Project Summary

This was a group project for my freshmen design course, with the goal of creating a chair out of a single sheet of foamcore board. This chair had to withstand 200lbs on the seat and 40lbs on the back rest. The only allowable materials were hot glue and the single sheet of foamcore board.

The purpose of this project was originally to introduce various aspects of the design process, but now a few years later I've found it's a good opportunity to practice CAD.

Overall, our group designed the best chair, far exceeding the load parameters, and vastly outperforming every other group.

Design Summary

This is the fully assembled chair prior to painting. Due to our material constraints, we opted for an interlocking grid base, which offered ease of assembly, and what we anticipated would be a highly rigid structure due to the abundance of fixed joints. The back of the chair is supported by 3 pillars, which interlock within the base structure, as well as behind the backboard with four sets of struts. These struts were added after the initial assembly was finished, due to a collective fear within our group that the chair may begin to twist during testing, causing the otherwise cantilever ends of the pillars to bend.

Results

Our chair was an awesome success throughout testing. The 200lb seat load was applied by having the heaviest adults sit on the chair, while the 40lb backrest load was applied with some rope near the top and center of the backboard, and measured with a spring scale.

Whereas many chairs failed even the initial base load tests, our chair was easily the most firm. No other chairs passed the backrest load test, although a few groups came close, however our chair easily surpassed the 40lb qualifying load, and was topped out at around 70lbs.

As we feared, at the highest backrest loads, our chair did begin to buckle at the base of it's back support pillars, as the struts designed to prevent this didn't extend far enough down. Luckily, this did not effect the overall structural integrity of our chair.

The video on the right, which is stitched together from a couple test clips, showcases this deformation in the final clip.

Post Analysis

Revisiting this project with some basic FEA skills, I decided to run a few test cases at the test backrest load of 40lbs and at 80lbs, while also mitigating the buckling at the base of the support pillars.

At 40lbs no remarkable deformation occurred, similarly to our actual chair. At 80lbs however, the back support pillars began to elastically deform. More careful analysis of critical stress points, as shown on the right, reveals that plastic deformation is very near to occurring where the support pillars meet the base of the chair.

This is the point where the bending moment on the support pillars causes the greatest amount of tensile stress, and due to the brittle behavior of foamcore in tension, these points would almost certainly have been the origins of tears in the pillars at greater loads.