Thread: How far to span a slab?

Originally Posted by White Cloud

I will validate, that I tend to use an apron and/or central spin . . . .

I assume you meant spine?
If so good idea.

Depending on the height of the spine (eg if you were to uses RHS) this can also help balance the look of having the legs further out.
Also a spine can have additional cross pieces attached to it along its length.
So instead of two cross pieces and 2 pairs of outboard legs like this you can have say 4 cross pieces and use just 2 central legs.
2 devil bolts per cross section and it's not going anywhere.

Bloody Hell
What are we building here -- a bar or a bridge to support a B-Double ???

The stair treads in the house I'm currently renting span around 1.8 metres and are only 2" thick construction pine.

get a grip -- for the bar as described, the leg placing is purely aesthetic.
The only structural issue is attaching the legs to the top.

Do all of us a favour. Place the Jarrah slab on the floor and then place 2 sticks under it. Now stand in the middle of the slab. You can add more weight by carrying some clamps or simply jump on it.

Come back and tell us the distance between the sticks that you are happy with.

Originally Posted by elanjacobs

From post 9

The principal still applies.

The slab is thick enough to support a reasonable load right out to its entire length. Aesthetics and stability are far more the issues here rather than strength.

Thanks BobL for catching my "typo"...I'm still getting the hang of this forums system. It seems I can't go back and edit old post, which is a habit I have grown accustomed to on other forums...I'll try to do better...

Originally Posted by ian

Bloody Hell
What are we building here -- a bar or a bridge to support a B-Double ???

The stair treads in the house I'm currently renting span around 1.8 metres and are only 2" thick construction pine.

get a grip -- for the bar as described, the leg placing is purely aesthetic.
The only structural issue is attaching the legs to the top.

LOL...A bar I thought...

Nevertheless, a stair tread I would suggest is perhaps a very poor comparison...Isn't it?

The typical stair tread has a riser supporting the tread above it. I have designed stairs over 3 m wide and the riser is sized accordingly for the span and load it is intended to support. Even in green wood construction they seldom have issues because of this and because they are assembled with traditional wedge joinery just as they have been for centuries...So, if they do move, loosen or otherwise misbehave they can be adjusted.

Originally Posted by Wongo

Do all of us a favour. Place the Jarrah slab on the floor and then place 2 sticks under it. Now stand in the middle of the slab. You can add more weight by carrying some clamps or simply jump on it.

Come back and tell us the distance between the sticks that you are happy with.

I agree Wongo, this is a good empirical practice to do for such projects. We will often, in the shop or classes with students, perform such exercises to, "get a feel" for how something may behave.

However, this exercise does not address all the potential challenges with such projects as bars, table, and the like in regards to long term performance of such a piece of furniture in my experience. Whether working in dry wood (or green which is my standard) wood can, and often does move, in all kinds of direction besides just sagging under load. I have seen them just as often have imbalances in interstitial zones for improper drying (aka case hardening) or from tension within the wood that does not reveal itself when I take it off the stump or mill...They can just as easily bow up too...not just bow (sag) down...

The apron, spine (thanks again for catching that typo BobL) and related sliding keys or other stiffening joinery common in Japanese, Korean and Chinese furniture assemblies do much more than just arrest "sag" and/or support the loads placed on the table...They also account for seasonal movement of the piece, reaction wood fiber, and a myriad of other things that can happen after assembly. Sometimes even after a great deal of time...

Regards,

j

Hi J

the particular stair treads are open -- i.e. they don't have risers -- and are pinned each end to the stringers. In essence each tread acts like a fixed shelf.

In terms of strength, Jarrah is around 1.5 to 2 times as strong as pine.


In terms of building a 450 mm wide bar with minimalist legs and no supporting structure -- such as aprons or a spine -- my best guess is that with 2 appropriately placed legs the slab could be reduced to around 35 mm thick and still not sag under any reasonable quantity of full beer cans packaged as slabs.

I get where your coming from Ian...I really do and I'm not suggesting it is wrong at all...

Nevertheless, I'm not sure I can feel comfortable just "winging it" with advice to folks without covering all the bases that 40 plus years of teaching and designing in wood has offered me.

Most folks here thus far, including the original poster Spelunx is only focused on "sag." This is not the only issue in good design for such a project, or should be. That's all I'm suggesting.

"Slab work" is becoming a big deal and quite ubiquitous around the globe currently. It is very much "in vogue," that is for sure, and since George (aka George Nakashima) and Mira his daughter (both acquaintances of mine) really began with this style much has happened for sure. As someone that was fortunate enough to "slab up" walnut that was (or is?) in the Nakashima collection and studied such contemporary works for a very long time, I'm just trying to suggest tha "sag" alone is not the only factor to consider at all when working with such projects...

Can a slab of wood, as described in this post span the suggested distance without any other support other than just 2 simple leg assemblies, without any other bridge work, spine, apron or other typical hidden supports that were (not always) in George's work...???

Yes it can...

And...I have also seen countless ones tear themselves apart, bow, move twist and also take all manner of odd action that one may not have accounted for. Many of us (George included) lost work because of it...I only share that because I can not see the slab in question, nor did I get to see the tree it came from, nor mill the log into bolt and slab myself...So feel it would be a disservice to give advise that did not suggest the other possiblities that do take place much more commonly than many selling, and building with slabs today seem to realize.

I'm not a "worry-wart" by nature, but I do tend to plan and design for things that can happen to a piece of wood and the furnition it makes when it is well outside my control or purview...especially when I don't know as much about the wood as I would typically like...

Your advice is reasonable, and I don't hold it in any negative light at all...It is not a complete advice however, nor a complete picture either, and other things can be at play here that I just wanted to share...

Respectfully,

j

I think we are singing from the same, or similar, song sheets.
Whilst I do appreciate George Nakashima's work too often people on these forums are overbuilding simple things like table and bar tops looking for "strength" where it is not required.

So I do wish people would spend a little time with the Sagulator tool.
For the slab mentioned, I'm getting an acceptable span of 2 METRES when the slab is carrying 100 kg every foot (30 cm) -- that's equivalent to 5, 20kg bags of cement per foot.

Reducing the slab thickness to 35 mm and the load to 60 kg per foot (roughly equivalent to 3 bags of cement per foot) I'm still getting an acceptable result.


Now a slab of beer weighs somewhere between 9 and 10 kilos, and in terms of area is bigger than a bag of cement, suggesting that you would perhaps need to stack the slabs 7 or 8 high to get around 60 kg per foot -- that is getting an awful lot of cans for a bar only 2.9 metres long.


But I think we agree that attaching the legs to the slab is the tricky bit -- get that wrong and the bar will collapse because the legs collapse.

Wow, just had a look at George Nakashima website.... that is amazing....

I have done a mock-up in scrap timber of one trapezoidal leg. I am planning to match the other side, and then run a 50 x 50 SHS bar through the bottom as a foot-rest. I am also making two outdoor dining tables to match, out of old jarrah floorboards. The idea is to make it all look similar. The trapezoid in the photo is at 93 and 87 degrees, so I am playing around with the same angles for the tables.

I may have enough SHS to run a spine down the middle if it would mean some insurance? IMG_20181015_174901 (2).jpg

Originally Posted by Spelunx

I may have enough SHS to run a spine down the middle if it would mean some insurance?

if you make the legs self supporting, especially in respect to leaning on one end and trying to push them over, you'll be fine.

as discussed above, the slab, all by itself, will be more than strong enough to act as the top of a bar

Originally Posted by Spelunx

Wow, just had a look at George Nakashima website.... that is amazing....

I have done a mock-up in scrap timber of one trapezoidal leg. I am planning to match the other side, and then run a 50 x 50 SHS bar through the bottom as a foot-rest. I am also making two outdoor dining tables to match, out of old jarrah floorboards. The idea is to make it all look similar. The trapezoid in the photo is at 93 and 87 degrees, so I am playing around with the same angles for the tables.

I may have enough SHS to run a spine down the middle if it would mean some insurance? IMG_20181015_174901 (2).jpg

Hi Spelunx,

Yes in deed...the Nakashima clan has set the bar high for the post modern period in free style wood furniture. George could be called the "Godfather" of slab work. Before him it was very rough and not of much craftsmanship at all, nor of very much aesthetic depth. After his work, which sits at the United Nations, the Vatican and many other grand locations..."slab furniture"...started to really take off.

Now, unfortunately, not done too well, by too many just believing they can grab a slab of wood and stick some legs on it and everything will be "hunkydory."

You seem to be taking the task much more seriously...Good for you!!!

Yes, to your question, any additional stiffening will add insurance and long term durability to the piece...

Regards,

j

Originally Posted by ian

... as discussed above, the slab, all by itself, will be more than strong enough to act as the top of a bar...

Will it be strong enough?

Is strength (in span..aka modules of elasticity and rupture) the only thing that folks really need to consider?

I would strongly suggest that strength in tension, compression and modulus of rupture or elasticity are only part of the equation whether dealing with a large structural timber or a slab for a piece of furniture...

I know from my own experiences, I would never be quick to suggest such a thing without first seeing the piece of wood in person...

Just looking at a chart doesn't tell me anything about a given piece of lumber...or the tree it came from for that matter...

Is this slab plane , rift, or quarter sawn?

Does it have any reaction wood in it?

What kind of reaction wood?

Is there crotch, large oblique knots, or other aberrant graining?

Does the designer and/or craftsperson making the piece understand the subtleties of wood orientation? Can they read wood and tell which end of the slab or plank is the crown of the tree or the root/base of the tree? Do they understand that "bark up" is often the best orientation for a piece of wood...accept when its not...LOL?

There is so much that goes into good woodworking that I often get really concerned when such generic advise is doled out without discussing or (at least) considering all these many other factors...

Respectfully,

j

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