Called yesterday by fellow looking for a table base for a slab of granite. This tabletop is 60″ x 42″ x 1.5″ solid granite, and he estimates it weighs about 450 pounds.
He wants a four leg base inset about 3″ from the top. Desired legs 4″ square at top, tapering to about 3″ square at floor. To be able to sit on chairs, the apron rail should be no more than 3″ wide. He is not willing to have a center leg.
Granite shop advises supports under the granite slab no further than 18″ apart. They suggested an X of diagonals between the legs, but the center lap joint would seem too weak for the weight involved.
Thickness of the legs would allow 2″ thick apron rails, but any mortises to join intermediate cleats to the rails would significantly weaken the rails (for example, a center cleat running between the end rails of the apron).
Has anyone successfully built a table for such a large heavy top, and would you be willing to share your design? My first thought is that a welded metal structure would be needed, perhaps resting on dadoes in the inner top edges of the rails, and I wouldn’t have any interest in guessing at a metal design.
Putting on my running shoes . . .
Thanks.
Replies
Don,
I have that piece of granite, although mine is 42 X 74.
I used 2 Ionic columns about 14" diameter spaced 36" center to center and 10.25" in from each side in the long direction and 12.25" in on the short sides.
When we moved about 4 years ago I added a 3/4" piece of plywood to connect the columns on the top
Except when my 6'2" brother-in-law leaned on the table when standing up once and the top started to shift I have never had and issue with it.
It's pretty hard to move 450 lbs.
ASK
I think you are on to an answer with steel reinforcement. Not only applied to the rails but "x" shaped channel to reinforce the half-lap in the center. A local welding shop should be able to fabricate the items.
A paint store could match the color of the wood - or the stone - to disguise the metal.
Frosty
“If you put the federal government in charge of the Sahara Desert,
in 5 years there’d be a shortage of sand.”
Milton Friedman
Chris and I built a table with a monster Bubinga Slab. It was for a client of mine to use on the outdoor patio, and wood legs don't work well for this. I designed a metal fabrication of a box frame with 3" steel posts.
http://forums.taunton.com/fw-knots/messages?msg=43946.1
In your case the side of the steel would need a wood facing, liquid nails is a natural for this, gives and sticks for exapansion issue. A tenon of steel into your legs should give plenty of strength as well, bolts through the tenon and plugged perhaps.
Attaching the granite is the hard part. I would use an angle iron as the top rail. You said the granite was to be 1.5" thick. Most granite is less than that, more like 3/4" and they apply a double thickness to the edge to make the detail. That may give you an extra 3/4" to use a 3/4" plywood top that the granite will glue to and to provide a uniform support.
Find a good metal fabricator, and charge for your time. I have a couple of other tables I have built with steel and granite for outdoor use as well.
AZMO
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-------(*)/ (*) http://www.EarthArtLandscape.com
Well, I would certainly use steel to support the top. But then, I work with steel as much as I do wood, so it's a no-brainer for me. There are lots of possibilities but the one you would probably be drawn to is to simply have a cross brace made out of steel flat bar, fully welded in the middle. Connect it securely at the legs and clad with veneer or solid stock that has had a deep dado cut into it.
Another possibility is to make a supporting sub-top entirely out of wood using the torsion-box technique. At three inches thick this would be very stiff. I would think that this torsion box wouldn't add any more weight than a steel member. The bonus is you get to do it all by yourself using material and methods you are familiar with.
I'll agree that 450 pounds sounds a little scary. But let's think in some different terms for a minute.
At 5' by 3.5', this slab is a total of 17.5 square feet. Which means that 450 pounds is actually only about 26 pounds per square foot.
Now think in terms of how a house floor system is built. Rim joists sit on the foundation. Floor joists are attached to the rims. And the flooring sits on the floor joists. (Most houses are required to have a 100 PSF rating, FWIW)
In this case, the table legs are the foundation. The visible aprons are the rim joists. Now all you need to do is add the "floor joists".
The compressive strength of wood in vertical applications is amazingly high. Four legs (well built, straight grained, etc) that are 2.5 to 3" square will hold several hundred pounds -- way more than 450. (Here's a surprise for you -- I'm a porker, and my wife and I weigh more than 450 combined -- yet our bed has only four legs, and they aren't 4X4's.)
The table aprons should probably be 6/4, as should the "joists". And I suggest that the "joists" be set on 12" centers.
And to avoid weakening the aprons, don't mortise the joists into the aprons. Instead, add a ledge to the inside of the aprons. Glue and screw it there. Then shape the ends of the joists to fit over the ledges.
Done properly, that will have all of the strength of mortised joints, without any weakening of the aprons.
I won't be laughing at the lies when I'm gone,
And I can't question how or when or why when I'm gone;
I can't live proud enough to die when I'm gone,
So I guess I'll have to do it while I'm here. (Phil Ochs)
Yes'm - I think you are dead on target here.
Joists 12" OC (long side - to - long side) to make a ladder assembly, resting on (and glued and screwed to) cleats that are glued and screwed to the rails. I can't see how that could possible fail to suppor the top. There are a lot of granite-top and concrete-top kitchen counters out there, and their cross supports are what - 16" or 18" OC? (whatever the drawer width is).
I think steel is the answer not so much for supporting the stone but in making a rigid "moment" connection to the legs. The legs are in effect cantilevers and 500 lbs of dead weight on top could be quite a strain on those joints trying to resist the table wanting to fold up. So I would suggest a frame of steel angles and welded angle or tube legs at the corners. The steel frame can then be clad in wood. Just make sure the connection between the legs and top is strong and rigid.And a small correction. Actually house are required to carry only a 40 psf live load. Added to that engineers usually assume a 10 to 15 psf dead load for the structure. Your number of 100 psf live load is for restaurants and corridors, which is why turing a house into an inn is not always possible.Peter
Thanks for the replies - several excellent points and suggestions. In no particular order . . .
Hadn't thought about the higher force on mortise and tenon joints. If the customer wants to stay with all wood apron and legs, I'll make the tenons as deep as possible, even though that would require some hand chopping. I'll also think about one or two 1/4" or 5/16" dowel pins in each tenon.
I'll suggest steel apron and leg assembly clad in wood, but don't think that will be received well.
Torsion box is an excellent option deserving serious consideration. I'd probably have multiple 1/2" thick layers to maximize the gluing surface, and use urea formaldehyde or epoxy (rigid glue) rather than white or yellow glue.
At first glance the cleat suggestion caught my interest, but after reflecting I have concern about the use of cleats. For example, if apron rails 3" high and use 2 1/2" high "joists" to support middle of the granite slab, the cleats can only be 1/2" high, which means the glue joint would only be 1/2" wide. Seems like the weight of the middle of the slab would be transferred to basically point loads where the "joists" rest on the cleats. Adding some screws would help. If slab does weight 450 as customer said (which may be an exaggeration as was pointed out) let's say at the worst 1/4 of the load would be supported by each long apron rail, and 1/4th by each of two interior "joists." That would be about 55 pounds on each end of the "joists" which seems a bit much for 1/2" cleat. Cut the "joist" down to 2" height and it may have too much flex?
Not putting away my running shoes yet. Thanks again for the feedback.
>>"Actually house are required to carry only a 40 psf live load. Added to that engineers usually assume a 10 to 15 psf dead load for the structure. Your number of 100 psf live load is for restaurants...."<<PArdon my mistake. But note that decks, in most juristictions, are required to have a 100 PSF rating.And I have no idea what you're talking about when you state that table legs are cantilevered, or that there is a tendency for atable to "fold up".While I agree that steel is much stronger than wood, it just is not necessary in the case of a table such as the one described by the OP. If it were, the kitchen cabinet industry would be hiring welders!
I won't be laughing at the lies when I'm gone,
And I can't question how or when or why when I'm gone;
I can't live proud enough to die when I'm gone,
So I guess I'll have to do it while I'm here. (Phil Ochs)
At my age quoting the building code is always iffy, so I did go and check the International Residential Code (IRC) which is in force in much of the country about your comment that:" decks, in most jurisdictions, are required to have a 100 PSF rating."Decks under the IRC are required to carry a 40 lbs per sq. ft. live load. Where I live in Minnesota they also impose a drift snow load which can triple the live load for portions of the deck...always a surprise. But there's nothing wrong with using the assumption of a 100 psf rating as codes are minimum requirements. It always amazes me how many 1/2" bolts it takes to provide that bearing where the deck is connected to the rim joist. The table legs are not just columns but have to resist forces applied sideways as they are bumped or dragged across the carpet. To understand the legs as cantilevers think of turning the dining room table on its side. The legs project like diving boards, a cantilever from the table top. Go out and press down on the leg at the very end and without too much effort you can stress the joint to failure where the leg attaches to the frame at the top. Making rigid joints between the legs and top is difficult in wood which is why so many tables have stretchers to brace the legs. So my point is that placing a lot of weight, whether the stone or someone sitting on a table top and then pushing sideways could cause the table to break apart by racking the joint. That's the fear at least.Attached is a sketch to suggest how by using a metal frame, clad in wood you could overcome the problem viewed upside down and in part.Peter
But that steel leg is still 'cantilevered', as you point out. And is subject to no less stress than a joint is wood.And welds that are at that stress point -- looks like about 2" of weld along a vertical line in your drawing -- would be the weakest point. Really good welds are almost as strong as the steel. Really good wood joints are stronger than the wood.And in recent years, there have been a few well-publicized deck failures. Perhaps you remember one (in the Chicago area?) that involved a three-story deck, and some fatalities. Building departments all over the US are realizing that a deck is sometimes used as a party space, with lots of people dancing.And they are adding local regulations to see to it that a crowded deck is not an overloaded deck.
I won't be laughing at the lies when I'm gone,
And I can't question how or when or why when I'm gone;
I can't live proud enough to die when I'm gone,
So I guess I'll have to do it while I'm here. (Phil Ochs)
The idea of the original question was how to make a strong, stable support for a heavy stone table top that can withstand only limited bending. Is wood up to the job, seemed to be the question.The reason many have pointed to steel is because its is at least 10 times stronger than the same dimensional piece of wood. So if strength is an issue use a whole lot of wood or a lot less if it just clads a frame of steel. I think we all understand that porches can be used as assembly spaces. But when you think about it, so can living rooms, dining rooms and kitchens. People at parties move about. If the live load is inadequate for one of these, it would be inadequate for all. If I remember correctly the porches (floors and railings) of the apartment in Chicago didn't break in two, but pulled away from the building as a unit and tipped. The code has been revised, whether because of this or something else to clarify the requirements for attaching a porch to a building. Fortunately wood structures usually give a lot of warning of failure and rarely collapse suddenly. They sag, creak, crack, char and slowly yield. Steel structures, pushed to the limits can just snap, like what happened with the interstate bridge in Minneapolis, or the balconies in the Kansas City hotel. So we try to understand the materials, cover our bets and then take our chances. Peter
I am a retired Paramedic from the Windy City, if it's the balcony I'm thinking of. They had a big investigation, and found there were 3 1/4" nails used where Lags should have been used. There were some lags used, but they were not the proper length or diameter. Without pulling a permit, there was no inspection. The guy who did the work was not licensed or insured. The media in Chicago tore him to pieces.
That collapse changed building codes all over the country. Chicago already had some weird problems with going way over the requirements of any codes accepted in other area's. Such as all wire must be in conduit, in residential construction. I do not think there many other places that strict.They still remember the great Chicago fire.
We responded to a house full of immigrants that were sleeping in the lower level of the house. It turned out that the furnace had a HUGE hole in the heat exchanger. The place was full of carbon monoxide, Im still amazed that anyone survived. We left seven victims at the scene to go to the ME's office. Someone told the Media that they had seen a truck from this certain company (I do not think I should put there name here, they went through enough already)parked in front of that house the day before. Well that company was the lead story on all the news for the next two days. That company had only been parked in front of that house. The Tech was doing a job like half a block away. There was barely any mention by the media of their mistake that had wiped out this guys business. This was the tragedy that caused all buildings to have CO detectors. Like I said Chicago go nuts over that kind of stuff.
Taigert
Thanks for confirming a somewhat distant memory. The last time that I detailed a LVL ledger beam fastened to a rim joist for an addition, the engineer said to make the connection with two 1/2" through bolts ever 16" on center. The addition was 20 feet wide so there was a lot more bearing on that ledger than most decks but it hints at what is required to make one of these connections.It used to be that lots of cities had their own set of codes, which led to great complications and mistakes in understanding requirements. The idea of having a national model code was to standardize laws to simplify design, construction and enforcement. The implementation of a national building code has been largely successful and Chicago may well be a hold out.Minnesota, like many states has a state building code which incorporates and modifies 30 some model codes. What is interesting is that a city in Minnesota is not allowed to require anything more than what the State Building Code requires. If they could what's the point of having a uniform code. Rural Counties in Minnesota were allowed to opt out of the State Building Code, because they claimed they could not bear the expense of checking plans and enforcement. There are stirrings to remove this exemption, forwarded not by anyone in the construction industry but the insurance industry. Seems insurance companies want to eliminate risk and one way to do is say, no you can't just fasten that deck to your house with a couple of 16d nails. Like you, I have found people just use buildings in unexpected ways. I worked on converting a 30 unit residential property that by code had a permitted occupant load of 90, housing over 300 people from Southeast Asia. Fortunately there were no disasters.
Well, the good news is that the granite only weighs about 350 lbs so your problem is not as severe as you thought.
In reference to how many lbs/sqft a house is built at, remember this, a sq ft is 144 sq inches. 4 table legs of 4x4 is only 64 sq inches. so you'll be placing 450lbs on less than 1 sq ft of area. The per sq/ft on houses is based on an equal load placed throughout the area, you can't concentrate it all in one spot.
>>"4 table legs of 4x4 is only 64 sq inches. so you'll be placing 450lbs on less than 1 sq ft of area. "No, no, no.In wood-structure houses, the floor is a system. The system includes the foundation, beams (if any are present), floor joists, joist blocking, subflooring, and flooring. Each of those components has a "job" to do.The flooring and subflooring have the job of taking point loads, and spreading them out over a larger area of the sub-structure.Think of an upright piano. That's a beast that has four (rather small) casters. Each caster probably has a floor-contact area of less than one square inch.(A refrigerator, or an occupied double bed are also good examples of this.)And the folks who wrote the building code knew that. But they knew that the code-required floor system would spread that load out over an area larger than the entire piano. So the PSF ratings specified in the codes, is not based on the residents spreading the load throughout the area -- that's the job of the structure.
I won't be laughing at the lies when I'm gone,
And I can't question how or when or why when I'm gone;
I can't live proud enough to die when I'm gone,
So I guess I'll have to do it while I'm here. (Phil Ochs)
Sorry but I've been doing homes and remodeling for 40 years. While a floor is a system, point loads must be carried thru to the foundation. Code requires doubling all floor joist under a whirlpool tub because the weight is too much for a small area. It's not a matter of collapse but rather sagging of the floor joists. They don't req. it under refrigerators, however my 40 year old house, where the ref sits near the middle of the floor joist span has sagged quite a bit over time. I'm not suggesting that the table is too heavy but you can't just look at it as the weight being distributed over the whole area.
The floor joist are also spaced apart usually at 16"o.c. If that load is placed between the joist you're sitting on plywood. I once saw a 4x4 bearing post completely crush the plywood under it to about 1/8" thick. It was a very heavy load but it can happen.trimjim
>>"I'm not suggesting that the table is too heavy but you can't just look at it as the weight being distributed over the whole area."Then why is no one suggesting that the table have more than four legs?Or perhaps legs with a larger cross-section?I ask because those are the only suggestions that would spread the point loads out over a larger area.And I'll add that this discussion has gone way to far, IMO. The original poster got good suggestions concerning how to build the structure that supports the stone top. And that (I'd guess) was the purpose.I'm done now.
I won't be laughing at the lies when I'm gone,
And I can't question how or when or why when I'm gone;
I can't live proud enough to die when I'm gone,
So I guess I'll have to do it while I'm here. (Phil Ochs)
Again I'd like to thank everyone for their input. Some of the discussion about building code adoption was a bit off topic. The amount of weight carried by the bottom of each leg, however, is a point I had not thought to consider, and it was well worth making, however it arose. It should be comparable to the "leg" loads of large, heavy sofas when fully populated during sports telecasts, but eating areas sometimes have softer floor surfaces (vinyl, cork come to mind). I'd want to recommend the customer not place leg buttons on the bottom, which was concentrate the loads even more.
After reviewing the provided information over several days, I've come to like the idea of a torsion box inside the apron rails and notched around the legs. The customer is a mechincal engineer, so he might be able to figure out the technical data for resin coated paper honeycomb at tricelcorp.com and spec a particular sandwich. Otherwise I'd offer probably three layers of 1/2" honeycomb paper with 1/4" baltic birch ply separators and outside surfaces. As this would be a uniform load bearing surface I'd consider supporting it with a 1/2" x 3/4" cleat glued and screwed to the inside bottom edge of the apron rails but also pocket screwing the torsion box (top or top and bottom) to the rails as well. If the torsion box fit snugly between the rails, the thickness of the box would I think also add some resistance to lateral forces when someone pushed horizontally on the tabletop.
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