Hi Folks,
The below was posted on the Knots woodworking blog. I am an amateur
windsor chair maker and there is a concept promoted by some as good design and by others as hocus pocus. I got to thinking that its essentially a truss or engineering question, and I am wondering if anyone out there could lend some opinion that would have some science behind it.
Here is what I posted on Knots:
There is a term call “pre-stressing” or “pre-Tensioning” that I think was first coined by Mike Dunbar. It has to do with the length of a chair stretcher inserted between two legs. If you think about two legs splayed out, with a straight piece between them, the legs are normally drilled with a mortise, and the length of the stretcher is the lenght between the legs, plus the dept of each of the mortises. The tops of the legs normally are mortised into the seat at the splay angle as well.
At any rate, the tensioning technique adds 1/4 ” or so to the stretcher length. When the stretcher is inserted it actually spreads the legs beyond the normal distance and effectively jams the legs sideways into the seat mortises.
I think Mr. Dunbar came up with this theory with just anecdotal information, where as the chairmakers that I know such as Curtis Buchanan, Pete Galbert, John Alexander, and others vehemently disagree and say that there is no science behind it, and in fact is wrong, and contrary to good design.
I know that there are many who support this tensioning theory, and I dont want to start a flame, but rather I would like to hear from an engineer who could comment on the loads and stresses and come up with a scientific approach to proving or disproving the theory. Any comments?
Replies
Pre-tensioning only works when you stress a material in the direction of it's highest structural capability - for instance, concrete his a compressive strength 10 to 50X higher than tensile strength, so you can pretension concrete (eg compress it) if it is going to experience tension.
The chair description of 'preload' is simply a way to take the play out of a poor joint when it is first sold, and will get loose over time anyway, plus it adds tension to a side of the wood already in tension. POS technique IYAM.
Agree with the contrary to good design school of thought, but then
Ever built a chair? If you place the legs in the seat first, such as with a Windsor, and then insert the stretchers, you have to push the legs beyond their final position just to get the stretchers in. Sounds like a good way to break the tenons in the seat, to me. Tenons in Windsors are typically wedged, so, they would be a weak joint to place additional side stress on during construction. Windsor legs are splayed in both directions. If you didn't compromise the seat tenon inserting the side to side stretchers, you'd have a second chance with the back to front ones. How would you control keeping the structure square and the legs sitting on the surface evenly? I don't think it takes an engineering degree to know when the cart is placed before the horse.
Beat it to fit / Paint it to match
My two cents (I am not an engineer, BTW) is that pre-tensioning or whatever, should counteract stress that will be put on something. For example, you put a beam in tension and cause a camber (arching) of the beam before you load it. That makes sense to me. But, the natural tendency of a chair leg that is already splayed slightly, will be to splay even more when a person sits on the chair. Forcing the legs to splay out before loading with a person only adds to the stress, and does not counter balance it, IMO. If there were a turnbuckle instead of a stretcher, or a twisted rope with a peg that pulled the legs inward, that would be good engineering, IMO, (though it would look like do-do).
No, it's a poor example of trussing or pretensioning, apples and oranges.
Now in my other trade, lutherie, a guitar neck may be made as such to have a built in back bow to counter act the string tension, or the fret slots cut thin and allow the fret tangs to cause a back bow. But being as that is not a scientifically repeatable task, the truss rod was added inside the neck to allow fine tuning to various string pulls.
By having a bowed rod that tries to get straight in a curved slot, you can apply the force where needed, using compression, but a chair rung or stretcher is not going to behave the same way.
Spheramid Enterprises Architectural Woodworks
Repairs, Remodeling, Restorations
PROUD MEMBER OF THE " I ROCKED WITH REZ" CLUB
Guess y'all tole' him
Ain't you an injun ear? Show us the math!!!Spheramid Enterprises Architectural Woodworks
Repairs, Remodeling, Restorations
PROUD MEMBER OF THE " I ROCKED WITH REZ" CLUB
Injun?! Why, I oughta . . .
Last night, I was on my yearly rant about disliking traditional, boring Thanksgiving food, and speculated to DW that just maybe the pilgrims had a Japanese guy or gal or two with 'em, and couldn't we please have sushi, if we were going to the trouble to make a huge special meal?
Then we pictured a fight between Squanto and the Japanese dudes about whether to bury the raw fish in the rows of corn, or to slice it up and serve it, "Not put fish in ground, dolt, wrap fish in seaweed and rice!"
Ah, well - don't think I'm getting my wish.
Forrest
We went though that a couple of years ago at Christmas. My mother put forward your argument and we settled on Indian food, spending the next several days making Curries, Dal, Nan, Raita - My mother inexplicably brought mashed potatoes.
That sounds cool - I mean, if yer gonna' do all the work, do something new.
Forrest
How do you keep sushi in one pc in a turkey fryer?Spheramid Enterprises Architectural Woodworks
Repairs, Remodeling, Restorations
PROUD MEMBER OF THE " I ROCKED WITH REZ" CLUB
Stuff it in a turduken.
A strong nation, like a strong person, can afford to be gentle, firm, thoughtful, and restrained. It can afford to extend a helping hand to others. It's a weak nation, like a weak person, that must behave with bluster and boasting and rashness and other signs of insecurity. --Jimmy Carter
The only advantage I see in the technique is that it assures that all the joints are tight to begin with -- all the tenons fully into their mortises. I've seen poorly constructed chairs where one joint isn't fully seated and that glue joint typically breaks first.