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Need Advice on a Frame

jpari's picture

Hi Folks,

     I need some advice on a table frame that I am currently planning.  The frame will be 19" W X 45" L.  The legs will be 4 cabriole legs with 2"x2" post blocks.  The frame will be 29 1/2" tall.  The apron will be a 4" sculpted apron.  There will be one additional  support in the center of the frame.  The frame will be made of hard maple.  The top will be a piece of 3/4" marble that weighs 11 lbs. per square foot.  Roughly 70 some odd pounds.  My question is can I build this frame out of 3/4" stock and will the 3/4" be strong enough to support the weight of the top, or should I go with perhaps 7/8" stock and would 7/8" be able to support the marble top or should I use some other thickness of stock?  I really need advice on this matter.  Thank you in advance for your help.

sk2tobin's picture

I don't have any direct (post #170790, reply #1 of 3)

I don't have any direct experience with marble tops, but my gut tells me your concern should be with the height of the apron not the thickness.  The marble top is going to put bending stress along the length of the apron (for example it will want to make the front apron sag in the middle)  making the apron out of thicker stock won't do much i don't think.  If anything I would make it taller if you are concerned it won't hold the weight.  Other things you could do would to be add additional center supports running from the back apron to the front.  Also you would want nice beefy tennons going into your legs.  However, the way you described the construction I think it will hold the 70 pounds just fine.  One way to test it would to be to take the apron after you mill it (or a scrap piece) and apply weight to it while it is standing on edge to see if the weight causes it to sag any.  Hope this helps.

RalphBarker's picture

"Think about The Force(s), Luke." ;-) (post #170790, reply #2 of 3)

The 70 pound weight of the marble top will be equally distributed around the top of the apron and the tops of the cabriole leg posts. The portion of the weight supported by the apron will, in turn, be transfered to the legs via the (presumed) M&T joints between the apron sections and the leg posts. Under static circumstances, the direction of that force initially will be straight down. I say "initially" because this is where the profile design of the cabriole legs comes into play. Since the feet of the legs fall outside of the axis of the leg posts, the tendency would be for the weight to splay the legs on its way to the floor. The more radical the arcs of the knee and ankle of the cabriole style, the greater that tendency. Thus, the primary role of the apron is to keep the legs together and resist that tendency to splay under the weight. Secondarily, the apron would serve to resist any flexing of the top caused by things being placed on the table. Marble isn't going to flex much, so we're back to thinking about the M&T joints which would see some radial torque by resisting the splaying.

Bottom line, I don't think increasing the thickness of the apron pieces by an eighth of an inch would gain you much in terms of overall strength. I'd keep the M&T joints as beefy as possible by minimizing the depth of the tenon shoulders. I'd also pay close attention to the grain direction in the legs, and avoid making the ankles too dainty. Essentially, the legs need to be strong enough to resist shearing the long grain under the weight of the top and whatever might be placed on it.

roc's picture

OK you got the civilized answers . . . now . . . (post #170790, reply #3 of 3)

. . . let me give you the "it's a jungle out there (and in here)" answer.

First . . . go here 

http://www.amazon.com/Nick-englers-woodw...

and read and look at his charts of the characteristics of various wood species.  Great info on hardness and bending strength etc.  Numbers you can plug into a spread sheet.  Then do what ifs such as what if I make that front 1/8 inch thicker.  I even have EXCEL spread sheet app on my iPhone.  See the iTunes app store.  Easier to use on your iPad or computer though.

I keep trying to go to this book for my answers

http://www.amazon.com/Understanding-Wood...

but then I give up and go to Nick Engler's book and find what I am after.

NOW

to the design aspect . . . are you ready ?

Forget the weight of the marble.  Who cares ?

Does that surprise you ?  Well here comes the ugly part . . .

design the table so that some 250 pound drunk git is going to sit on it . . . or worse some silly 180 pound home maker/maid is going to stand right in the middle of it to hang some Christmas or party decorations.

Do you see where I am going with this ?

OK I gotta stop for a while . . . the mental image is just too horrible and ghastly to envision and type at the same time.  Lion and zebra time.  I can't look !

Bob help your ankles.

roc

Give me six hours to chop down a tree and I will spend the first four sharpening the axe. Abraham Lincoln ( 54° shaves )