Marble dining table help please

[Gary Ragatz]

Since the table is going to be fixed-length, you have lots of flexibility to strengthen the support structure. I think I'd make the aprons pretty thick (maybe 8/4) and create a lattice of solid stock with half-lap joints to span the aprons. Attach the lattice structure to the aprons with mortise/tenon or sliding dovetails. Should be no problem with the top racking

If you're going with the L-shaped legs, I'd take @Jon TenEyck's idea a step further and fill each of the corners with a good-sized triangle of solid stock the same depth as the aprons to provide some extra racking resistance for the legs.

[andrew whicker]

You could laminate carbon fiber (or steel, but that would require hardware) into the apron or apron / leg connection point.

[John TenEyck]

With that in mind maybe a torsion box would be the best approach. Light but very stiff.

John

[johnny means]

I see. Excellent analogy. Thank you.

So would you forgo the plywood altogether and use cleats to affix top as another person suggested?

A couple of dabs of silicone in some key spots is all that's necessary. as said earlier the mass of the top will do most of the work. I'm guessing a top this size would be in the 400 lb range. It really takes a lot to get that moving. I'm also assuming it never gets moved.

[Jamie Buxton]

OP is in Los Angeles -- earthquake country. Along the San Andreas, the major ground motion is usually horizontal.

[Don Stephan]

My suggestion would be first to pay a couple respected marble fabricators to specify how the top should be supported and attached.

If the customer already has the marble who is responsible for getting it on the tabletop and secured? If the marble top is glued to the table will it be possible to move the table out of the house and into another should the desire arise?

[Paul Bent]

Once the marble was positioned, I would silicone a few stops to the underside of the marble and butted (no silicone) against the inside of the framework. Top would be removeable and easy enough to cut off the silicone stops if needed to relocate.

[Curt Putnam]

A true torsion box would provide all the support you need and could be hidden behind decorative aprons. With a nonslip surface that slab will not move.

[Frank Pratt]

I'm not a fan of the L shaped legs your client wants. They look cheap to me. But if you have to use them I would add some solid blocking at the top to reinforce the leg to apron joints.

John

Yikes! I agree, they look really bad.

[Malcolm McLeod]

A marble top that big will weigh several hundred pounds. Any sagging in the middle will lead to cracking. I would double or even triple up on the support length wise. Cross members will be much less helpful.

Once the marble was positioned, I would silicone a few stops to the underside of the marble and butted (no silicone) against the inside of the framework. Top would be removeable and easy enough to cut off the silicone stops if needed to relocate.

A true torsion box would provide all the support you need and could be hidden behind decorative aprons. With a nonslip surface that slab will not move.

My folks did a USAF ~3yr tour in Germany; Italian visit yielded a set of green + black-veined Carrera marble slabs - including a coffee/cocktail-table sized piece. All are relatively thin IMO :: ~25mm. But all have survived to date; this involves their inclusion in several iterations of furniture thru many years.

Highlighted above are some key take-aways:
As surface area increases, the thickness 'ratio' drops for a given thickness and it will crack if not well supported. Lots of good advice above on healthy 'ladder' or torsion box frame. Didn't see it mentioned, but you might look at taller tapered 'keel' - parallel to side apron, but centered - out of range of knees. The tall keel has greater mass-moment of inertia = less flex.

Don't want a wood frame around the edge? My experience is that even smaller pieces (10x20") are heavy enough to stay in place with normal house-hold handling, so hot glue locating blocks to the slab that constrain movement. Just flip the top upside down, place the leg/frame, and glue the blocks touching the aprons, keel, etc. If you go with a torsion box, cut pockets in the top layer of the box to receive the glued-on blocks. Just lift the slab off the frame as needed. Blocks can also be removed easily and allow for re-purposing the slab (maybe a value-add selling point?).

Mr. McDottie might consider how the slab is transported as well. My folks kept the shipping crates in the attic thru several houses and many years; tops were pulled off furniture, individually crated, and shipped separate from the frames to their next house. The largest crate finally got trashed due to very limited storage space, so I just built a new crate to support a recent move. Again, maybe a value-add option?

[Malcolm McLeod]

Imagine a ladder suspended across a gap. You really wouldn't expect the rungs to flex a whole lot. The plywood substrate would actually add more flexion than it would prevent, seeing as how it wouldn't support it's own weight.

Recommend you not ignore the 'rungs' tho'! They serve to unify the rails. A point load gets spread across more of the structure. (In case it's not obvious) IMO ditch any plywood except as the top/bottom of a possible torsion box solution.

[Ollie McDottie]

With that in mind maybe a torsion box would be the best approach. Light but very stiff.

John

Thank you all for these excellent insights. Man, this community never lets me down.

So to sum up — no glue needed for top, just a thick bead of silicone on top of apron and/or members of torsion box, allowed to set before putting on the marble slab.

As for apron, looking at about 4” wide either 8/4 or 10/4 stock.

I like the idea of a torsion box. Would I build it out of the same stock at the apron/legs? White oak? That would be quite expensive, perhaps ash or poplar could work as well?

Instead of mortise and tenon, will dominoes suffice for connecting to apron? I’ve never made a torsion box, doesn’t seem terribly difficult, but any and all insights, as always supremely appreciated.

And I’m working on convincing client to go with 3-4” solid square columns for the legs instead of the l-shaped. If I can convince her that it more structurally sound, she will readily opt for it.

[Randall J Cox]

you might want to try and calculate the weight of that top if you know its thickness. I'm sure there are tables out there that give marble weight in cu inches or whatever. It has to be more than 200 lbs. I have a small coffee table with a marble top and the top alone is really heavy. Maybe 65 lbs.

[Prashun Patel]

Throwing this out there... would the client consider a seam in the center of the table for the marble? This will provide stress relief at the expense of aesthetics.

The taller you can make the long aprons, the better the resistance to flex. Shaker trestle tables are able to use quite thick lengthwise supports that are in the center of the table, so they aren't seen and don't interfere with the sitter's knees. This of course means you need a way to catch and transfer that load by the side assemblies vis-a-vis thicker aprons, closer spaced legs, or center supports.

To be honest, when I am in situations where flex is a threat and at odds with the aesthetics, I opt for low profile steel or iron supports. You can use angle irons or i bars or even flat bar stock mounted vertically. If the client will pay and weight is an issue, carbon fiber is an option - but I have not used this for a table, so cannot vouch.

[andrew whicker]

I agree. I Googled it and a 1.5" thick marble slab that is 40" x 96" is 530 lbs. 2" thick is 700 lbs.