# Table design - table top dip/tensile strength



## C M C (Feb 6, 2012)

Hi guys, I have a question which i hope you can help me with.
I am making myself a work bench/table in hardwood (in ash or oak depending on price) but as a reasonably new wood worker and having only really worked in softwoods I have some doubt that the design is totally sound.

The hardwood lengths will 2700mm with a 100x100mm profile (x10 lengths in total to make up the width). The lengths will be joined together by 16mm steel bars threaded through the depth of the table top at regular intervals (7 in total). See images

The 2 lengths at either side of the table which are attached to the legs will be joined with a Mortise&Tenon join which a steel bar will also penetrate PLUS 2 smaller pins next the this to prevent any movement within the join. Attached you will see my sketches which i hope explains my issue in more detail. 

What i would like to know is 'will the table top dip in the middle under weight or is the profile thick enough for this not to happen. My concerns are that I have no overhang with the table top so physics isn't exactly on my side. The table need to carry upto 80kg. Aesthetically the table will not be changed in design only the hidden engineering! Otherwise it would be too easy!

Looking forward to hearing you comments and many thanks in advance!


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## cabinetman (Jul 5, 2007)

I would just use glue and clamps, no threaded connectors. There should be no deflection.











 







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## woodnthings (Jan 24, 2009)

*yes it will sag*

Maybe not much, but over time it will. Why not add a center brace, or support? A storage chest would work and need not be attached to the table , just shimmed up to give support. I use salvaged (used) 2 drawer legal file cabinets, full extension roller drawers and the correct depth. They make great storage. I have about 8 of them under several benches.  bill

BTW Have you considered a Torsion Box design?


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## cabinetman (Jul 5, 2007)

woodnthings said:


> *yes it will sag
> *Maybe not much, but over time it will.


What do you base your opinion on? Interesting...you state like a fact that approx 4"x4" (3.937") Oak or Ash glued and clamped for a table top will sag.












 








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## C M C (Feb 6, 2012)

Thanks so far guys.

Can you let me know what will be the reason for the sagging? The joinery&construction or just the weight/length?

I had thought about a torsion box design but i wanted to try something a little different this time, Something a more substantial. 

The torsion box direction this is my Plan B/C but for now at least I would like to explore this design and try to make this work somehow. 

Even though an additional support seems logical I'm also trying not to have to depend too much on additional supports attached or not (as suggested) even though this is a great idea for additional storage.

Hmm... any other ideas before I try another approach?


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## woodnthings (Jan 24, 2009)

*your bench will be fine*

It will sag, over time maybe only a few thousands, but that's the nature of beams. If you want it "dead" flat then that may not be possible. Even steel beams sag of their own weight over a certain span and need to be pre-stressed. For a work bench as you describe, it won't matter. The added mass of the bench will resist forces from planing and such. A back panel will resist "racking" which is your main issue I see in the design you propose. The leg joints are all in "shear" and the moment arms are very short relying not so much on the structure, but rather the glue surface and the pins in the joints. 
Deflection in beams, whether round, square, rectangular, hollow or solid is an entire separate and complex subject.
I have only a very limited knowledge. ... a few strength and materials classes at the University level. They involved considerable Calculus equations, not my favorite subject. Sorry. 
FYI:
http://www.ehow.com/how_7705958_calculate-deflection-stress-beams.html

Amazon.com: DEWALT DCF885L2 20-Volt MAX Lithium Ion 1/4-Inch 3.0 Ah Impact Driver Kit: Home Improvement

The principle of a torsion box is that it is comprised of 2 "skins" separated by a grid. The lower skin becomes a "tension" member, the upper is in "compression. The grid members are both in compression and tension and the glue binding them is is in "shear"....basically. The grid keeps the skins separate and that's how it it is so rigid. I have one that I made that's 30" wide 10 ft long and 4" deep and a work and outfeed table. Also supported on file cabinets... just sayin'.
Here's a link: http://www.finewoodworking.com/PlansAndProjects/PlansAndProjectsarticle.aspx?id=28855

 bill

BTW I see that your thickness dimesion is 4" or 100 mm. How do you plan to cut that material? A 10" tablesaw won't work, so a 12" table saw will be necessary. You could rough cut it on a bandsaw then plane and joint it smooth enough to properly glue it.


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## cabinetman (Jul 5, 2007)

C M C said:


> Thanks so far guys.
> 
> Can you let me know what will be the reason for the sagging? The joinery&construction or just the weight/length?


I don't think it will. I've made conference tables with lesser sections that did not deflect. One was a table in Red Oak for my lawyer in the 80's made up of 10/4 on edge, 12/4 in height. Glued up was very rigid. Made as a Parsons table, had no deflection. Was one beast to move around and deliver.










 







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## C M C (Feb 6, 2012)

Great news!
Hey thanks guys for all your thoughts, comments and links on this project. This is the first time I have used the forum and I found it very informative.
I'm really not looking forward to moving the table once it's built but i guess once it is in place it'll be staying put for a good while.

In answer to 'cutting the material' question by woodnthings: I'm tempted to order the material cut and planed to size so i can just concentrate on the other aspects of the table (not mentioned in this thread). Plus I can hire a workshop space here in the Netherlands with machinery for half a day or so if i have any major issues.

For the join: I was thinking of cutting the joints with my tenon saw. It has a deep enough cut. I might need to limber up first though... Good idea/bad idea?


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## C M C (Feb 6, 2012)

I might actually reduce the thickness so as to keep costs down and then I don't need to hire any other tools.


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## C M C (Feb 6, 2012)

I though perhaps the 2 pins would reduce the chance of excessive racking?


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## C M C (Feb 6, 2012)

one final question...
Do you think if i add a shoulder to the tenon, it might help some?


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## C M C (Feb 6, 2012)

by that i mean a 'stopped or blind tenon' ..oops


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## C M C (Feb 6, 2012)

...and get rid of the bridle completely


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## cabinetman (Jul 5, 2007)

C M C said:


> ...and get rid of the bridle completely



This joint will be substantial if well fitted and glued. Two spaced fasteners would help.










 







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## C M C (Feb 6, 2012)

Thanks and best wishes!


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## C M C (Feb 6, 2012)

If others wish to contribute to this thread please feel free to do so.
I will not be purchasing the materials until next week sometime.
Bye for now!


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## Rebelwork (Jan 15, 2012)

If we were on the moon no bracing would be required but since we are here on THIS rock gravity is the evil one here. A simple table is one thing but as a work bench weighted by tools,lumber etc is going to play a big part of the top stability:yes:....


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## C M C (Feb 6, 2012)

... i'm sorry I do not understand you (?)


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