# How strong is this joint?



## sneggysteve (25 Nov 2019)

My wife likes the look of this range of furniture and wants me to make a couple of things. However I am worried about the strength of the z joints. She doesn't want this desk but it it the best item to show the joint - the full range of furniture is at - 

https://www.furnitureplusonline.co.uk/z-oak-small-desk

Any ideas on the actual joints - I can't see any dowels, bolts etc. Are they just mortice/tenons or dominos?

Thanks


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## MikeG. (25 Nov 2019)

I'm the ultimate proper-joints advocate, but there is no way I'd make something like that just using wood. There is no joint, nor joining system, which can make that strong enough. I would definitely be inserting some steel in there, probably cut out of plate and inserted inside the structure by building this as laminations.

Personally, this offends my aesthetics. I dislike anything not looking like it is strong enough to do the job it is designed to do.


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## AJB Temple (25 Nov 2019)

I have seen this kind of thing done as a sliding dovetail (could be a stopped dovetail) which will resist the joint pulling apart from downward pressure. 

However, I would not risk it without hard wood and substantial frame dimensions, and even then I would not want to put a lot of weight on it. 

Absent that, I agree with Mike.


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## novocaine (25 Nov 2019)

Assuming there is a mortise and tenon in there, then it's an OK way of jointing but still a rubbish way of balancing the forces applied. you can see the rear top joint is separating on the picture. 
Basically It's a bell crank which will increase the torque applied to the joints, you have a small amount of force directly down (from the weight at the rear of the desk) and a large amount of torque (from the weight at the front of the desk, where you rest your arms). stupid way of doing it that means the frames need to be massive to take the load. 

I'd love to do the calculations but I haven't got the time today.

Edit: what Mike said.


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## Rich C (25 Nov 2019)

I agree with Mike, I'd be wanting a welded steel frame in the z shape and then effectively clad it in wood to get the strength there.


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## thetyreman (25 Nov 2019)

an X shape on the sides would be better, stronger but how it currently is might look good but it's going to be weak when you start pushing down on the top and the front is pretty much unsupported, also the balance of weight will be strange and it's likely to want to tip forwards every time you lean on it.


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## novocaine (25 Nov 2019)

this is about the only way I'd be happy with an all wood "Z frame". 





purely because of the width of the "leg" shifts the forces inboard considerably making it more like a 2 leg table.


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## Phil Pascoe (25 Nov 2019)

Rich C":m62u0ifs said:


> I agree with Mike, I'd be wanting a welded steel frame in the z shape and then effectively clad it in wood to get the strength there.


If you look at the oak example given it's made from engineered timber - ideal for hiding the steelwork.


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## Rich C (25 Nov 2019)

phil.p":204uvre8 said:


> Rich C":204uvre8 said:
> 
> 
> > I agree with Mike, I'd be wanting a welded steel frame in the z shape and then effectively clad it in wood to get the strength there.
> ...


I did notice that. :wink:


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## Inspector (25 Nov 2019)

Search for Zigzag or Zig Zag chair for similar pieces that were made in 1934. An Aussie built one using the first Dominos when they came out there at least a decade ago. Your wife's desires could be met with Dominoes, finger joints and probably others. Not something that fits my taste but....


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## AJB Temple (25 Nov 2019)

Didn't realise the picture expanded up. It's a steel frame clad all round I suppose.


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## Chris152 (25 Nov 2019)

Maybe have a look at Rietveld's variation in his zig zag chair




'It uses only four elements which are joined together using a system of dovetailing, all joints are placed at optimum locations for load bearing since it has no legs.' 
https://the189.com/design/zig-zag-chair ... -rietveld/
Obviously, the trick in his design's to include the supporting blocks.

edit - sorry, just read your comment Inspector.


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## novocaine (25 Nov 2019)

Not sure whats traditional about wood over metal frame but apparantly its made with traditional techniques. I think it might not hide any such frame its just over priced laminate construction claiming to be solid oak. Think if i were interested on it id email asking if its soild or laminate over frame.


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## sneggysteve (25 Nov 2019)

Thanks everyone - you have confirmed my concerns.

I am waiting for a reply from the manufacturers via the store. Will up I get a response.


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## sneggysteve (25 Nov 2019)

Well here's the reply !!!!!

"Good Afternoon

Further to my previous email.
Please note the joints are Butt joint, glued and screwed.
No metal in the joint. They've been selling these for years. Common sense is the key. i.e. Don't stand on the end of the table.

I hope this helps."


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## novocaine (25 Nov 2019)

So back to Mikes orginal post. No tar.


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## Benchwayze (25 Nov 2019)

When I see designs like this, I am reminded why jointing methods from 300 years (or more) ago, are still used today; but then I am an old fuddy, who is stuck in a rut... Yes? 

John (hammer)


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## thetyreman (25 Nov 2019)

Benchwayze":2k59dhqv said:


> When I see designs like this, I am reminded why jointing methods from 300 years (or more) ago, are still used today; but then I am an old fuddy, who is stuck in a rut... Yes?
> 
> John (hammer)



+1 I agree, traditional joinery and designs work best, why re-invent the wheel?


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## Chris152 (25 Nov 2019)

Can't say i'm a fan of that diagonal, but sometimes it's good to sacrifice some strength for interesting form? There are stronger materials than wood, but we still work with wood presumably because we like it - doesn't necessarily make it best for the work an object has to do. And i definitely think playing with possibilities is a good thing - sometimes it throws up something good, even something better. Tho it usually doesn't.
My 2p.


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## Hornbeam (25 Nov 2019)

novocaine":2wrp4mg3 said:


> this is about the only way I'd be happy with an all wood "Z frame".
> 
> 
> 
> ...


 So at what stage does a joint change.
If you look at the z design with plywood ends then if instead it was made out of solid with a loose tongue bridle joint at the top and bottom it would be just as strong. I think that the answer depends on how wide the diagonal is, what type of wood is used and how heavily loaded the table will be. Unfortunately somebody will almost certainly sit on it. Is this joint any worse than a sliding dovetail on a pedestal table, not forgetting how strong some glues are (compared with early glues). I dont think metal would add much due to different expansins, difficulty bonding etc


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## Jonathan S (26 Nov 2019)

I would do some research looking at how the Scandinavians do there Z design.
We have 4 of these..... https://www.google.com/url?sa=t&source= ... 4749920709

Children and adults have used and abused ours for years and there still rock solid, I often use them as a hop up.

I guess there is something we can learn from the Scandinavians.....no I didn't mean we could learn from IKEA's woodwork! [SMILING FACE WITH OPEN MOUTH AND COLD SWEAT]

Sent from my SM-J530F using Tapatalk


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## MikeG. (26 Nov 2019)

Jonathan S":13rrlwr6 said:


> I would do some research looking at how the Scandinavians do there Z design.
> We have 4 of these..... https://www.google.com/url?sa=t&source= ... 709........



There is a huge difference between yours and the monstrosity in the OP. In particular, the angle is much steeper (ie closer to 90 degrees), and the load (ie the weight of the occupant) is at the mid-point of the base rather than right at the back. The latter point effectively halves the stress on the joint, and the former does too. In other words, there is approximately (by eye) a quarter the stress on that joint than the one in the opening post.


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