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Take a close look at the arch in the above image. In its shape is the secret of its stability. If you hold a length of string loosely, its dependant curve will resemble that of the arch. The shape is called a catenary and you can see it in all kinds of structures, both tensile and compressive. Suspension bridges, spiderwebs, the lines between ship and pier. The catenary's signal feature is that the force involved in holding it up is aligned with a tangent to the curve, at all points. What this means is that there's no sideward force, as there would be with a semicircular, Roman arch. I knew none of this when I made my first arch. The shape came from my fingers and only when I visited the Exploratorium in San Francisco did I learn why that arch worked. The legs lean inward, increasing their lean, but the increasing sideward load is countered by an equal lean in the other leg. Gravity is a sand sculptor's friend, holding the sand together. This is a basic shape for any sand sculpture that flies. Being a compression structure, the catenary arch requires sufficient cross-section to bear the imposed load. This is true along the arch's entire length, and I've had them collapse due to soft spots in the legs. It's a balancing act. The legs must be gradually thinned so that no one area overwhelms another. The bases must be thick enough, in terms of the number of grains of sand--coarse sand requires relatively thicker legs--to hold up the whole piece. The catenary arch is a starting point. You can use it to make other shapes in your sculpture, the basic principle being to make sure any undercut shape curves sufficiently to balance the loads. A straight slanting sculpture element will tend to fall, but a little curve to give it more compression will help it hold. There are limits. The cross-section of the whole piece must be adequate to carry its own load and that of any element depending upon it for support. Elements must lean on each other strongly enough to hold the sand, but not so strongly that one pushes the other over. The limits of compression loading are strongly affected by the sand you're using. Coarse sand is very difficult. Fine sand will make delicate structures, and sand with silt in it, as long as the silt stays in place between grains instead of settling into layers, will do almost anything. | |||||
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crossect.htm 1999 February 14