Help with the calculations: can a flat steel bar can hold this table?

  • #1
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Hi everyone! Brand new here. I'm sorry if this isn't the right place to post this but I am terrible with math/calculating anything so I thought I would reach out and ask! No harm done, hopefully.

I'm building an 8' x 3' x 2" live edge walnut slab table and am trying to create some steel legs for the table.

I'm attaching a photo of the legs that I would like to create. It's quite a curved looking structure HOWEVER the steel flat bar is only the black bit you can see. There are three of them in one structure. (The curved bit will be brass or copper...not adding much to the structural integrity of the table.)

I plan on making two of these structures for the entire table.... So 6 steel flat bars in total.
Will 6 of the 2" x .5" x 27" steel flat bar be enough to hold a 250 to 300 lb table?
I'm concerned about warping or bending as the legs will be the standard 27" tall!

Secondly, if this makes the entire table much too heavy.... Is it possible to substitute aluminum flat bar in (same dimensions) instead of the steel?

I am not clear on the PSI of these two metals and I've had a heck of a time trying to figure it out.


Thanks in advance!!
Sincerely,

my sanity.
 
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Answers and Replies

  • #2
berkeman
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Welcome to the PF. :smile:

Your images didn't post. Try using the Upload button in the lower right of the Edit window to attach PDF or JPEG images to your post or a reply. Also, this is just a table, right? Not a deck or someplace where people will stand? Thanks.
 
  • #4
kuruman
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Have you considered the practical aspects of your design? This table should easily seat 3 and maybe 4 diners along the 8 ft. side plus 2 at the ends. Will there be enough leg room for 8-10 people under the table?
 
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  • #5
sophiecentaur
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@Mads W Cool design which would set off the walnut well. Whenever I make a structure (never as posh as your's though!) I always find I have overdone the thicknesses of components and the thing weighs a ton, so I can appreciate your concern about weight at this stage.
I am not clear on the PSI of these two metals
Are you referring to the ability to just support a straight vertical load? That would be enormous and a single length of 1/4" bar could probably support 1/6 of the table. But that's not the issue. What's most important is how well it will resist lateral forces. It should 'feel' stiff enough not to worry the sitters and it must be rigid enough not to allow the wine and coffee to slop about when someone nudges the table. You must also expect to want to climb on the table at some stage. No wobbling allowed. This is actually an important bit of the design and it would be pretty difficult to know what would or would not be acceptable without some experimentation. Even detailed calculations wouldn't deliver what you want to know. You could look at existing designs and see what's standard practice with skinny table legs. The heavy top would make things more difficult. Your style of design is often used for glass tops, which are much lighter.

I think the shapes are inherently fairly strong as the planes of the vertical strips will mutually resist flexing. The strength will be improved greatly if you use the table itself as an active part of the structure. The (invisible) arcs at the top should be fixed at mid points as well as at the vertices. Perhaps the curves would be better replaced by straights or even angle section. (Cheaper too).The main weakness could be in the ability of the bases to rotate and twist the prism shape and there's nothing that can inherently stop that except the presence of the legs at the other end - which should always be there under normal circumstances.
It is pretty easy for you to calculate the weight of the frame. You should do this first and compare it with the weight of that great slab of walnut. It may not be too relevant, in the end.
 
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  • #6
CWatters
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How about making the whole thing from steel and giving it a copper finish? Painting, cladding, plating or patinating it?
 
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  • #7
sophiecentaur
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@Mads W
Why not visit some furniture stores? There will be nothing in them as nice as your idea but the size of steel that's used for legs and the firmness of tables will give you a good idea about what's actually needed.
Also, is it possible to make up a small scale model of the proposed arrangement with plastic strip for the legs? Your local DIY store may well have plastic strip which could be cut and bent and the joints could easily be pop-riveted together, helped by washers to protect the plastic from the rivets. Or you could use suitable adhesive. If you have the skills to attempt fabrication of the real thing, this would not be a problem and, even without too much skill and not many tools, a model would not be too hard. A model would indicate the modes that it could flex in.
 
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  • #8
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Wow, thank you everyone for such quick and thorough replies!! I was not expecting that! You all have blown my expectations to bits.

In terms of lateral force... An idea I've had is to add a matte black aluminum/steel plate to the base of each structure. Essentially I'd weld the steel or aluminum to the base, powder coat the entire thing black and attach the decorative copper afterwards.
The plate will make sure the structure cannot twist and it will also ensure that each 'leg' will not gouge into my floor!

(This design gets heavier and heavier the more I think about it. However I've not seen many designs like this ((now I know why, haha)), so I am determined to find a way!)

As well, I believe I'll inset an aluminum/steel plate or pieces into the underside of this table to connect the two structures and give the length of the wood some support so as not to warp over time. Most wood tables have 'C channels' inserted along the length anyways so I may as well use this to my advantage.

I am able to manufacture a prototype. I will build one first out of plastic strip or some sort of siding (great suggestion!) Once I can tweak that into something strong I will attempt it out of aluminum or something cheaper. I do have an entire machine shop at my disposal (otherwise this idea would have been dismissed ages ago!).

I have also thought of copper plating or powder coating a single structure.... logically that is the best way! And i won't say no to it, if this ends up being a pipe dream. If you all haven't already noticed, I am very stubborn and there MUST be a way to make this work! Yes, it will be much harder and much more work... But that's the fun part of attempting something new. :)

I believe my main concern right now will be the weight of this whole thing! Would this structure be just as able to hold weight with aluminum flatbar (or even thin aluminum square tubing) instead of steel?

Again, thank you all for this. If it ends up coming to fruition I will make you ALL some table legs that may or may not weigh 400000000 lbs!
 
  • #9
sophiecentaur
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I do have an entire machine shop at my disposal
Ain't you the lucky one?
A lightweight plastic version will answer and raise a number of questions for you but I'm sure it's the way to go.
Good luck with the project, Do some sums about the weight of parts. I doubt if the legs will be too heavy, compared with a nice thick slab of hardwood.
 
  • #10
jrmichler
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It won't be that heavy. That top is 8 X 3 X (2" / 12") = 4 cubic feet of wood. Walnut can weigh up to about 40 lbs per cubic foot, so 4 X 40 = 160 lbs. Search density of wood for more information on wood weight. You can calculate the weight of aluminum or steel legs if you know that aluminum weighs 0.10 lbs per cubic inch, and steel 0.283 lbs per cubic inch.

Supporting the weight of the top is the least important strength requirement. The table top support system (fancy term for legs) needs to support the top plus a large person perched on one end plus somebody pushing the whole thing sideways. I suggest making a prototype leg out of steel or aluminum, fastening it to a 3 foot square piece of 3/4" plywood, then doing some sitting and pushing tests. I'm an engineer, not an artist, so I like aluminum or stainless steel for legs. No coating to chip off, never rust, can be finished to a polish or satin finish, lasts forever. If you decide to go with aluminum, get a recommendation for a strong, weldable alloy. Not all aluminum can be welded. Aluminum is available in grades from weak as butter to stronger than many steels.

If you fasten channels to a 2" thick top, be aware of expansion due to humidity. You could fasten your legs directly to the top if you weld some pads to the legs and use wood screws. That top will be rigid, so you may need leveling feet under the legs. Even if the table is built perfectly straight, it will be setting on a floor that is not perfectly straight. Here is a source of leveling feet: https://www.mcmaster.com/feet.

Looks like a fun project.
 
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  • #11
JBA
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The main weakness could be in the ability of the bases to rotate and twist the prism shape and there's nothing that can inherently stop that except the presence of the legs at the other end - which should always be there under normal circumstances.
With regard to the susceptibility to twisting, this can be an important element even though it shouldn't be an issue with both stands sitting on the floor, consider what might happen if someone decides to rotate the table to a different direction and only picks up one end to do so.
 
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  • #12
Tom.G
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My first impression was that buckling of the vertical part of the legs might be a concern, but probably @jrmichler has already thought of that. If not, maybe making the verticals I-beam or box-beam would solve it.
 
  • #13
sophiecentaur
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My first impression was that buckling of the vertical part of the legs might be a concern, but probably @jrmichler has already thought of that. If not, maybe making the verticals I-beam or box-beam would solve it.
One nice part of the design is that the planes of the strips (large dimension) in the three legs have components in the direction of any applied lateral force. This gives an impression (illusion) of a weaker structure than it actually is. The plastic model should reveal basic problems.

You can compare the design with the simplest, tried and tested, construction with four box section legs, one at each corner (standard cheap meeting room type. 1" box section, welded to a rectangular frame seems to do the job fine so I think the 2" X 0.5" strip is unlikely to be a problem.
 
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If weight is a concern, I recommend keeping it low enough that 2 people can easily lift it to reduce injury risk. Earlier I nearly crushed my manager at work moving an insanely heavy table
 
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  • #15
sophiecentaur
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If weight is a concern, I recommend keeping it low enough that 2 people can easily lift it to reduce injury risk. Earlier I nearly crushed my manager at work moving an insanely heavy table
Tut tut. Had you been on a 'lifting' course.?Health and Safety will getcha if you don't watch out!
But what's the use of a table that's at knee level? You can't get close enough to sup the soup.
 
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  • #16
jrmichler
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My first impression was that buckling of the vertical part of the legs might be a concern, but probably @jrmichler has already thought of that. If not, maybe making the verticals I-beam or box-beam would solve it.
You are correct, I did think of buckling. For a very short time. That frame could be made of 0.25" X 2" wood. A wood frame would not buckle under normal loads, and most woods are 15 to 30 times more susceptible to buckling than steel. The critical load cases are, as has been mentioned above, pushing sideways and twisting by picking up one end and moving sideways. Analyzing those cases by hand calculations would be difficult. Such an analysis would be easy using FEA, however getting the boundary conditions correct would be a challenge. That's why I recommended building a prototype.

That table will not be easy to pick up. The top alone will weigh on the order of 150 lbs, and the frame will add 50 to 100 lbs more or less depending on material and size.
 
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  • #17
Tom.G
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That table will not be easy to pick up. The top alone will weigh on the order of 150 lbs, and the frame will add 50 to 100 lbs more or less depending on material and size.
That weighs almost enough to be a billiard (pool) table. Any plans for that @Mads W ?
 
  • #18
jrmichler
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Friends. Strong ones. Several of them. And free beer after the job is done.
 
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  • #19
sophiecentaur
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Having a live edge doesn't mean the whole area pdf the table top needs to be full thickness. A terrible waste of lovely walnut, perhaps but you could rout out the majority of its underside area to form a recess 1" depth and that would almost halve the weight.
You could use 1"X 1" angle, to form a frame, joining all six legs. The planned 2" X 0.5" strip could be made thicker and wider, perhaps. Your CAD package will allow you to check the aesthetics of the chunkier design. A thinner veneer 'box' of copper could be wrapped around thicker steel legs. But the edge in the existing design does look nice.
 

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