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Author Topic: Through tenon or not  (Read 738 times)

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Offline iosborne

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Through tenon or not
« on: December 02, 2021, 09:47:40 PM »
Greetings,
I am building a wrap around porch with post and beams of rough sawn 8x8 green white oak. I curious about the relative benefits and detriments of through tenons vs stub tenons and those in between for the post tenons.  I see the merit of not removing too much wood from the beam but also of having enough relish at the end of the tenon.  I am not planning on stub tenons by the way although I am a newbie here so please don't teazle me. ;)

Offline iosborne

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Re: Through tenon or not
« Reply #1 on: December 04, 2021, 01:31:16 PM »
I guess I should clarify a bit.  In Sobon's Timber Framed House he's calling for post top tenons out of 8x8 that are inch and a half off the reference face and inch and a half in width with a height of 3 7/8 inch.  Isn't that undersized shouldn't they be 2 inch off and 2 inch wide?   Is there a reason for this? Why not make the tenons centered on posts? To keep away from heartwood, or simply for ease of layout?
Further, for a one story structure, why not make the tenons longer? Would this weaken the beam significantly? 

Offline Jim_Rogers

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Re: Through tenon or not
« Reply #2 on: December 04, 2021, 04:39:59 PM »
The posts in that frame and book, were made out of oak. Some builders only do 1 1/2" tenons in oak as it is stronger then softwoods.
The layout of 1 1/2" off the reference face is a standard layout so that you can use your framing square to offset the layout lines. And then again to check the offset to be sure you have it right:


 

In the above picture you can see the 1 1/2" tongue of the framing square hanging down on the tenon cheek and the tri square is on the surface of the timber. When you slide the tri square over and it hits the framing square you know you've got to trim a little bit more. It is the way we check our work to be sure it is correct.
Lots of people center tenons on post. But when you do that, you don't have an easy-to-use tool on hand to check the tenon cheek is in the correct spot. 
The post top tenon has to be long enough to prevent the plate from rolling off the top of the post from the horizontal push of the rafter thrust. The peg also prevents the plate from rolling. And with all the weight of the roof system pressing down on the plate, there is a very low risk of the plate rolling off the top of the post. In order for the plate to roll the tenons have to break off the top of the post. The pegs have to break, also. Can it happen, sure. Will it happen. Most likely not.
Normally an 8x8 post with a 2" tenon offset 2" is usually 4 or 5 inches long, up into the plate.

Jim Rogers
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Offline iosborne

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Re: Through tenon or not
« Reply #3 on: December 07, 2021, 12:13:47 PM »
Thanks for the clarification Jim.  Reading through many of your posts has been very helpful. Another curiosity, how often do you try and keep your posts oriented to their growth. If I have a post that when standing up, it would be opposite to the way it grew, is that much of a problem? I know with longbows that is not good but keeping joints out of knots and defects in the timber was my first priority.  

Offline Jim_Rogers

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Re: Through tenon or not
« Reply #4 on: December 07, 2021, 01:20:00 PM »
 Another curiosity, how often do you try and keep your posts oriented to their growth?
I don't believe it matters at all. Some times when we have a "jowl" post, that jowl is where the tree met the stump. So those can be in the frame upside down as to the way they grew.
Jim Rogers
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Offline Don P

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Re: Through tenon or not
« Reply #5 on: December 07, 2021, 07:05:32 PM »
That's a hoot, its something I have pondered a time or two. Sawing through a series of truncated cones then end loading it, which direction is stronger. Stress running into or out of the column. Then branches, if you've ever torn the entire branch stub out of a log and examined the swirl of the grain, there is certainly an up and a down.

So just in the last week or so I posed it to a young engineer friend in the course of a lumber grading conversation, I expect an answer any decade  :D

One thing I've thought about with porch posts is if they are upside down checks in the knots are better oriented to drain rather than catch water.

In reality those may matter in a high performance thing where you are looking for particular characteristics and are running closer to ultimate strength. For building timbers the difference in strength is well below the factor of safety in assigning strength values to grades and species.
The future is a foreign country, they will do things differently there - Simon Winchester


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