Which joint is stronger?

[Flyover]

I have a small project that involves joining 2x4s. When it's done, force will be applied downward from above (direction of the arrow), and I'm wondering which joint will be strongest in that situation: the dado (A) or the rabbet (B). The dado removes less material, but the rabbet maintains the full 3.5" thickness all the way across.


(My own hand drawing; not to scale, obviously. Note that in both cases the lower 2x4, the one going across, is meant to be oriented vertically.)

 

[Jeff Handy]

I vote B, much more gluing surface.

 

[Steve123]

Without dimensions, your question is impossible to answer with any certainty.

If you are concerned with strength only in the direction of the arrow, then the dado can be a quarter inch deep, and the A is far stronger than B.

Jeff is correct that B has more gluing surface and gives more strength in direction perpendicular to arrow, but as I read your question, you are concerned with loading in direction of arrow only.

 

[Jeff Handy]

Meanwhile, although the intended load will be straight down, in the real world there will probably be other forces that might easily bust version A apart.

 

[Flyover]

For the most part force will be straight down but yes, of course in the real world there will be some forces in other directions as well. The down force is the main thing I'm concerned about mitigating though.

I do plan to glue the joint, so non-endgrain gluing surface is something to take into account too -- otherwise a quarter-inch deep dado would be the obvious choice.

The wood is all 2x4s (so, 3.5" x 1.5" cross section). I left out dimensions of the cuts because I can adjust those depending on what would be (optimally) strongest based on y'all's recommendations.

 

[Flyover]

Actually let me adjust what I said above: in terms of which forces I'm concerned about mitigating, the downward force from above (Y-axis) is probably 90%, the force along the horizontal plank (X-axis) is 2%, and the force perpendicular to the joint (Z-axis) is 8%.

 

[nealtw]

 

[Snoonyb]

I vote for "B", because the, "force will be applied downward from above (direction of the arrow)," arrow should be rotated 90 degrees to afford a true representation of the loading, represented by "force".

 

[bud16415]

I would go with “C” a very shallow 3.5 wide rabbet cut into the bottom and the top piece left flat on the end. Then drill and screw from the bottom into the end grain of the upper piece and install some long deck screws. Actually seeing as I don’t know what it is for I would skip the rabbet and just join the pieces with the screws. Not removing any material will weaken the bottom piece the least, plus any sharp corners will cause a stress riser in that area.

 

[Flyover]

@Gary: are you saying I should use a floating tenon? I have some dowels floating around...

@Snoonyb: I'm not sure what you mean by "rotated 90 degrees to afford a true representation of the loading".

@bud16415: I like the shallow dado idea (I assume that's what you meant). I'd like to try not to use screws, especially not screw into endgrain.

This is for a stepstool. I'll upload a sketch of what I have in mind later today, and point out the joint in question.

 

[Snoonyb]

"@Snoonyb: I'm not sure what you mean by "rotated 90 degrees to afford a true representation of the loading".

From your OP;"When it's done, force will be applied downward from above (direction of the arrow),"
"Note that in both cases the lower 2x4, the one going across, is meant to be oriented vertically.)"

Since the, as drawn, horizontal member is actually, in application, a vertical member, that would make the member with the arrow in place, actually a horizontal member, so, force applied downward could only be depicted by rotating the direction of the arrow, 90 degrees.

 

[Flyover]

@Snoonyb: Oh I see what you mean. No, I meant vertically as in, the 3.5" side of the 2x4's cross-section will be oriented up and down. The grain is still going left and right. I thought this is what is meant when they say a board is oriented vertically.

 

[Steve123]

If these are 2x4's and the dado is 1/4" deep, then the horizontal member in A is 60% stronger in bending, than in B. That would be just the strength of the horizontal member.

If they are glued well, the vertical member probably adds some stiffness to the joint. I commonly use PL Premium when I need maximum strength. I find the glue joint is commonly stronger than the parent material, with PL Premium. So you could say that B is effectively one piece. The joint itself is expected to be stronger in B than A, due to the greater gluing surface and more readily clamped tight over entire surface. So in the end, I would go with B.

 

[Flyover]

I think I'm leaning towards B as well. It occurs to me I could make the rabbet more like 60/40 rather than 50/50 too if I want. Would that add strength?

 

[Michael Armstrong]

I love floating tenons.

 

[mabloodhound]

If you are not going to use any mechanical fasteners (screws) then definitely 'B' is the choice.

 

[Flyover]

Well, my router decided it won't hold a bit properly; it skids and stalls and kicks and then lets fly chunks of wood I had no intention of removing. Cost me hours of work today. So, I will not be doing B. I think the floating tenons are looking good...

 

[Jeff Handy]

Buy a new cheap router at Harbor Freight, and use plan B.

A router that kicks and stalls and throws wood is not a router, it is a piece of crap trying to put you in the hospital.

 

[Flyover]

It might have been the fault of my quick change collet, come to think of it. I've been having problems with it for a while. I'm going to uninstall it and try attaching bits the old fashioned way, see if that solves the problem.

 

[Flyover]

For whatever it's worth, I went ahead and made the 1/4" dadoes. So far it seems to have worked fine. In the process of gluing everything together, will post pics when it's done.