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A Tale of Two Tenons (or "Which One Will Be Stronger?")

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Forum topic by garbonsai posted 01-10-2017 06:47 PM 909 views 0 times favorited 12 replies Add to Favorites Watch
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garbonsai

154 posts in 1532 days


01-10-2017 06:47 PM

Topic tags/keywords: question ash joining

I’m currently in the planning stages of building a fairly sizable work/storage table that needs to hold a lot of weight — 1,250 to 1,750 lbs., give or take, distributed mostly evenly over the 96” long by 24” wide counter-height top and bottom shelf. The legs — all 8 of them — will be 3” x 3.5” solid Ash. The skirts will be 3” x 1.5” solid Ash, and there will be extra supports of the same dimensions running width-wise between each set of legs.

I plan on using mortise and tenons to join the skits and supports to the legs. I’ve spent a good many hours flipping through the various recommendations for sizing mortise and tenon, and between those and the need for strength and the materials I have on hand, I think I have it figured out.

My question is, which is going to be stronger: a double tenon, each of which is 2-1/8” wide, 1-3/4” deep, and 1/2” thick (which leaves a 1/8” cheek on the bottom and sides, a 1/4” space between the tenons, and a 3/4” haunch on the top), or a single tenon which is 2-1/8” wide, 1-3/4” deep, and 3/4” thick (which leaves a 3/8” cheek on the bottom and sides, and a 3/4” haunch on the top)? I plan on drawboring them either way…

The attached image is just to show the two tenons — the skirts and legs are only 12” long because I’m focusing on the joinery.

-- Do not taunt Happy Fun Ball.


12 replies so far

View Dan Hulbert's profile

Dan Hulbert

107 posts in 196 days


#1 posted 01-10-2017 07:04 PM

Have you considered just building it the way shown at the left end of your drawing? Two boards sitting on two boards, held in place by two more boards. The proper name for the joint escapes me, but I can’t imagine any cut tenon would be stronger for the vertical load you expect.

-- Dan

View garbonsai's profile

garbonsai

154 posts in 1532 days


#2 posted 01-10-2017 07:10 PM

@Dan: Bridle joint? My concern there is racking forces, as the strength of the joint would then be almost entirely dependent (at least at the top — the bottom rail would be captured) on the glue and pins holding it together. Maybe I’m wrong? This is a new undertaking for me… :)

-- Do not taunt Happy Fun Ball.

View eflanders's profile

eflanders

136 posts in 1427 days


#3 posted 01-10-2017 07:18 PM

Racking is an issue to consider and thus I would employ as much glue surface as possible. However, using a diagonal brace in addition to your chosen joint method would be the option I would employ for the loads you are talking about.

View Clarkie's profile

Clarkie

405 posts in 1418 days


#4 posted 01-10-2017 07:21 PM

The bridle joint will hold it’s own and is the proper joint for that application. I’ve used it on a few work benches over my time and have had very good results.

View rwe2156's profile

rwe2156

2401 posts in 1058 days


#5 posted 01-10-2017 07:41 PM

That’s a LOT of weight and IMO if you don’t either beef up the apron, use support brackets, or put additional leg in the middle it will sag.

I don’t think there is any weight bearing advantage to a double tenon. I think its mostly structural.

Stretchers are necessary to add stability and prevent racking.

-- Everything is a prototype thats why its one of a kind!!

View garbonsai's profile

garbonsai

154 posts in 1532 days


#6 posted 01-10-2017 07:47 PM

@Dan, @eflanders, @Clarkie: Should I give the bridle joint any sort of a cheek to account for seasonable movement of the rail/apron? Like so…?

@rwe2156: As in, an additional leg (for a total of 5 on each side)? I wish I had my SketchUp file for the table itself with me right now… I currently have 4 legs on each side — one on each end, and the other two distributed evenly between them. I’m starting to think I should learn to weld and go that route for this particular project… :)

-- Do not taunt Happy Fun Ball.

View garbonsai's profile

garbonsai

154 posts in 1532 days


#7 posted 01-10-2017 10:31 PM

Okay, here’s the current design for the bench, shown from the bottom to show the support structure. Thoughts?

-- Do not taunt Happy Fun Ball.

View bbasiaga's profile

bbasiaga

867 posts in 1572 days


#8 posted 01-10-2017 11:56 PM

I think the thickness of your top and shelves is key here. You want it to distribute the load to the legs as evenly as possible, to take the pressure off the aprons and whatever joint you decide to use.

If you did bridles glued and pinned them, that joint won’t come apart from the load. To prevent racking, if you can attach sheets of plywood across the back and the sides (shortening your overhanging shelf, that will help a ton too. Otherwise you could build in cross braces on both sides and the back.

Another thought, you could make full width tenons by laminating the boards together. Meaning you’d take the thicnkess of the apron as shown, cut to the length proper for the bridle joint, then glue two thinner pieces (one on either side) to create the tenon shoulder. You’d get the advantage of the full size tenon/bridle and the extra strength and racking resistance of the shoulder, but at the expense of more weight and cost. It may help to have the extra beef there for the loads you are talking about anyway.

Just some thoughts. Hope they help.

Brian

-- Part of engineering is to know when to put your calculator down and pick up your tools.

View ColonelTravis's profile

ColonelTravis

1364 posts in 1471 days


#9 posted 01-11-2017 02:57 AM

Upper limit for this table is 1,750 lbs., give or take? That is a lot of weight even with eight legs. You’d be putting the equivalent of a 1985 Honda Civic on that thing. (Seriously.)

I’m not an engineer and can offer no good ideas for this, sorry. I hope you can find a solution. Just curious what you plan to use it for?

View clin's profile

clin

631 posts in 573 days


#10 posted 01-11-2017 05:55 AM

Concerning the original question, I think the single tenon will be stronger, especially considering your plan to use drawbores. For racking loads, it’s the shoulders that support this with the glue holding the shoulder tight. The shoulder is in compression and the sides of the tenon and its associated glue joint is in shear. The glue is strongest in shear. So having a lot of tenon glue with little shoulder doesn’t balance well. Having just 1/8” of should is a lot of pressure on a small area.

The advantage of two tenons is more glue area, but with drawbores, you don’t even need any glue. It’s the pegs that hold things together.

Having said that. I would NOT rely on the M&T joint to prevent racking in this case. While it’s going to be quite strong, you are talking about some significant weight. And I’d error on too strong. I would reinforce all corners with gussets or diagonals. And where possible, perhaps full panels for sides, back, and internal panels (between fornt and back legs).

As far as the weight on the legs. You have more than enough support. While 1,750 lbs sounds like a lot, a 3” x 3.5” leg has 10.5 sq-in of cross section. Even if ALL 1,750 lbs were on one leg, that’s just 1,750 lbs/10.5 sq-in = 167 PSI. Based on one online source, ash has a compression strength with the grain of 2,300 – 7,400 PSI. So you’re no where near close to crushing the legs.

You’d have to run a buckling calculation, but at 3”x3.5” and assuming about 36” tall, I don’t think there is any concern for a leg buckling, even with all the weight on one leg. Assuming even distribution, you’re looking at about 220 lbs per leg. Which is just nothing for legs this size.

I agree with others that controlling sag between the legs is something to give some attention to. But if your 1,750 lbs is spread evenly in the 6 areas between legs (3 on the top and 3 on the bottom), that’s about 300 lbs per section. Quite a bit, but not crazy high. And with nearly the equivalent of a 2×4 frame, it won’t sag a lot. But, I’d consider adding support through the middle of each shelf section. Like another skirt piece running the length through middle.

Of course if the top and shelf are especial thick, they may not sag much anyway. But if they are just something like 3/4” plywood, I’d put a more framing under them to support them.

Assuming plywood or similar, where wood movement isn’t an issue, be sure to tightly secure the top and shelf to the frame. This will help keep these from sagging.

-- Clin

View Rick M's profile

Rick M

8666 posts in 1957 days


#11 posted 01-11-2017 06:25 AM

Clin is on it. The strength of the table is in the legs and a thick top will be necessary to help distribute weight. The aprons are there to prevent racking and help distribute load but they are not holding the weight. It’s the shoulders of the tenons that prevent racking and height will matter. IMO, 3” is too short, you need to be up around 5-6” tall aprons and the tenon should be no more than 1/2 the thickness because you need those shoulders. I don’t think you will have any trouble supporting the weight as long as it doesn’t squirm around.

-- http://thewoodknack.blogspot.com/

View a1Jim's profile

a1Jim

115619 posts in 3154 days


#12 posted 01-11-2017 12:44 PM

Depending on what you want the table to look like why not just use 2×6 construction grade lumber and joist hangers?

-- http://artisticwoodstudio.com Custom furniture

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