Why Roman Concrete Outlasts Ours

Recently, there’s been a flurry of news surrounding a new paper which examined the mineral structure of concrete samples taken from a 2000 year-old Roman breakwater. The articles range from measuredly pointing out it’s carbon efficiency, to extolling it’s  near-mystical properties. The fact that these structures are still intact after millennia, while ours often decay to the point of uselessness after less than 50 years, obviously raises some questions. Namely, was Roman concrete better than ours? Why does ours fail so quickly?

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New Southern Pine Design Values

On June 1st, new design values for southern pine lumber came into effect. These results are based on full-scale testing of various lumber sizes, and supersede the interim results that went into effect last year, which only affected 2″-4″ sized lumber. The kick in the teeth is that the new values show a sizable decrease in capacity for compression, bending, and tension, with reductions ranging from 10-30%. More information can be found at the SPIB site.

Allowable design values vs. actual strength at failure.
Allowable design values vs. actual strength at failure.

The changes are the result of the large-scale destructive testing of thousands of pieces of southern pine lumber. Wood is a highly variable material, and so requires a large number of samples to reliably establish safe design values. This sort of testing first began in the late 1800’s, and is conducted every so often by lumber testing organizations. Testing standards have changed over the years, but currently must follow ASTM D 1990. Testing organizations must be certified by the American Lumber Standards Committee. There are currently seven organizations, which are responsible for various regions and wood varieties. Southern pine lumber is covered by the Southern Pine Inspection Bureau.

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Using The Tools You Have

MacGyver: World’s Greatest Engineer

Engineering is a game of optimization under constraints. Problems are never just “design a beam that can span a hundred feet“, but “design a beam that can span a hundred feet, is made of concrete, weighs less than 40 tons, and is less than five feet tall.”  Or, more likely, “design a beam that can span a hundred feet as cheaply as possible”. Problems with only one requirement are easy to solve – it’s the ones with multiple, sometimes conflicting requirements that require clever solutions.

One of the most important of these requirements is “…and design it using only these tools“. This isn’t something that shows up in the design contract, but it’s a necessary reality. The tools humans have invented so far, be they wrenches or word-processors, are a limited subset of what’s theoretically possible to accomplish. And the tools any given engineer will have available are a limited subset of that. Much like MacGyver, we can’t solve engineering problems any way we’d like. We have to use whatever junk happens to be lying around.

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An Introduction to Graphic Statics

As I’m so fond of mentioning, engineering design required the use of a number of creative methods before the invention of calculators and computers. Some of the most important and widespread of these were graphic methods of analysis. Graphic methods essentially translate problems of algebra into geometric representations, allowing solutions to be reached using geometric construction (ie: drawing pictures) instead of tedious and error-prone arithmetic.

Unfortunately, these methods are slowly being forgotten. It’s extremely rare to ever see them used, outside of a select few occasionally taught in structural analysis courses. But understanding how, and more importantly why, they work unquestionably makes for a better engineer.

To remedy this, this post will lay out some of the basics of graphic statics. If there’s interest, more posts on more advanced methods will follow.

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