1. Sustainable Chair Project
Blake Johnson
Abstract— The goal of this paper is to obtain a complete
understanding of the concepts required in the design of a
sustainable chair out of a weak and flimsy material. The general
concept and thinking behind the project, as well as the analysis
of the reason behind the success of the project is the essential
purpose of this project. Statics was the main mathematical base
to the entirety of the project, as well as a small knowledge of
materials and deformation. The idea of tension vs compression
was made apparent during class and directly applied to the
design. The ability to realize how each part was going to react
with one another was the most essential ability to obtain during
the process of the design.
I. INTRODUCTION
The chair was the first project in the Mechanical Design
class. It involved the design of a single material and involved
no other material in the entire design process: which also
excluded mending material. The design chosen was a design
that focused on limiting material by using the smallest
measurements required; this was due to the fact that one of
the requirements was to keep the design under 500 grams.
The chair dimension requirements were to use a minimum
of 360 mm wide, by 360 mm deap, by 480 mm high chair.
Therefore the design used was a seat that was a perfect
circle with a 360 mm diameter, and legs that stood 480
mm high.(see Fig. 3). The most important requirement in
the project was that the design had to withstand the weight
of Professor Spenko when he sat on the chair, and leaned
back into the back rest.
II. CONCEPT GENERATION AND EVALUATION
The concept that came to fruition was one that kept the seat
at it’s minimum requirement in order to avoid large bending
moments, which were easily acquired due to the weakness
of the material at hand. On top of this there was an attempt
to keep the seat at a height minimum as well in order to
prevent a moment within the supports. The demensions were
not the only requirement in the design, because the weight
of the design was also something that was required to stay
under 500 grams. Due to the fact that the material was so
weak, much of the focus was on the base and support of
the chair. With these known requirements, a pugh chart was
used in order to determine which original drawing had the
best design. The largest weight of this pugh chart focused on
the weight of the structure, and how stable the structure was.
Then the dimensions were lightly taken into consideration as
well. (see Table 1)
III. ANALYSIS
The design used was almost exactly one of the original
ideas drawn up before the design process even began. (See
Fig. 1. Picture taken of the fully assembled chair, just moments after the
chair successfully withstood the test.
fig. 2, Drawing 3) The supports used were designed to cut
out as much excess weight as possible, while still keeping
the integrity of the supports intact. This was done to allow
the stress lines of the design to flow through the supports
and to prevent massive buildups of stress in different areas.
The original design used only had two supports cross over
each other, but as we moved further along in the design
process, there was a realization that that design was going
to easily twist and bend in the supports. This was the
biggest problem that was present in the entire project. With
the material used, massive bending moments throughout the
support system would have been catastrophic. There was
also an issue in the stress analysis of the structure. (see
fig. 3) There was a large stress concentration in the seat
itself. And the stress analysis showed that there was not
enough material in the support system. In order to detour
the seat from collapsing and large bending moments from
happening, supports running at a forty-five degree angle
from the original supports were designed and used. The

2. Drawing 1 Drawing 2 Drawing 3
Fig. 2. Three original drawings chosen for the project.
TABLE I
PUGH CHART
Criteria Wt Dwg 1 Dwg 2 Dwg 3
Mass 3 +1 +1 -1
Height 1 +1 -1 0
Seat depth 1 +1 -1 +1
Seat Height 1 +1 -1 +1
Stability 3 +1 -1 +1
Total +5 -3 +2
Weighted Total +9 -3 +2
dwg - Drawing, Wt - Weight
original two standing supports interlocked at a 90 degree
angle, and the added supports formed a perfect square
between all four quadrants that stood from the floor and
interlocked with the seat. These added supports interlocked
into a second base that interlocked into the original standing
structure. Most of the material used throughout the entire
design was in the base and supports. This formed another
obstacle where we had to use very limited material, yet
make a structure that could resist the weight of a person
leaning on it.
When the backrest was first designed, there was an
emphasis to not change the original seat and design, yet by
further inspection, it was realized that there would not be
enough material to resist shear by interlocking the backrest
with the seat. Therefore, the design was changed, and the
supports were cut out to fit the backrest throughout the
entire underside of the seat. This had a negative effect on
the supports of the chair, but since the backrest support was
still resting on the support and the seat was still resting
on the backrest, the structure showed little deformation
due to the weight exerted on it. Another positive of this
design was that when the backrest was leaned on, it caused
a moment about the pivot point on the back of the chair,
and helped bare the weight of the professor. Since material
is almost always less prone to shear in tension than it is
in compression, this helped prevent the chair from collapsing.
The final issue encountered was that when the weight
analysis was completed, the chair was at 499.4 grams. Even
though this was under the required 500 grams of material,
this was just an estimate. It was found through other trials
done by other groups, that the estimation was off by about
ten grams or so. Which meant that the estimation would have
been about ten grams under the actual weight of the design.
So just before the project was put through the cutting phase,
there was an adjustment to the design. In order to limit the
amount of weight, it was necessary to run the stress analysis
in order to determine the stress concentrations and where to
avoid cutting weight. (see fig. 3). As you can see, there was
a large build up of stress on the outside of the seat, but that
build-up was concentrated, for the most part, on the seat
itself; not the underlying supports. It was decided that the
least amount of material needed was in the cross supports
that helped prevent torque in the original design. There was
a truss system added to help prevent twisting in the design.
When the new assembly was put through the stress analysis,
it was observed that there was no added stress concentration
in the design. On top of this, one of the cross supports on
the backrest was negated. This design feature helped shave
10 grams off of the original design estimate, which ended
up being essential. When weighed just before the test, the
chair weighed in at 496 grams.
If the requirements had changed so that the chair had to
withstand a greater load, then there would have had to have

3. Fig. 3. Stress Analysis, along with lines of force
been adjustments to the design itself. The size of the chair
dimensions would actually not have to be changed. What
would would have to be adjusted is an increase in the amount
of material used in the supports. And potentially a little more
material directly underneath where the supports interlocked
with the backrest. When the chair had to bear weight, the
area directly underneath these points had a slight deformation
which caused the rim of the seat to loses stability.
IV. EXPERIMENTAL RESULTS
When the design was tested, the chair seemed rather
reliable. There was very little cracking noises coming from
the chair that would have indicated failure somewhere in the
chair. There was slight deformation directly underneath the
supports for the backrest that rested directly on top of the
main supports. This was because of a failure in recognition
of force buildup in a small area when the design had to be
changed for the backrest. Also there was slight deformation
in the seat due to the bearing of weight. But this was more
of a warping effect than a failure in the design. And the
final design flaw recognized was the backrest where the pivot
point was. Although it did not break, the backrest started
to bend, and did not return to its original design position.
This was due to limited resources, and an attempt to limit
materials. The backrest was too thin at the top, and did not
have enough of a ribbing by the pivot point where all of the
torque was going to build up. Outside of these slight design
flaws, the chair was a complete success, and if done again,
would have required minor adjustments in order to prevent
these minor flaws from occuring again.
V. DISCUSSION
To start with the design, there was an emphasis from the
beginning to limit the mass of the structure. Therefore the
first tactic in reducing the weight was to use the smallest
dimension requirements. Secondly, the seat was designed to
be a circle rather than a square, which also made it easier to
support the outer edges with less material since the radius
was constant. This also made it easier to design from the
inside out. The original support system was going to involve
only two large interlocking legs. When designed, there was
a cut out that started from the outer corner and connected
tangentially to a large circle. The reasoning for the circular
design was to allow the force lines to transmit smoothly
throughout the entire structure while also preventing mass
buildups of forces in concentrated areas. Then, there was a
realization that the supports were not going to be able to bear
the torque about the center, so there were supports added.
These supports interlocked with a second set of stands that
interlocked with the original supports at a 45 degree angle.
In order to limit weight and keep structure integrity they
were cut with a large radius that extended from the center of
the new support to the very edge. The final weight limiting
component was used on the supports that created a square
support system was cut through the center of it. These cuts
extended through 75 percent of the height and about half of
the width. In order to prevent massive pressure buildup, these
cuts were made with semicircles that had the same diameter
as the cuts, and in order to keep some structural integrity, a
truss system was designed in order to help prevent twisting
about the center of the supports.
VI. CONCLUSIONS
The sustainable chair was a project that required engi-
neering capabilities and knowledge to design. There was
a necessity to adapt to certain situations and issues in
the design process. There was an effort to bring statics,
mechanics of materials, and innovative ideas and combine
them together in order to create a light weight chair that
could withstand a relatively astounding amount of weight.
This project is directly related to the real world, and what
engineers do on a daily basis. The ability to work on a
deadline, given a ”budget” and to directly apply everything
learned up until this point to a project is something that is
going to be entirely what this profession has been, and will
continue to be about.