Domain: ketchum.org
Stories and comments across the archive that link to ketchum.org.
Comments · 9
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Re:Aeroelastic flutter
Here's an older paper:
http://www.ketchum.org/billah/...
The distinction is drawn at the end of part III. Seems to me to be pure semantics. If the bridge were driven to flutter at a self-resonant frequency then yes, it was a resonant phenomenon. Does causing a wine glass to emit a tone by running your finger around the rim constitute a resonant phenomenon? The variation of the driving force being at the resonant frequency is caused by the wine glass vibrating at that frequency already, as was the variation on the torsional forces on the bridge being caused by the bridge already vibrating.
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Re:Strawman
I'm quite confident that in my "introduction to engineering" class back in 1985, the professor explicitly used the word resonance when showing us the movie of the bridge collapse.
A quick search finds evidence textbooks have indeed called the phenomenon that brought down the bridge 'resonance':
https://en.wikipedia.org/wiki/...
If you want to skip wikipedia and go right to the citation:
http://www.ketchum.org/billah/... -
Re:Bridge failure
The Tacoma Narrows bridge apparently was not designed not to collapse - the designer failed to factor in the high wind speeds in the Tacoma Narrows and the resulting resonant effect on the structure into the bridge design.
Before you re-write history, check the news reports of the day. It wasn't a very windy day. The bridge was stable at much higher winds. The moderate wind and the direction was just right to produce a resonant feedback. It wasn't high winds that too the bridge down. It was steady mild wind that kept putting more motion into a resonant system.
References;
http://www.pbs.org/wgbh/nova/bridge/meetsusp.html
At the time it opened for traffic in 1940, the Tacoma Narrows Bridge was the third longest suspension bridge in the world. It was promptly nicknamed "Galloping Gertie," due to its behavior in wind. Not only did the deck sway sideways, but vertical undulations also appeared in quite moderate winds. Drivers of cars reported that vehicles ahead of them would completely disappear and reappear from view several times as they crossed the bridge. Attempts were made to stabilize the structure with cables and hydraulic buffers, but they were unsuccessful. On November 7, 1940, only four months after it opened, the Tacoma Narrows Bridge collapsed in a wind of 42 mph--even though the structure was designed to withstand winds of up to 120 mph.
http://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge
The wind-induced collapse occurred on November 7, 1940 at 11:00 AM(Pacific time), due partially to a physical phenomenon known as mechanical resonance. [4]
And for sake of balance here is a modern study stating it wasn't resonance but instead a negative feedback;
http://www.ketchum.org/wind.html
" . . . in many undergraduate physics texts the (1940 Tacoma Narrows bridge) disaster is presented as an example of elementary forced resonance . . . Engineers, on the other hand, have studied the phenomenon . . . and their current understanding differs fundamentally from the viewpoint expressed in most physics texts. In the present article the engineers' viewpoint is presented . . . It is then demonstrated that the ultimate failure of the bridge was in fact related to an aerodynamically induced condition of self-excitation or "negative damping" . . . This paper emphasizes the fact that. physically as well as mathematically, forced resonance and self- excitation are fundamentally different phenomena.
The one common thread in all the above is it was not a high wind that took the bridge down. It was the feedback pumping energy into the motion. -
This stuff isn't new
Galloping Gertie was the first large-scale project to incorporate this material.
http://www.ketchum.org/tacomacollapse.html -
Re:A reverse scenario
Er, well, the building basically exists, but the students in your story do not.
The building is Kresge Auditorium. It was designed by Eero Saarinen, one of the most famous architects of the 20th century. He also designed (e.g.) the St. Louis Arch and the TWA Terminal at JFK Airport.
A somewhat biased but detailed view of Kresge Auditorium is available here. As you can see, no PhD theses are mentioned.
The building's roof is a single thin concrete shell. The original design was very ambitious, such that the roof was to be supported only at the three points where the shell contacts the ground. The design was later changed so that the mullions in the large banks of windows would bear some of the load.
Kresge Auditorium was one of the first buildings of this type. More thin shell concrete structures available here. -
Re:Nano-cables can in principle be made at scale nAh, so tens of thousands of years away instead of tens of years away?
Eh, show me an engineering project that's used them. Until then, it's more "magic business beans" (I like the current IBM commercials! They're still the deposed evil empire, but I like the commercials.)
Something as easy as fusion power has remained 20 years away for fifty years now. Until you can buy it or make it, it's a science project, not engineering. (I'd worry about the stability of the composite matrix over time, radiation and stress, but that's just me.)
I'll say it again: we need a new space transportation system within ten years.
Beanstalks are a nice idea, but scaling up from a centimeter to 100,000 KM is a few orders of magnitude, plus other things we might not be aware of yet might cause trouble. (I'm sure that we knew all about bridge building in 1940. Or did we? I doubt tidal resonance would ever be a problem, right?)
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It makes senseI think this is a perfect example of what a good engineering program is all about. Engineering isn't just about what's best, it's about learning from the mistakes of others while gleaning the best elements of their designs for future use. As a Waterloo Mech grad, I think back to the basic engineering teaching tenets of "This is good design" (e.g. The F-117A) and "This is bad design" (e.g. The Tacoma Narrows bridge - I LOVE THAT MOVIE).
This new course can be looked at from both perspectives. C# is state-of-the-art, easy to use (it's case-insensitive just like VB!), has a great support organization behind it, and will undoubtedly achieve good market penetration. On the other hand, it's very new, still has flaws, has security holes, and is generally not quite ready for prime time. Since these students are the ones who will be coding the next generation of languages (face it, a lot of them will probably end up at MS), it's better that they should be familiar with what's out there now, and what's wrong with it.
On the other hand, I'm now a six-year Java vet, and I have no intention of switching...
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Re:Software Engineering at MicrosoftWell, that's an oxymoron for you!
You do realize that this is considered to be engineering.
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Of course it sucks....The analogy between engineering programs and engineering buildings is a reasonable one, and I've seen it used before. But there's an implication everybody seems to have overlooked.
If making a complex program is anything like putting up a large building, then we shouldn't be suprised if most programs are seriously flawed. We've only been doing software engineering for a few decades (somewhere between 1 and 12, depending on how you define the concept). Builders and architects have been honing their skill set for for several thousand years. And they still screw up occasionaly. You can argue that such failures are tragic, but are necessary for engineering to advance.