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Thread: Tennon Strength Question

  1. #1
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    Tennon Strength Question

    I'm still pretty new at chopping mortises and cutting the tennons that go into them, so I practice whenever I get the chance. Right now, I'm putting together a rack to store boards on. I've ripped some 2x4s into 1 1/2 x 1 1/2. I've chopped some 1/2" wide 1" deep mortises into them. Extending from the mortises will be 1 1/2" x 1 1/2" x 12" (13" if you count the length of the tennon). I'm planning on putting a small diagonal between the vertical and horizontal, maybe 3" out the length of the horizontal. These will be attached to the wall studs. I have 3 levels, approximately 12" apart. I'm guessing I can store up to around 12 2x6s on each level. I don't have any question on the strength of what I'm building, but it got me to thinking.

    Is there a way to determine how much weight a tennon, supported only on one end, can hold by itself before additional bracing is needed. In other words, in my construction, how long could I make the horizontal without the diagonal below or for a particular length how much weight can it support?

    I hope I worded that clearly.

  2. #2
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    Quote Originally Posted by Bill McCarthy View Post
    I'm still pretty new at chopping mortises and cutting the tennons that go into them, so I practice whenever I get the chance. Right now, I'm putting together a rack to store boards on. I've ripped some 2x4s into 1 1/2 x 1 1/2. I've chopped some 1/2" wide 1" deep mortises into them. Extending from the mortises will be 1 1/2" x 1 1/2" x 12" (13" if you count the length of the tennon). I'm planning on putting a small diagonal between the vertical and horizontal, maybe 3" out the length of the horizontal. These will be attached to the wall studs. I have 3 levels, approximately 12" apart. I'm guessing I can store up to around 12 2x6s on each level. I don't have any question on the strength of what I'm building, but it got me to thinking.

    Is there a way to determine how much weight a tennon, supported only on one end, can hold by itself before additional bracing is needed. In other words, in my construction, how long could I make the horizontal without the diagonal below or for a particular length how much weight can it support?

    I hope I worded that clearly.
    You have several thing going on here at the same time, but if you are looking at the ultimate strength of a single tenon, and assuming it is glued or pinned so it won't rotate or pull out of the mortise, the calculation is simple. The tenon's cross sectional area multiplied by the shear strength of the wood. If rotation or pullout are possible, then the bending moment on the tenon becomes an issue. The shear strength will vary with the variety of wood being used, and while this information is available online, in the case of small tenons especially, reality may vary.

    The surest way to know is the oldest way. Make a life size mockup, load it, and break it.

    Stan

  3. #3
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    Quote Originally Posted by Stanley Covington View Post
    You have several thing going on here at the same time, but if you are looking at the ultimate strength of a single tenon, and assuming it is glued or pinned so it won't rotate or pull out of the mortise, the calculation is simple. The tenon's cross sectional area multiplied by the shear strength of the wood.
    You can get reasonable stating point numbers for shear strength for a range of species from the wood database. It's important to realize that wood is pretty variable as engineering materials go, so if you go the analytical route you'll want to divide the calculated strength by a big number like 3x.

  4. #4
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    No matter what you get from the fancy calculations know this, those tenons are too short. In your application I would make them 1 1/2 long. The reason for this is not strength per se, but more rigidity (less sag). Sag will be the thing that causes the biggest concerns, that's the best benefit of the diagonal brace.

  5. #5
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    One small but useful tip if you plan to make any Chippendale or similarly constructed chairs is this: Where the horizontal seat rails go through the legs,do not worry about making the mortise come out ON CENTER on the back side of the legs. If you do that,the tenon will have to be somewhat cross grained,and weaker. Make your tenons straight with the grain,and let them come out off center as seen on the ORIGINAL CHAIRS. This will make the tenon much stronger,especially when someone rocks the chair back onto its 2 rear legs. It was the usual practice in the 18th. C.,in door making as well,to let tenons go clear through the part they were being attached to. There are a few exceptions. One being in the case of table legs where two tenons from the apron are going into the legs at 90 degree angles to each other. In this case,the tenons are sawn off at their ends at 45 º,and the mortises are chopped so deeply that they meet in the center of the legs. The tenons then butt up against each other in the center of the leg. I have made my tables with HAUNCHED tenons for maximum strength. You will have to look up that term. I don't have any here to photograph. They are all glued up inside the tables.

    The 18th. C. chairs were made in this fashion. And rightly so. I'm not a chair maker,but being in the same building as the cabinet maker,I saw plenty of furniture and chairs being made. My terms for parts here may not be correct,but I hope you get the idea: Never make a tenon at an angle to the grain of the wood,or it will easily break off under stress. Of course,chairs take more stress than stationary case pieces,which never get tilted,or sat upon by overweight people.

    P.S.: I just Googled HAUNCHED mortise and tenons and got illustrations. Not exactly like the ones I used,but I didn't look at the numerous other sources offered.
    Last edited by george wilson; 07-26-2017 at 8:53 AM.

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