A personal experiment into the physical inversion of structures
Tension – Compression. The two primary physical forces we deal with. Tension is pure and self-stabilizing, compression is darker because it brings with is the issue of buckling.
While studying arch structures at undergrad level, I stumbled across a special form of arch, the catenary. A catenary is the shape formed by a hanging chain in tension. I then read that if you inverted a catenary, you would get the shape of an arch that would support itself in pure compression under its self-weight – there would be no bending to deal with!
It was Robert Hooke who coined the phrase “As hangs the flexible line, so but inverted will stand the rigid arch.” Naturally, this 2D hanging chain concept can be developed into a 3D shape. I ventured further into this and found that there was a plug-in called Kangaroo for Grasshopper that allowed you to experiment with these form-found shapes – the geometry of the structure is ‘found’ by the loads on the structure.
With the help of Imperial College’s lecture series on Rhino+Grasshopper, I managed to develop the Grasshopper script below that does the form-finding work for a pure compression shell.
The beauty and structural purity of these forms left me with a desire to make physical models of them – so I did! These were the steps:
- Find some old fabric
- Cut it out into a shape you would like to test
- Mix up a cement slurry and saturate the fabric in the slurry
- Fix the corners (or other ‘boundary conditions’) to a flat board with something – I used drywall screws
- Invert the board and support the board off the ground while the cement slurry hardens
- Turn the board right-side up and remove the fixings and your structure
Warp and weft at 45 degrees to the edge
Warp and weft at 90 degrees to the edge
Getting messy with the fabric and cement slurry. Beneath is the baseboard used to fix and hang the fabric while the cement slurry hardened.
These are the finished shells once removed from the base board and turned the right way up. The left structure had the warp and weft at 45 degrees to the edge.