Wood table base for a granite top

[boojovi]

Hello, I have found this forum to be very useful in my research for my new project and finally decided to create an account and post a question.

 

I am trying to design and build a wooden table base for a granite table top that is 72x47 weighing in at over 400 lb.

I did read and re-read about the desk posted here

After all my research I have finally come up with this design.

 

 

The base is to be 64x39, allowing a 4 inch hang over on all sides.

 

i would like to use these legs, 3.25 in diameter

 

 

 

My primary goals are safety and stability, I know this table won't be moving far, if at all.

I'm hoping for constructive criticsm of the design, tips for helping hold the weight beyond just 4 legs, and any thoughts on the legs.

Thank you for your time and comments.

[G S Haydon]

Hi boojovi,

 

My gut tells me the structure may not be strong enough. I think you might have to add stretchers and rails lower down (this type of idea) and really beef up the joinery. Without the extra strength imo the design you show could be dangerous.

I wait with interest to see what others think

[Guest]

I believe that GS is right on.  You are asking an awful lot of simple mortise and tennon joints to maintine integrity with that much weight.  I also did not see corner brackets attaching the legs to the aprons.  This, too, will give more strength to the table.

 

Dan

[Freddie]

Im just looking at the dimensions of the granite top and have a bad feeling about the base. Even with integral mortise and tenon, no lower support would pose a lot of stress on those joints. The wieght of the top will transfer down the legs, looking for some path of relief. Like haydon said, a lower stretcher system would help transfer load and the stress of all that downward force

[wdwerker]

A corner brace at 45 degrees with a screw into the leg will help stiffen things up. You can make a lower stretcher "Y" shaped at both ends to reduce it interfering with chairs. If you do not want to do a lower stretcher then increase the ends of the aprons. Did you notice the desk had arched aprons? Wider on the ends and tapers to,less in the middle. Just increase the width of the last 5" of the aprons.

[wdwerker]

And most important thing is do not try to slide the table even an inch! Pick it up , move it and sit it down ! The desk is probably safer because it sits near a wall. A table in the middle of a room usually gets pushed one way or another. Pushing will stress the apron to leg joint and all the weight will make it stick for a second before it moves, that will lead to disaster!

[boojovi]

Haydon, thanks for the idea about the stretchers, I had originally planned for a central column, about 6x6, do you think this could be an alernative?

 

Tex, I was planning corner brackets, how big do you think they should be? I found some that have one, up to three screws going into the aprons

 

werker, i like the Y shape for the stretcher allowing seating at the ends, this also gives me a lower surface to add some embellishments

 

Thanks for all of the ideas and comments.  I am trying to do as much research as possible before diving into the deep end.

[G S Haydon]

My gut says the bases of the legs need to be connected to form a rigid cube of sorts. A center column could work but it might look a bit odd on a traditional build.

 

+1 wdwerker on the deeper aprons. All joinery will need to be over the top to avoid problems. 

[Nick2cd]

I guess I don't completely understand why everyone is saying the base is inadequate to support such a load. It seems to me that most of the weight will be transferred straight down the legs, no? Is it the lateral forces that are concerning? I can see where lateral force with that much potential energy from the heavy top could create serious stresses on the MT joints. I'm not trying to start an argument, just merely looking to better understand what everyone else is seeing.

[Tpt life]

My brother in law weighs 310 lbs and is six foot six. One stumble is all the lateral force it would take. The mass of the granite contains enough inertial mass to stop slowly. The force is amplified by the leveraging aided by the length of the leg. This is not scientific proof with measures of force just hypothesis.

[Byrdie]

Without something creating resistant force at the bottoms of the legs, the amount of force pushing down from the weight of the top will cause the legs to move away from that downward force.  The legs will want to splay out.  If the aprons or corner braces can generate enough force pulling the legs in against the downward force you may be successful.  

[G S Haydon]

I guess I don't completely understand why everyone is saying the base is inadequate to support such a load. It seems to me that most of the weight will be transferred straight down the legs, no? Is it the lateral forces that are concerning? I can see where lateral force with that much potential energy from the heavy top could create serious stresses on the MT joints. I'm not trying to start an argument, just merely looking to better understand what everyone else is seeing.

 

Hi Nick,

 

It's all about safety. The lateral forces are the problem. All this job is relying on at present is the aprons joining into the top of the legs. One big shove and it could/ collapse at best breaking the granite and breaking the joinery, at worst crushing toes or even worse squashing a child. Sounds over dramatic but I would not be able to sleep at night if I made the table in its current form.

[Nick2cd]

Quite understandable. I see what the masses are concerned with now. Thanks for the clarification

[wdwerker]

If a 300 lb man falls on a cheap mass produced table neither will fare well. But the cheap table probably won't break any bones when it hits the floor either.

[tdale51@yahoo.com]

What about using draw-bore M&T joinery on the aprons?  If it worked to hold barns and ships together for hundreds of years wouldn't it be sufficient for a table?

[Freddie]

Barns are also sheethed tdale, preventing the structure from racking. Too much leg below the m&t that has potential to sway.

[pghmyn]

Increase the size of the aprons to about 5" like wdwerker said. Also, my suggestion would be to add some cross bracing at angles under the top - in addition to some beefy corner blocks.

 

The reason I say to add cross bracing is to give it stability from moving at all in the X and Y directions. If the table was to move in those directions over time (even just a little bit), and then eventually loosen the joints, then there could be bad news.

[boojovi]

 Thanks for all the input!  I have scrapped the four legged design that seemed very unstable from the start.  I agree that lateral forces and weak joinery are the problem.  Credit to my wife for coming up with this design, using several 4x4 to create a large solid leg at each end, then covering it with a thin sheet of wood for decoration.  Also added a little stretcher to help with lateral forces going the long way of the table.  Comments and suggestions are welcome. 

 

[Tpt life]

If you are happy I think this design to be sufficient. It seems from a design standpoint that you really changed dramatically. Please be certain that you are satisfied with the design and not overly swayed by our nay saying:-)

[Beechwood Chip]

Will you have enough leg room for a chair at the end, with the 4" overhang of the granite?

 

You went from a design that was light and airy, to a Sherman tank.  As CS said, if you like the design it will certainly be strong enough.  Personally, I don't like the solid supports - I don't think they go with the Victorian spindles on the stretcher.  I think you should decide whether you want to go modern, rustic, Victorian, or something else.

 

If you like it, then it's fine.  We're in the realm on esthetics now, so it's all subjective.  These are just some ideas.

[boojovi]

We are planning on putting some embellishments on the wood blocks, including some carvings and a border, so I think it will lean more victorian, but my main concern was always safety.  When we measured, we thought about 9 to 10 inches for knee space under the table, meaning that the legs will be 6 inches from the edge of the base on the ends to allow for leg room.  With a width of 47 on the granite, a 39 inch base with each leg about 27 inches long to allow for leg room on the sides.

 

Please keep the comments coming, stylistic or safety/structural, I believe the best products come from ample discussion and consideration of all possible avenues.

 

edit: How should I attach each 4x4?  Or maybe attaching to the structure above is enough?

[Tpt life]

http://atlasantiques.wordpress.com/2010/06/04/victorian-to-1930s-massive-dinner-or-library-table/

I know we could drown you with ideas so please tell us when you've had enough. I like the curved nature of the semi-solid ends of this table if not the design exactly. I think it could be simplified to taste or competence.

[wdwerker]

If you go with the solid 2 leg approach make the internal frame behind the aprons a bit differently. Add 2 long pieces that will bear on the corners of the legs connected with crosspieces about 12" apart to make an internal ladder of sorts. Then the apron frame ends can go out to the full width attached to the internal ladder and the long side aprons attach to the ends. This will let the weight of the cantilevered ends be transferred to the corners of the legs.

I would cut the top of the legs down on the side about a 1/2" or so and attach the internal ladder to this ledge with some large screws. This would carry the weight straight down. It will also make the legs removable to ease moving the finished piece into the room for final assembly.

You could use the original legs with an " H " shaped stretcher to lighten the appearance and then decorate it with spindles if you like.

Just suggestions for structural and functional consideration , make the appearance suit your personal tastes.

Is the granite top 3 Cm thick (1 1/8" ) ? Or thinner? A 4" overhang is too big if it is thinner.

[dwacker]

My cabinet assembly table is sitting on a design similar to the second box design made from only 1/2 plywood with no internal framing except more ply "webbing" Its a old surplus 48x96x8 granite surface plate. I don't know what it weighs but my guess would be at least 3000 lbs my little fork lift won't lift it. Held fine for 20 years of leaning on it.

[ksqrdks]

Hello,  I have made around 40 tables for granite tops.  I am currently sitting at our kitchen table which is almost a slab of granite at 54"x106".  Here are my learnings and I hope it helps.....

1)  Table legs, the stouter the better.  Nothing less then 3" and they should never taper to less than 2".  I use beefy 6" legs when the table starts getting any considerable size.

2)  Put a piece of plywood on the top and secure it to all of your support framing.  I cut the plywood to be the exact dimension as the top.

3)  Secure the legs in the corners FIRM.  This means through the plywood and the internal cross supports.  The first cross supports should start so that you can secure them to the table legs as well.

4)  When the slab gets too big you really should consider a center column.  When the table is big and you really should have it, you will never notice it.  The center support column will in almost all instances make it rock solid.

5) 4" overhand seems about perfect to hide the plywood and saves on finishing.

 

Attached is a pic of the underside of my kitchen table.