Slope Stability of Pretzel Bags

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We civil engineers have been reduced to button pusher status. We have good minds, and they are in the backseat. Job ads ask if you know certain software packages. We should use the mind God gave us and not rely on all this computer stuff so much. In my practice we use Mathcad and Roark’s Formulas for Stress and Strain and AISC and ACI 318-2008. I taught Statics and Dynamics, Strength of Materials, and Surveying at the college level. Also I have performed forensic analysis on soil-related failures. Charles Steinmetz, the electrical engineer, put GE on the map a long time ago. The defense rests.

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Thank you for this article. Your comments are spot on. I am a bridge engineer with a state DOT. I believe that in many ways the engineering profession has become too entrenched with the “numbers” and our fascination with the pursuit of precision.

As a well-seasoned engineer, in my position I routinely have to remind the younger engineers to take a step back and think about the ultimate goal of building a well-designed structure that can be accurately constructed and that has details that do not inherently cause maintenance problems in the future.

Sometimes a little light goes on, and they realize that endless hours spent with their mountains of numbers do not make them better engineers. The numbers are just tools, not the ultimate goal of engineering. Their ideas, engineering judgment, and ability to put that into a set of plans and specifications are what we need. Those gold nuggets are not mined from the numbers, but from experience, exercising their judgment by learning from their own accomplishments and mistakes and those of others.

The real battles are fought on the construction side. Fabrication and constructability issues many times control member types and sizes. In addition, consideration must be given to access for inspection and repair. And sometimes, just plain old experience shows that making certain members smaller or thinner will ultimately be the cause of problems in the future.

One fact that all state bridge engineers will proclaim today is that thanks to those engineers, in years gone by, who used those old simple conservative design methods and specifications, our bridges have enough load capacity to stay alive today under the massive loading cycles they have endured in the years since and we continue to call upon today. Our bridge infrastructure is in poor condition, but the situation would be a lot worse without that extra capacity due to their conservative methods.

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I have an old book by Steinmetz whose pages are unmarked except for a page deriving a formula for sizing flywheels to give enough stability to electrical alternators that is given a check mark. Electrical engineering makes more use of “higher mathematics,” I believe, than civil engineering.

The relationship of engineering to mathematics is a love/hate one. I believe but cannot prove that the prominent role mathematics has always played in formal university-style engineering is partly due to the fact that mathematics was always an important part of a classical university education and followed classical mathematical lines, so that areas of immense importance to engineering such as numerical mathematics were relatively neglected (Silas Holman, a professor at MIT in the 1890s, wrote that in calculations not exceeding 20 operations, 5 place logarithms are generally sufficient, and he also wrote that at least half the effort expended in computation is wasted effort).

I remember being marked wrong and admonished by a math teacher when I used infinitesimals in calculus, even though I got the right answers. I found calculus taught in an old textbook that way; I used it in preference to what was being taught in my assigned textbook, and I liked it a lot since it made solving problems easier and, to use current jargon, “transparent.” Since that time, mathematicians have proved that infinitesimals have a rigorous justification.

I discovered a bug in an early computer program when, in manually plotting a train speed distance curve, I found the train accelerating on a steep upgrade. The bug was due to an instability in the computer program, so depending on the initial conditions, half the time the program would converge to the wrong value, which if the grades were moderate, would escape casual inspection. The job was done over with a slide rule. The result was not as precise as a computer solution would have been because of the necessary approximations, but more importantly, it was reasonably correct.

All that said, someone said the role of mathematics in engineering is to provide insight, not numbers.

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Ah, the life of an engineer. Who else would be mesmerized by such an event? It brings back fond geotechnical engineering memories.

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I had a teacher (yes, I call him a teacher) in college who said if all you want to do is work with formulas and computers, then you were no more than a technician. Anyone can plug and chug to get numbers. The true application of physics, math, education, and experience makes an engineer.

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I believe that the superaccuracy of today’s computer design programs does in fact encourage the design of “anemic” structures … those that theoretically work on paper but are trimmed so close to the bone in required sections for the purpose of “economy” that they represent serious future risks. Changing load, weather, seismic, potential damage, and other unexpected conditions can bring a seemingly well-tuned and -proportioned structure into the “red zone” quite unexpectedly.

I remember, in years gone by, the “sacrifice steel” or “mill overrun” that was incorporated into structures in excess of the required cross-section, especially on riveted plate girder railroad bridges I had encountered, which served to keep these structures functional as time and usage took their toll—their very mass withstanding the service demands imposed upon them.

In this age of “deferred maintenance,” superslender members use high-strength steel and concrete and have potential anomalies (fatigue failure, crack propagation of welded members, posttension failure, etc.) that can affect a structure. … Perhaps a compromise should be made in our perception of the “economy” of a minimized computer-designed structure by factoring in the costs of repair or replacement due to potential losses of service life. The key word—experience—should effect a correct balance between what the computer dictates and what history dictates.

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I have a meeting, every Tuesday morning, at a cafe, where a bunch of small business owners meet for business-to-business marketing. I am The Engineer. The chairs in our meeting room tend to fall over backward when people get up to speak, especially if they have put their pocketbook on the back, or a heavy coat. Each time, I say, “Factor of Safety = 1.” It’s interesting that some of these people are actually starting to say it before I do—and that after so many repetitions of what that means from me, they actually know what they are saying.

I’m working on explaining the return on investment of preventive maintenance, too.

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I am not a railroad bridge engineer, but it may be that the seeming ability of old railroad bridges to stand up to present-day traffic may be due in part to the fact that the old steam locomotives delivered hammer blows to the bridge and emitted “smoke,” more accurately described as “acid rain,” to the bridge members. The railroad bridge community had to design for these adverse conditions no longer present.

The old-time, but not too old-time, railroad engineer also had the advantage that with time, some designs would be shown to be failures; thus, a bridge could be “designed” just on the basis of selecting a “successful design” from a “catalog” of successful designs. This may be equivalent to someone attributing a long life to behavior when in reality a large part is due to inheritance.

Sometimes adding extra material can actually make things worse. For example, an unneeded stiffener in a plate girder can serve as a locus of corrosion. With respect to explaining the economics of preventive maintenance, one can cite “a stitch in time saves nine.” For popular presentations, I think “margin of safety” is a better term, and 0 means all the margin has been eaten up.

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I am not an engineer, but I am fascinated by the subject. Yes, engineering is very interesting to nonengineers when they are exposed to it. Your pretzel bag grabbed my attention so fast, as I will be interviewing an engineer on the subject shortly. I have a small trade magazine, and on this month’s issue I used a Slinky image, outside the box for a magazine but a useful tool as you all know to show wave equation analysis.