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Tensegrity

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Jinal Doshi is a technical designer at DCI creating structural systems for high-rise, residential, mixed-use, office, and hospitality projects. He has a natural interest in structural engineering and enjoys discussing design concepts, the behavior of material, and integrity of different systems. Jinal also enjoys sharing his technical knowledge and experience, which led him to writing his own educational blog called "Structural Madness."

Tensegrity, also known as tensional integrity or floating compression, is a structural principle based on the use of isolated components in compression inside a net of continuous tension, in such a way that the compressed members (usually bars or struts) do not touch each other and the pre-stressed tensioned members (usually cables or tendons) delineate the system spatially. This term was actually invented by an American inventor Buckminster Fuller.

The principle’s strength is hidden in the name of “tensegrity” itself. Tensegrity is coined from the combination of two words – "tension" and "integrity." A structure featuring tensegrity supports loads through compression bars placed between a network of tensioned cables. You can also imagine the concept by picturing “floating” compression in the sea of tension. In most cases, the primary path of load transfer from the top to ground is in the form of tension cables – the compression elements are just present to keep the shape intact.

What makes these structures so special compared to traditional frame structures? A simple answer lies in the beauty of their form-finding nature. Whenever a load is applied at any single node of the structure, the structure adapts itself to that load and deforms slightly to adjust itself to that load.

In a traditional frame structure, each member – say a beam and a column – have their specific roles and tend to resist local loads in a predetermined manner. The beam is loaded, resisting forces through internal tension and compression stress. The beam tries not to deform under that load. But, as soon as the demand on these local members exceeds their capacities, they start to deform and quickly fail not long after this increase in demand.

Tensegrity bears load by giving way to it - a very Zen approach. Every outside force, no matter how small, minimally deforms the whole structure until it is once more in balance. This means that tensegrity is continually “failing” form - but it is failing in its entirety, so even a rather large force can cause only a rather small deformation. Consider it this way –when a drop of water falls in a bucket full of water it disturbs the entire volume of water rather than a local area where it has fallen. The energy is dissipated in the entire volume. Tensegrity is the structural equivalent of the same concept. A load applied to the structure, anywhere, deforms the whole structure globally rather than causing any local overstressing (remember the term floating compression in a “sea” of tension). This same sea of tension helps in dissipating the load through the entire network of the structural system.

The deformation “failure” is ductile, meaning there is no sudden “breaking point” until the breaking point of all the weakest members bearing that particular load is reached. The system is constantly and consistently failing, or giving way. As opposed to a conventional load-bearing structure, deformation will be minimal until a certain point is reached, at which the system more or less suddenly fails to bear its load and buckles, crumbles, or shears. It’s because a traditional structure tries to absorb the energy of the load locally rather than dissipating the load globally.

Because the primary load path is carried through tension cables, we know that the stiffness of cable increases as the tension increases. Therefore, as the structure gets bigger, the forces in tension cables are bound to increase. In other words, as you increase the amount of applied force, there will be an increase in the stiffness of the tension members. As the stiffness of the members increase, the stiffness of overall structure increases, which eventually makes them stronger.

Conventional buildings are designed to carry loads in compression. We know that the compression member loses stiffness with an increase in loads. As you increase the load, it slowly deforms and suddenly gives up and buckles. This means that as the loads are increased, a column gets softer, which in turn reduces the stiffness of the structure. But in the case of tensegrity structures, the major load is carried through the tension cables. Increasing tension will thus make the structure stiffer, similar to post-tensioning effects in a slab or a beam.

Another question is the applicability of such structures. Tensegrity designs are most often used in many stadium roofs, bridges, and sculptures. Here are a couple of examples:

1. La Plata Stadium, Argentina

2. Kurilpa Bridge, Australia

This is not exactly a tensegrity bridge but it does utilize an innovative use of the hybrid tensegrity in its horizontal spars. The bridge would be classified as a tensegrity cable stay structure.

While tensegrity is not a widely used structural solution, it could serve as an innovative design tool for roofing systems for large buildings such as parking garages in the future.


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About the Author

Jinal Doshi is a technical designer at DCI creating structural systems for high-rise, residential, mixed-use, office, and hospitality projects. He has a natural interest in structural engineering and enjoys discussing design concepts, the behavior of material, and integrity of different systems. Jinal also enjoys sharing his technical knowledge and experience, which led him to writing his own educational blog called "Structural Madness."

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