Thread: Jarrah zigzag chair

Teejay,

I take your point that your calculations relate only to static load. No doubt my chair might be at risk if it were flopped down upon hard by a 250 Kg person; but I think your calculations clearly show that is perfectly safe under normal conditions of use by normal people.

Rocker

Originally Posted by Rocker

Teejay,

I take your point that your calculations relate only to static load. No doubt my chair might be at risk if it were flopped down upon hard by a 250 Kg person; but I think your calculations clearly show that is perfectly safe under normal conditions of use by normal people.

Rocker

Have i missed something? or did Teejay tell you this in private. If so could you let us in on the testing procedure and exactly what would have failed if a fattie just plonked down on it

Lignum,

I did ask Teejay in a PM to do the calculations for me, and he was kind enough to spend considerable time and effort doing them and explaining them to me. I summarized the results that he gave me in post #27. No actual testing was done; Teejay based his calculations on the dimensions of the parts of my chair and the known properties of jarrah. As he points out, however, his calculations refer to a static load, not to a load applied with some momentum. Hence, if a 250 Kg person flung himself down on the chair, he might well apply a dynamic load that would be roughly equivalent to a static load of 500 Kg.

But all this is pretty hypothetical; no 250 Kg person would be likely to fling himself down on my chair.

Rocker

Hi there,
Here are details and few scans from the book I mentioned in a previous post:

Der Kragstuhl (The Cantilever Chair)
by the Stuhlmuseum Burg Beverungen
ISBN 3-923854-24-2 Germany 1986

Cheers,

I've enjoyed reading this technical debate concerning joint strength for chairs.

Although I don't want to hijack this thread, I remembered that I had seen work by a California based craftsman who specialises in chairs and rockers. His name is John Moldoven. Here's a link to his web site where he shows some of his work. Of particular interest relating to this thread is a few shot he shows on his chair joints.



http://www.jmoldovan.com/products.cf...ategoriesID=29

Don,

Those joints look pretty much standard for a Maloof-style solid-seat rocker. Of course those joints don't need to be able to withstand anything like the stresses involved with the joints of a cantilevered chair.

My hunch is that the legs would break on my chair before the joints failed. Only a test to destruction would settle the matter conclusively; but I am not willing to sacrifice a perfectly good chair merely to ascertain its ultimate strength. What is quite clear is that my chair is easily strong enough to withstand normal use.

The testing of the pinned and keyed mitre joint at the Bitingmidge get-together showed that that joint could withstand a static load of at least 300 Kg; and I am convinced that the pinned bridle joint is stronger still.

Rocker

Australian Woodworker rejected an article on my chair on the basis that they were not able to satisfy themselves that my chair would stand up to the dynamic loads that it might experience in the longer term. I suppose it is understandable that, in the current litigious environment, they don't want to put themselves in jeopardy of a lawsuit. But I am still convinced that, if properly built out of a suitable hardwood, the chair would be perfectly safe, and would last for many years.

Rocker

Originally Posted by Rocker

Australian Woodworker rejected an article on my chair on the basis that they were not able to satisfy themselves that my chair would stand up to the dynamic loads that it might experience in the longer term. I suppose it is understandable that, in the current litigious environment, they don't want to put themselves in jeopardy of a lawsuit. But I am still convinced that, if properly built out of a suitable hardwood, the chair would be perfectly safe, and would last for many years.

Rocker

Rocker I think that is more a reflection on their mag than anything. If they were interested why didnt they build one and have it tested. Any woodworker with a basic understanding of good joinery would realise it is a safe chair that you made. Have you submited it to AWR? you should as its a terrific chair. Included here is the pic of the sliding dovetail miter i did when the initional discussion of your join was happening. With no glue this is a very strong. Im not sure of the ratio as it was quickly done by eye just to make sure it would work

Lignum,

I did try AWR too. They had no reservations about the safety of the chair, but said they didn't want to publish a second article on a Rietveld-style chair so soon after their original article. This didn't seem a very valid reason to me either, but that is their prerogative.

Rocker

G'day Rockster
Can ask a question? It may have already been covered in the old posts but I can't see it. I weigh 95 KG - sometimes 90Kg, sometimes a 100kg (diabetes is a bu gger). Would your great looking chair hump my frame repeatedly. To my mind the design, to a layman, wouldn't seem to support my weight over a long period of repeated use. You could probably set up some kind of repetitive apparatus in your shed and let it load/unload for say 6 months to see how the joints stood up. Then you could provide the results to the pinheads at the mag. I reckon it looks great anyway. Nice work.
Cheers
Mike

Shedhand,

I don't see why my chair would not be able easily to stand up to a person of your weight. Even if the glue failed on the joints, they should still be sound because the pin alone would hold the joint together. However, I have no intention of making the chairs commercially, so I see no point in proving to myself that the chair is safe by a destructive test, when I am confident of that anyway.

Lignum,

I think your sliding dovetail joint, particularly if pinned, would be even stronger than my bridle joint, but it would be hard to make it except by using skillful Dark-side techniques, whereas my bridle joint can easily be made using a tenoning jig and dado set.

Rocker

Originally Posted by Rocker

Australian Woodworker rejected an article on my chair on the basis that they were not able to satisfy themselves that my chair would stand up to the dynamic loads that it might experience in the longer term. I suppose it is understandable that, in the current litigious environment, they don't want to put themselves in jeopardy of a lawsuit. But I am still convinced that, if properly built out of a suitable hardwood, the chair would be perfectly safe, and would last for many years.

Rocker

That is a pretty common reason for rejecting articles Rocker. They want to keep their magazine fresh and with it. The last thing they want is someone coming up with a story that is too similar to one they just had.

You could try resubmitting it in 6 months or so

Studley