Destructive testing of a zigzag frame

[David Dundas]

At a recent woodworkers' get-together held in Brisbane, Australia, we tested a zigzag frame to destruction; the intention was to ascertain the ultimate strength of the frame, which has a similar consruction to some of my zigzag chairs. The frame's 22.5 degree miter joints have an aligning domino and twin reinforcing spline keys glued into saw kerfs with epoxy. The cross section of the lumber (Tasmanian blackwood, which has similar properties to American walnut), was 35 mm x 40 mm (about 1 3/8" x 1 5/8")

The first photo shows two woodworkers with a combined weight of about 500 lbs climbing onto the frame, which easily supported their weight.
The second photo shows four woodworkers with a combined weight of about 800 lbs standing on the frame, which supported their weight for a few seconds before collapsing.

On one side of the frame, the joint itself remained intact, but the wood split. On the other side, the joint itself was torn apart, with the keys being pulled out of their kerfs. In this case, perhaps the joint was starved of epoxy because of the tight fit of the keys in their kerfs.

The test showed that a cantilevered chair with well-made joints should easily be able to support the weight of even an obese sitter with a wide margin of safety.

David Dundas

[Eddie Darby]

Do you have any 'after' photos?
I sure am amazed at the torture it took to bring it down!
Hope that all landed safely!

[Bob Childress]

The test showed that a cantilevered chair with well-made joints should easily be able to support the weight of even an obese sitter with a wide margin of safety.

David Dundas

That's me, David, an obese sitter wide a wide margin.

[Mike Duffy]

Does anyone know where can I get detail plans for that Z chair?
Thanks.

[David Dundas]

Eddie,

Here are a couple close-up pics of the ruptured joints. As mentioned above, one of the joints is still virtually intact, while the other broke along the glue lines.

Mike,

You can find a copy of my article on a zigzag dining chair here . I suggest, however, that you omit the brass pins, which I now believe weaken rather than strengthen the joints.

David Dundas

[Gary Keedwell]

That's me, David, an obese sitter wide a wide margin.

Ahhh...Those Aussies know how to party!!!
Gary

[Jim Becker]

David, very interesting test case! Thanks for posting it.

[Mike Duffy]

Mike,

You can find a copy of my article on a zigzag dining chair here . I suggest, however, that you omit the brass pins, which I now believe weaken rather than strengthen the joints.

David Dundas

Thank you.
I'm going to put it on my "to do" list.
Mike

[Danny Thompson]

The most recent Woodcraft magazine contained detailed plans.

Unfortunately I cannot tell you the exact issue # because, as I just learned, it was taken to the recycling center (!) over the weekend by the future-former LOML!

[David Dundas]

There is a You-tube video of the destructive testing of miter joints here: http://www.youtube.com/user/bitingmidge . The delay in posting it is due to the fact that I have had open-heart surgery in the interim. I am happy to report that I have recovered well from it, and now look forward to a new lease of life.

There is an error on the video: the splines reinforcing the miter joints are actually 1/8" thick (i.e. 3.2 mm, not 6 mm, as stated).

David Dundas

[Michael Wildt]

Hi David,

Good to hear you are feeling better.

I read the thread with great interest. I noticed that the dining chair joint design is different from the chair that you tested. What would be your comments on the domino joint versus the splines ?

Thanks,
Michael
(Wood novice)

[David Dundas]

Michael,

As you have noted, my Z dining chair's miter joints are made with three reinforcing locking tenons, rather than splines. An individual miter joint made in Tasmanian blackwood with reinforcing tenons (rather than a frame) was tested; it failed under a load of about 230 lbs, so a chair made with that wood could be expected to fail under a load of about 460 pounds. My Z chairs are made in jarrah, which is stronger than blackwood, and are able to support the weight of three men weighing a total of about 500 lbs without damage.

The frame shown in the video was reinforced with twin splines, which provide a larger glue area than the locking tenons. It failed under a load of 365 kg (803 lbs). I used twin splines to reinforce the miters that join the legs to the rockers of my zigzag rocking chair, so I am confident that that chair is easily strong enough to support the weight of any sitter who could fit between its arms.

David Dundas

[Chris Padilla]

Good stuff, David, and thanks for posting and glad to hear about your new lease on life! Are you pictured in any of the photos or are you like me when my family goes on vacation and I'm NEVER in any pictures beause I'm the one taking them! LOL....

[David Dundas]

Chris,

I am the one wearing white shorts, standing on the far end of the board. Unfortunately, my videographer cut off the torsos of the testers so as to ensure that the whole of the frame was visible.

David Dundas