Cardboard Chair

I did this project just for fun. I wanted to make a sturdy chair that an unsuspecting person wouldn’t realize was made of cardboard.

Design

I started off by figuring out what kind of chair I wanted to make. Not a table chair that gets scooted around all the time, and nothing too fancy that it’d be difficult to create. I settled on a lounge chair. It wouldn’t be moved around often, and the lounged position helps distribute a persons weight over more of the chair making it structurally simpler to design.

I took some measurements of myself (I’d be lounging in it the most right?) and spec’d the chair out based on those measurements. The chair would be ~32”Lx 20”W x 32”H.

Structure

Then I did some research into how strong cardboard is. Luckily, the cardboard industry rates cardboard using an “Edge Crush Test.” The ECT units are lbs/inch (note that’s linear inch not square inch). Typical single wall cardboard is 32 ECT. Knowing this, I could find how many inches would be needed to support some amount of weight. There were some complications. Being a lounge chair meant the weight was more evenly distributed, but also meant I had to guess what that distribution would be. I somewhat arbitrarily chose a 12” x 2” area at the bottom of the seat to represent a butt carrying ~65-70% of a person’s weight. I also had to figure someone would plop down on it, resulting in momentary spikes in apparent weight. All in all, after accounting for the weight distribution, I ended up going with 13 frames (480 linear inches) of 32 ECT. I know, I know, 15,000 lbs of capacity?! But when we look at just the butt area, for someone my size with a little bit of a safety factor, it actually ends up right at that 32 lb/in limit (see figure at right).

Build

I chose to attach the cardboard accordian-style for two reasons. First, the vertical connections here gave added stability to the individual frames and as a result the chair as a whole. Second, I added ease of setup and take down as a priority. The string running through the frames and terminating in buttons at both ends make setup much quicker, limiting the accordian from overextending. Additionally, they keep the outer sidewalls from flapping freely since there is no other mechanism holding them in place (yet).

The spacers serve a few purposes. They keep the frames equally spaced, helping even out the loads between them all. They also serve the purpose of maintaining the chairs width, countering the string-provided load. Last, and most importantly, they help keep the chair upright. Without these spacers, the rolling motion of the chair is only supported by the shear strength of the accordian connections, which are only stapled and not very strong. Each set of spacers has two connections to the chair, which in turn are connected by a flat section. The chair’s rolling motion results in a shearing force on this flat section, a force the flat section is able to counteract.

A piece of cardboard is scored to follow the chair’s curves and laid over the top, significantly helping distribute the weight. This was actually a nice surprise - I wasn’t planning on doing this, but had extra cardboard. Then a layer of foam and finally a fitted fabric cover.

Result

It’s definitely a comfortable chair. My sewing skills (well, lack of) gave away that this chair was homemade but no one guessed it was made of cardboard. Fun project and easy chair to pack up and take places - as long as they’re dry…

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