Breaking Load of 1"x12"x30" marble

May 24, 2010 7:17 AM

Breaking Strength of Marble. How much weight can a sheet of Tennessee pink marble support? Sheet is 1"x12"x30". It and two others like it will be the base of an octagonal plinth which will support a concrete urn weighing about 200 lbs empty, maybe 400 lbs when planted. Plinth is entirely built of this marble and hollow inside. I am open to suggestions for interior supports, currently thinking of gluing a rod to underside of marble and extending it below ground surface.
posted by fkeese to Home & Garden (12 answers total)

It depends. It can support practically unlimited weight in pure compression. It can support surprisingly little weight in tension or bending. I have a hard time envisioning exactly what you're doing. You've got a plinth that will sit on it or you've got a plinth you're _making_ from it? A picture would help.

Assuming you've got a large plinth sitting on top of a sheet of marble and are concerned about the sheet of marble breaking... If that's the question, you'll be fine if the earth underneath it is very firm. Tamped earth is good.

If there's an air gap under the marble, the distance between supports becomes the critical factor. Is it unsupported for a full 30 inches? How wide the urn is matters because it tells you how the load is distributed. 400lbs in the center of a 12" wide slab supported 30" apart would be pushing your luck I suspect, but doing the math requires a better mental picture.
posted by pjaust at 7:30 AM on May 24, 2010

Kinda need more info. On the face of it, a slab of marble, one inch thick, with no imperfections should be able to carry a load of 400 pounds if it is not cantilevered too far. I'm having trouble picturing "an octagonal plinth" made from three pieces of marble. Are you cutting these pieces into more (8) pieces? Just how does this thing go together?
posted by Old Geezer at 7:34 AM on May 24, 2010

You may also want to talk to someone at a monument company; they do this sort of thing all the time for gravestones and such and so should be able to give you some guidance.
posted by TedW at 7:43 AM on May 24, 2010

It will easily support much more weight than that. Marble is pretty strong. If you want to be on the safe side, add an extra support in the center of the sheet of a brick or something. Where will this be sitting - on dirt, concrete or floor?
posted by JJ86 at 7:53 AM on May 24, 2010

How about a SketchUp model? You mention three pieces as a base for an octagonal plinth. Hard to put together a mental image of three pieces and an octagon.

The answer must also consider the source of this marble — not as originally quarried but has it come to you second hand? If it has been exposed to weather, for example, it will be weaker than a piece that has been protected. For that matter, will this be outside? In a climate where freezing can occur?

More questions than answers here, I know, but listen to pjaust, a 1" slab can be weak in bending.
posted by Dick Paris at 8:25 AM on May 24, 2010

Are you stacking three octagonal pieces on top of each other, perhaps pyramid style? That's what I'm envisioning.
posted by MrMoonPie at 8:55 AM on May 24, 2010

Response by poster: Sorry for the confusion. To simplify, I have a span of 30" over which I want to place a piece of Tennessee pink marble which is 12" wide and 1" thick, with no imperfections visible. This must support an urn weighing 200 lbs empty. the urn itself has a base 16" round. How close is this marble to breaking under the load? The project is an outdoor octagonal plinth utilizing 8 marble verticals about 12" high and wide both, resulting in the 30" span, floored with the 1"x12"x30" piece in question, along with two others, one on either side of it. So, the whole plinth is constructed of this 1" thick and 12" wide marble. All joints glued up with best silicone caulk. Totally empty inside, and sitting on the lawn. Sure I could build it of cinder blocks and clad it in marble but I still would like even a ballpark range of weight this span of marble will withstand. And while we are at it, what might be used as a quick and dirty support going between the marble and the ground?
posted by fkeese at 9:33 AM on May 24, 2010

No way is hollow marble, on edge, glued together, going to work for this. The marble might be sufficiently strong if your engineering is perfect and it's all compression load. And if it's on solid, perfectly level ground.

But the moment that the neighborhood cat jumps up on it and wobbles it, it's going to break.

The tensile and torsional strength of any stone leaves a great deal to be desired.
posted by Netzapper at 11:17 AM on May 24, 2010

An easy way to get a useful answer would be to ask a cabinetmaker if you can stand on a marble counter top of the dimensions suggested. The supports are critical. The constraint type you want to ask about is "simply supported".

Here is an interesting discussion about simply supported marble beams. Clearly, the material is OK in bending given a great deal of care.

I took a few of the properties data from the link and a couple other places, and if a) they're all good; b) the material is perfectly homogeneous - a bet I wouldn't take - I get you're nowhere close to the tensile yield, 11 ksi, with a 200 lb load on a simply supported chunk of marble of the dimensions shown. There's no way I would design anything on that calculation. Brittle materials are too weird. The way I would go about your task is to make a subfloor suitable for outdoor use, then put all the marble on that.
posted by jet_silver at 12:15 PM on May 24, 2010

I say just set the marble on top of some concrete blocks, then the pieces making up the octagonal plinth aren't responsible for carrying any weight and the top marble is well supported. That would be very quick and dirty.
posted by orme at 5:41 PM on May 24, 2010

I misunderstood what you were trying to do but in reality the least of your design problems is going to be the load capacity of the top slab. The sides of the plinth will either buckle or topple before your slab breaks. You need something hidden to be the structural element like bricks (stick with something solid. Silicone caulk is not a "glue", it is a sealant. It will have no holding power with the weights you are attempting to use.
posted by JJ86 at 6:04 AM on May 25, 2010

Response by poster: As a postscript to this question may I add that the project was successfully completed, that is to say, an 8 sided marble base, hollow, topped with 3 marble slabs, all glued together using best clear silicone bathtub caulk, and set to rest on level grassy ground. A large heavy concrete garden urn placed on this plinth has filled with rainwater once or twice and when full weighs maybe 400 lbs. All is well with plinth and urn, they have not so much as even subsided into the wet soil. I thank you all for your collected wisdom on this question, those who thought it would or could succeed were correct!
posted by fkeese at 11:55 AM on November 14, 2010