Engineering for Tables

      A discussion of the stresses applied to a heavy table in use, and what they imply in terms of appropriate dimensions and construction details. March 28, 2012

Question
Is there a chart somewhere that indicates how big a leg to use on a table of a certain size to support a specific weight?

Forum Responses
(Furniture Making Forum)
From contributor M:
I do not know if any charts are available, but I have found that leg size is not as important as the rail to leg joint and the strength of the rail itself. If you have an idea of the weight that a table may need to support, and provide some info on the material that will be used, plus the table construction method, maybe we could give some suggestions.



From the original questioner:
Thanks. The table would be 30" x 40", approximately 35" high. 4 tapered legs, minimal apron (designer requested 3"; we might nudge her towards 4"), aprons tenoned into mortises in legs. Cannot have a bottom shelf or stretchers. We could add corner braces, or additional support beneath the top if necessary. She wants a marble top, thickness undetermined, but at 3/4" thick about 125 lbs. Possible they'll choose 1 1/4" thick, so more weight. I was trying to see if (beyond proportions/aesthetics) the legs needed to be any particular size in order to hold this weight. It just seemed there must be some formula I could apply to figure this out regardless of size of table, and adjust for weight of top?


From contributor G:
Formula




From the original questioner:
Thanks. Based on the formula, and approximate weight of my table + top, I can see that each of the 4 legs would exert a downward force of 17.05 Newtons on the floor, with a corresponding force exerted upward on each leg. I am still at a loss to as to how to translate this into how thick a leg needs to be, as that does not seem to be factored into this equation, which would offer the same result whether the legs were straws, or tree trunks? Is there really no standard or rule of thumb for how big a leg (contacting floor) needs to be used to support a given amount of weight?


From contributor M:
Not that I build a ton of tables, but I have not had any failures due to weight with the ones I have built. I usually size my legs by appearance or aesthetics, then make a judgment call on whether or not the table will support its own top plus the load that may be placed on the top itself. Sounds like you are building a well made table. Mortise and tenon is my preferred joint and I use them in exterior shutters and tables. I test a lot of my work by building shop tables and watching their performance, then using what I learn in making dining, end and coffee tables, etc.

What size leg are you thinking about using and what wood species? I believe the 3" apron would be fine, and look better than a 4" on a table this small. You could easily get a 2" tenon, unless you choose to go the full 3". We make a lot of our tables from heart pine, usually 1 1/4 x 2 or 1 1/4 x 3. On a small table we would go with the smaller size and taper down to 1 1/4 x 1 1/4 (approximately) at the floor end. I would not hesitate to put the marble top on this size leg.

One thing is most people do not put a lot of extra weight on a marble top. Plus we see a lot of antique tables in homes and shops that have small legs carrying marble tops. Most that were made correctly and have been cared for are still working quite well.



From contributor K:
I think what you want to know and what you are asking are two slightly different things. A small cross section of wood will support a tremendous (tree-mendous?) amount of weight in strict support before cellular collapse. I will go out on a limb and guess that a 1" x 1" piece of white oak will support 500 lbs or more before it fails. Possibly 2-3 times that - imagine a 1x1x1 cube of white oak, and loading the end grain by hydraulic press. I can see 1500 lbs, easily. Hoadley's book"Understanding Wood" will have this info.

However, in a table, lateral forces play the dominant part. The table has a stone top, is loaded with food, dinnerware, etc, and someone decides to scoot it over a foot for more chair clearance on one side. Not pick it up and move it, but scoot it across the floor. Scooting that table 12" will stress the leg to apron joints and also cause the legs to flex. The 1" x 1" cross section leg will shudder and shake, and if the grain is less than continuous along the length, may separate. A 4" x 4" leg will not flex at all.

In a static situation, both will support the weight, but the flex of the 1x1 legs will cause the table to feel wobbly and insecure long before a fatal scoot. Note that antique dealers, conservators and fine makers and collectors all move such things with care and knowledge, while the average user just needs to get three more people seated over there.

Your quest is for balance of form and function. What will surely satisfy the function may not be a pleasing form, and the perfect form may not satisfy the demand for function. As the designer, you must weigh the merits of any design on the twin scales of form and function to arrive at a successful design.

One way to do this is to look for historical precedent. The classic Hepplewhite leg tapered on two surfaces to half its cross section as it hit the floor. This gives good heft at the joint, where flex would be least tolerated, and gets thinner and more graceful as it approaches the floor, where the leverage is the least and the need for appearance dominates.

In short, there is no formula for this, thankfully. As a result, there are thousands of solutions, many are both attractive and functional. Vive la differance.



From the original questioner:
Thank you very much! Both of your responses made great sense and I am comfortable moving forward. Appreciate your time and expertise!

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