One Odd trick improves members' stiffness

  • Yes I’d use it
  • No I’d not
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So apparently using a plate inside a mitre joint (Design C) improves its stiffness by 34% for 2% more weight. Anyone seen this used before? Would Jason want to do an experiment based on this? Would it still work with a hidden plate instead of a protruding plate? Would you use this now that you are aware of it?


this last one only has 47% displacement of A while only having 79% the mass

Well that’s interesting. I’d love to see this tested in the real world by @Fireball_Jason.

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I’m going to dub this a “mitred bulkhead” style joint rather than just “design c”.

I’m thinking that with a hidden plate sized slightly below where the weld is supposed to be and tacked on that it won’t actually take any longer as you’d be filling the same spot with the same weld just now it’s a three joint weld rather than two. As it is drawn it would take two welds instead of one per mitre so probably take a bit longer.

I’d love to see it tested against all featured configurations as well as tubing bent with and without cerrobend to see which is stronger. Also test against a butt corner joint since it is a simpler joint but probably weaker because of that.

I would use it someone wanted to pay for all the extra work. Would like to see how this compares to a step up in wall thickness or size.

I wonder how much the extra work would cost the customer? I’ve never had to purchase a custom fabrication so I’d not know. To me it seems like two extra steps to make it happen which is no more than adding an end cap. Cut bulkhead to size, tack in place. Proceed as normal after that. Sure you’d have to account for the thickness when cutting the mitred tubes in the first place but that is normal already.

This is how I’d size the bulkhead for a hidden joint:

literally just 40% more material than an end cap. Tack corners and proceed.

This is what I saw. Miter joint that needs a piece placed between the two parts.
I am working off shop drawings for this project. Now if I need maybe one or two of these I could just cut pieces of flat stock with a cut off tool in a grinder.
If I need more than 2 I have to draw in Cad and burn and maybe have to touch up with a grinder. Now I have to tack in place and weld, not a big deal but does take more time.

I have never ran across this before and doubt I ever will as the company’s I work for aren’t doing anything structural.

I wonder how often this type of joint is used by engineers. I know one I can reach out to and see what he says.

Remember everything can look pretty easy on paper!

I will report back! if I remember :rofl:

Right now in my life Memory is the tool I am lacking, and its out of stock where ever I look for it!

How could it be 40% more than a end cap? What you are showing is a end cap? or am I missing something?

I saw the 34% increase in stiffness but did not pay attention to the small 2% increase i weight.

This is really cool. What’s the length and size of the tubing in the simulation? What amount of force is applied in the simulation?

The main problem I see with this technique is not the extra work, it’s the public understanding of this method.

A customer could see two parts and decide that it doesn’t look strong or well built, because they don’t have never seen this type of simulation being run or used in the real world. A gusset is much more intuitive.

The results with B,D,F in relation to C are difficult to believe.

Looking forward to it!

Blackboard time:

The endcaps for these two tubes would be 1x2 roughly but since the mitred bulkhead reinforcing plate is diagonal it would end up 1x2.8 roughly. Ergo 40% more. Yes the original picture is just an endcap since I haven’t made a mitred tube endcap before. I intended to show how much smaller the plate would be compared to the wall thickness of the tube yet still register on the corners for tacking. It would essentially be a permanent backing strip for mitre joint so you can have a full penetration weld without worrying about crud falling into the tubes.

In the Video but also in Part 1:

25mm square tubing with 2mm wall thickness. It seems to be a single load case of two forces of 675lbf simultaneously in X and Z which results in a 955lbf vector. I’m sure the gusset would be better in a Y axis loading case but does almost nothing for torsion experienced from the X force. @Unknown has a bunch of interesting stuff on their blog.

Agreed! Probably why I’ve never seen it employed. It seems @Unknown came up with it themselves. I’d love to see some Shop Science testing all of these. As well as a normal butt joint since it kinda has a bulkhead itself. With and without endcap and whether tube Y or tube Z is the large one.

@too what is the link for “unknown” supposed to go to, I don’t see it?

okay, your explanation makes more sense and more believable to me now, the new diagram helped a lot and also the explanation of the force being in X and Z axis.

here is a side view for lengths of the tubes since Unknown didn’t list the dimensions in those directions.

Doh, blogger’s handle is Unknown and I no computer gud.


Ok this was the problem I did not pay attention to what you said. I just saw 40%.
you are correct a joint cut 45 degrees is approx. 40% longer than one cut 90.
This was all on me for not slowing down and reading what you wrote.

I will try to do better in the future, but if the future is any sign of the past, were in trouble! :rofl:


So I missing something, this one has no mitered joint?

Now on these here the L shapped piece looks like it mounts to a vertial surface like a wall correct?
and if it does, how much load is there on the joint that is mitered?

Now I am thinking like this, I mount two of these on the wall (reference A) pointing up just like in the picture. I use these to support 1000 lbs. of tube steel. Isn’t almost all the load, at the wall connection?

The picture of a blue L a few posts up shows how this study was conducted. Load was only applied to the little plate and fixed to the wall plate. No load was applied directly to the tubes like your case.

This one was a demonstration of what an ideal case would be if there was no obstruction as shown in the green picture a couple posts up. If you must work around an obstruction then you’ll have 30% more weight while being 40% flimsier using the Mitred Bulkhead or 113% flimsier without reinforcement.

Ok this has gone way over my head. I would need to spend alot more time to understand this.

Just for fun I showed this to a friend and here is what he had to say. And his conclusion is way past what I understand also.

Interesting, I could see where it would benefit the rigidity of the connection IF the members used were a thin walled tube. In the example they are using 14 ga (which I would consider thin walled) In that case adding the plate would brace the tube walls against local buckling. The shorter the connected members are the more benefit you would get globally.

In the model shown the members appear to be quite short. Maybe 2 feet? If it was longer… say 6 feet you would get very little benefit from the plate connection. Using the mitered joint also helps because it effectively increases the section modulus and moment of inertia of the member at the joint. Just looking at the output it’s hard to say much more as there are other things that could be at play in the models.

Is it worth the extra effort to get the increased rigidity? I would say not… but I’ll let you decide.

Looking at the output they are getting .58” of deflection in 24”(assumed). Personally I would say that is way to much and not something I would sign off on.

Typical allowable deflection for steel… depending on what it is supporting would range from L/360 to L/180 where L = Length in inches. Let’s say we use the low end tolerance of L/180…. If the frame were designed to L/180 the deflection in the frame would be 24”/180= .133” or 1/8” so say you add the plate and it now deflects 0.087” you did all that extra work and improved the deflection by 1/32”. You improve your deflection ratio from L/180 to L/276… so if the spec was L/240 maybe you do it… but you are doubling the welds spending time cutting plates and prepping for 1/32” deflection decrease.

Remember the model analysis was done using short members where you get your greatest benefit…. So the longer the members extend away from the connection the less you benefit.

@too I take it you are working as a engineer, or going to school to become one. I applaud you for that ! I Love the trades and everything that has to do with them.
I Also like the discussions about better ways to do things, we all learn from that.

This was interesting even though it was way over my head, but don’t stop posting neat stuff just because of that.

Thank you and carry on!