12mm beech sheer strength

[archetypeLLC]

I need to build a fixed ladder for a loft. I'm only about 9mo's into my XL and wanted to see if two tenons per side of rung would be adequate for a 24" wide ladder out of 5/4 w/oak. Can I get away with just the sheer strength of 12mm tenons w/o dado's in the rails? Total of 4 per rung.

Any actual data on 12mm sheer?

Thx!!!

 

[epicxt]

I'm building two ladders like that for different jobs right now and I don't think I'd trust just the domino tenons. I'm dadoing in the treads. Others may have more knowledgable input but my view is that I can't  predict the weight of the heaviest person who will use them so I'm assuming the worst.

 

[jmbfestool]

I made two ladders for two kids bedrooms to get up to the 2nd floor in the house.  About 3 meters long

I made them out off birch ply and dominos.

It's been over year.  Adults used them.  Trades man been up to work.

So far no signs if failure.  I'll see if I can a picture

 

[mattfc]

Beech has a shear strength of ~14N/mm2

so for a 12mm domino, its cross section is approx. 12x26mm=312mm2
Shear=14x312=4368N

To convert Newtons to KG divide by 10 approx., so 1 domino should shear at ~400KG. This is in perfect conditions, runner of tread may split, but also load should be distributed over 2 dominos, one at each end of the tread, so in theory 800kg UDL, put a 75% safety discount and you still have a load of 200KG per tread

Personally I think the domino would rip out the tread before shearing itself for the domino to become the weakest point you would want the domino to only be 1/3rd of the tread height, meaning you would need treads 36mm thick!

 

[jmbfestool]

Found some pics!

Thats how we did it.

[attachimg=1]

[attachimg=2]

[attachimg=3]

 

[wow]

Trying to figure out what the third Domino slot is for - is there a bracing piece on edge under each step? It isn't shown in any of the photos that I could see...

 

[jmbfestool]

I'll upload another photo I have

 

[kmdwoodwork]

I made a garage loft ladder about 3 years ago using 2x construction lumber and 10mm dominos, i can't remember but i think i used 6 per step and it is still working fine and it also held up to the fat man jump test when i first built it.

 

[abgoto]

Beech has a shear strength of ~14N/mm2

so for a 12mm domino, its cross section is approx. 12x26mm=312mm2
Shear=14x312=4368N

To convert Newtons to KG divide by 10 approx., so 1 domino should shear at ~400KG. This is in perfect conditions, runner of tread may split, but also load should be distributed over 2 dominos, one at each end of the tread, so in theory 800kg UDL, put a 75% safety discount and you still have a load of 200KG per tread

Personally I think the domino would rip out the tread before shearing itself for the domino to become the weakest point you would want the domino to only be 1/3rd of the tread height, meaning you would need treads 36mm thick!

That's great information, thanks for posting it.

Extrapolating from here, 5/4 stock is 25mm thick, so if you follow the guideline of the mortise being no wider than 1/3 the thickness, you would be using 8mm dominos.

Cross section of the domino is 8x26 = 208mm2
Sheer strength is 14 x 208 = 2912N or approximately 296kg per domino

You could put three dominos per side (6 dominos per tread) for added strength.

Based on some experiments performed by Paul Marcel, I recommend putting glue on both the domino and in each mortise.

 

[jmbfestool]

Trying to figure out what the third Domino slot is for - is there a bracing piece on edge under each step? It isn't shown in any of the photos that I could see...

Found the other picture.  I need to get my photos more organised lol

[attachimg=1]

 

[archetypeLLC]

Thanks for all of the great info!

If 1/3 mortise thickness to stock is gospel, it looks like I'm hosed either way unless I used 6/4, other than adding a cleat.  Even if I added 3 per side, do you guys think it would actually blow out a white oak tread? 

 

[wow]

jmb

Thanks for the extra picture. That helps a lot to understand it and it looks very strong!

l

Thanks for all of the great info!

If 1/3 mortise thickness to stock is gospel, it looks like I'm hosed either way unless I used 6/4, other than adding a cleat.  Even if I added 3 per side, do you guys think it would actually blow out a white oak tread?

Why not make up a quick one or two step section and try it? You should be able to figure out pretty quick if it would work or not

 

[mouppe]

I don't think the tenon would be the weakest part of the assembled ladder. I think the risk of the oak tread splitting along the grain would be far higher than the domino shearing, particularly if you use a 10mm domino.

To mitigate this possibility I would offset the tenon in the tread slightly to have more meat above the tenon than below.

I did this with long bookshelves once and I felt more relaxed about the result.

The narrower you make the tread the stronger it will be too.

I don't want to contradict other posters, but my feeling is that if you use too many dominoes you will weaken the tread not strengthen it. I would use two max per tread end to prevent any pivoting of the tread.

5/4 white oak should be plenty strong enough for this ladder, and I would probably use 2 x 10mm dominoes.

 

[jmbfestool]

I don't think the tenon would be the weakest part of the assembled ladder. I think the risk of the oak tread splitting along the grain would be far higher than the domino shearing, particularly if you use a 10mm domino.

To mitigate this possibility I would offset the tenon in the tread slightly to have more meat above the tenon than below.

I did this with long bookshelves once and I felt more relaxed about the result.

The narrower you make the tread the stronger it will be too.

I don't want to contradict other posters, but my feeling is that if you use too many dominoes you will weaken the tread not strengthen it. I would use two max per tread end to prevent any pivoting of the tread.

5/4 white oak should be plenty strong enough for this ladder, and I would probably use 2 x 10mm dominoes.

I agree,

I do the same with shelves I lower the dominos to allow more meat above the domino. 

Also agree to many dominos doesn't mean it will be stronger its about getting the happy medium.

Saying that proper glued domino should bond with the piece all round and become as one with the workpiece you dominod into.

 

[archetypeLLC]

Why not make up a quick one or two step section and try it? You should be able to figure out pretty quick if it would work or not
[/quote]

That's a good idea

I don't think the tenon would be the weakest part of the assembled ladder. I think the risk of the oak tread splitting along the grain would be far higher than the domino shearing, particularly if you use a 10mm domino.

To mitigate this possibility I would offset the tenon in the tread slightly to have more meat above the tenon than below.

I did this with long bookshelves once and I felt more relaxed about the result.

The narrower you make the tread the stronger it will be too.

I don't want to contradict other posters, but my feeling is that if you use too many dominoes you will weaken the tread not strengthen it. I would use two max per tread end to prevent any pivoting of the tread.

5/4 white oak should be plenty strong enough for this ladder, and I would probably use 2 x 10mm dominoes.

Totally agree and that was going to be my next question..... just give the top side a bit more meat.... I think I'll do a quick mock up before I commit.

I've only posted just a few times, but everyone in the FOG world has been more than helpful. Great community here.

Thanks again!

 

[Birdhunter]

Neat project. My gut feel as a retired engineer is to use 2 tenons with most of the step stock above the mortise. If I was worried about sheer failure, I'd add a triangular support below each step at each side. But, I tend to over build things. I'm not into delicate.

 

[monstrol]

That looks stonger than heck! The brace underneath looks plenty beefy.  I would bet the oak will break first if you really go after it.