General Rules for Joinery Design

[baryon]

THANKS Jim--- The mortises have not been cut yet.    You are very helpful  and down right knowledgeable. !!     Baryon

[rjwoelk]

Why do timber framers work with big timbers?

They are bent that way. :D

[Magicman]

Hello rjwoelk, and Welcome to the Forestry Forum.

[dad2nine]

Jim is there a general rule for depth of a given mortise or length of tendon?

[Jim_Rogers]

Yes, but it is not a simple answer.
It all depends on the location of the tenon and mortise. Each joint has to be reviewed as to how much stress or load is on that joint.
Then the tenon has to be long enough to provide enough "relish" beyond the peg hole to have the strength to not "blow out" the tenon and split it along the grain.
The basis for this is something like at least 4 diameter's of the peg beyond the peg hole. But it could be more. I'd have to look up the standard rule, but I don't have time right now to do that.

Jim Rogers

[1234wood]

Hi jim? I plan on building a saw mill building   24x30  10 x10 beams pine vertical  post set on concrete pillars with strong ties  in concrete  10 ft spaceing with rafters not sure on size probably 7/12 or should I have someone draw it up I need a 21 ft opening  for logs near middle of 30 ft side of building

[Jno wood]

Yes, but it is not a simple answer.
It all depends on the location of the tenon and mortise. Each joint has to be reviewed as to how much stress or load is on that joint.
Then the tenon has to be long enough to provide enough "relish" beyond the peg hole to have the strength to not "blow out" the tenon and split it along the grain.
The basis for this is something like at least 4 diameter's of the peg beyond the peg hole. But it could be more. I'd have to look up the standard rule, but I don't have time right now to do that.

Jim Rogers

I am following along all these rules and based on this I have come to the following conclusion:
I have a 6" post, and a 4X6 knee brace with 3/4" pins.
This means:
1. the tenon should be 1" thick (1/4 the thickness of the knee brace)

2. the tenon needs to have 3" of relish (4x 'the pin')

3. the tenon need to be 1.5" from the edge (2X pin)


So in this case the tenon is 1" thick X 4.5" deep

Did I get it all right?

Some more notes:

- By the "peg rule" this peg actually should be 1/2 x tenon thickness or 1/2" and this would change all the numbers - 3" deep tenon???

-I have read that relish should be 3.5d - 7.5d for softwood

[Jim_Rogers]

There are exceptions to every rule. Understanding when the apply does take some research and experience.
On a 4x6 knee brace we do 2" flush tenon and 1" peg.

Also, we normally make the tenon length 3".
Recently in a discussion with another designer he felt that 3" brace tenons were too short. I believe he uses 4".
Braces are strong in compression and that's their job. Lack of pegs in braces have been found in many old barns and structures.
Some people say that the peg in a brace tenon is there to just hold the brace in place during raising.

Jim Rogers

[Jno wood]

Thanks Jim. One thing about the project I am working on is all the joints are under compression, so it should be a very forgiving project as far as pins go.

 

[darksmokepunchr]

In general I don't see a use for pegs in tenons.  The data I've read all talks a lot about peg failure under shear, and the pegs are weaker in shear than the wood in tension, so I understand.


I favor cutting tenons as half-dovetail, and using a wedge of a harder wood.  The wedge is under compression and the tenon is under tension.  For knee braces this has to be on the outside, which means your dovetail might be on the top edge and lifted by the wedge, which is unusual; and for a horizontal member with the wedge applied vertically, you'll want the wedge to be sufficiently tapered and to be driven in from above so it doesn't just fall out if it shrinks some.

If the wood shrinks a little, brace it, remove the wedge, cut a new hardwood wedge, and drive that in as a replacement.  The building should be designed such that a load-bearing member won't just fall out if un-wedged, and such that the building won't just collapse if you pull a brace:  it should be safe but not preferable to pull the wedge without first temporarily bracing everything up.

That said, I don't have a way to analyze all this except by dead reckoning.  I'm aware that some folks who do this for a living also use wedges rather than pegs nowadays, so at least I'm not inventing some obvious advancement that everyone else somehow missed.  That's a good sign.

What do you all think re wedges versus pegs?

[apratti]

Dude - That is a very handsome joint. Not sure I want to recreate it for knee braces but *DanG! It's great as shown as beam to post. Japanese? Tell me about the mortise cut angle trick bc I may have some slop with that joint if it's not through the post.
Thank you!
M Pratti  

[Angusga]

I've been around timber frames for years.  I've only built one myself though, and it was done with scribe rule.  I'm working on a deck/porch now with square rule layout.  I  have a question about Knee braces.  I'm measuring from reference faces on one side of post timbers that have braces coming in from both sides.  I'm thinking to inset the braces on the other side by measuring the same way, but from the non reference side.  My perfect timber dimension is 7.5".  Is there a problem with doing it that way?  here is the front view timber layout plan.
 

[cmiller]

Is there a rule for how long the tenon should be? What is the rule the distance along the tenon to place the peg?
Thanks
Clint

[Jim_Rogers]

Is there a rule for how long the tenon should be? What is the rule the distance along the tenon to place the peg?
Thanks
Clint

I'm researching this, I will post back when I have an answer.
Jim Rogers

[Jim_Rogers]

The timber framers guild engineering council has these two documents.

Sometimes that are easy to understand, sometimes not.

Maybe DonP can shed some light on this topic.

Jim Rogers

[Jim_Rogers]

and

[rllintz]

Hi Jim,
Is there a general rule of thumb for length of brace? For example if my span is 12' between posts should I use two opposing 48" braces or is 36" ok or is it mostly aesthetics? Also, if I'm putting a window or door centered in between 2 posts and the span is 8' do I still need to put braces in?

Thanks Jim

[Jim_Rogers]

Hi Jim,
Is there a general rule of thumb for length of brace? For example if my span is 12' between posts should I use two opposing 48" braces or is 36" ok or is it mostly aesthetics? Also, if I'm putting a window or door centered in between 2 posts and the span is 8' do I still need to put braces in?

Thanks Jim

You can use braces that have 36" legs for that span. 
If you have other braces at the ends of that wall you don't need to put braces in with the window or door. If you don't have any other braces in that line, then make the braces 2' or something that will fit. Good luck with your project.
Jim Rogers