Previous Page [ To Contents ] Next Page

Amy C. Edmondson
A Fuller Explanation
Chapter 15, From Geodesics to Tensegrity: The Invisible Made Visible
pages 245 through 249

Nature's Example

Look around; nature's been using tensegrity all along. Humanity was able to overlook this structural truth for thousands of years because tension tends to be invisible. Seeing rocks sitting on the ground and bricks piled upon bricks, we have developed a virtually unshakable "solid-things" understanding of how Universe works. The ubiquitous tension forces, from gravity to intermolecular attraction, tend to be more subtle:

      ... at the invisible level of atomic structuring the coherence of the myriad atomic archipelagos of a "single" pebble's compressional mass is provided by comprehensively continuous tension. This fact was invisible to and unthought of by historical man up to yesterday... there was naught to disturb, challenge or dissolve his "solid things" thinking... (7)

      The inseparable partnership of tension and compression does not mean that the two forces are the same. On the contrary, fundamental differences are the basis of their successful interdependence.

      Compression is local, discontinuous, says Bucky. When we load a column, we push it together. If we push a thin column too hard, the column will buckle like a banana; there is no other way for the stressed member to yield. For this reason, compression members are subject to an inherent limit to their length relative to their cross-sectional area, called the "slenderness ratio." The development of stronger materials has increased that ratio only slightly over thousands of years: from a maximum of 18: 1 for stone columns in ancient Greece to approximately 33 :1 for modern steel. The limit is not as much a result of inadequate materials as of geometry: compression is directed inward, and hence eventually forces the overstressed column to buckle. Compression fights against the shape of a strut.

      Tension, on the other hand, pulls apart. The direction of tension serves to reinforce the shape of the stressed member. Pulling straightens; pushing bends. As a result, assuming stronger and stronger materials, there is no inherent geometrical limit to the length of a tension component. While the capability of compression members has remained more or less the same, tension materials have improved by leaps and bounds, and significant advances continue today.

      Compression was the sole basis of man-made structures until the tensile strength of wood was exploited, enabling for the first time the construction of structures light enough to float on water: simple rafts, initially, followed by progressively more sophisticated rowing and sailing vessels. But at 10,000 pounds per square inch, the tensile strength of wood was still overshadowed by the 50,000 psi compression strength of stone masonry. Not until 1851 saw the first mass production of steel—with a tensile strength equal to its compression-resisting capability of 50,000 psi—was tension finally brought into parity with compression, explains Fuller; "so tension is a very new thing." This development enabled the Brooklyn Bridge in 1883 and ushered in a whole new era of tensional design. Scientists rapidly created metal alloys of greater and greater tensile strength with less and less weight, ultimately leading to jet airplanes and other previously inconceivable miracles. A new material called carbon fiber, with an unprecedented 600,000 psi, was responsible for a recent "miracle". In 1979, Paul MacCready pedaled the first human-powered aircraft, which he called the "Gossamer Albatross," over the English Channel; two years later he repeated the journey with a completely solar-powered plane. The only reason he was able to accomplish this feat, emphasizes Fuller, was that the extraordinary tensile strength of carbon fiber allowed the plane to have a wing span of 96 feet, while weighing only 55 pounds; you could hold it up in one hand. But the newspapers didn't mention the carbon fiber, he declares sternly; "nobody talks about this invisible capability." This beautiful example of "doing more with less" was a favorite for Bucky, who never forgot being told by well-meaning adults during his first eight years, "Darling, it is inherently impossible for man to fly."

      Driving toward a specific observation about Universe, Bucky describes theoretically ideal structural components for each of the two forces, as suggested by their different characteristics. Why will a short fat column not buckle under a compressive force that easily breaks a tall thin column of the same material and weight? Geometry governs the situation as follows. A system is most resistant to compression in one direction, namely along an axis perpendicular to its widest cross-section, in which case, its vulnerable girth is as strong as possible. This is the neutral axis. The wider its girth, the more impervious a column will be to compression. Therefore, a short fat column is better able to resist buckling than a tall thin column of the same material and weight, because the latter lacks sufficient resistence perpendicular to the line of force. Similarly, if a compression force that will easily break a long column (a pencil, for example) is applied perpendicular to its length, that column will be unharmed (Fig. 15-9a, b). That much is common sense, but in less extreme situations an understanding of the "neutral axis" is necessary to enable the prediction of exact results.

Compression strength in columns
Fig. 15-9
Click on thumbnail for larger image.

      Next, imaging loading a slightly malleable cigar-shaped column; compression causes the girth to expand, forcing the column to become progressively more spherical. This transformation suggests a candidate for the ideal compression component (Fig. 15-9c).

      A sphere is the only shape in which every axis is a neutral axis, which is to say, a sphere's width is the same in every orientation. Therefore, this shape resists compression from any direction; it cannot buckle. Hence the ball bearing. This tangible example illustrates the ideal design for compression.

      What about tension? Evidence of longer, thinner, and ever more resilient tension materials suggests that there's no inherent limit to length. Fuller takes this a step further: "May we not get to where we have very great lengths and no cross-section at all?" He answers his own question, "This is just the way Universe is playing the game." Gravity is that invisible limitless tension force. "The Earth and the Moon are invisibly cohered..."; the tension cable has reached the limit case in thinness: it's nonexistent. "You have enormous tension with no section at all." A splendid design! The solar system is thus a magnificent tensegrity: discontinuous compression spheres (i.e., planets) are intercoordinated—never touching each other—by a sea of Continuous tension. "Every use of gravity is a use of...sectionless tensioning," Fuller continues, observing that "this is also true within the atoms: true in the macrocosm and true in the microcosm" (645.03-5).

      By mid-twentieth century, it was clear that the design plan of Universe involves islanded compression and continuous tension, but up until then

      man had been superficially misled into [thinking] that there could be solids or continuous compression. .. Only man's mentality has been wrong in trying to organize the idea of structure. (645.04)

Previous Page [ To Contents ] Next Page