12 pm »
First, thanks for the quick and thoughtful responses.

Brad - I've been reading through this forum like crazy...hoping to avoid asking questions that have already been answered.  For now I'm building a simple 12x16 shed so I can get some practice.  Eventually I hope to build a monitor barn (looks like I should probably cough up the dough and get with an engineer).

Jim - Yes, 8" depth.  Thanks for the info on center vs edge of peg hole.  I was assuming a through tenon would be stronger, but didn't know for sure. 

Thanks a ton...I'm sure I'll be posting again

Offline Brad_bb

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Re: tenon design
« Reply #4 on: January 05, 2017, 12:07:58 am »
...hoping to avoid asking questions that have already been answered. 

That's not likely to happen.  It's ok when the same questions come up.  We'll either refer you to a past post that covered it, or try to answer it over again.  No problem. 

With the shed, go for it.  when you get to the bigger project, yes get the engineer - like Fire tower.  A lot of timberframers use Fire tower.  Good group.  http://www.ftet.biz/
Anything someone can design, I can sure figure out how to fix!
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Offline Heartwood

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Re: tenon design
« Reply #5 on: January 05, 2017, 09:49:59 am »
What Brad said.
The 4 peg diameter relish is a recommendation from the Timber Frame Engineering Council (see the Guild's Timber Framing #79, March 2006) but assumes some tension loading and varies with species. How much needs to be calculated based on the design, and then compared to the shear parallel to the grain (Fv) capacity of the area of the shear planes beyond the peg hole (see Chappell or above article). It's very hard and impractical to achieve 4 diameters relish on many tenons, especially in pine. There is always some tension applied during raising, but joints without tension in service (such as braces) don't even need a peg. For minor members such as girts and braces I do 3" tenons, 4" on post tops and major members with no tension in service (remember the mortise weakens the mortised member, so no deeper than necessary) and through tenons on members with some predicted tension (tie beams actually doing some tying).
The best answer will come from an engineer.