Wednesday, October 10, 2012

The Rise of the Arch

The virtue of the true arch, whether used for an opening into or a hollow space within a structure, is that in ages before steel came into use as a weight-bearing member, the arch could make a wider opening and/or a greater hollow than any other form of support, be that columns holding the horizontal lintel of a gate or supporting a roof. And the secret the ancients discovered was that stone is easily cracked and broken when weight above it causes it to stretch (tensile force) but resists weight much more effectively if the weight causes it to compress instead (compressive force). For this reason, before the discovery of the arch, huge temples were forests of tall columns, not much more than 10 feet apart—although nicely aligned.

The earliest arches, known also as false arches, emerged as far back as the second millennium BC. They attempted to reduce the tensile stress on bearing members by extending the supporting columns stepwise into the opening, as shown by the illustration of this early kind of arch, known as a corbel. Corbelled arches predate the true arch by two thousand years and are found all over the world, including ancient South America.

The Etruscans invented the true arch, also shown as a diagram. By using wedge-shaped stones, they converted tensile stresses on the stone from above into compressive stress, so that the force exerted on the stone is transferred laterally to the columns—as shown in the illustration. The Romans learned the art, perfected it, and then applied it with enormous exuberance to every kind of building, including enormous aqueducts. The true arch was solidly in place by the second century BC. The later flying buttress (mentioned on this blog here) is an extension of the principle that manifests in the arch.

In contrast to stone or brick steel stretches easily under weight while retaining its integrity. When steel beams began to be deployed as weight-bearing members in the nineteenth century, the functional role of the stone arch was diminished. Not that such did not continue to be built—indeed they still are. The massive deployment of steel in buildings is, of course, the consequence of plentiful supplies of cheap fossil fuels. As these sources of energy are exhausted, the arch, no doubt, will once more become an important technology—the enabler of generous interior space.

I bring this post today by way of illustrating a point I made yesterday about technology. Technology cumulates even as large social structures decay, die, and others are then born in turn. The corbelled arch survived the decline and fall of Sumer and Akkad; the Roman arch survived the decline and fall of Rome and may rise in the future. How the old is passed on to the present is illustrated neatly in two YouTube videos. The first is titled “How I Build Stone Arches,” by Mike Haduck (link). It’s hands on. Amusingly, it illustrates how modern and ancient technologies cooperate—when Haduck uses an electric saw to shape his stones…and then consults an old book that illustrates arch-building early in the twentieth century. The second, “History of Visual Technology: Stone Construction and the Arch,” presents an excellent tutorial on the fundamentals involved in this art—and how it has evolved over time (link). The arches closest to our house? Why, they are part of our house (built 1929). We walk beneath them each time we go somewhere.

The stresses in matter can, by careful observation, be made to harmonize beautifully. The stresses in large aggregates of people—not quite so effectively. But the human experience is highly layered. As we leave the upper levels where the compressive stress of union and the tensile stress of freedom are most visibly in conflict, great constructs eventually give way—while at lower levels useful tooling is never quite lost even if knowledge about it thins out and has to be recovered from old books using Xerox machines.

To this, as a card-carrying word-maven, I need to add a caution. Architecture does not have its roots in “the arch.” In that word the root is archon, the Greek for “master” or “chief”—chief builder. The arch has its root in the Latin arcus; it stands for the “bow.”
Images from Wikipedia link for the corbel and link for the true arch.

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