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Author Topic: General Rules for Joinery Design  (Read 38880 times)

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Offline Jim_Rogers

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General Rules for Joinery Design
« on: September 06, 2005, 10:25:02 am »
Recently a timber framer suggested I post these "General Rules" for joinery design, that I learned about recently at a Joinery Decisions workshop class.

Here they are:


There are 8 general rules to timber framing for joinery design.
The "Golden Rule": Design joints to do the same structural task assigned to the loaded timber without putting the capacity of the receiving timber at risk.
1). Cut the joints and arrange the fastenings so as to weaken the pieces of the timber that they connect as little as possible.
2). Place each abutting surface in a joint as nearly perpendicular as possible to the pressure with it has to transmit.
3). Proportion the area of each surface to the pressure with it has to bear, so that the timber may be safe against injury under the heaviest load which occurs in practice and form and fit every pair of such surfaces accurately in order to distribute that stress uniformly.
4). Proportion the fastenings so that they are stronger than the loads that are anticipated in the pieces that they connect.
5). Place the fastenings in each piece of timber so that there shall be sufficient resistance to the giving way of the joint by the fastenings shearing or crushing their way through the timber.
6). Select the simplest forms of joints, and obtain the smallest number of abutments.
7). Both the tenon and the mortise should be shaped to be parallel with the grain of their respective members.


Most traditional layout was done with the layout tool at hand, which was the framing square. And as it has two parts, the body (the 2" wide part), and the tongue (the 1 1/2" wide part), most layout was one of these two dimensions.
Understanding and using layout faces or reference faces helps us to correctly layout, cut and join timbers so that every piece of the frame lines up to the others and it all goes together correctly.
Each timber has a reference/layout face and that face depends on what the "general frames rules" are, these rules were established by the master framer or frame designer.
But some standards usually always apply. For example, all gable end bents have to have joints flush with the outside of the building. All roof joints have to be flush with the top plane of the roof slope. All wall joints have to be flush with the outside of the walls, all floor joints have to be flush with the top surface of the timbers supporting the floor. These are general timber framing layout rules that apply to most if not all frames.
Some of the interior bents and other interior parts can have different layout faces and rules depending on the master framers plan and none of them are wrong, if it all goes together right.


Other standard rules for joinery decisions are that tenon size should be one quarter of the timber thickness. That means a 8x8 should have a 2" thick tenon and a 6x6 should have a 1 1/2" thick tenon.
 
Pegs should be one half the tenon thickness. So a 2" tenon should have a 1" peg, and a 1 1/2" tenon should have a 3/4" peg.
Now sometimes a 1" peg will take away too much wood and therefore a smaller peg may need to be used as long as it's strength is high enough as to not fail during it's load carrying capacity.


I hope this is helpful to you.
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Offline logmason

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Re: General Rules for Joinery Design
« Reply #1 on: September 06, 2005, 12:22:07 pm »
What is a bent? The link above this post, glossary, will not open on this computer, and my dictionary is no help.

Offline Jim Haslip

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Re: General Rules for Joinery Design
« Reply #2 on: September 07, 2005, 02:23:08 am »
Good Info Jim... thanks


Offline Joel

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Re: General Rules for Joinery Design
« Reply #3 on: September 07, 2005, 09:48:02 am »
Jim...

As usual, an informative post from you. Thank you!

Joel

Offline Jim_Rogers

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Re: General Rules for Joinery Design
« Reply #4 on: September 07, 2005, 12:46:00 pm »
The glossary, I believe, is in pdf format. This requires a pdf reader program. Most commonly used is acrobat reader. You can go to their website and download a free reader and then you'll be able to view the glossary. And you can print it for future reading and use when not "online".
I would suggest you get a reader program. I'm sure you can find one at www.adobe.com/acrobat, but if not let me know and I'll find the correct website address.

A "bent" is an assembly of timbers. Like, for example, a gable wall (with or without rafters). It is usually assembled flat on the deck and then raised up to a standing position. The raising up can be by hands, crane, ropes and tackle, forklift, or other methods.

Jim Rogers



Whatever you do, have fun doing it!
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Offline jpgreen

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Re: General Rules for Joinery Design
« Reply #5 on: October 29, 2006, 11:35:14 am »
Thanks Jim.

Very good info. 

It really does take a newbie a lot of study for this stuff to all come togher and sink in..  :P
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Offline Jayson

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Re: General Rules for Joinery Design
« Reply #6 on: November 30, 2006, 08:41:25 am »
Jim you are the bomb. Your info is always great. I hope to meet you someday. Please let me know if you are working on anything down south. I would love to help. Jayson

Offline slidecreekdan

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Re: General Rules for Joinery Design
« Reply #7 on: December 18, 2007, 08:43:02 pm »
Yes, I must say, Thanks again.

Offline woodwright

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Re: General Rules for Joinery Design
« Reply #8 on: December 30, 2008, 02:13:54 pm »
Recently a timber framer suggested I post these "General Rules" for joinery design, that I learned about recently at a Joinery Decisions workshop class.

Here they are:


There are 8 general rules to timber framing for joinery design.
The "Golden Rule": Design joints to do the same structural task assigned to the loaded timber without putting the capacity of the receiving timber at risk.
1). Cut the joints and arrange the fastenings so as to weaken the pieces of the timber that they connect as little as possible.
2). Place each abutting surface in a joint as nearly perpendicular as possible to the pressure with it has to transmit.
3). Proportion the area of each surface to the pressure with it has to bear, so that the timber may be safe against injury under the heaviest load which occurs in practice and form and fit every pair of such surfaces accurately in order to distribute that stress uniformly.
4). Proportion the fastenings so that they are stronger than the loads that are anticipated in the pieces that they connect.
5). Place the fastenings in each piece of timber so that there shall be sufficient resistance to the giving way of the joint by the fastenings shearing or crushing their way through the timber.
6). Select the simplest forms of joints, and obtain the smallest number of abutments.
7). Both the tenon and the mortise should be shaped to be parallel with the grain of their respective members.


Most traditional layout was done with the layout tool at hand, which was the framing square. And as it has two parts, the body (the 2" wide part), and the tongue (the 1 1/2" wide part), most layout was one of these two dimensions.
Understanding and using layout faces or reference faces helps us to correctly layout, cut and join timbers so that every piece of the frame lines up to the others and it all goes together correctly.
Each timber has a reference/layout face and that face depends on what the "general frames rules" are, these rules were established by the master framer or frame designer.
But some standards usually always apply. For example, all gable end bents have to have joints flush with the outside of the building. All roof joints have to be flush with the top plane of the roof slope. All wall joints have to be flush with the outside of the walls, all floor joints have to be flush with the top surface of the timbers supporting the floor. These are general timber framing layout rules that apply to most if not all frames.
Some of the interior bents and other interior parts can have different layout faces and rules depending on the master framers plan and none of them are wrong, if it all goes together right.


Other standard rules for joinery decisions are that tenon size should be one quarter of the timber thickness. That means a 8x8 should have a 2" thick tenon and a 6x6 should have a 1 1/2" thick tenon.
 
Pegs should be one half the tenon thickness. So a 2" tenon should have a 1" peg, and a 1 1/2" tenon should have a 3/4" peg.
Now sometimes a 1" peg will take away too much wood and therefore a smaller peg may need to be used as long as it's strength is high enough as to not fail during it's load carrying capacity.


I hope this is helpful to you.

Offline Brad_bb

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Re: General Rules for Joinery Design
« Reply #9 on: May 21, 2009, 10:07:56 am »
So Jim,
If you have a timber(s) that is 10X10, should a tennon be 2.5 wide or remain at 2"?  What if it's hardwood?
Anything someone can design, I can sure figure out how to fix!
If I say it\\\\\\\'s going to take so long, multiply that by at least 3!

Offline Jim_Rogers

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Re: General Rules for Joinery Design
« Reply #10 on: May 21, 2009, 10:44:05 am »
Well the rule says it should be 2.5, and I've seen larger tenons and mortises in big stuff.
However, being hardwood you maybe able to make it just 2" but each frame design needs to be reviewed and understood before making a general statement like that......
Whatever you do, have fun doing it!
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Offline Brad_bb

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Re: General Rules for Joinery Design
« Reply #11 on: May 22, 2009, 10:27:48 am »
Understood,  I was just wondering how common it is to have larger than 2" in tennon in 10" stock, especially when it's hardwood.  The stress and shear area needs to be calculated/sized for each joint for each frame.
Anything someone can design, I can sure figure out how to fix!
If I say it\\\\\\\'s going to take so long, multiply that by at least 3!

Offline moonhill

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Re: General Rules for Joinery Design
« Reply #12 on: May 26, 2009, 07:12:31 am »
As the name suggest, General Rules, there is always an exception, that is where I get into trouble or find myself ecstatic with a new solution.

As an example the tie beam pushed out from the gable plane to allow a boarding trench.  Or a 3"x5" brace on a 9"x12" post using 1-1/2" mortice in soft wood.

Tim
This is a test, please stand by...

Offline Aikenback

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Re: General Rules for Joinery Design
« Reply #13 on: December 11, 2009, 10:32:18 pm »
Hi, one problem I ran into was a beam at the end of a deck which ran on a 45 degree back to the house. the post it was attached to was set square to the rest of the deck. with the beam and tenon entering the post at 45 degrees the tendency for the tenon to shear off along the grain and be ruined was discovered at fitting and we doubled the tenon thickness to 4". luckily we had 10x10 posts for the deck.

I'm wondering If there is a common rule for this. The engineer didn't pick up the potential problem on my drawings. By the way the project is done and looks great, but being my first traditional timber job it was stressful not knowing how it all would turn out, like driving in the fog.

I was wondering if I could get advice on theory and technique here. After falling out of love with construction after 20 years, I've been looking forward to doing alot more of this. What would i do if the post couldn't handle a wider tenon. The posts run through the deck and incorporate the railing.
................Blake
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Offline Jim_Rogers

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Re: General Rules for Joinery Design
« Reply #14 on: December 12, 2009, 09:24:40 am »
a beam at the end of a deck which ran on a 45 degree back to the house.

You never stated what size this beam was. And in order to give you some advice, I'd need to see the whole joint.

Jim Rogers..
PS. As mentioned there are exceptions to every rule. Oh yea and welcome to the forum.
Whatever you do, have fun doing it!
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Offline Aikenback

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Re: General Rules for Joinery Design
« Reply #15 on: February 11, 2010, 06:21:53 pm »
i'm sorry it took so long to reply, i dont know how to upload the picture. i also have a pdf of the joint in question. the beams are 8x16 with a 12'' high x 2.5'' thick tenon thru to the the center of a 10x10 (5'' incl. housing). The post runs thru to form the railing post above. 1-1/4" housings. A short beam runs into the last post and supports two short joists much like a hip rafter would in a roof except it's a deck and therefore like a floor. there's a trianglular section of the tenon that is susceptible to shear off with the grain. if I can become less of a caveman with the computer, I could show it.   
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Offline Jim_Rogers

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Re: General Rules for Joinery Design
« Reply #16 on: February 11, 2010, 06:31:08 pm »
Send anything you want to me and I'll post it for you.
Whatever you do, have fun doing it!
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Offline Jim_Rogers

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Re: General Rules for Joinery Design
« Reply #17 on: February 12, 2010, 08:54:12 am »

(here is a picture that Blake sent me of his project) More of his work in my gallery under Blake Todd Album, some very nice work, indeed.


Whatever you do, have fun doing it!
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Online Dave Shepard

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Re: General Rules for Joinery Design
« Reply #18 on: February 12, 2010, 07:59:20 pm »
The mortises in one house I looked at had 2 3/4" layout on a post of about 9"x10" section. All of the posts on the Dutch barn I'm working on now are 2 1/4" layout. They range from about 6"x9" to about 9"x12 1/2". I wonder if the size of the framers square might have something to do with that sometimes. :D
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Offline Aikenback

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Re: General Rules for Joinery Design
« Reply #19 on: February 14, 2010, 12:51:50 pm »
I have a question regarding peg placement. I've read the "general rules" and have the "timber framer's workshop" book by Steve Chappell and run everything I want to do through an engineer, but the more I know, the better. (for me, anyway).

For a 1" peg in the through posts in the deck I did there seems to be little tolerance for placement. I believe I needed about 2-1/2" of relish and a minimum distance of 2" from loaded edge of post. The specs from the engineer required 2-1/2" from loaded edge. This means with the tenons butting into each other there was no ability in the joinery designed to increase the relish just for peace of mind. Unless I were to offset the tenons and lap them on either side of each other in order to run the tenon past the centre(Canadian spelling) of the 10" post. How common is this?

Also, what about the housing? If I were to set the pegs at 2-1/2" from face and then remove the housing, the face edge of the peg and the end of the housing only has 3/4" of "meat"  between the post and the end of the beam. What would happen with an 8" post?

I had alot of confidence in this regard here because the posts were wider than the beams which gave me the full 2-1/2" from the face on each side of the beam.

Should one consider a spline joint instead?

Is this tiresome answering similar questions from different people over the years?

Am I cheating by asking questions here instead of taking a design course?

Should I just trust the Eng.?

Should I just stick to nails and 2x4s? (please no!!)
no whining.