Solving Stress Problems with Soap Bubbles

Serious engineering work.

Calculating the stresses in a member, and determining if they are within it’s capacity, is one of the main tasks of a structural engineer. Often this is accomplished by making simplifying assumptions that allow the use of relatively simple mathematical methods. But for anything other than the simplest shapes under the simplest loading conditions, these approximations don’t work. In these situations, the difficulty of calculating stresses ratchets upward, and requires solving second-order partial differential equations.

Today, thanks to modern computers and tools such as finite element analysis, solving these sorts of problems has become relatively trivial. But before these aids existed, complex stress problems were simply too difficult to solve mathematically. Because of this, alternative methods had to be devised for working out the stresses, methods that didn’t rely on solving intractable equations. One of the more ingenious of these was the soap-film method for calculation torsion stresses.

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The Infrastructure Report Card: What “Deficient” Actually Means


ASCE’s 2013 infrastructure report card was recently released, and can be found here. Once again, the results are dismal. Our infrastructure received a “D+” overall.

Unlike almost everything else that engineers do, the infrastructure report card garners a fair bit of notice. In any discussion of government funding priorities, the state of our infrastructure is frequently brought up, and the infrastructure report card along with it. Because it has such high visibility, there’s also been some skepticism about the accuracy of the ratings. It’s been suggested that the ASCE might be exaggerating the extent of the problem – “juking the stats”, as they say – since a worse infrastructure means more work for engineers.

Because the report card covers a broad swath of infrastructure projects, we’ll just look at one particularly noticeable portion – the bridge section. The salient stat here is the percentage of structurally deficient bridges. The easiest way to pad these numbers would be the inclusion criteria – what qualifies as a “structurally deficient ” bridge. So what does it take to qualify? Are the stats on the level?

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