Joining big legs to a torsion box nested in a very long outdoor table...

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Forum topic by butterknife posted 11-04-2015 04:39 AM 1241 views 0 times favorited 13 replies Add to Favorites Watch
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9 posts in 903 days

11-04-2015 04:39 AM

Topic tags/keywords: question tip cypress farmhouse outdoor table parsons torsion box hangar bolt

Hey there all you woodbutchers! Long time listener, firsttime caller.

I finally decided to register and be more than a lurker. I have a “big” commission, my first really and I’m stuck. And the more I fiddle with the plan, the more confused and stuck I get.

It seems like it should be so much more simple than I am making it. So perhaps you pros could lend an ear and point out the obvious to a novice? I would be very grateful for anything you can offer.
So, please bear with me. I am going to try and make this as short as I can and semi-intelligent.

I am in the last 30% or so of a build on a Parson’s style, rustic farmhouse table intended for the outdoors in Los Angeles.

The kicker is, the table is 14’x33”x29”. Add to that: two nesting ~11ft benches and i have myself a quite unwieldy job for a single set of hands. But I have managed so far.

here is the general plan in sketchup:
original size:

I am using cypress, in the rough, from a local lumber yard out here. It will be weathered with vinegar/steel wool then given a latex paintwash.

On to the confusing bits:

In order to reduce sagging overtime and deflection from big dinners and elbows, I built a 12ft torsion box that will be nested up under the table top and enclosed somehow behind the apron and between the legs. The torsion box dimensions are as follows:

12ft long
21in wide
4.5in deep

constructed of 1/2in mdf skins and webbing and 3/4inch mdf sidewalls. it weighs somewhere in the neighborhood of 100-120lbs.

standing up on edge just for the pic’s sake

I thought I could notch out a section of each leg to accept each of the 4 corners of the torsion box, then use 2 hangar bolts and some reinforcing hardwood (to be secured inside the torsion box on each corner) to secure the legs directly. BUT, because I plan on using haunched M/T to attach the apron members to the legs, I start to run into issues of clearance and usable meat to drive in the bolts. (The legs are 3.75×4.75, laminated cypress)

here is my attempt to visualize how that might work in sketchup:
orginal sizes:

all angles are from above the leg/apron

you may be scratching your head at the small tenon with stub dowels. That is because I came up with a setup that I think is probably overcomplicated and begging for disaster since the measurements/clearances dont have enough play to allow for mistakes.

here is a grab of the torsion box (represented by just a corner segment showing the inside hardwood pieces and bolts.
each bolt would clear the tenons and hanches/stub dowels by 1/4 in or less… which makes me really hesitant to call this idea a success.

original size:

SO…. I feel like I am painting myself into a disastrous corner and I am hoping that there is some very simple solution to all of this. Perhaps I can find some sort of hardware that wont be too obvious and stick out too much to attach directly to the legs without removing material from the stock? Maybe I can notch out the legs but not fasten the torsion box to them, instead allow it to “float” within the confines of the apron (assuming I build some sort of cage or shelf to hold it in place on the apron members. Maybe I can change the orientation of those hangar bolts, or use a single one either vertically into and along the length of the leg … or counterbored diagonally into the leg member?

On top of all that, once I nest in the torsion box, how will i be able to attach the table top to the apron effectively? the torsion box will be in the way. I suppose I could fasten the long rails with pocketscrews on the inside, drop in the torsion box, attach the legs to the box and apron, then finish the short apron members with pocket screws on the outside... but that feels sloppy.

I have no clue if any of this makes sense. It confuses the hell out of me, so feel free to ask for clarification… if you made it this far, that is.

Thank you all very much for any and all help you can offer. I really appreciate it! and again, im sorry for the length and confusing description.

13 replies so far

View HokieKen's profile


4747 posts in 1106 days

#1 posted 11-04-2015 01:59 PM

You could stick with your original design but just make the apron-leg joints regular blind mortise/tenons. Then run screws through the legs and aprons into the hardwood in the torsion box. Even better, if you haven’t skinned the top of the torsion box, use bolts with nuts/washers inside the torsion box. Counterbore your bolt/screw holes and fill them with dowels. Then you have a fairly simple joint that just looks like a pinned tenon.

I’m a little iffy about the stability of a table that long with 4 independent legs though. I think if it were me, I might consider a trestle design for the legs and maybe a stretcher run up the middle between them if there’s room for it. But your legs are pretty beefy so maybe the apron will be sufficient to prevent racking.

Just my $.02!

-- Kenny, SW VA, Go Hokies!!!

View AandCstyle's profile


3027 posts in 2225 days

#2 posted 11-04-2015 02:03 PM

Butter, I’ll take a stab at this to get the ball rolling. If the torsion box (TB) were narrower or the table wider (you would only about an inch on each side), you could leave the legs full dimension and hang the TB from the table top with screws. You would need to slot the holes in the top of the TB to allow the top to move seasonally. The TB will provide the rigidity for the top and the top will hold the weight of the TB. This will give you room to connect the top to the aprons using wood buttons or metal figure eights. If you want, you could attach cleats to the inside edge of the aprons for the TB to rest on. Good luck and welcome to LJ.

-- Art

View butterknife's profile


9 posts in 903 days

#3 posted 11-04-2015 05:20 PM

Thanks Kenny and Art! Thats great input from both of you.

It is starting to look like I have a torsion box for workshop now because I made a poor decision in using MDF for an outdoor application. All over I am finding that mdf, even sealed well, will eventually succumb to moisture, even in dry Los Angeles (thanks to the looming el nino)

So, instead of rolling the dice, I think I will cannibalize the box for a new shop assembly table and some infeed/outfeed for various machines that need it.

As for the table, I think I am going to double the apron thickness and join them with double M/T, add a couple stretchers between the side apron members and some half lapped cross members to make the structure taught, but willing to twist a little with the uneven ground it will be on. As a bonus, I dont have to worry about the thing swelling up and disintegrating.

If i knew how to weld, im sure I could save myself a lot of work – but thats an issue for another day.

thanks for the welcome! lots of knowledge in here!

rough lesson well learned

View HokieKen's profile


4747 posts in 1106 days

#4 posted 11-05-2015 08:10 PM

Oh yep. I missed the outdoor part in your OP. MDF definitely doesn’t play well with water. Hate you had to find that out later rather than sooner but glad you found out before your customer did. Good call not using it! And it makes excellent shop tables so it’s definitely not a total loss.

-- Kenny, SW VA, Go Hokies!!!

View ChefHDAN's profile


1057 posts in 2817 days

#5 posted 11-05-2015 08:47 PM

BK, glad you already got the MDF news, I know CA is dry, but MDF outside would be a no no.

Your welding comment reminded me that I once reinforced a long suspended shelf span with an angle iron. I plunge cut a kerf with my circ and a straight edge to put 1 side of a 1/8”x1”x48” angle iron into a 2×4 horizontal box frame. Measured to meet the through holes and then sank screws in from the edge to secure the angle iron. The shelf was connected to the wall at the back and only chained to the ceiling at each corner, put a boatload of weight on it and never saw a sag in it.

-- I've decided 1 mistake is really 2 opportunities to learn.. learn how to fix it... and learn how to not repeat it

View butterknife's profile


9 posts in 903 days

#6 posted 11-05-2015 09:19 PM

no worries Ken -

it was an incredibly green thing to do, diving in headfirst without thinking to ask the obvious questions like that. the funny thing is, I know that mdf isnt good when it comes to moisture. but i figured I could seal it up and call it a day.

oh well. like you said, not a total loss.

Chef… thats a really smart idea. im somewhere between that and just adding some beefy stretchers and a few cross supports. the metal could save me a lot of extra cuts

View ChefHDAN's profile


1057 posts in 2817 days

#7 posted 11-05-2015 09:43 PM

Remember the size and weight thing too…

Are you delivering or is the customer picking it up?

-- I've decided 1 mistake is really 2 opportunities to learn.. learn how to fix it... and learn how to not repeat it

View butterknife's profile


9 posts in 903 days

#8 posted 11-05-2015 10:28 PM

Its a local delivery and cost will be covered by the customer.
I am prepared for heft. a bit of steel will be less than or equal to the massive torsion box i was already prepared for.

just looking into hot rolled steel angle and it looks like i could get a 20ft section of 2”X2” @ 1/4” thick, split that into 2 10ft sections and attach to the back of each side apron member with panhead bolts, concealing them by laminating the apron members after countersinking the heads into one plank, then gluing the second over the top.

does that make sense? and would 10ft sections be sufficient over the 12ft span? i would think it is fine and should make those apron beams very rigid, nullifying hte need for center stretchers.

something like this… will this work?

View butterknife's profile


9 posts in 903 days

#9 posted 11-05-2015 10:29 PM

View butterknife's profile


9 posts in 903 days

#10 posted 11-05-2015 10:34 PM

heres a supplemental shot…

View rwe2156's profile


2883 posts in 1448 days

#11 posted 11-06-2015 01:00 PM

I was going to question the MDF but you’ve already got that. Good decision.
Besides that, I don’t think a torsion box would stay flat in that long unsupported span.

I really think the design of the table is flawed because that is an awfully long top.
Most tables this long will employ a trestle design, not a standard perimeter apron design.

Perhaps this is a consideration? There is a certain egonomic advantage to not having legs in the corners.

I’m assuming you can’t change the design, so my $.02:

Apron/leg attachment: I don’t see the advantage of the dowels I would just make it one tenon.

To support the middle of the long apron, you could use support leg with a trestle, or put a stretcher between the legs on the ends and use an angled support to another stretcher between the aprons.

You can also use wider 2X material for the aprons.

You can attach the top in any number of variable methods that allow for movement, maybe Z clips?

You will need a couple stretchers across the width no matter what you do.

And make sure you use a waterproof glue ;-)

No matter what you do, in an outdoor situation that top is going to move.
Cupping is going to be the biggest issue, even with a breadboard.
I would keep the boards down to 6” wide or consider leaving a small gap and forego the breadboard.

Also, the breadboard looks way too wide and unsupported in the sketch.
Bottom line I think your difficulties stem directly from the design.

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

View HokieKen's profile


4747 posts in 1106 days

#12 posted 11-06-2015 03:15 PM

The angle should work well as a stiffener. If it were me, I think I’d laminate the boards first then lag bolt the angle to them. Nuts will eventually come loose with expansion/contraction of the wood.

I didn’t really pay attention until rwe brought it up, but how are you planning to do the breadboards?

-- Kenny, SW VA, Go Hokies!!!

View butterknife's profile


9 posts in 903 days

#13 posted 11-06-2015 05:02 PM

unfortunately, the customer wants what the customer wants. He wants a super long supper table without the central support. If i were a smarter man, I would have talked him out of it – but im not. And now I am learning that it might have been a mistake. But I will continue forth with the plan as is and reap the rewards or consequences :)

I planned on using zclips for the top and the design as been adjusted to place support the breadboard over the apron. Short of that I will add some tusks under the breadboard to give it additional elbow weight support. Again, not ideal, but what is?

I also think I may sandwich flat bar steel at 4” between the apron boards prior to joining. I will make that decision today.

The more I learn, the less i know – truer now than it has been in a very long time for me!

thank you all for the input

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