Wednesday, 19 October 2011

Fabrication Oriented Parametrics

I was recently invited to the city of Changsha, China to conduct a 2 week workshop with Architecture students from Hunan University and also build a pavilion within certain cost/size/material/fabrication/time constraints. We were 3 tutors: Yu Du (Zaha Hadid Beijing), Shuojoing Zhang (UN Studio Shanghai) and myself, along with support from the Hunan University staff. This post is a documentation of what we built and how we got there.

Conceptual Design

The design brief was to design a pavilion outside the main DAL (Digital Architecture Lab) studio space with some seating within volume constrains of 6m x 3m x 3m, but mostly act as a sculptural piece that demonstrates a parametric design-to-fabrication process. The form we came up with was a single sculputral doubly curved surface that formed seating and a notional canopy.

Initial shape making: some exercises in basic aesthetics


Once the base surface was frozen, a series of panellization exercises followed with the constants being that the in-house laser cutters were the only fabrication technique available, and plywood would be the most feasible material to procure and fabricate. So keeping in mind that planar panels out of plywood was the only way to go, we zeroed in on skewed hexagonal panels that formed gaps between them everytime the curvature was too high.

Flattening the panels achieved dual objectives: easier fabrication and a porous structure

Custom Detailing

Figuring out the panellization is one thing, figuring out how they're suspended in space is quite another. It was decided that the panels would be held by a triangular meshed network of thin steel cables, which in turn would be supported by laser cut wooden profiles fixed to a steel frame -- so far so good. Now came the part where we had to manage to fix panels to this cable network with a joint that allowed flexibility to move/rotate panels in all 3 axes. We 'invented' a custom detail that did exactly this:

Prototypes of the 'flower detail' being tested

The elongated slots in the plywood panels allowed transverse movement, while the circular slots in the steel discs allowed for lateral movement. Each hexagonal panel was to be fixed at 3 alternating corners, forming the 'plane' of that panel. Based on this information, three elongated slots were modelled into each panel and a numbering sequence was deviced to minimize chaos during fabrication. The production of laser cut drawing was automated from the model, so the script laid out the panels in an orderly manner which made it easy to stock them after they were cut.

Generating a fool-proof numbering sequence in a hex-grid is tricky

Fabrication Optimization

The laser cutters that we were working with had a bit of a problem: they were exceptionally slow at cutting curves, and even if we gave them segmented polylines, they would take a significant amount of time stopping at each junction and starting on the new segment. While the panels were all straight lines, the text became the killer. So we ended up inventing our own font that minimized the number of turns it takes to write a number, and obviously there were no curves. It's not the prettiest, but it saved about five minutes of laser cut time per panel, and there were 650 panels in total.

The machine friendly font setup in Grasshopper

The most labour intensive task in the entire assembly process was that of getting the cable mesh right. Cables ran in 3 direction (being a triangular grid), but each length segment on every cable was different. All our coding expertise could only go so far as to automatically generating excel files in correct sequence marking each cable name and segment length. We color coded and tagged each direction differently, but that only helped to some degree.

Patterns in numbers: a screenshot of the excel file containing cable lengths.

The cables had to be manually marked, cut and piled, and then clamped to the structure. The photos below should illustrate what a process this was.
A carpenter making post-it tags from the excel files

The good ol' T-square. Cables being measured and tagged

Nobody wanted to be the one unrolling this back into straight lengths.

So far so clean: adding alternate cables in 1 direction looks fairly clean and simple

But it starts getting complicated fairly quickly

Reaching a point of near-incomprehensibility when done

 The single biggest lesson learnt during the cabling process was to never repeat such an elaborate 'every-length-is-unique' setup, and adapt quickly to the skill of the workmen working on it. Finally, below's a slide show of the completed canopy.

Here's some sequential shots of the fabrication process.
Construction Sequence 1: Never assumes the walls to be vertical

Construction Sequence 2: Never assume the ground to be horizontal

There's also a short video of the Rhino+Grasshopper setup that generated all the final design geometry and construction data: