structures zoo–desktop tensegrity

Our rough schedule for the Structures Zoo course is to go from heavy to lightweight, introducing students to more and more exotic structural types as we go (sort of like going from the goats in the petting zoo up through the penguins, orangutans, and eventually the lions…) We’re closing in on the end of the semester, and the last lab or two should be doozies, but the more recent one definitely turned a corner as we went from membrane structures to what I think of as networked structures.

The paradigm for these is the geodesic dome, invented by (or, really, rediscovered, packaged, and marketed by) Bucky Fuller. The evolution of the dome through his initial attempts at Black Mountain College (and, I think, at Chicago’s Institute of Design, but that’s another post), shows a gradual refinement from structures based on flat members arranged in “great circles” that failed, to geometric patterns that arranged nodes around a spherical surface, connected by multiple linear elements. The result was a structure with vast redundancy–like a monolithic concrete structure, a true geodesic presents a nearly incalculable number of potential paths for gravity and lateral forces to take. As a result, it functions very much like a shell structure, but with nearly all of the weight removed.

Exotic enough, but all of those elements are sized to take either tension or compression. A particular variant of the geodesic experiments, (pictured at the top) involved figuring out how to organize the linear elements so that compression and tension could be isolated–in other words, placing the heavier, compressive elements only where they were needed and rendering the rest of the dome in thinner, lighter, tensile elements–cables.

The instigator of this idea was Kenneth Snelson, who went on to a career as a sculptor and consultant for all sorts of projects employing this principle. But Fuller also co-opted it, calling it “tensegrity” and (depending on who you believe) claiming it as his own. While geodesics were a bigger business success, tensegrity ranks higher on the exotic structures spectrum because of the uncanny openness the principle creates:

Super weird-looking, right? But also completely stable. If you start at the base, you can see that the structure is based entirely on triangulation–think of it as a three-dimensional truss. Each compression element has its ends stabilized by three cables, fixing those two points in space. As long as you can trace those cables through other, similarly fixed points back to the foundation (the nice, boring equilateral triangle at the bottom), the whole thing is stable–at least mathematically.

First steps–none of the spaning elements are stable in the (roughly) left-right plane yet, but this group is figuring out that those sticks need to be fixed top and bottom…

So, on the desktop, using wood stirrer sticks for compression elements, regular string for tension, and duct tape to make joints, you can very quickly and simply build a network of triangulated members, finding stability where you can and fixing points by watching where the structure is floppy or where it feels fixed. Student teams started with sterilite boxes, and quickly realized that the secret to building these is that you have to go down to go up–that some tension elements want to pick up gravity loads by reaching down (thus ensuring that they’ll always be in tension), while you can use the compression elements to gain height–with impressive results. A second ‘family’ of cables stabilizes the compression elements’ upper ends, fixing points of triangles to give the whole structure geometric stability.

One team went with a precedent study, building an analogue version of the structure that held up the (now-demolished) Georgia Dome. You can see the “go down to go up” principle at work here as the structure “climbs” a series of (red) compression elements to gain height. Again, there’s some missing rigidity in the short axis, but they’ll get there.

These get impressive pretty fast–here’s Rob admiring that first team’s finished product, a cathedral-like tower of sticks that seems to float almost by magic, even if you know the trick (also, in the background, ace knitterbot sculpture by ace ISU digital fabrication team, for inspiration).

The Georgia Dome is still among the best examples of the principle at work in practice, and its limitations are apparent when you start poking at these models–even with all of the triangulation you’re dealing with inherently flexible structures, and once you realize that you have to design for wind-related lateral forces and uplift in addition to gravity the deflection issues get more difficult. But the results are, to say the least, visually compelling, and tensegrity’s material efficiency is remarkable.

So, what’s lighter than light? That’s this week’s lab…