Interstate 35W Bridge in Minnesota Collapses

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Postby Shapley » Fri Aug 03, 2007 2:14 pm

We can discuss the details of the engineering design processes for this SF dip if you wish, but it will be a somewhat technical conversation. In summary, it was not malicious, but it was a result of what were known as the best engineering practices at the time and the limitations of the tools and information available to the designers.


I've not suggested that it was malicious, I think it was done out of a cost-
consciousness inherent in the undertaking of a large-scale project such as the Interstate Highway System. I would also expect that the design loadings of interstate bridges was lower than the actual loadings they experience. Metal Fatigue, while known, was not fully understood at that time (if it can be said to be fully understood today). Material quality control probably wasn't as good as it is today, either. It is quite simple to get by with substandard materials in a bridge that designed with a safety factor of 2, harder with a safety factor of 1.5, and probably impossible with a safety factor of 1.2. Again, I'm not sure of the criteria used in design of the interestate highway system, but a look at the design of a 4-lane interstate bridge compared to a 2 lane bridge of turn-of-the-centrury design reveals a significant difference in design, tilting (again in my unqualified opinion) in favour of the turn-of-the-century design being superior. Not to mention that they are generally more aesthetically pleasing. Look to the Brooklyn Bridge as an example, or to any of Brunel's bridges in the United Kingdom.

My belief is that the dip in safety factors was borne out of an arrogance of engineering. There was an established thinking in the post-war world that anything could be accomplished better, cheaper, and faster with 'new' technology. We had defeated armies, harnessed the power of the atom, and built a fleet of ships and planes faster than had ever been done before. We were bold, we were daring. We were ignorant. The technology that won the war was built to outlast the Germans, not to outlast the generations. Centuries of engineering expertise that had taught us to overbuild, overbuild, overbuild was tossed on the rubbish pile. We could build better bridges with half the materials, we had the technology, or so we thought.

To be fair - they have held. They simply are designed as disposable architecture. Designers at the turn of the century designed their bridges to be monumental - to last for generations. Designers of the post-war era designed for the here-and-now. Any engineer worth his salt knows that what goes up must come down, and that a structure is built as a time delay to control the time frame until the eventual return to Earth. the engineers of the turn-of-the-century and the engineers of the post-war era merely disagreed as to what a reasonable timeframe should be.

V/R
Shapley
Last edited by Shapley on Sat Aug 04, 2007 4:30 pm, edited 1 time in total.
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Postby dai bread » Fri Aug 03, 2007 4:51 pm

Interesting comments, Shap., and they bear out what's happening here with our harbour bridge.

The original structure was designed & built in the 1950s, and opened for traffic in 1959. It was a 4-lane bridge, and soon proved to be woefully inadequate in capacity.

During the late 1960s, the extensions known as the Nippon Clipons were added, giving another 4 lanes, 2 per clipon. They were designed on totally different principles from the original rigid structure. They were designed to flex under load, and it's quite alarming to be under the bridge and watch the big steel beams moving down as a truck goes over the top.

The result after 40-odd years is that the clipons crack regularly, are inspected almost daily, and repairs are common. The original rigid structure is still as sound as the day it was built. At least, I haven't heard otherwise.
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Postby piqaboo » Tue Aug 07, 2007 3:56 pm

There's a fascinating book on the balance in engineering between using new technology etc to its abilities, and staying with the tried and true.

I believe the title is "To Engineer is Human".

"To Engineer is Human" link
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Postby analog » Thu Aug 09, 2007 9:25 am

They were designed to flex under load, and it's quite alarming to be under the bridge and watch the big steel beams moving down as a truck goes over the top.


Failure analysis is a fascinating niche field of study, figuring out why things broke. In the last thirty years the progress in miniaturizing computers has made it possible to measure and analyze the movements of machine parts like bridge beams, and do some predicting. The railroads started in the seventies instrumenting a railcar to check the condition of their track, and in in the nuclear plant we monitored the structural health of our reactor internals remotely. This bridge worry seems a natural application for vibration signature analysis. As a structure weakens its natural frequency and probably its damping too just have to change. That's easy to measure and will probably become part of routine bridge monitoring.

There's a TV documentary on building of the new Missouri river bridge going into Alton Illinois, I think called "Superbridge". It's a modernistic spiderlike suspension bridge. There's a haunting scene where the construction superintendent says, to the effect: "Anybody can build a bridge that'll stand a hundred years. It takes finesse to build one that'll barely stand." Anyone who remembers the line more precisely feel free to correct me.
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Postby piqaboo » Thu Aug 09, 2007 9:54 am

One of the points the author makes in TEIH is that when calculators came along, people kind of forgot about significant figures for a while.
They believed the calculators.

Computers make it much easier to evaluate many factors simultaneously during design. And of course, failure analysis exists to identify those factors no one thought were significant initially, aka the unknown.
The author gives lots of examples of huge failures, such as airplanes that fell out of the sky, and how they contributed to our knowledge of failure types.

I think in bridges built in the last 4 or so decades, we'll find the failures are due to maintenance failures rather than design flaws. Somehow, the ongoing costs of things are always underestimated.

I;ve not been keeping up with the news :oops: . What was the final death toll attributed to the bridge?

I find it interesting that at least one section that collapsed was entirely on land, and that the bridge broke in such tidy sections (all straight ended). Selma's favorite commute is designed to break in chunks, with the middle three designed to float so they can be hauled out of the way leaving the channel free for GBGB passage.
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Postby Shapley » Thu Aug 09, 2007 3:38 pm

I;ve not been keeping up with the news . What was the final death toll attributed to the bridge?


The last figure I saw had the known dead listed as 5, with 8 missing.
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Postby barfle » Thu Aug 09, 2007 3:55 pm

They found two bodies today. As of this writing, the total is seven dead and at least six missing and presumed dead.
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Postby Trumpetmaster » Sat Aug 18, 2007 12:57 pm

We flew into Minneapolis on the way home....
Saw the collapse from the plane as we were coming into Minneapolis.

Horrible site....
Words just cannot describe it....
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Postby BigJon@Work » Tue Aug 21, 2007 1:15 pm

Yes, Shap, arrogance is a much better word than malice.

I've done failure analysis for about 8 years. Kept me on my toes.
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