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Before geodesic domes appeared on the scene in 1948, the dome of St. Peter's Cathedral in Rome, with a diameter of 150 feet, was unchallenged as the largest architectural clear span. 150 feet must have been seen as a fundamental upper limit, a sort of divine zoning law. Ultimately, the inherent stability of triangles cannot alone account for the geodesic dome's ability to span unlimited distances with no interior supports-nor for its unprecedented strength-to-weight ratio.
The rest of the explanation lies in an understanding of tensegrity, Fuller's contraction of the two words tension and integrity. "Tensegrity describes a structural-relationship principle in which structural shape is guaranteed by the... continuous, tensional behaviors of the system and not by the discontinuous exclusively local compressional member behaviors" (700.011); Fuller thus introduces a discussion of the interplay of tension and compression forces in Universe. His term also refers to the inescapable co-occurrence of tension and compression, while its first syllable emphasizes the too often overlooked role of tension.
All systems consist of some combination of tension and compression forces. The two are inseparable.
All systems? What about a simple piece of nylon rope pulled at both ends between two hands? Isn't that pure tension?
Try to visualize the experiment: pull as hard as you can on both ends and notice what happens to the thickness of the rope. Its diameter shrinks slightly, betraying the invisible compression force around the rope's circumference, or perpendicular to the applied tension. Even in this simple example, unplanned compression is inevitable.
The same is true in reverse. Applying compression to a column introduces surface tension around its girth. Pushing from both sides along the long axis of a strut causes the outside surface to stretch, albeit slightly. Tension and compression go hand in hand, as simultaneous complementary functions; however one or the other usually dominates a given situation.
641.01 No tension member is innocent of compression, and no compression member is innocent of tension...
641.02 Tension and compression are inseparable and coordinate functions of structural systems, but one may be at its "high tide" aspect, i.e., most prominent phase, while the other is at low tide, or least prominent aspect. The visibly tensioned rope is compressively contracted in almost invisible increments of its girth dimensions... . This low-tide aspect of compression occurs in planes perpendular to its tensed axis... .
We thereby draw the distinction: our rope is a tension element; the loaded column, a compression member. (6) Effective design must balance the two interdependent forces in preferred ways.
The apparently insurmountable limit to the clear-span of a structure existed because the interdependence of tension and compression was not fully understood. In general, the history of construction reveals an overwhelming dependence on compression. Our concept of building has been inseparably tied to that of weight; early humanity piled one stone on top of another, and we continue to employ the same single strategy, fighting gravity with sheer mass. But compressional continuity has its limits, such as the impossibility of achieving spans greater than 150 feet. Any larger dome would collapse under the force of its own weight. Moreover, although architects may not have reflected on the principles of tensional integrity, necessity apparently forced them to add a powerful iron chain around the base of St. Peter's dome; the outward thrusts from all that compression needed further restraint.
Fuller decided that a better approach was needed than that of slapping on a bandage at the end. Following nature's example, tension must be designed into the structure at the start. In fact, tension must be primary.
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