1. City College of New York School of Engineering
Mechanical Engineering Department
Spring-2014
Mechanical Engineering I 6500: Computer-Aided Design
Instructor : Prof. Gary Benenson
Student : Mehmet Bariskan
Interpretive Paper #1 : Role and Development of
Engineering Judgment

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This is a report comprising some of my notes and thoughts after reading three articles, in order: a
critique of engineering failures titled “How Engineers Lose Touch” by Eugene S. Ferguson. The
Second article, an impressively comprehensive breakdown of the details of FEA titled “The
Question of Credibility” by Jack Thornton. The third article, “The Mathematical Disposition of
Structural Engineers” by Julie Gaingsburg, who offers a very thorough analytical study of
engineers in practice.
As engineers, we should always try to reduce the amount of errors with our knowledge and from
experience that is belongs to us or other engineers. Although it is not always possible, it is great
to have someone who is capable of solving problems with mathematics, as well as his or her
judgment. Failure to recognize the power of the judgment in engineering has led us to
catastrophes such as Minnesota Bridge collapse, Tacoma Narrows Bridge collapse, the space
shuttle challenger accident, and other fatal accidents.
Therefore, the responsibilities of engineers should beyond the mathematics and calculations. In
Ferguson’s paper, there are some examples of critical failure. These failures are due to faulty
judgment rather than faulty calculations. For instance, in the 1979 accident at the nuclear power
plant at Three Mile Island. The level of the coolant in the reactor vessel was low because a relief
valve remained open while, for more than two hours after the accident began, an indicator on the
control panel said it was shut. When the solenoid was off, he assumed, the valve would be closed
while it was open. The designer didn’t show the position of the solenoid, and it has caused the
accident. Another example On July 3, 1988, the ship shot down an Iranian civilian airliner,
killing 290 people. The Aegis system had received IFF (Identification, friend or Foe) signals for
both military and civilian planes, yet the ship’s radar indicated only one plane, and the decision
was made to destroy it. These examples indicate that the lack of judgment of the designer due to
inability to see the real-life situation of the application of the system. During the design process
of the Aegis system and reactor system, the designers failed to realize that the end user are
human beings and that such crucial decision cannot be made within such a short amount of time.
It is wise to learn from the history of engineering practice. We might not be able to become
professional in every engineering field, but we certainly cannot allow ourselves to neglect
relevant knowledge from other engineering fields. We were able to obtain such valuable
knowledge in the class toward the end of the semester. The FEM programs such as Solidworks
have limitations, and possible design mistakes that lie within them. It is also important that if a
profession becomes too inward looking, it might causes faulty judgment during the design
process. One of the examples in the Ferguson’s article illustrates the point very clearly. The
collapse of the Tacoma Narrows suspension bridge was due to the lack of consideration in
aerodynamics effect on the bridge. If the designer had followed traditional ways of designing the
bridge, without omitting the vertical stiffening on the deck, the collapse would have never

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happened. It was because of Ammann’s idea of building bridges with this concept that led to the
collapse, said David Billington, an unorthodox professor of civil engineering at Princeton
University. In this case, judgment was necessary in the sense that knowing beyond one’s
engineering profession is very important.
From all the history of failures due to faulty judgment, we have realized that judgment is
obtained through personal experience, as well as events throughout the history of engineering. In
this new era, we should not only learn how to use modern software to solve problems, but also
learn how they are developed, and what limitations they have. In addition, we can always expand
our views in engineering by simply striving to understand how things work in our real world, and
realize that not all engineering problems are readily solvable.
Engineers are responsible for every aspect of the analysis. An engineer should have a good
understanding of the science pertaining to the field as well as experience that will provide
intuition to make proper assumptions when definite information is limited. Gainsburg speaks of
these traits of an engineer in her article “The Mathematical Disposition of Structural Engineers.”
As quoted from Gainsburg, “The first and most indispensable design tool is judgment. It is
engineering and design judgment that not only gets projects started in the right direction but also
keeps a critical eye on their progress and execution.” “It is judgment that separates the significant
from the insignificant details, and it is judgment that catches analysis from going astray” (p.121)
Also, as quoted from Gaingsburg, “, “it emerged that, for some engineers, personal experience
with elements and lab-test photos were powerful resources for judgment.” And “Experience just
seeing, being on a job and seeing what’s been built and knowing beyond just the theoretical.” In
other words, the sense of engineering judgment can be possibly improved by simply seeing the
object, interacting with it, and learning to “feel” it. When I compare these quotes with my
experience in this course, I totally agree with these sentences. In this course, we had chances to
observe various types of mechanical systems before starting the projects. Especially in the FEM
project #2, we have investigated the many pulley systems to get an in depth idea of how they
work, and I have conducted my experiment based on those observations. The boundary
conditions of the model were determined through engineering judgment rather than just a theory.
I have used all my effort, but the solution was not converging at the beginning. As Gainburg
said, it was going to go to astray. After then, I have remembered the sentence that Professor
Benenson was keep telling us “have you looked at your data or model” then I compare my 3-D
model and the real object that we have observed. Pulleys were being forced from the motor, but
it was obstructed to turn by a belt. I have changed my boundary conditions, according to this
observation and I had a converged solution.
In the Thornton’s paper, there are some ideas about the FEA belongs to its developers,
practitioners and customers. As quoted from Thornton, according to FEA software developers,

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“It is unfortunate but too many engineering managers do not understand the vast complexity of
the physical world that FEA addresses.” Before this class, I was in the group of engineer who
don’t understand the complexity of the physical world that the developers of the FEA were
mentioned in Thornton paper. Also, as quoted from his paper, “Engineering problems often come
to us with an expectation of getting a number from an analysis, or just verifying a number they
already have. Unfortunately, a number is just a point and cannot tell you very much.” Then, I
have recalled the FEM modeling Project#2. I was studying to find the maximum stress with the
different radius of fillets. The professor of this class has put a note on my paper after he has
reviewed it. “These results are not meaningful. Did you look for convergence?” Unfortunately, I
have just made one study of each fillet, and my results were not meaningful. Either professor’s
opinion or the software developer’s opinions were right. Looking for a specific number shouldn’t
be our target. Usually one or more mathematical curve is needed so that any single number is
evaluated within some context. According to FEA practitioners, “Understanding FEA means a
journey into stress, strains, fatigue, materials science, and the myriad of mathematical ways their
interrelationships are represented, including kinematics.” In my whole life in engineering
especially with this class, I have noticed that the engineering is not just a number. It is feeling
and understanding the meaning of the problem and it is a journey through the numbers.
I have been using SolidWorks to make a design for manufacturing process since 2005. A lot of
my colleagues kept telling me, I don’t need to take a Computer Aided Design course since I have
already known to make a design. But this course has given me the idea that how a student
becomes an engineer. We have learned many opportunities to get and develop an intuitive “feel”
of how the real object fails, with the aid of touching and seeing these objects in the class. We are
also trained to make engineering judgment, in which we seldom do in other courses. Since the
class title is computer aided design, we have been exposed to both the advantages and
disadvantages of the application of SolidWorks.