Tensegrity

The principle

Tensegrity is a structure whose integrity lives in its tension, not in its stacking

Tensegrity is the structural principle in which rigid members in compression never touch one another and are held in position by a single continuous network of members in tension. That is the entire definition. The compression members are islands. The tension is the sea they sit in. Cut one cable anywhere in the net and the object does not sag at that spot, it fails everywhere at once, because no part of it was ever resting on any other part.

The word belongs to Buckminster Fuller, who contracted tensional integrity into tensegrity and was granted US Patent 3,063,521, Tensile-Integrity Structures, on 13 November 1962 (Fuller 1962). The first object belongs to Kenneth Snelson, an art student who built one in December 1948 and then spent sixty years explaining that he had. Both of those sentences are true. This page will carry both, because the history reads better when nobody has to lose.

Why does a library about the nervous system keep a file on a sculpture? Because tensegrity answers a question that classical anatomy answers badly. Anatomy draws the body as a stack. Vertebra on vertebra, block on block, load descending through bone until it reaches the floor. Tensegrity draws a different object, and the different object behaves the way living bodies are actually observed to behave.

Black Mountain, 1948

A summer session in North Carolina put a twenty one year old in the room with Buckminster Fuller

Kenneth Snelson, born 29 June 1927 in Pendleton, Oregon, arrived at Black Mountain College in the summer of 1948 as a second year art student from the University of Oregon. He was twenty one and he was there to paint. The session had been organized by Josef Albers, and the roster around him that summer included Willem de Kooning, John Cage and Merce Cunningham. Buckminster Fuller turned up two weeks late, a last minute substitute for a professor of architecture who never came.

Fuller spent that summer trying to raise the first geodesic dome out of venetian blind strips. It would not stand. The students named the collapsed thing the Supine Dome. The failure did nothing to slow him down. He lectured for hours at a stretch on structure as an accounting of forces rather than a catalog of shapes, on tension and compression as the only two things there are, on the universe as a system that balances rather than a set of objects that lean.

What did Snelson take home from that? Not a method. A question. He went back to Oregon in the autumn with a head full of forces and no instruction whatsoever in what to build. That gap is the whole story. Fuller supplied the frame of mind. Snelson supplied the object, alone, in a state where nobody was watching.

December 1948

The Early X Piece stood eleven inches tall and nothing inside it touched anything else

Through the autumn of 1948 Snelson worked through a run of small studies. In his own later account the sequence went from modular elements hinged one to another and stacked vertically, to modules suspended one to the next by thread slings. Each step removed a little more contact. Then in December he removed the last of it. Early X Piece is two wooden X forms held apart by nylon tension lines, measuring 11.5 by 5.375 by 5.375 inches, and the two X forms never touch.

Why should that matter to anyone? Because until that object existed, every structure human beings had ever built passed compression through contact. Arches, columns, trusses, cathedrals, bridges. Something pushes on something else, and the push travels down a continuous chain of touching parts to the ground. Early X Piece breaks the chain. Its two rigid parts are strangers. They are held in exact relationship by nothing but string under tension.

Pick it up and turn it over. Nothing inside shifts. Nothing settles. It does not care which way is down, because its shape was never a consequence of gravity. That indifference is the signature of every structure that followed.

Summer 1949

Fuller turned the sculpture over in his hands and understood immediately what he was holding

Snelson returned to Black Mountain in the summer of 1949, the session Fuller himself directed, and he brought the plywood X-Piece with him. In his 1990 letter Snelson wrote that Fuller had plainly not grasped the thing from the photographs sent ahead (Snelson 1996). He was struck by it. He held it. He turned it over and studied it.

Then Fuller did the characteristic Fuller thing. He announced that the configuration was wrong. The modules, he told Snelson, ought to be shaped to the central angles of a tetrahedron. That instruction tells you almost everything about the man. He did not simply admire the object. Within minutes he was redesigning it into the geometry he had already committed his life to, because Fuller ran on tetrahedra the way other people run on habit.

On 22 December 1949 Fuller wrote to Snelson and acknowledged, in writing, his original demonstration of discontinuous pressure, which Fuller wrote as com-pressure, together with continuous tension. That letter survives. It is the reason this argument has a documented answer rather than two men remembering a room differently. Whatever came afterward, Fuller committed the credit to paper within six months of first seeing the piece.

The naming

Fuller compressed tensional integrity into one word and the word carried the credit with it

Fuller coined tensegrity around 1955, contracting tensional integrity into a single term. Naming is never a neutral act. Before the word, the object was a curiosity a young sculptor had made. After the word, it was a principle, and principles belong to whoever states them. In Synergetics, published in 1975, Fuller set the definition out formally at sections 700.01 and 700.011 (Fuller 1975).

Read the definition below and notice what is in it and what is not. It is exact. It is elegant. It generalizes far past sculpture, all the way to atoms and solar systems, which is precisely what Fuller wanted from it. It also contains not one proper noun. Snelson understood the mechanism better than anyone. His complaint was never that Fuller had removed an object from his studio. It was that a name functions as a claim, and that if the word is yours, people will assume the idea was too.

Snelson himself never wanted the word. He preferred floating compression, which is a better description and a worse brand. He was also more modest about the principle than Fuller ever was, arguing that its real value lay in what it revealed about structure and beauty rather than in engineering efficiency, and describing his own works as force diagrams standing in three dimensional space.

The dispute

The fair account names both men, and neither of them loses much by it

The documented record supports a clean split. Snelson built the first tensegrity structure. Fuller named the principle, generalized it, patented it and made it famous. Everything painful in this story sits in the gap between those two sentences.

The grievance has a chronology. In January 1951 Fuller published the structure without naming Snelson. Snelson recalled asking him about it and being told, in what has become the most quoted line in the whole affair, that he could afford to remain anonymous for a while. Fuller filed his patent on 31 August 1959 and it was granted on 13 November 1962, and Snelson is not named anywhere in it. Public acknowledgement finally arrived in 1959, when Fuller invited Snelson to the Museum of Modern Art to see a tensegrity mast in an exhibition titled Three Structures by Buckminster Fuller, curated by Arthur Drexler, running from 22 September 1959 to 1 March 1960. Snelson made the fuss. The credit followed. It had taken eleven years.

The argument then ran for decades. In 1980 Fuller answered a one page letter from Snelson with twenty eight pages of his own. In November 1990 Snelson wrote to the French engineer Rene Motro, who was assembling what became a special issue of the International Journal of Space Structures on tensegrity, published in 1996, and set the record out in full (Snelson 1996).

Being fair to Fuller costs nothing and is closer to the truth. He appears to have experienced ideas as belonging to the universe rather than to persons, which is an appealing philosophy right up to the moment it lands on a twenty two year old with no reputation to spend. He credited Snelson privately in 1949 and verbally in lectures for years, and failed to credit him in print at exactly the moments when print was what counted.

Being fair also means noticing what Fuller actually contributed. Snelson made an object. Fuller made it a principle, carried it from a table top to masts and domes and finally to a general claim about how all structure works, and gave it a word compact enough to travel. Without Snelson there is no first tensegrity. Without Fuller it stays in a gallery in Oregon and nobody outside the art world hears of it. Take either man out and this page has no subject.

The patents

Three inventors patented the same idea within three years under three different names

Between 1962 and 1965 the same structural idea was patented three separate times by three men who agreed on very little. Fuller filed Tensile-Integrity Structures on 31 August 1959 and was granted US 3,063,521 on 13 November 1962 (Fuller 1962). David Georges Emmerich filed French patent 1,377,290 in 1963, published in November 1964, under the name Construction de reseaux autotendants, self tensioning networks (Emmerich 1964). Snelson filed on 14 March 1960 and was granted US 3,169,611, Continuous Tension, Discontinuous Compression Structures, on 16 February 1965 (Snelson 1965).

The priority question runs back further still. Karl Ioganson, a Latvian constructivist working in Moscow, showed a piece usually called Gleichgewichtkonstruktion, an equilibrium construction of struts held apart by tension wires, at the OBMOKhU exhibition in 1921 (Gough 1998). Emmerich argued in his 1988 monograph Structures tendues et autotendantes that Ioganson had the principle first. Snelson rejected the equivalence while conceding the influence, accepting that the Russian constructivists shaped the art he came out of and denying that Ioganson had built the same thing. Ioganson built something very close and it went nowhere. Snelson built it in 1948, kept building it for life, and it went everywhere. Priority and consequence are different questions and it is cleaner to answer them separately.

Motro later produced the definition that settles the technical argument without touching the human one. A tensegrity system, in his formulation, is a system in a stable self equilibrated state comprising a discontinuous set of compressed components inside a continuum of tensioned components (Motro and Raducanu 2003). No names in it at all. Snelson, for his part, chose a patent title with no poetry and no branding, and it remains the most honest description of the principle any of the three produced.

The physics

Pre-stress is what makes a scattered network behave as a single object

A tensegrity structure is pre-stressed, which means every element in it is already loaded before anything is done to it. In a stacked structure the members sit unloaded until a load arrives from outside. In a tensegrity the cables are taut and the struts are pushing outward against them while the object sits on a shelf doing nothing. That resting load is the pre-stress, and four consequences follow from it.

First, load sharing is global. Push on any node and the tension in every cable in the network changes at the same instant. There is no such thing as a purely local event. Second, stiffness is tunable. Raise the pre-stress and the whole network stiffens without a gram of material added. Drop it and the network goes slack and imprecise. Third, the response is non-linear. The structure yields easily at first and then stiffens sharply as it deforms, which Fuller described as the ability to yield increasingly without ultimately breaking. Fourth, it oscillates. Strike a tensegrity and it rings rather than thuds, because energy entering at one point travels the tension net as a wave and the entire object takes part in absorbing it.

Hold those four in mind. Global load sharing. Stiffness set by pre-stress. Yield then stiffen. Whole system oscillation. Now think about what a living body does when you push on it, when it braces, when it is startled, when it is exhausted.

Needle Tower, 1968

Sixty feet of aluminum standing on a fourteen inch footprint

Snelson built Needle Tower in 1968 out of aluminum tubes and stainless steel cable. It stands 60 feet high, roughly 18.2 meters, and it meets the ground across about fourteen inches. Joseph Hirshhorn gave it to the Hirshhorn Museum and Sculpture Garden in Washington in 1974 and it has stood in the garden since. Conservation completed in April 2010 replaced the upper section with Snelson supervising. A second version, Needle Tower II, also from 1968, went to the Kroller-Muller Museum in the Netherlands in 1971.

None of the tubes touch. Stand underneath and look straight up and the struts resolve into a six pointed star, because each layer of three alternates its helical orientation against the layer below. The tower sways in wind and comes back. That is the point of it. It is not braced against weather, it negotiates with weather, and it survives loads that would snap a rigid mast of the same height and footprint, because it never tries to resist as one stiff line. A structure that gives everywhere a little outlasts a structure that gives nowhere at all.

Tensegrity and the model

What tensegrity gave the Unified Model of Tone

The Unified Model of Tone takes this architecture as load-bearing and states it in its own voice. A living body is held in shape by distributed tension rather than by stacked compression, and tone is the state of that tension. Bones float within a continuous tension network built from every soft tissue in the body: muscle, tendon, ligament, joint capsule, fascia, dura, and the connective tissue investments of every organ. That network is pre-stressed. No single tissue carries the architecture. The architecture is the integrated tensional state of all of them.

Here the model departs from the account it inherits, and the departure is the point of this page. Biotensegrity, as it has been argued in the literature, is a claim about load. The model adds a claim of its own: the tension network is the geometric self-registration of the body, which is to say that its integrated tensional state is how the body knows its own shape. The network does not send messages about bodily geometry to some other system that reads them. Its current organization is the current bodily geometry.

Follow the consequence, because it is the same consequence the physics section names. A local change in tension does not stay local. It redistributes through the whole system. Stress one corner of a tensegrity structure and the entire structure adapts to preserve balance and function. This is why the model expects remote effects that a purely local view finds puzzling. The familiar examples are a plantar fascia that influences headaches, a big toe injury that alters gait at the opposite shoulder, cranial work that reaches the sacrum. Those are consequences the tensegrity principle predicts rather than independently established facts.

The architecture runs down to the cell. Donald Ingber established in 1993 that the cell is itself a tensegrity structure, with microtubules as compression elements and the actin cytoskeleton as tension elements (Ingber 1993). Force applied at the tissue scale travels the soft tissue continuum to the cell membrane, where it changes ion channel behavior and gene expression. Touch does not only move tissue. It changes the account the body is keeping of where it is.

The credit boundary is exact. Neither Snelson nor Fuller made a claim about human physiology. Snelson made sculpture and insisted on it, calling his works force diagrams standing in three dimensional space rather than engineering proposals. Fuller made a general claim about structure that reached from the atom to the solar system, and human beings were incidental to it. The architecture is theirs. Reading the body as a pre-stressed network whose tension is tone is the claim of the model, and neither man is enlisted to endorse it.

The handoff

The architecture existed for a quarter of a century before anyone applied it to a spine

Reading the human body as a tensegrity structure is a separate development with its own author, its own timeline and its own evidence. That work belongs to the orthopedic surgeon Stephen Levin, who took the principle out of the gallery and into anatomy from the mid 1970s onward and named the result biotensegrity (Levin 2002). He has his own file in this history and the argument is made there rather than here.

What matters at this point in the sequence is the order of events. A physical object demonstrating continuous tension and discontinuous compression sat on a table in Oregon in December 1948. It was patented three times over by 1965. It stood 60 feet tall in a museum garden by 1968. Only after all of that did anyone look at a vertebral column and ask whether it had been drawn wrong for four hundred years.

Two men, one object, one word, and an argument that outlived them both. Snelson made a thing that had never existed before. Fuller gave it a name compact enough to travel the world. The principle survived the quarrel intact, and it remains the cleanest available answer to the question of why a living body does not behave like a pile of stones.

What the record shows

The dated record credits the object to Snelson and the principle to Fuller

• December 1948. Snelson built Early X Piece, two wooden X forms held apart by nylon tension lines and measuring 11.5 by 5.375 by 5.375 inches. The two forms never touch.
• 22 December 1949. Fuller acknowledged in writing the original demonstration by Snelson of discontinuous pressure together with continuous tension, within six months of first handling the piece (Snelson 1996).
• 13 November 1962. Fuller was granted US Patent 3,063,521, Tensile-Integrity Structures, on an application filed 31 August 1959. Snelson is not named anywhere in the document (Fuller 1962).
• 16 February 1965. Snelson was granted US 3,169,611, Continuous Tension, Discontinuous Compression Structures, on an application filed 14 March 1960 (Snelson 1965).
• 1921. Karl Ioganson showed an equilibrium construction of struts held apart by tension wires at the OBMOKhU exhibition in Moscow. Emmerich argued in his 1988 monograph that Ioganson had the principle first, and Snelson rejected the equivalence while conceding the influence (Gough 1998).
• 1968. Needle Tower stands 60 feet high, roughly 18.2 meters, on a footprint of about fourteen inches, and none of its tubes touch one another.
• 1993. Donald Ingber established that the cell is a tensegrity structure, with microtubules in compression and the actin cytoskeleton in tension, which places the same architecture at the scale where tone reaches gene expression (Ingber 1993).