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I am trying to design a real cardboard chair for a project I have in my engineering modeling course. The chair should be able to support a weight of 250 pounds. I think we are free to use any type of cardboard in the project

How can I use available cardboard to make suitable legs? Would cylinder tubes be stronger than rectangular box legs made of glue layers of cardboard (possibly corrugated)?

Does anyone have experience using cardboard in props or floats that would have an idea on what works for supporting heavy weights?

Catija
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method
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    First and foremost would be: How big is this supposed to be? Life size or a scaled model? The chair is made from cardboard? How much weight is it supposed to handle? What are you allowed to use as far as hardware and supplies? Glues at least I would hope. Cylinders (done properly) will distribute stress better then squares for sure. Some more information would be useful. Thick cylinders with a thick base would make a chair easily. The back could just be for show. comfort could be subjective. What is the actual criteria for this project of yours? – Matt Oct 04 '16 at 02:31
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    method, this sounds like a question better suited to our Engineering site... but be very wary of sounding like you are asking folks to "do your homework" for you. Asking about these structural concepts works well on that site,but questions should start with a background of what you know and tried to lead into a very specific question seeking a bit of guidance about the obstacle you are trying to overcome. Good luck! – Robert Cartaino Oct 04 '16 at 13:14
  • Since you've not asked on [engineering.se] that I can tell, I've tentatively opened this question for our stack, with some minor modifications. If you ask on Engineering, we can close/remove the question here. –  Oct 08 '16 at 19:28
  • Resin impregnated cardboard may be cheating but is worth looking into. A honeycomb structure for the seat might be interesting – Chris H Jan 03 '17 at 11:51

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You don't specify what kind of processing is permitted on the cardboard, if any, but if you have relatively free rein I'd recommend investigating papier mache - it has been used historically for furniture making so could represent a very viable option for making chairs out of cardboard.

Options range from a very basic and functional version made from essentially cardboard tubes glued together, to very ornate items such as seen here:

From papiermache.co.uk

At the most complex end of the scale, paper and cardboard can be glued in layers to make a material which is almost as strong as wood, and can be cut, carved and sanded in much the same way. This is very time consuming and can require specialist equipment so it's not particularly practical for hobby applications, but it goes to show the kind of strength that can be obtained.

Glorfindel
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Tara Hanratty
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Look into the several cardboard stools which are commercially available. And read the limitations on those as far as weight is concerned.
A google images search 'cardboard stool' will reveal many stools and several chairs.

This link leads to a stool on a commercial site, very basic two sheets of cardboard crossing and a third sheet on top as seat, they rate it to 150 kg (330 pounds), and adults rather than just kids.

If you have plenty cardboard you can do a couple of tests with heavy weights but as long as you have the card board in its strongest position (vertical) and with enough body (thick corrugated board or folded and used with more fields thinner corrugated boards) you will be good for your weight limits.

As far as I remember a box as people use to move house will rate about the strength when just folded shut, adding tape or glue to fix the shape will add strength and you might be able to cut away part of the sides opening up the structure.

An other approach would be to stack cardboard, any kind, to the needed height and use that for the base. Cutting out parts of the sheets will give the impression of legs while still keeping enough strength.

Commerically made cardboard tubes will be strong enough for a chair, even fairly thin ones, but the problem will be how to attach them to a seat or frame for the top of the chair.
In work we use the stretch wrap film that goes around pallets when loaded and that comes on tubes of about 2"(5 cm) diameter, about 20" lengths (50 cm) (From memory, might be off a bit) and those will make sturdy legs if you can fit them into your frame or seat strongly enough.
Those tubes should be available from any company near you that wraps their loads.

Just using glue to attach a tube to a plain flat field of cardboard is likely going to result in a failure while using or even already in the building stages.
You are more likely to get it working when you carve a connection out of the cardboard of the seat or build up a core where the tube/leg slips around.

Willeke
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  • Carpet shops may also have tubes available. Even poster tubes could take the weight and are cheap to buy, With all tubes you're likely to need to cut them very square -- maybe make an oversize mitre box for the cutting. – Chris H Jan 03 '17 at 11:49