# Help with strenght of this western red cedar table



## Arckon (May 25, 2014)

Hello all,

I would like to have some second opitions about my project.

Im building a wood and concrete table and would like to know if you guys think my table structure will be strong enough to handle the concrete table top which will weight around 350lbs.

Structure is made from 4x4 and 2x4 western red cedar. All joints are Tenon and mortise that will also be glued. There will also be rectangular aluminium brace (90* corner) on each corner. For added stability amd strenght im thinking of placin a plywood sheet underneath the concrete (sitting on top of wood structure) OR either placing 2 more 2x4 cedar beams on the structure to support the tabletop.

Ideas, opinions ?

Thanks for your help!

Arckon


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## AndyCotton (May 13, 2014)

350 pounds is a lot of weight, it might not be a bad idea to do both. 


If it wasn't for having to eat, sleep, and work, I'd never come out of my shop.


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## amckenzie4 (Apr 29, 2010)

I can't picture what you mean by "There will also be rectangular aluminium brace (90* corner) on each corner." Do you mean a piece of square tube somehow screwed to the 4x4, or do you mean a piece of angle iron on the outside corner of the 4x4, or what?

I don't think plywood under the concrete will do much to add strength. More cross-braces will help spread the weight, but I'm not sure that's what you want: you want that weight coming straight down into the posts.

350 pounds IS a lot of weight, but 4x4 posts are exceptionally solid against compression. I'd have two concerns:

1) Water absorption from the bottom. You don't say whether this will be indoors or outdoors, but red cedar and concrete implies it's meant to withstand some weather. With the bottom of the posts against the ground, they'll rot a lot faster than they would otherwise. There may not be much you can do about it, but it's something to think about.

2) Racking. Racking is side-to-side force on the table, and I'm not convinced you have enough support against it. across the narrow dimension you should be fine, but in the long dimension I'd be nervous about having only one cross-piece at the bottom. If you want to leave the sides mostly open, I might do two lengthwise stretchers six or eight inches apart in the middle. I'm inclined to over-engineer, but I'd be nervous about having the thing start to fold over lengthwise.


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## Arckon (May 25, 2014)

amckenzie4 said:


> I can't picture what you mean by "There will also be rectangular aluminium brace (90* corner) on each corner." Do you mean a piece of square tube somehow screwed to the 4x4, or do you mean a piece of angle iron on the outside corner of the 4x4, or what?
> 
> I don't think plywood under the concrete will do much to add strength. More cross-braces will help spread the weight, but I'm not sure that's what you want: you want that weight coming straight down into the posts.
> 
> ...


Thank you so much for your input.

1- Thanks for pointing that out. Yes indeed, there will be plastic mold under the wood legs to prevent rotting. It will be and outdoor table. I will also ''seal'' the cedar with this


2- Sorry for the confusion. I meant something like those small metal corner bracers like this one but stainless steel. So if i understand correctly..you would add more of those long pieces. I have a design where there will be a wood piece running along the middle of my concrete tabletop. Hence, i cannot put a lengthwise piece right underneath. Do you think this design would work (i added 2 more 2x4 lenght)

Would you go even further and add those X shaped support beams ?


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## amckenzie4 (Apr 29, 2010)

I don't think adding more braces at the top will help: the table-top will strengthen that, and I assume it will be glued to the base somehow.

My concern is to make sure the two pairs of legs at the ends don't get nearer to or further from each other. So I would add the long braces to the bottom of the legs, not the top.


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## Arckon (May 25, 2014)

amckenzie4 said:


> I don't think adding more braces at the top will help: the table-top will strengthen that, and I assume it will be glued to the base somehow.
> 
> My concern is to make sure the two pairs of legs at the ends don't get nearer to or further from each other. So I would add the long braces to the bottom of the legs, not the top.


argh i see. I wanted to avoid that so people sitting around the table with chairs wouldnt have to deal with those brace but i guess thats still minor compared to a 400lbs slab dropping on them. 

Thanks


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## amckenzie4 (Apr 29, 2010)

You could still set them near the middle, I'd just do two instead of one. If it's 36" wide, maybe center them 14" in from each side, and low enough that people can rest their feet on them.


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## j10c3y25 (Jan 8, 2014)

I think people vastly underestimate strength of many materials. Your design as it was would hold 350 lbs like it was nothing, especially with m and t joints AND corner braces. The garden box I built had 2x4's screwed into an L for legs and they support 1500 lbs of gravel, water, and soil when its full. As for preventing against racking, I don't think those stretchers are going to do much for you. If you really wanted to add structural support you could put triangular bracing in the top corners, but you may lose a little leg room. Since you're doing mortoise and tenon anyway, why not use drawboring? Seems like that would be cheap insurance for rock solid strength.


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## amckenzie4 (Apr 29, 2010)

j10c3y25 said:


> I think people vastly underestimate strength of many materials. Your design as it was would hold 350 lbs like it was nothing, especially with m and t joints AND corner braces. The garden box I built had 2x4's screwed into an L for legs and they support 1500 lbs of gravel, water, and soil when its full. As for preventing against racking, I don't think those stretchers are going to do much for you. If you really wanted to add structural support you could put triangular bracing in the top corners, but you may lose a little leg room. Since you're doing mortoise and tenon anyway, why not use drawboring? Seems like that would be cheap insurance for rock solid strength.


This is probably a good point. My concern may be in the "It's not overengineered enough!" category.

Like I said, I'm sure the legs will hold the weight. My only concern is what happens to the structure when someone shoves it across the deck.


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## j10c3y25 (Jan 8, 2014)

amckenzie4 said:


> Like I said, I'm sure the legs will hold the weight. My only concern is what happens to the structure when someone shoves it across the deck.


Who do you know that can "shove" a 400 lbs concrete slab across anything? Lol. Not trying to start an argument or anything, but just like with any design usage is going to be the biggest factor. To move this thing will take at least 2 people lifting from the sides and walking it somewhere, and it being set down unevenly or lifted unevenly (supported on only one or two legs) is going to be the largest stress it will ever experience at one time. This problem should be pretty well alleviated with diagonal bracing and common sense when moving it. The only thing left to worry about is creep and fatigue, and as long as the weight is spread fairly well across the span (I would go ladder style cross bracing instead of the long stretchers) that should be fairly minimal.


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## Al B Thayer (Dec 10, 2011)

j10
Your first drawing is enough. That's how I would build it. The concrete when dry will be self supporting in the center. If not, you did something wrong. Very easy to add rebar length wise and your set.

Best off if you make the M and T joint as strong as you can. Skip the corner braces out of metal. Not going to make a hills bit of difference. 









Hard to find any joint much stronger than this one. 

Al

Nails only hold themselves.


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## amckenzie4 (Apr 29, 2010)

j10c3y25 said:


> Who do you know that can "shove" a 400 lbs concrete slab across anything? Lol. Not trying to start an argument or anything, but just like with any design usage is going to be the biggest factor. To move this thing will take at least 2 people lifting from the sides and walking it somewhere, and it being set down unevenly or lifted unevenly (supported on only one or two legs) is going to be the largest stress it will ever experience at one time. This problem should be pretty well alleviated with diagonal bracing and common sense when moving it. The only thing left to worry about is creep and fatigue, and as long as the weight is spread fairly well across the span (I would go ladder style cross bracing instead of the long stretchers) that should be fairly minimal.


I watched a group of friends move a 300 pound rock by pushing it over a dirt surface. A 400 pound table over a wooden or concrete table isn't that absurd. I worked with a couple of people who could each easily lift 150 pounds or so... I could see the two of them deciding it was easier to lift one end and pull the table across a surface than "walking" it across.


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## Al B Thayer (Dec 10, 2011)

So is this going to be another beer table?

Al

Nails only hold themselves.


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## judgment (May 22, 2014)

Al B Thayer said:


> j10
> Your first drawing is enough. That's how I would build it. The concrete when dry will be self supporting in the center. If not, you did something wrong. Very easy to add rebar length wise and your set.
> 
> Best off if you make the M and T joint as strong as you can. Skip the corner braces out of metal. Not going to make a hills bit of difference.
> ...


A Mortise and Tenon joint with wedges. How common is this type of joint compared to a normal M&T with no wedges? How do you determine when wedges are indicated? Thanks, also sorry if this is considered hi-jacking. I can start a new thread if necessary.


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## Al B Thayer (Dec 10, 2011)

judgment said:


> A Mortise and Tenon joint with wedges. How common is this type of joint compared to a normal M&T with no wedges? How do you determine when wedges are indicated? Thanks, also sorry if this is considered hi-jacking. I can start a new thread if necessary.


The joint can be made blind for a cleaner look. I don't know how common it is but I use them where I need too. If I don't have a situation where I want the absolute maximum strength. I just glue in the tenon and be happy with it. 

The wedged tenon will give the joint a small dovetail effect. It makes movement just that much harder. You can insert the tenon and then drive in the small wedges and then cut the extra off and sand them flush.









In the chair above I needed maximum strength. Both joints going each way are thru wedged. Kind of hard to see the wedges. 

Al

Nails only hold themselves.


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## judgment (May 22, 2014)

Do you fabricate the wedges yourself or is that something you might buy in bulk?


...On my dang phone


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## Al B Thayer (Dec 10, 2011)

judgment said:


> Do you fabricate the wedges yourself or is that something you might buy in bulk?
> 
> ...On my dang phone


Just make them from the same wood your working with. These wedges only expand the tenon about a 1/16" on both ends.

Al

Nails only hold themselves.


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## Arckon (May 25, 2014)

j10c3y25 said:


> I think people vastly underestimate strength of many materials. Your design as it was would hold 350 lbs like it was nothing, especially with m and t joints AND corner braces. The garden box I built had 2x4's screwed into an L for legs and they support 1500 lbs of gravel, water, and soil when its full. As for preventing against racking, I don't think those stretchers are going to do much for you. If you really wanted to add structural support you could put triangular bracing in the top corners, but you may lose a little leg room. Since you're doing mortoise and tenon anyway, why not use drawboring? Seems like that would be cheap insurance for rock solid strength.


Thanks for the help and everyone else as well. I think ill go with my first plan as posted initially. Ill definitely use triangle brace at the top as well. 

I managed to do the tenon and mortaise but had to adapt and some of them no longer respect the 1/3 and 1/2 rule of thumb. Still very close so hopefully i didnt sacrifice a lot of strenght. Will probably post some pictures tonight. I just need to undercut so all my joints look nice now.


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## Arckon (May 25, 2014)

Im going to use those to hold the table legs to the deck :










as well as those kind of bracket here and there:


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## Arckon (May 25, 2014)

Al B Thayer said:


> So is this going to be another beer table?
> 
> Al
> 
> Nails only hold themselves.


 More like an outdoor patio do-it-all table. Like this one: https://img0.etsystatic.com/006/0/6523064/il_fullxfull.360524138_shs3.jpg


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## Gus M (Jun 1, 2014)

first drawing was fine.
its very common design.


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## Arckon (May 25, 2014)

Heres some images of the almost finished table. All theres left to do is to re-sand a little bit the base as well as apply a final coat of sealer. Finish joining the piece of wood into the concrete and then install the concrete on top. Im very pleased!


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## Jetmugg (Apr 28, 2014)

OK - the world is dying to know.... how did you make the concrete top? Is it reinforced with steel? Fibers in the concrete? ect....


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## Brian T. (Dec 19, 2012)

350lbs is like two grown men sitting on any table. I don't believe that you need to tie it down on the deck = it isn't going anywhere and, some day, you may want to move it. 4x4 legs are good, some spans of support underneath are good.

My biggest concern is residual moisture in the joints and wood/concrete interface.


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## bauerbach (Mar 25, 2012)

Im curious, why the wood timber in the concrete.


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## Arckon (May 25, 2014)

Jetmugg said:


> OK - the world is dying to know.... how did you make the concrete top? Is it reinforced with steel? Fibers in the concrete? ect....


Hello Jetmugg, 

For the concrete itself i used Quickrete 5000 with Chengs Outdoor pro formula which contains all the necessary ingredients to make quickrete 5000 a good concrete for outdoor uses. It does contain a bit of fibers. I also reinforced the concrete with steel like usual.

I then cast the tabletop upside down in a melamine foam. I chose not to vibrate the mold very much in order to keep as many air bubbles as possible. This table is a gift and that person wanted air bubble.


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## Arckon (May 25, 2014)

bauerbach said:


> Im curious, why the wood timber in the concrete.


The tabletop you see is actually upside down. Once flipped. The wood inlay will be a part of the table design. Its a piece of juglans that is almost as long as the concrete and runs in the center of the tabletop (much like a river). Its not my idea but something ive seen done here and there and wanted to give it a try. The look is very nice. Quite amazing in fact.


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## Mort (Jan 4, 2014)

Did you coat the wood inlay with anything? If not I'd be curious to know how long it would last.


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## Arckon (May 25, 2014)

Mort said:


> Did you coat the wood inlay with anything? If not I'd be curious to know how long it would last.


I used circa 1850 exterior varnish to finish the piece of walnut


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## Arckon (May 25, 2014)

UPDATE: Project is finally over. Im very proud of the result. Especially considering this was my first time working with both wood and concrete. I must admit building absolutely everything on my own was daunting but i managed to do it. Even tho its not perfect its still very nice for amateur work.

Concrete table with wallnut insert and wester red cedar base.



THanks all for the help provided. Much appreciated and i wouldnt have been able to do it without the precious comments ive got here.


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## FrankC (Aug 24, 2012)

Looks like your four legged buddy is happy with it too. Nice job, you should be proud of it.


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## Arckon (May 25, 2014)

UPDATE: its been a while and just in case anyone cares i thought i would give you a small update. The table is still in very good shape after spending 2 years outside. Winters are especially harsh here (quebec). The only problem was the walnut. It lost its colors after a few months. I replaced it with cedar. Otherwise the table base and the concrete are holding up quite well.


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