Table design questions (again)...

[Steve Wurster]

A while back I posted a question about a design for a kitchen table, and Gene Davis was more than helpful in coming up with some good ideas for me (thanks again Gene!). I've finally gotten time to work on this again and have some questions about the material sizing and joinery.

I've attached a Sketchup drawing of the base (based heavily on Gene's design), as well as a picture of the table that inspired the design. On my table the legs are going to get "roughed up" with rasps to make them look a little more "natural". The top will be live-edged so the leg treatment is going to complement that. The top will be about 72"x44" and about 1.5" thick. The space between the legs will be 40". The vertical leg pieces are angled 6 degrees left and right. The leg assemblies themselves will be fully vertical and will not angle away from each other at all.

The leg pieces are all 2" thick (i.e. along the long dimension of the table). The feet are 3.25" tall (by 38" long); the vertical leg pieces are 2.5" wide; the mid-leg stretchers, the main stretcher, and the cleats at the top are all 2" wide/tall. Everything will be solid material, so no glue-ups. The joinery will be all blind mortise and tenon (i.e. no thru-tenons); they will all be pinned, but I've never done drawbore. The mortises are all about 3/4" thick by 1.5" to 1.75" in width. The length / depth of the mortises will be about 1.25".

My concerns are with respect to the material sizing and joinery. Is this base design going to hold up to the needs of a dining room table? I don't think I'm concerned about the mortise and tenon joints all that much, although I would like to hear opinions on those as well. My main concern is whether that lone stretcher can handle the stress that will be placed upon it. I was wondering whether I should have a second stretcher along the top of the legs (I do have the material; no worries there). I figure that would help resist movement and would allow for more anchor points for the top.

Thoughts, concerns, and opinions are more than welcome.

Thanks,
Steve

[Prashun Patel]

It seems like a reasonable design.

If it were me, I'd drop the middle stretcher to between the feet, to avoid knee-knocking. I'd also add a center stretcher at the top between the 'cleats'.

I am sure there are calculators for this, but personally, I'd only go stretcherless on the top that long on a 2" top. That's not science, it's my instinct.

[Steve Wurster]

Prashun,

Good to hear from you, and thanks for the advice. I'm definitely going with that upper stretcher.

I don't think my wife likes the idea of the other stretcher being down between the feet. At about 44" wide there's at least 20" from the side of the table to the stretcher. I would hope that would lessen the chance of banging your knees. I'll bring up the idea with my wife and maybe do a quick mockup or take some measurements.

Thanks.

p.s. Saw your post about your desk being shown in New Hope. I'm going to have to make my way over there (only live about 15 minutes away). Congrats!
p.p.s. I still love my bandsaw!

[Jamie Buxton]

If you look at your proposed design from the side, what you see is a big open rectangle. It has very little resistance to wracking. If you put a brace from one corner to the opposite corner, the table gets much much stronger. One brace would look funny, but you can get the same effect by moving the horizontal stretcher up to top, and running braces from it at an angle down to the crossbars on your trestles. As an added benefit, you reduce the hazard of knee-knocking.

[Jamie Buxton]

Here's the concept. Much less wracking, and much less knee-knocking.

bracedtablelegs.jpg

[Steve Wurster]

Jamie,

Thanks for your input. I'm not sure my wife would like the design you came up with. My original design was more along the lines of a traditional trestle table and she wasn't too keen on that one. I'll pass this by her though and see what she thinks.

Ignoring the potential knee issues (which I'll look at when I get home), would my design with the addition of a top stretcher but no diagonal pieces be sufficient to avoid racking?

[Jamie Buxton]

.. would my design with the addition of a top stretcher but no diagonal pieces be sufficient to avoid racking?...

Think it through... Without the top stretcher, the table top itself connects the tops of the two trestles. With the top stretcher, you have both the table top and the top stretcher connecting the tops of the two trestles. Not much difference, is it? And in both versions, you still just have a big open rectangle, with little resistance to racking.

[Steve Wurster]

Think it through... Without the top stretcher, the table top itself connects the tops of the two trestles. With the top stretcher, you have both the table top and the top stretcher connecting the tops of the two trestles. Not much difference, is it? And in both versions, you still just have a big open rectangle, with little resistance to racking.

I was thinking of having both the top and middle stretchers. That is, the design would look like the attached. Isn't this pretty much a typical trestle table design?

My wife is definitely not keen on the diagonal pieces. She is possibly okay with the idea of lowering the middle stretcher down to the bottom. Doing that might result in a change to the design of the legs, so she's looking online for other tables to use as inspiration.

[Prashun Patel]

I suggested the top stretcher As sag insurance and to provide more attachment points for the top to the base. In this capacity doesnt it function more as an apron, not a rackerstopper, jamie?

[Malcolm McLeod]

...with little resistance to racking.

Personally, with nice tight 2" M&T joints, I think your initial sketch is plenty strong. Now quit reading, unless you like engineering data-vomit.

+++++++++++++++++++++++++++
Everything is relative ...and all design is a compromise. There is always some resistance to racking in most joints, but it is almost totally based on the strength of the joint itself.

A looonngg time since Structures class, but if I can mimic the prof, any frame has 2 failure modes: joint failure and beam failure. Joints concentrate stress, so are typically the weak point.

A frame made of 3 beams hinged together at the ends (a triangle) still doesn't collapse on itself, even tho' it's joints have already 'failed'. It collapses only when a beam fails.

A 4-beam frame made with the same hinged joints just flops around. It must also have rigid joints to be structurally sound. So 'triangles' are stronger for a given weight because it minimizes the risk of joint failure (everybody has seen a crane mast). But if weight/cost isn't an issue, you could use M&T in 12X12 legs and stretchers as per your initial design and I think even the cynics here would allow it might be strong enough for the intended use. Even with that big rectangle.

Your proposed table doesn't use hinges for joints, so even a big open rectangle is not the end of the world. Just make sure the joints have the strength to resist expected loads, as does your example table design. If your legs and stretchers get light and thin enough, the joints get progressively weaker, and you need to start thinking in terms of triangles. A wider (tall) stretcher servers this purpose, with basically an internal triangle. You can also make the stretcher/leg joint wider right at the joint (i.e. 'flare' it out onto the leg). The wider this joint becomes, the more racking resistance it has within the joint itself.
-- Enjoy.

[Steve Wurster]

Personally, with nice tight 2" M&T joints, I think your initial sketch is plenty strong.

The joints aren't 2"; the material is all at least 2"x2" and the joints are all at least 3/4"x1.5" wide/thick.

[Malcolm McLeod]

The joints aren't 2"; the material is all at least 2"x2" and the joints are all at least 3/4"x1.5" wide/thick.

Understood. My opinion stands.

[Steve Wurster]

Understood. My opinion stands.

Thanks. I think I like your idea of having the stretcher flare out into the leg. If it were up to me I would probably have more smooth transitions in the joints (in essence, curves), but my wife really likes the "straight line" look of Mission / Arts & Crafts style furniture. Of course this table will have a live-edge top, because she really likes that too.

Another question: Would having 2 stretchers placed further apart across the top and possibly lapped into the cleats be a better solution than just having 1 centered stretcher across the top that's joined with M&T?

[Gene Davis]

Use the design you show first here, and start cutting wood.

Don't overthink this. A 1.5" slab is plenty stiff enough. Make the top first and set it atop two sawhorses and try it out. You'll see.

[Kent A Bathurst]

Use the design you show first here, and start cutting wood.

Don't overthink this. A 1.5" slab is plenty stiff enough. Make the top first and set it atop two sawhorses and try it out. You'll see.

Alternative phrasing: Don't overdrive your headlights.

Absolutely correct. Not only sound enough for a dining table, it will be sound enough to dance on. Heck - a dry-weight Mini Cooper would be under 700 lbs per leg - I would be more than comfortable with that.

When that top is attached to the top-level cleats, nuttin' is going nowhere in terms of lengthwise. The M+T joints at the top of the legs into the cleats will prevent any "splay" - those angled braces in one concept add nothing useful. If you want the top-most stretcher - fine, do it. Will be hidden, and won't add anything, but harmless enough if that is what you want. Waste of wood and labor, though, is my thinking.