Thread: Destructive testing of Rietveld z-joints

Pic 1-4 show the results of the destructive test of the mitre.

Pic 5 shows a photo of the test applied to the dovetail joint. The dovetail seemed to fail much easier than the mitre. There are two possible causes: (1) the stock was longer, so the vise had more leverage, and (2) the joints were not that well made.

I conclude that the mitre joint would be adequately strong for a z-style side table, possibly with a few mitre keys used for good measure. Along with the fact that they are easier to make than the 45° dovetails, I've decided to use the mitres.

Are you working for Mythbusters?

Originally Posted by Redgy

Are you working for Mythbusters?

Probably about the same degree of scientific rigour...or just boys having fun wrecking stuff.

Zenwood,

You didn't mention what glue you used, or what the wood was, although it looks like pine. I am sure that if uou use a suitable hardwood and epoxy glue with keys to reinforce the mitres, the table would be plenty strong enough to survive normal use.

Rocker

Whats a Rietveld z-chair look like Zenwood ? (is that a stupid question )

Be interesting to get some idea of the force employed to break the joint. It seems to me that a chair must take enormous strain. The length of the timber pieces in your experiment were relatively short. As I remember from Physics, the length of the 'lever' increases the force at the fulcrum exponientially. Hence the force you developed in the vice could easily be applied by a big fat sitter. Say he weighed 120 Kgs, and he sits on one side of the chair. Thats a force moment of 120Kgs at the end of a 400mm (or so) lever. Also he has increased kinetic energy as his bum is moving and must be arrested by the chair, so the maximum force that applies is at the moment the chair stops giving and trys to accelerate back its normal position. As I remember that about doubles the effective weight, hence we've got 240kg mass at the end of a 400mm lever. I thing there's a calculation for this, I'll try and find it.

Originally Posted by apricotripper

Whats a Rietveld z-chair look like Zenwood ? (is that a stupid question )

See the linked thread for Rockers interpretation. Otherwise do a search for Rietveld and see a lot of info.


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Corbs

I found a site that gives calculations on leverage.

http://www.engineersedge.com/calcula...e_levers_1.htm

Using this info, and its very rough as there are many variables, it seems that 120kg on a 400mm lever can exert in excess of 8 times its weight at the fulcrum. This is for static weights. Given the bum is descending and must be arrested then accelerated back to a point of equilibrium, that could double the force.

Rough estimation says the the force evident at the chair joint can easily (momentarily) exceed one tonne! Thats gotta be a tough joint.

Safety Factors.

Dead load 1.25

Live load 1.5

Impact load 2.0

Best to keep dead things off the chair - might stain it . Any lump descending will exert an effective load twice their real weight :eek: - hmmm.

Cheers

Originally Posted by Rocker

Zenwood,

You didn't mention what glue you used, or what the wood was, although it looks like pine. I am sure that if uou use a suitable hardwood and epoxy glue with keys to reinforce the mitres, the table would be plenty strong enough to survive normal use.

Rocker

The glue was Aquadhere PVA+ (crosslinking PVA), the wood was oregon. I intend using jarrah for my side tables. I think jarrah, plus say 4 keys, and PVA+ should be fine for side tables.

Bodgy,

The chair will get a good work-out at the Bitingmidge gtg, when I shall be happy to let the heaviest person present sit on it and shift around in a normal fashion. I won't encourage anyone to jump up and down on it, though.

I am convinced that the bridle joint, reinforced by a screw, is quite strong enough to withstand normal use, but not, of course, gross abuse. I am trying to imagine how the joint would fail, if tested to destruction. I believe it would be necessary to exert a force strong enough to snap one of the 35 x 40 parts of the chair, since I believe the wood would fail before the joint did.

According to statistics quoted in Bootle's book, American white oak has a modulus of rupture of 105 MPa and a Janka hardness of 6.0 kN. The comparable numbers for jarrah are 112 and 8.5, and for hoop pine, 90 and 3.4. I would not want to make my chair out of a timber that had numbers lower than 100 MPa and 5 kN.

Rocker

Originally Posted by zenwood

The glue was Aquadhere PVA+ (crosslinking PVA), the wood was oregon. I intend using jarrah for my side tables. I think jarrah, plus say 4 keys, and PVA+ should be fine for side tables.

Zenwood,

I would be wary of using PVA, which, as Mick has mentioned in another thread, is liable to creep. Since a joint in a zigzag chair or table is under considerable stress, creep might prove to be a problem. I believe it would be worth the extra expense of using epoxy for the joint.

Rocker

Originally Posted by Rocker

Bodgy,

The chair will get a good work-out at the Bitingmidge gtg, when I shall be happy to let the heaviest person present sit on it and shift around in a normal fashion. I won't encourage anyone to jump up and down on it, though.

Rocker

Look forward to the results. Not knowing the build and body fat indices of the Bitingmidge attendees, will this be a stringent test?

The figures I quoted are certainly inaccurate due to the chair joint not really functioning as a true lever, and the flex in the wood mitigating the peak loads, but I still find it amazing how much force the average chair needs to withstand. As you say, the wood should split before the glue gives up.

I trust there will be no unfortunate injuries to what is a sensitive part of one's anatomy

Originally Posted by apricotripper

Whats a Rietveld z-chair look like Zenwood ? (is that a stupid question )

Apricot,

Rocker's z-chair was inspired (as is my z-table) from the zig-zag chair designed by Gerrit Rietveld in 1934

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