May 22, 2010

Needle Tower

Slim and graceful, Kenneth Snelson's Needle Tower stretches 60 feet into the sky. The structure looks too delicate to stand so tall, but it's strong enough to withstand severe storms.

Erected in 1968 beside the Hirshhorn Museum and Sculpture Garden on the National Mall in Washington, D.C., this tapered framework of aluminum tubes and stainless-steel cables is an example of a tensegrity structure. The tubes aren't connected to each other. Instead, cables thread through the tubes to hold the assemblage together in perfect balance.

Snelson discovered the underlying principle for such structures in 1948, advocating the term "floating compression" to describe the balance between tension and compression and, in his sculptures, between flexible cables and rigid tubes. R. Buckminster Fuller (1895-1983) coined the word "tensegrity" (combining "tension" and "integrity") for the same idea, and his term stuck. Snelson refers to weaving as the "mother of tensegrity."

Snelson defines "tensegrity" as follows: "Tensegrity describes a closed structural system composed of a set of three or more elongate compression struts within a network of tension tendons, the combined parts mutually supportive in such a way that the struts do not touch one another, but press outwardly against nodal points in the tension network to form a firm, triangulated, prestressed, tension and compression unit."

Snelson's Needle Tower delivers a wonderful geometrical surprise when you venture underneath and look up to see a striking pattern of six-pointed stars.

This pattern arises naturally out of the requirement that each layer of a tensegrity structure consist of three compression elements (tubes). The sets of three alternate, giving the impression of a six-pointed star as you look up the tower. Snelson's sculptures often show this kind of symmetry.

The elegance of Snelson's tower suggests its use as an aesthetic alternative to conventional communications towers. But tensegrity structures are fairly elastic and flexible. They sway in the wind, which may not be ideal for the antennas and dishes that would top such structures.

Needle Tower recently required some conservation work. A video shows a crew of museum exhibit staff raising the repaired sculpture back into place on Hirshhorn Plaza, clearly demonstrating the structure's strength and flexibility.

Photos by I. Peterson

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