joining legs to a nested torsion box in an outdoor table...

[Jamie Buxton]

I would probably use a piece of 1/2" exterior plywood. We used to make long beams in residential construction with two 2x12 boards with 1/2" plywood sandwiched. That gave us 3 1/2" in width, which fits standard 4" board widths and it is strong.

You're missing the point, aren't you? The steel flitch plate is for strength, not thickness. Steel is way stiffer than plywood.

[Jamie Buxton]

...and since the facility i found locally that sells this stuff only sells in 20ft spans, would it be considered logical and safe to go with 10ft of reinforcement on either side instead of the full 12'1" that the beam spans?...

Yes, you should be okay. In fact, there might be a little advantage in holding the steel short of the beam ends. The steel may rust (if it ever rains in CA again). If the steel extends over the legs, rust stains might drip down the leg faces. But if the steel stops short of the legs, any rust drips would fall to the ground.

[lowell holmes]

You're missing the point, aren't you? The steel flitch plate is for strength, not thickness. Steel is way stiffer than plywood.

Really, I'm not missing the point. I just happen to think (from experience) that the beam I suggested would do the job without the corrosion of steel.

The steel will work and that's probably what he should use. Be prepared for rust though.

Has anyone calculated the loads on the beam in order to size it. That's where you should start. There is a book,by Harry Parker, SIMPLIFIED ENGINEERING FOR ARCHITECTS AND BUILDERS, available from Amazon. Spend a little time in that book, It is available from Amazon.

[Matteo Lorenzo]

I would probably use a piece of 1/2" exterior plywood. We used to make long beams in residential construction with two 2x12 boards with 1/2" plywood sandwiched. That gave us 3 1/2" in width, which fits standard 4" board widths and it is strong.

Yes, you should be okay. In fact, there might be a little advantage in holding the steel short of the beam ends. The steel may rust (if it ever rains in CA again). If the steel extends over the legs, rust stains might drip down the leg faces. But if the steel stops short of the legs, any rust drips would fall to the ground.

interesting thought. I was reading about how flitch-plating was less optimal these days due to the fact that its more cost effective to sandwich marine ply which gives a similar strength and costs less ( and you can drive nails in)
but for my application, I think steel is going to be the right route to take. thanks for the suggestion though! I did a cursory calculation on the sagulator and came up with an acceptable deflection result even without steel at a nominal load. I honestly think the steel is overkill, but I am a fan of overkilling it when underkilling it is a possibility.


And thanks Jamie, thats kind of what I was thinking ... and I honestly think that 10 feet is way more than sufficient, but ive never done anything like this so ... have to ask!

Wouldnt it be smart to coat the steel in some sort of krylon spray inhibitor or something prior to implanting it?

thanks guys

[Matteo Lorenzo]

Thanks howie - this is definitely a mixed bag of heart and sapwood - probably all newgrowth. I am going to coat the leg ends with epoxy as suggested in an earlier post, give the whole thing a few good coats of spar and I will also instruct the customer to keep it covered with a tarp. I still like to reiterate that this will be in dry dry Los Angeles - and while we do have a stormy winter ahead, its rare and I take some solace in that fact.

[Jamie Buxton]

...Wouldnt it be smart to coat the steel in some sort of krylon spray inhibitor or something prior to implanting it?...

Yeah, trying to coat the steel would be a good idea. There are anti-corrosion paints, like Rustoleum. There's also truck-bed coating. Ugly, but strong, and it sticks to steel very well. You can buy spray cans of the stuff.

[Jerry Miner]

Yes, coating would be good. Any good metal primer.

No adhesion necessary. Use plenty of screws. As a non-engineer, I would use something like a pair of screws every 12" or so.

10' length should be fine.

I'm surprised to hear that marine ply has a similar strength to plate steel. If you believe it, then use the ply.

[lowell holmes]

Yes, coating would be good. Any good metal primer.

I'm surprised to hear that marine ply has a similar strength to plate steel. If you believe it, then use the ply.

Obviously marine ply does not have the same strength as steel. However, double 2X12 or for that matter 2X10 with 1/2" plywood between the 2X's has been used a long time and the resulting beam is strong. The beam with steel is stronger, but if the wood filler is strong enough, it is easier to use. It will rot, but it does not rust.

It also is a beam design that has been around a long time.

[Matteo Lorenzo]

I just wanted to follow up and thank everyone involved in this thread for all of your insight and great ideas.

I managed to finish the table without incident, delivered and installed today to a happy customer.

For what its worth, I ended up going with the flitch-plated aprons that were doubled in thickness and connected to the legs with pegged double M/T. The connection is very rigid in the end and the table is really heavy and solid.

For the flitch-plated apron side members, I routed an 1/8th pocket in each 4/4 piece of cypress and sandwiched the steel into that slot prior to glue/screw. I also used a fair bit of epoxy to really seat it in there.
There is zero (noticeable) sag and I reckon that table could support a LOT of weight... the top will surely fail before the frame if there is a neighborhood dance party on it.

so, again, thanks so much for everyone's participation. it was absolutely priceless to me.

as promised, some snaps...

resident butterfly that checked my work daily
resting on a flitch-plated apron member after joining the two slats. 4in X .25in x 10ft plates embedded in both side apron members

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double M/T connections to leg stocks. you can see the steel plate here as well

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angle showing the joined apron from leg to leg and no sign of sag. this was loadless, but I have put quite a bit of weight on the top to test and was not able to create any sag.
it was probably overkill, in the end, but id rather that than underkill!
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and the finished piece at the new owners house

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[Glen Flint]

Matteo... I've been losing sleep over how I'm going to build a torsion box table for a 10ft dining room table.... the usual method for a flat work surface in a workshop is not going to work. Nathan Day Design has accomplished this, but I'm not sure how they affixed the legs to the torsion box. In my internet searches, I stumbled apon this thread. I actually kind of like your first idea, with a nested nook for the torsion box to snugly reside. That was my working plan and I still might go with that.

[Lee Schierer]

Great advice so far-ditch the MDF, Cypress is excellent outdoors. My suggestion is in line with others to double ALL the apron thicknesses and add 2 stretchers in between the side aprons (double thick also) which could then extend through the end aprons to add some support to the breadboard ends of the top. You can add some design elements to the "tusk" tenons for appearance and clearance sake-like an ogee for example, or just a straight angle cut. Please let us know how this ends up (with pictures).

If you double the thickness of the stretchers, it has less effect than adding 1" to the height of the stretchers. The 1" of height will also add less weight to the table than doubling the width of the stretchers.

[Thomas McCurnin]

OK, since its already more or less built, I'll have a contrary opinion. It may fail, but oh well, I've never learned anything from doing it right, only by making mistakes, so let's carry this project through as is and make it work. Call me an optomist.

I'd just attach your legs in the corner in one of the hollow spaces. I'd attach them with construction screws from the aprons and shim them out to meet the cross pieces in the torsion box and add some more constructions screws from the interior. Then I'd brace the legs underneath with some more 3/4 wood, stupid steel L brackets or small shelf brackets. Glue the legs with epoxy, which probably won't help much, but what's the harm?

Lower mortise and tenon stretchers would be nice, but for dining that probably wouldn't work. Maybe an angled stretcher with an angled shoulder from the leg up to the bottom of the torsion box, or heck, go simple and add a 3/4 x 3 x 18" angled stretcher screwed into the leg then screwed into the underside of the torsion box apron. OK, the table structure is done.

Now let's figure out how to make the wood work. I'm in Los Angeles too (PM me if you want to get together) and I would seal the MDF both sides with multiple coats of a worktop varnish or polyurethane. Several coats, like minimum 4 (it will soak up the first two coats). The more the merrier. Then I would add a paint, go with farmhouse green, that would be nice, again several coats. You might have to use rattle cans for the underside of the torsion box for the varnish and paint.

Add your top, seal it with varnish, and seal the legs with multiple coats of varnish as well.

Buy a 20mil custom cover for this rascal and keep it covered.

My guess is you'll get a decade out of this.

[Jim Becker]

Matteo... I've been losing sleep over how I'm going to build a torsion box table for a 10ft dining room table.... the usual method for a flat work surface in a workshop is not going to work. Nathan Day Design has accomplished this, but I'm not sure how they affixed the legs to the torsion box. In my internet searches, I stumbled apon this thread. I actually kind of like your first idea, with a nested nook for the torsion box to snugly reside. That was my working plan and I still might go with that.

What you need to do is to plan for the legs in the design of the torsion box so you have structure available to provide the support needed. For a 10' table, where you put them is going to come into play, too as it will influence the internal structure so that it's in line with the legs for the same reason a "regular" table often employs aprons between the legs. If the legs have a simple rectangular/square top section, you can build in pockets for them to slip into snugly which means it can also be easy to knock down for moving in the future. Etc. Think it through.

[James Jayko]

I read 38% of the original post and my only thought is “this is too complicated by half,” and “outdoor MDF is a disaster.”

[Thomas McCurnin]

I'm not the OP, but one guess is that he wanted the look of a thick 3-4" slab without having to pay for the lumber or fuss with the weight. I have to admit that the torsion box is beautiful, but too complicated for my tastes and yeah MDF outside would be a problem.

I think I'd just put kiln dried straight grain Douglas Fir 2x4s face to face and skin that monster with a decent hardwood. Actually, for an outdoor table, there is nothing wrong with Douglas Fir and 4x4 legs, like an outdoor workbench. Heck, its just an outdoor table, not an heirloom piece.

But since its already built, rather than second guess his methods, I wanted to see if he could make it work as designed.