Need help on wooden mitered bench strength using Dominos and splines

[DarrenB]

I'm currently building a wooden sitting bench as a xmas gift for my wife.
I think I will get it done in time but I need some help in Domino configuration for the mitered joint between the sitting part and the sides, or to see whether I need a more robust joint than just Dominos.

I'm building it out of an old ash kitchen table. The dimensions will be 110cm wide (43"), 40cm deep (15.5") and 45cm high (17.5"). The material is 4cm thick (1.6").
It will look like the bench in the picture below (found it on Google nwwoodgallery.com)
Two full grown persons must be able to sit on it without it breaking apart from the skewing force on the miter joint, and because its a xmas present I would like it to hold forever!

My questions:

1. Will the Dominos be enough to reinforce the miter joint and keep the bench from collapsing?

2. Should I go many smaller Dominos (6, 8mm) or fewer large ones (10mm)?

3. How many Dominos would be needed to support the joint?

4. Will the splines help in strengthening the miter joint together with the Dominos or will I run into the risk of weakening the whole joint if I have too many cuts?

5. I was contemplating a solution in which I use the spline cuts to drill a horizontal hole into the sitting part of the bench from within the spline. I could push in steel rods and then hide the holes with the spline. Would that cause problems when the wood is moving? Should I push in long dowels instead?

6. Any other ideas to reinforce the miter joint?

Thanks!

The source:
http://www.nwwoodgallery.com/sites/default/files/styles/detail_page_main/public/furniture/_DSC0041_0.jpg?itok=5PDE75Nv

 

[Holmz]

The finger joints give a lot of bond area to spread the load.
They seem like the best choice.

 

[Owego]

The finger joints give a lot of bond area to spread the load.
They seem like the best choice.

I agree with Holmz.  The potential racking forces on those miter joints are pretty high.  The finger joints  span the whole miter profile, tenons would be limited to the center of the miter profile.  I'm not a structural engineer, but I think points of highest tension are at the apex of the miters - that's where the finger joint has the advantage.

You could give the domino approach a try (I'd use the biggest that fit the 1/3 rule) and space them twice the width of a domino apart.  Then put a diagonal brace (maybe two) underneath each joint.

 

[Birdhunter]

I'm very conservative in my approach to building things that people sit on. So, I would sneak a couple of triangular braces under each end. I'd also go with finger joints. I don't think Dominos are a good approach.

 

[Holmz]

The finger joints give a lot of bond area to spread the load.
They seem like the best choice.

I agree with Holmz.  The potential racking forces on those miter joints are pretty high.  The finger joints  span the whole miter profile, tenons would be limited to the center of the miter profile.  I'm not a structural engineer, but I think points of highest tension are at the apex of the miters - that's where the finger joint has the advantage.

You could give the domino approach a try (I'd use the biggest that fit the 1/3 rule) and space them twice the width of a domino apart.  Then put a diagonal brace (maybe two) underneath each joint.

I believe that one would be better off with 2 spaced as far apart as possible, so maybe 4 or 5-mm ones 2-3 mm in from each edge.
The you have ~37-mm between them.
One can compute the compression and tension forces when it is racked.
Basically there is a huge amount of leverage, and as the spacing goes to zero the leverage goes to infinity.

This is possible to do with Dominos, it just it is a tortuous way to do it...  IMO.

You could do a test piece and get a buttock landing on it at an angle to test it.
I worked with a fellow once that was morbidly obese. He used to hover towards the toilet seat and the leg strength failed about 8" up from the sound of it.
He ended up shearing 3/8 bolts off the wall eventually.
If it is designed for him it would suit even Jabba the Hutt.

 

[Owego]

Holmz,  I assume you mean to put pairs of 4 or 5-mm dominos across the short face of the miter.  I don't think this will work as the outer domino will have only a tiny amount of stock for a mortise.  Or maybe I don't understand (wouldn't be the first time).
Tom

 

[DarrenB]

So when you say finger joints to do you mean box joints or the actual splines? Sorry for the confusion but english is not my mother tongue  [embarassed]...
...and the correct term may be keys in the case of the bench?

What do you think about the 5th point in my post? Is that an alternative?
I was thinking that I could route/table saw the spline channels but before gluing the splines/keys into the slots I would pre-drill holes in the channels and use some long and thick wooden screews. This way I would have the structural benefits of steel. Drilling the holes horizontally into the sitting part would also reduce the skewing forces and let the longer slab "rest" downwards onto the sides.
The added benefit would be that the screws would also pull everything together in the glue up reducing the need for complicated clamping.
The keys/splines would then be cut to compensate for the screw head and cover the screws all together.

The question then becomes will all the re-inforcements actually weaken the overall strength of the joint? I.e. if I use Dominos, screws AND splines/keys.

 

[Holmz]

Holmz,  I assume you mean to put pairs of 4 or 5-mm dominos across the short face of the miter.  I don't think this will work as the outer domino will have only a tiny amount of stock for a mortise.  Or maybe I don't understand (wouldn't be the first time).
Tom

I think it is what I mean..
It seems like a bastardisation of a way to use a Domino.

In the olden days the old Craft'sman table saw had a piece of Maple I put on about 4-5" high and 1-1/2 thick.
You cannot do finger joints with a track saw, and a domino would become a career.
One could cut all the cheeks with a pin on the fence. and the then move the pin and cut the other side's cheeks.

 

[junk731]

Darren I have made  a lot pieces in that style of bench using an 80 degree ( 2 40 degree miters) miter joint and for that width 4 10mm dominos. No failures or complaints from customers. Now to qualify this method I would not use it with softwood like pine or wood much thinner then what your using. Too many people get caught up in overkill joints when simplicity works best.

John

 

[DarrenB]

Darren I have made  a lot pieces in that style of bench using an 80 degree ( 2 40 degree miters) miter joint and for that width 4 10mm dominos. No failures or complaints from customers. Now to qualify this method I would not use it with softwood like pine or wood much thinner then what your using. Too many people get caught up in overkill joints when simplicity works best.

John

So John, just to confirm what you're saying: You mean that for my project 4 10mm Dominos per side will be sufficient?

I found this video that would support that idea in that case. He seems to be building a pretty solid bench with no other joint reinforcements than Dominos.

 

[tiralie]

I would defer to John's advice, but I do like the look of the splines and would use them. Dominos are great for hidden joinery like this.
Since the design does not dictate hidden joinery, the hardwood splines as shown in the picture will work. Cutting them in may be a bit tricky though.
When I saw this posting, my first concern was deflection of the seat. So I went to Sagulator and entered your data. With  100 lbs per foot load on 1.6 inch thick (white) Ash, 15.5" wide, the deflection of the bench seat is .01" or 0.003 in per foot which is minimal.
Tim

 

[Owego]

Darren I have made  a lot pieces in that style of bench using an 80 degree ( 2 40 degree miters) miter joint and for that width 4 10mm dominos. No failures or complaints from customers. Now to qualify this method I would not use it with softwood like pine or wood much thinner then what your using. Too many people get caught up in overkill joints when simplicity works best.

John

John, There's nothing better than actual experience.  Thanks John.  What is the thickness of the stock for your benches  ?

Tom

 

[junk731]

The thinnest that I have made is 1 1/4" and the wood was Shedua. I prefer stock between 1 1/2" and 1 3/4", usually what you get from 2x material after planing. I also use a slow cure epoxy for glueup as opposed to any Titebond type product. Gives the the needed time to make sure joints are tight and clamped properly for maximum strength.  As Tim stated he likes the look of splines, my preference is always to the uncluttered clean waterfall look of a continuous slab. I absolutely detest butterflies that are seen.

John

 

[tiralie]

My preference is always to the uncluttered clean waterfall look of a continuous slab.

I do too actually, but the splines in that photo of the bench the OP posted seemed to work design wise.

I absolutely detest butterflies that are seen.

Yea don't get me started. I think it's over used and frankly some of slabs used in Nakasima inspired "tables" are so bad (checked, sapwood, crotch etc.) that it completely distracts you from enjoying the piece of furniture. It's fashionable, there's not a ton of labor in it and it sells...
Tim