joinery strength tests

[Malcolm McLeod]

Bill,
Ian & Art have let slip the dogs of Mechanical Engineering, so I'll add my $.02.

Its relatively easy to do a force diagram - simply a stick representation of the piece with distances and applied forces. (If its a chair, who will sit in it? A grandchild, or your favorite NFL tackle?) Generally, for any design the torque loads are more critical, so calculate force X distance and sum them around a given joint.

Figure out what joinery method you want to use & the magazine load ratings will get you close.

[Art Mann]

More than one person has stated or implied that test-to-failure joint testing is not valuable because it doesn't allow for every possible joint loading geometry. I say that is ridiculous. Simple racking tests will be a very good predictor of joint reliability in most furniture building and other applications. If an individual has some other primary type of load - like pure shear or perhaps pure tension - then the racking test is only moderately useful for comparison purposes - not definitive but possibly better than nothing. I think it is also pretty silly for people to imply that that testing is typically biased by the investigator toward one method or another. If you compare several of these studies, they all come to pretty much the same basic conclusion. This includes the ones done by the tool manufacturers.

[Pat Barry]

I wish someone would do a test of 'how much pounds-of-force are required for an application'.

That is, for the average bench, or chair or table, how strong should the joint be? Then I could calculate how many dominos and dowels I need or whether a stronger joint is required.

These discussions tend to get into the apples-to-oranges flawed design of the test. But I'm much more interested in knowing what's good enough for what application.

I'm learning over a time, but it'd be neat to have a reference so I could be comfortable when building something new.

There are too many variables to consider to make this sort of a rule of thumb thing that anybody can use for any situation. Take a chair for instance - who is going to use it. Are they wigglers or rockers? And by the way, strength tests are just initial point testing. Has anyone ever seen a life test done? Unlikely. With your joints, if possible, make them as large a glue surface as possible. This is a case of more is better so if in doubt look for ways to increase the area.

[John TenEyck]

I'm pretty sure Finite Element Analysis (FEA) has been done by several folks for a chair, table, etc. of interest. Those studies would show what the applied forces are under simulated load conditions. Short of that, the comments above about simple bending and torque calculations would get you in the ballpark of the forces. Once you have the force you can design the size of the joint required to withstand that force based on the shear strength of the glue and surface area of the joint. The required cross sectional area of the frame components would be similarly calculated based on the properties of the wood to be used. This is what engineers do all day long, and I was proud to do my part for several decades - in another discipline. There's a science and methodology to it. That no one (of whom I'm aware) has published an article on the whole process is rather astounding, but I bet it's been done somewhere, perhaps a furniture manufacturer who, rightly so, has kept the info. proprietary.

Without such information people follow what's been shown to work well over time. Some push the envelop to make very light and graceful designs - and have failures. Others are super conservative and over build - with ugly proportions the result. The only way you really make progress is to do the engineering analysis and testing required to understand what's going on. We didn't get to the moon by intuition.

John

[ian maybury]

Hi Art. Guess i'm not saying that a simple test can't be broadly indicative of something - the resulting information if correctly interpreted and applied with caution may well prove very useful. It likely beats the hell out of having none at all. One issue though is that the choice of the best jointing method is generally a multi-dimensional and quite highly nuanced call, and simple tests seem unlikely to fill in all the blanks.

That said my caution relates primarily to the human factors.

As implied in the original i've seen mag tests (none mentioned here) the configuration of which somehow ended up favouring one jointing system over some of the others.

Then there's the more than reasonable likelihood that the maker of a given system will if handed a positive result trumpet a one dimensional version of it from the rooftops - no matter how qualified it was in reality.

Finally there's the likelihood that many punters will resist looking past this, and as a result walk themselves into difficulty...

[Jim Dwight]

I'm a mechanical engineer too. So when I look at tests like this I consider them data but I also realize some of the limitations. So when I see ~50 lbs difference in a floating tenon versus a traditional tenon when both seem to be the same size, I don't get excited. I don't think the tests are good enough to determine a 50 lb difference. If the joints are made properly and there is no difference in the material (which will never be the case with wood) there should be no difference. But there was a little suggesting either some variation in the wood or possibly incomplete glue application.

The butt joint being as "strong" as it is also seems like a test anomaly. We all know they aren't real strong joints. The cope and stick has more area so it should have been stronger. And a stub tenon is even worse? Maybe these joints were not made very well. Or they didn't get enough glue.

I don't think dowel joints are poor but I've seen them fail in older furniture (chairs) and I think it may be the fact that you don't get a lot of long grain to long grain gluing area. That may be a bit different with beadlock, I'm not sure. It looks a bit like a wavy loose tenon which should work better. The other issue dowel joints could have is with the glue failing (perhaps because of seasonal movement) and then loading being applied to individual dowels. But still, this type of joint also works well in a lot of applications. Tage Freid (sp?) used it a lot on his fancy stuff. It is not a bad joint.

I mentioned the size of the parts partially because I am sure it affected the domino result. 2.5 inches width in an inconvenient in-between size if you use loose tenons 1 inch wide. 2 leaves little room outside of them and 1 isn't really enough. Do a test with 1.5 inch wide parts and with conventional M&T joints that also have 1 inch wide tenons and I am confident that the domino would be right there with a conventional M&T joint. If you were making something with 2.5 inch width parts and needed a strong joint, however, the results suggest you should cut down some dominos or make a custom loose tenon rather than relying on a single domino. An experienced woodworker would come to the same conclusion looking at the joint, however.

I think this is useful data but it can't be extended to cover every situation and needs to be reviewed in light of other data and experience. Where other experience and logic say the data is suspect, additional tests are indicated (like the butt joint versus stub tenon results). But generally, I think the data is interesting and somewhat useful.

[John TenEyck]

I would get a little more excited if the tests had used 25 or 30 specimens per sample set. With 5 specimens the variance would have to be almost zero for the there to any real difference between many of those results. I once set out to do a bunch of joint testing because I still had access to our universal test machines at my former employer, but I couldn't get enough people interested in making the specimens to actually move forward. Now I no longer have that access, and frankly, I've lost interest in doing the work. I've settled on my own joinery approaches, usually loose tenons custom sized to the application. After 30 years with no failures I'm fairly confident they work well. I've pushed the envelop on several occasions, too, fully expecting some of them to fail. To my surprise, none yet have. As an example, I built some chairs with loose tenons where the front leg to rail joints are open at the top. If the glue were to let loose there is nothing to prevent the leg from falling off. They are now 20 years old and a couple of them get daily abuse. Glue is amazing stuff.

John

[Phil Thien]

I wish someone would do a test of 'how much pounds-of-force are required for an application'.

That is, for the average bench, or chair or table, how strong should the joint be? Then I could calculate how many dominos and dowels I need or whether a stronger joint is required.

These discussions tend to get into the apples-to-oranges flawed design of the test. But I'm much more interested in knowing what's good enough for what application.

I'm learning over a time, but it'd be neat to have a reference so I could be comfortable when building something new.

I think Prashun is getting to the crux of it.

There are few applications for simple right-angle joints that will be subjected to tortuous loads. And there are lots of acceptable solutions (a simple diagonal brace, for instance).

And I don't deny that a conventionally-made M&T or half-lap is going to be about the strongest right-angle joint.

But if you're making (for instance) a table where each side has two legs connected at the top and bottom by an apron and a stretcher (respectively), now the very design of the structure is going to spread forces across multiple joints. An assembly made of multiple comparatively weaker joints becomes quite strong in this way. It becomes difficult to apply racking forces in such a way that they a single joint is isolated and compromised.

Two 2x4's nailed together at the ends (in a right-angle) makes for a pretty weak joint. And yet many of them together form the houses in which we live (and they're quite strong). And that again is because loads are spread across many joints.

I have tool stands made of two-by material held together with biscuits and glue. I can't apply enough racking force in order to break one, not in the way in which the things were intended to be used.

[Mike Henderson]

I'll just comment that you can make Dominoes wider. Just cut one side of the dominoes and glue two together to make whatever width you want. It's easy enough to make the mortise any size you want by doing multiple plunges. I've done that for loose tenons on chair repairs.

Mike

[Chris Friesen]

The other thing that nobody has mentioned is that these were brand-new well-made joints. Things are going to behave differently a few years down the road after humidity fluctuations and such. A drawbored M&T starts out weaker than a non-pinned one, but will continue to hold tight even if the glueline cracks.

[Brian Holcombe]

More than one person has stated or implied that test-to-failure joint testing is not valuable because it doesn't allow for every possible joint loading geometry. I say that is ridiculous. Simple racking tests will be a very good predictor of joint reliability in most furniture building and other applications. If an individual has some other primary type of load - like pure shear or perhaps pure tension - then the racking test is only moderately useful for comparison purposes - not definitive but possibly better than nothing. I think it is also pretty silly for people to imply that that testing is typically biased by the investigator toward one method or another. If you compare several of these studies, they all come to pretty much the same basic conclusion. This includes the ones done by the tool manufacturers.

Art, I think they should be tested in practical application. It's my understanding that Danish chair manufacturers will test their chairs by rocking them until failure....the tilting backward in your chair type thing that your mom would yell at you about....they do that until the chair fails.

the takeaway from that test is not how long those joints will be in service, but instead that they can hold up a lot of weight. I don't intend on building a table that will hold 2000lbs and the practical side of me knows that if a table were put under such load that it would break the moment it rocked.

so being that I have reasonable stress requirements I'm back looking at joinery that will stay together for many years and frankly I'm not convinced that it is a domino or a dowel or whatever else floating tenon.

If you look at chairs from fifty years ago, where did they fail? The glue died and then the chair came apart.... I expect Most often if they broke it was actually because they became loose and weren't taken out of service. What stays together is often the joinery that is allowed to move in unison with all of it's parts.

[Michelle Rich]

Good
golly!
The reason I posted this info on the 3 methods was because when I posted my experiences with the
Beadlock so many asked how it compared to the other 2. I posted this one to answer their questions and not to decide which method , whether M&T or pocket holes were superior..just these 3 jigs/methods.

[Mike Henderson]

If you look at chairs from fifty years ago, where did they fail? The glue died and then the chair came apart.... I expect Most often if they broke it was actually because they became loose and weren't taken out of service. What stays together is often the joinery that is allowed to move in unison with all of it's parts.

My experience with repairing chairs is not that the glue fails - especially with chairs that used dowels. When you take the joint apart, what you see is wood attached to the dowel (with the glue). The failure is that the wood fails - it comes apart from the main part of the wood. There's just too much stress for the amount of glued surface area.

Especially with dowels, another tip off is that the hole is now too big for a replacement dowel. If you were to use dowels to repair the chair, you'd have to drill out the dowel holes and put in the next larger size dowel. A better repair is to use a loose tenon.

Many chairs continue in use after the joint fails, held together by the corner blocks. The chair "rocks" but people keep using them for quite a while before they finally decide to get the chairs fixed.

Mike

[Roger Pozzi]

Good
golly!
The reason I posted this info on the 3 methods was because when I posted my experiences with the
Beadlock so many asked how it compared to the other 2. I posted this one to answer their questions and not to decide which method , whether M&T or pocket holes were superior..just these 3 jigs/methods.

I find the info very interesting. And, I have been a Beadlock fan, and user since I first saw it demonstrated on the TV show "Cool Tools" when it was hosted by David Theil.
And, I must add, as I've noticed on most of the posts about tools or techniques, everyone will jump on board to justify their beliefs and try their best to dismiss the OP's original posting.
So, Michelle, I will say that I understood what you posted, and was glad to see the refernce to FWW magazine, and your thoughtfulness to share that information. Thank you!!!

[Pat Barry]

Keep in mind that as you build your chairs you just don't copy the joinery designs of the past because they won't hold up for long under today's load conditions.