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Basic mortise questions

Started by SleepyDog, August 08, 2022, 05:44:14 PM

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SleepyDog

I've looked through the forum and read a book but haven't been able to locate a few answers:

1. What is the minimum distance between mortises (this is for a post if it matters)?

2. Is there a minimum, maximum, or optimal length for a knee brace (the one in question is for a post to eave plate)? 


much appreciated!!!

Ljohnsaw

1. Explain that a little more.  Are you wanting two posts next to each other?  Are you putting a post on top but not directly?

2. Looking at it from a Physics/Geometry point of view, the longer the better.  Short braces give the two parts more leverage to break things.  But, too long just doesn't look right.  I opted for long since this frame is so big.  It is 12' from sill to beam for reference.  The engineer ran this through his computer stress program and he really like how it worked:



 
Note that the posts stop at the beam.  I have some 2x8s strapped to the post that were used to lift the beams in place.  Also note that your eyes are not playing tricks on you.  The braces are NOT at 45°.  My frame uses 3-4-5 triangles for braces and roof pitch (52.7° IIRC).  It gives better head room near the interior walls/posts and better openings for doors and windows.
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Ford 545D FEL, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 42" capacity/36" cut Bandmill up to 54' long - using it all to build a timber frame cabin.

SleepyDog

Thanks for your response! 

I will have two bents, each made up of 8x8 posts to make a 8x10 lean-to garden shed. 

I originally asked the distances between mortises because I wasn't sure how they should be "stacked" on a post. The mortises on these posts are for:
1. thru tenon of beam
2. brace for beam
3. brace for eave plate on adjoining face of post

The plan is to have a very slight slope on the roof to put a green roof on. To make that happen, I don't want the mortises to be too close together vertically (whatever that might look like). But it seems like I can make the brace for the long eave plate long enough to make it happen so it is below the other mortises. 


Ljohnsaw

What about making a tenon on top of the post to fit, say, the cross (tie) beam and then a mortise in the face about 6-8" down for the eave beam (or vice-versa)?
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Ford 545D FEL, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 42" capacity/36" cut Bandmill up to 54' long - using it all to build a timber frame cabin.

Don P

To answer the question from a structural grading view. A hole is viewed the same as a knot. A mortise that is 1/4 the width of the face and well away from any edges is a #1. Move that mortise to the edge as in a lap and notching rules begin to apply as well but the effect on the timber is far greater. It would be conservative to reduce the design size of the timber to the smaller remaining section. 

When there is more than one knot in the stick what do you do when grading? 1st determine if they are in the same section. That is a little vague, usually interpreted as in the same ~6" of length. An engineer would probably say something like "within 45 degrees", and being a dumb nail banger on an 8x8 I'm going to stay 8" away if possible. If in the same section combine defects when grading. If both mortises are in the same section and nicely centered, 1/4 width of each face, I would combine them and say that the defect cumulatively removes 1/2 of the net section. Use the design strengths for #3 material.

Jim_Rogers

I have been doing some research to try and find answers to these questions.
For you and for me.
I am a member of the Timber Framers Engineering Council, and as such I can and have asked the group of experienced timber framing engineers.
I have asked if there is a document in our library of documents that addresses these questions. There is no document that answers these questions directly.

And if there was not a document what would be some advice.

I have received several emails. One also said to call him on the phone so we can discuss the situation.

I called that engineer and we talked for 11 minutes about these questions.

1) of course the standard answer on this is "it all depends." First of all are we talking about two mortise in the post in the same plane? Like with the plate. Or are we talking about a mortise with the plate and a mortise 90° to the plate?

If the braces are in line with the plate and one is going up and the other is going down to a timber like a sill, he said he has seen the mortise being continuous for both braces. And the two braces butting against one another inside this long mortise.

If the two mortises are 90° from each other then there should be a distance between them so that the post is not weakened by removing too much wood in one location.

One engineer's email reply said that the distance between them of wood (with no wood removed), should be the distance of the shorter of the two mortises.
Usually a 45° mortise pocket for a brace would/could be 8" or so.

If the two brace pockets/mortise are 90° from each other then at least 8" so that you're not weakening the post 

Question 2).
There is a common answer that the longer the better. 

I asked if he had ever heard of the 1/3 rule. That is 1/3 of the length of the post. For example an 10' post then a 3' brace.

His reply was that normally it is taken on a case by case basis.

So the bottom line is that there is no one single answer to this question.

Everyone who replied to my email said basically the same thing.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

SleepyDog

I'm grateful for the responses, this is helping me immensely. In sum:

1. (distances between mortises)
-I'll keep them apart, the distance of the smallest mortise as they will be 90° from each other. One brace for the tie beam, the other for the eave plate. 

2. (lengths of braces)
The longer the better but 1:3 (brace length : post length) is a good rule of thumb to follow. 

Jim_Rogers

The distance from the bottom of the plate to the top of the tie should be at least 8". I like to use 12".
because of this:







 


In the above view you can see why you should stagger your peg holes. Not vertically aligned.

Jim Rogers

 
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

SleepyDog

I'll be sure to offset that distance appropriately. 

1. Should the mortise spacing between the braces for cross beam and plate in the diagram provided (braces not pictured) also be at least 8"/the smallest length of the mortise?

2. Should I prioritize the bigger brace for the beam or plate? I imagine whichever is bearing the most load?

Don P

Jim's pictures show tension perp failures, tension perpendicular to grain. This is the most unpredictable failure mode in wood, there is no design value for most species. 

From the code reference on such things, the NDS;
3.8.2 Tension Perpendicular to Grain
designs that induce tension perpendicular to grain shall be avoided whenever possible (see references 16 and 19). When tension stress perpendicular to grain cannot be avoided, mechanical reinforcement sufficient to resist all such stresses shall be considered (see references 52 and 53 for more information).

Jim_Rogers

Quote from: SleepyDog on August 15, 2022, 08:06:28 PM
I'll be sure to offset that distance appropriately.

1. Should the mortise spacing between the braces for cross beam and plate in the diagram provided (braces not pictured) also be at least 8"/the smallest length of the mortise?

2. Should I prioritize the bigger brace for the beam or plate? I imagine whichever is bearing the most load?
1) Yes.
2) Yes, again. 
Jim Rogers
I recently saw a picture on facebook, of a frame where they had not completed the siding or completed the attaching the posts to the concrete. and a large storm came through their area, and blew the frame off the supports.
The post broke at the tie beam due to the two peg holes being aligned. And one was very high in the mortise.
I've tried to capture that picture, but I can't find it again. I'll search again if I can find it, I'll post it here.
If your tie beam tenon needs two pegs, they should be stagger horizontally so that they are not in the same grain line. And they should be low on/in the mortise.
A tie beam is going to shrink, vertically. If the peg is high up in the mortise, then the weight of the tie beam and any load it supports, is going to push down on the upper peg. This force could split the tenon inside the mortise. This split could weaken the joint. I feel it is best to allow the tie beam to shrink down by placing the pegs lower on the joint.


 
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

SleepyDog

I'm tracking about 1½" from shoulder and 2" from bearing surface on a knee brace for the first peg. 

How far above and offset (away from the shoulder) should the second peg be? 

Jim_Rogers

I only use one peg in the tenon/mortise for a brace.
If you mean another joint, tell me the joint you're asking about.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

SleepyDog

Yes, for the tie beam where two pegs are called for (I'm also unsure when two is preferred over 1) - how is the peg placement decided? 

Jim_Rogers

When a tie beam shrinks it gets shorter. The gravity takes over and that why it gets shorter. 
By taking apart older building where there was a tie beam peg hole up high on the tie beam tenon they have found that the peg held the tie beam up and with the weight of the beam and the load it is carrying, the tenon had split at the peg hole to the end of the tenon.
The timber framers guild engineering council has recommended that the peg holes should be lower to allow to tenon to shrink down and not split as it does so.

My engineer has instructed me, in one house design, that the hole spacing should be like this:


And these are the notes:

 



I disagreed with these dimensions because I felt that the peg hole was too close to the end of the tenon. Even though the tenon was not as shown above, but a full through tenon.
Traditionally, a peg hole is 1 1/2" off the shoulder of the housed tenon, 2" off the side of the timber if no housing is used. And 2" up from the bottom.
The second peg hole, sometimes suggested by the engineer, would be another 2" up and another 1 1/2" off the shoulder. Thus being 3" off the shoulder.

I believe I wrote about this before.

So, when the timber framers engineering council did some "joint busting" at the national convention, I took some "tie beam to post" sample joints to the test area, that I had cut and prepared for busting.

One pair of timbers (post and tie beam) represented my idea of the correct traditional peg hole placement. And one pair of timbers representing my engineer's peg hole placement.

I expected that my joint would be stronger than the other joint.

The put both into the test busting machine and pull them apart until complete failure.

Both joints failed completely identical. A very surprising way to me.
I expected the "new" joint to fail by the tenon relish to blow out beyond the peg hole, as the peg hole was (in my opinion) to close to the end of the tenon.
The relish did not blow out. On either joint.
What failed was the pegs. All four of the pegs failed in the same manner. They bend around like a "U" and pulled out. Some sheared off at the tenon/mortise face.
I videotaped both tests.
Both joints failed at around 6425 lbs of force. One failed about 200 lbs more or less than the other. 

The bottom line is that the frame design has to be engineered to determine if one peg or two, or more should be used to secure the tie beam to the post.
The function of the tie beam to the post joint has to be stronger than the thrust generated by the rafters.

Good luck with your project.

Jim Rogers

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

way up north

Quote from: Jim_Rogers on August 31, 2022, 12:37:57 PM
When a tie beam shrinks it gets shorter. The gravity takes over and that why it gets shorter.
By taking apart older building where there was a tie beam peg hole up high on the tie beam tenon they have found that the peg held the tie beam up and with the weight of the beam and the load it is carrying, the tenon had split at the peg hole to the end of the tenon.
The timber framers guild engineering council has recommended that the peg holes should be lower to allow to tenon to shrink down and not split as it does so.

My engineer has instructed me, in one house design, that the hole spacing should be like this:


And these are the notes:

 



I disagreed with these dimensions because I felt that the peg hole was too close to the end of the tenon. Even though the tenon was not as shown above, but a full through tenon.
Traditionally, a peg hole is 1 1/2" off the shoulder of the housed tenon, 2" off the side of the timber if no housing is used. And 2" up from the bottom.
The second peg hole, sometimes suggested by the engineer, would be another 2" up and another 1 1/2" off the shoulder. Thus being 3" off the shoulder.

I believe I wrote about this before.

So, when the timber framers engineering council did some "joint busting" at the national convention, I took some "tie beam to post" sample joints to the test area, that I had cut and prepared for busting.

One pair of timbers (post and tie beam) represented my idea of the correct traditional peg hole placement. And one pair of timbers representing my engineer's peg hole placement.

I expected that my joint would be stronger than the other joint.

The put both into the test busting machine and pull them apart until complete failure.

Both joints failed completely identical. A very surprising way to me.
I expected the "new" joint to fail by the tenon relish to blow out beyond the peg hole, as the peg hole was (in my opinion) to close to the end of the tenon.
The relish did not blow out. On either joint.
What failed was the pegs. All four of the pegs failed in the same manner. They bend around like a "U" and pulled out. Some sheared off at the tenon/mortise face.
I videotaped both tests.
Both joints failed at around 6425 lbs of force. One failed about 200 lbs more or less than the other.

The bottom line is that the frame design has to be engineered to determine if one peg or two, or more should be used to secure the tie beam to the post.
The function of the tie beam to the post joint has to be stronger than the thrust generated by the rafters.

Good luck with your project.

Jim Rogers

Jim Rogers
This is great info. Thanks Jim.

Don P

There are several code refs that get into parts of this, this is another that Jim's pic brought to mind. From the AITC manual, I think this chapter is a free download on the website. In one way or another these are all tension perp failures. The connector, or connection design, is stressing the beam in tension, perpendicular to the axis of the beam.







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