Tensegrity

Tensegrity is a portmanteau of tensional integrity. It refers to the integrity of structures as being based in a synergy between balanced tension and compression components.

The term "tensegrity" was first explored by artist Kenneth Snelson to produce sculptures such as his 18 metre high Needle Tower in 1968. The term 'tensegrity' was coined for Snelson by Buckminster Fuller. Fuller is best known for his geodesic domes, which he developed based on concepts explained and demonstrated by Snelson through sculptures. The term "synergetics" may refer more abstractly to synergetic systems of contrasting forces.

Concept

Tensegrity is the exhibited strength that results "when push and pull have a win-win relationship with each other." Tension is continuous and compression discontinuous, such that continuous pull is balanced by equivalently discontinuous pushing forces. Buckminster Fuller explained that these fundamental phenomena were not opposites, but complements that could always be found together. Tensegrity is the name for a synergy between a co-existing pairs of fundamental physical laws; of push and pull, and compression and tension, or repulsion and attraction.

If one pushes a ping-pong ball on a smooth table with the point of a sharp pencil, the ball would always roll away from the direction of the push, first rolling one way then the other. Push is divergent. On the other hand if attaching a string to the ping pong ball with tape, and pulling it creates convergence. No matter how other forces might influence the ball to roll away from you, the string would always bring it to you more and more directly. Pull is convergent. Similarly, pulling a trailer uphill with a car, the trailer will converge toward the same course behind the car. If the trailer begins to sway, increasing pull by increasing acceleration can dampen the swaying motion. Driving downhill however will cause the trailer to push, and the trailer will show tendencies to sway from side to side.

The rubber skin of the balloon can be seen as continuously pulling (against the air inside) while the individual molecules of air are discontinuously pushing against the inside of the balloon keeping it inflated. All external forces striking the external surface are immediately and continuously distributed over the entire system, meaning the balloon is very strong despite its thin material. Similarly, thin PVC plastic membranes such as a plastic bags are often stronger when loaded rather than unloaded.

Applications

The idea was adopted into architecture in the 1980s with David Geiger designing the first significant structure -a competition

hall for the Summer Olympics of 1988. The Georgia Dome, which was used for the 1996 Olympics is a large tensegrity structure of similar design to the aforementioned

competition hall.

Theoretically, there is no limitation to the size of a tensegrity. Cities could be covered with geodesic domes. Planets and stars (Dyson sphere) could be contained within them.

"Tensegrity is a contraction of tensional integrity structuring. All geodesic domes are tensegrity structures, whether the tension-islanded compression differentiations are visible to the observer or not. Tensegrity geodesic spheres do what they do because they have the properties of hydraulically or pneumatically inflated structures."

As Harvard physician and scientist Donald Ingber explains:

"The tension-bearing members in these structures – whether Fuller's domes or Snelson's sculptures – map out the shortest paths between adjacent members (and are therefore, by definition, arranged geodesically) Tensional forces naturally transmit themselves over the shortest distance between two points, so the members of a tensegrity structure are precisely positioned to best withstand stress. For this reason, tensegrity structures offer a maximum amount of strength."

Biology

The concept has applications in biology. Biological structures such as muscles and bones, or rigid and elastic cell membranes, are made strong by the unison of tensioned and compressed parts. The muscular-skeletal system is a synergy of muscle and bone, the muscle provides continuous pull, the bones discontinuous push.

Stephen Levin's web site is a good start for further information on this topic.

References

Donald E. Ingber, The Architecture of Life, Scientific American Magazine, January 1998
Buckminster Fuller, SYNERGETICS—Explorations in the Geometry of Thinking, Volumes I & II, New York, Macmillan Publishing Co, 1975, 1979
Seeking the Gift Tensegrity Dr. Timothy Wilken TrustMark 2001

External links

Kenneth Snelson's site with an excellent article on the theory and development of tensegrity as well as pictures of his sculptures from desk top pieces to 90 foot towers.
Tensegrity in a Cell -- This interactive feature allows you to control a cell's internal structural elements. From the research department of Children's Hospital Boston.
Stephen Levin's Biotensegrity site Several papers on the tensegrity mechanics of biologic structures from viruses to vertebrates by an Orthopedic Surgeon.

Tensile architecture | Buckminster Fuller