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Thread: Question for you engineer types

  1. #1
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    Question for you engineer types

    question.jpg
    Which one of these designs will be stronger? Or are they the same? My gut is telling me B will be stronger, but my gut has no real engineering experience.

    Thanks.

  2. #2
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    I'm not an engineer and I've never slept at a Holiday Inn Express. But I'll give my .02

    If the grain is running the length of parts A and B then I'd say part B is stronger with the load straight down on the tip of the parts. I could picture part A splitting and failing.

    How is the load exerted the same on A and B? Wouldn't the load slide off of A and hang straight down on B?

    Anyway that's my free opinion.
    Confidence: The feeling you experience before you fully understand the situation

  3. #3
    Quote Originally Posted by Jerry Bruette View Post
    I'm not an engineer and I've never slept at a Holiday Inn Express. But I'll give my .02

    If the grain is running the length of parts A and B then I'd say part B is stronger with the load straight down on the tip of the parts. I could picture part A splitting and failing.

    How is the load exerted the same on A and B? Wouldn't the load slide off of A and hang straight down on B?

    Anyway that's my free opinion.
    I agree. Grain direction would be critical.
    Lee Schierer
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  4. #4
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    Quote Originally Posted by Lee Schierer View Post
    I agree. Grain direction would be critical.
    Yes - of course it would. Should have included that. And figure nothing is going to slide off - load is at the very front edge only. And assume the wood will not split - I'm wondering about the M&T joint - which would fail first, all thing being equal.
    question.jpg

  5. #5
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    Not enough info and not clear enough info.
    You show the tenons being and extension of triangle shape, is that the actual shape, or are you doing standard rectangular tenons?
    Are they shouldered tenons?
    You say assume the wood won't split, why?
    The weakest part A is the potential for it to split with weight applied at the tip, if the grain is horizontal. Different if the grain is sloped.
    How are you attaching the weight?
    Is it from a point or spread out?

    I need more info to understand the application.

  6. #6
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    Hey Mark - appreciate the help and comments.
    Not enough info and not clear enough info.
    Agreed - framing the question is often the hardest part.
    You show the tenons being and extension of triangle shape, is that the actual shape, or are you doing standard rectangular tenons?
    Tenons would be rectangular
    Are they shouldered tenons?
    Yes
    You say assume the wood won't split, why?
    Because I'm interested in which shape has the greatest strength: with the slope running up from front to back, or the slope running up from back to front.
    The weakest part A is the potential for it to split with weight applied at the tip, if the grain is horizontal. Different if the grain is sloped.
    I said to assume the wood won't split because I'm interested in which joint would have the greatest chance of failure.
    How are you attaching the weight?
    Like I said, not an engineer, but can we figure it's a point load at the very front that will not slip off?
    Is it from a point or spread out?
    Point.

    It seems to me that all things being equal, the pivot point might make a difference. In A, the pivot point is in the same plane as the tip where the force is applied, but in B it's at the bottom of the triangle. Maybe that makes no difference to joint failure, and it's simply a design decision. If the glued joint is stronger than the wood, then grain direction would be critical to assembly failure, not the slope of the triangle. If this were for a seat in a chair, for instance, perhaps grain direction would be more critical than the shape of the joint?
    If you don't stand for something, you'll fall for anything.

  7. #7
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    Bill,

    I think this drawing might help explain the forces at work. In Triangle A most of the resultant force is applied at the top of the tenon due to the larger tensional forces. In Triangle B, the large compressional forces apply more force at the base, thus reducing the strain on the tenon. Disclaimer: It has been almost 50 years since I studied any of this, so it may be the product of a mushy brain. But I still recommend the engineering school down the road from you in Terre Haute.

    Wayne

    Force on tenon.jpg

  8. #8
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    thanks for taking the time to clarify for me, Wayne - much appreciated. Maybe my gut ain't so stupid after all.
    If you don't stand for something, you'll fall for anything.

  9. #9
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    Science is just a possible explanation of what your gut knows.

  10. #10
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    Bill, Cantilevers in wood are exceptionally strong, with or without glue, Knowledge of cell structure, understanding the forces that will be applied, Joint design, proportions, grain direction are what determines the strength.

    Years ago i did a lot of experimenting with oddball designs playing with joinery. I made a few Nakashima chairs and a bunch of weird variations, including a rocking chair that most people were afraid to sit in. In the photo of me working on a machine you can see behind me my lumber rack, its a half lap bridle joint, glued and screwed, I stacked a lot of heavy lumber on those. In the second photo you can see an angled joint that i was working on, when it was done i decided to make it into a chair the chair seat is not glued and can be slid out, but easily support a person. You see the Nakashima chair variation that i did with arms, you can put a stick across the arms sit on it, the arms will easily hold your weight.


    2-113 (1).jpg2-Profil95.jpg3-pedChair.jpg1-Profil9a.jpg4-Profil20.jpg3-table chair.jpg6-Profil29.jpg7-089.jpg

  11. #11
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    Inspirational!

  12. #12
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    Another line of thought.

    Assuming grain direction is parallel to top face.

    If point is weak (where force is applied) then option A should tolerate more force. Force is at an angle in case of A, increasing the force at which tear will start.

    Let's say point won't break. In this case both should be equally strong. Net torque generated would be equal.

  13. #13
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    Nice work Mark. And I have an oddball design in mind, and I think I'll just get started and see where it goes. Thx for the info Mark - and everyone else that posted.
    If you don't stand for something, you'll fall for anything.

  14. #14
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    Bill, you're welcome, and i hope that we can see your project.


    I don't know actual difference would be but i think that the longer and narrower the levers the less the difference.

    The ratio of tension to compression forces changes as the lever are made longer and the overall difference the two lever styles becomes smaller.
    The tension, compression and lever length are different in each lever, but it looks a lot like they balance each other out to some degree.

    In the sketch that i have done you will see that the lever in B is slightly longer than A; Although being above the pivot point there is some of that force directed on an angle to the base, but overall, in practical terms, i don't see that it would make any measurable difference which one you employed. The wood grain and joinery would have more effect.

    Designing joinery with an understanding of the tubular nature of wood is most important. Endgrain is far stronger than side grain and can easily crush the cell walls at pressure points. Mitered pressure points provide a far stronger and balanced joint intersection.

    lever.jpg

  15. #15
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    Bill, back in the old days ( prior to the internet) I didn't take many photos of stuff i made, i guess there really wasn't much point as there was no one to show them to anyway.

    I found a another photo of a prototype chair design, it had round posts legs and triangle brackets for the seat supports and feet, they were mortise and tenon construction.

    melanies table2a.jpg

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