[This post is part of the Metropol Blog Series.]
Early in 2000, a young restauranteur presented an idea for a huge “Cafe” at Center Camp. His premise was “the larger the structure, the more coffee would be sold,” so if it was big enough it would much more than pay for itself. However, even if this formula didn’t prove out, a grand central meeting space would still be a fine community asset.
A 3/4 sphere of glued toothpicks was the model for a one to two hundred foot high dome, this to be built of timber bamboo shipped up from Mexico. The proposed structure was evaluated by our City Designer, Rod Garrett.
This design proved not entirely practical, as it would have an enormous surface area compared to the usable area within its footprint, and might roll through the city like a giant potato masher in high winds. Further, we had no expertise in building high in the air with bamboo, possibly having to import a crew from Asia. Lastly, the bamboo would simply explode into cracks and splinters in the extreme low humidity and heat of the high desert.
Rod was asked to come up with an alternative design. Without any knowledge of precedents for very large temporary structures built quite inexpensively in the middle of a vast desert having occasional hurricane force winds, this took some study.
It seemed prudent to design a structure which could be taken down, transported, stored and reassembled every year; therefore, it should be composed of durable and modular, replaceable elements. As the winds could come from any compass direction, the structure should be able to effectively disperse potentially great forces.
This resolved to the idea of a large diaphragm, a round ring pulled into compression by a membrane which distributed loading from any point. Being slightly peaked in the center, the deflected wind would tend to hold everything down. However, with the outer ring elevated for access, it could become a giant Frisbee.
Bedouin tents came to mind, developed over eons in similar conditions. So, in profile, another line was angled from the post supported outer ring down to the ground, but would remain partly open for access. This would anchor the structure in place, as well as deflect the wind upward.
Next came the materials. They should be lightweight and relatively ‘soft’. This for safety, should anything come apart and fly away. Shade cloth was chosen for its strength, reasonable cost, and because it was somewhat porous to the wind, reducing loads and lofting.
Instead of using rigid materials, a tensile cable structure was conceived. Anchored into the ground outside, it would form a network of double lines radiating in from the outer ring to a post-supported inner ring, and from there down to ground anchors (the locus formed by these cables became a central design feature). This open circle would rapidly release pressure differentials between the inside and outside of the structure. It would also elevate and open up the space, referencing the ancient dome of the Pantheon.
With that all in mind, the practical realities were laid out in the computer. In choosing a 12 feet by 24 feet module for the shade cloth, the 96 such panels emanating from a center circle of 48 feet produced an overall diameter of 250 feet. This made the radial span of the cable almost 100 feet, so intermediate cabling attached down to anchors added form and structure, and reduced ‘bounce’ from wind. It was a grand space, a footprint of 45,000 square feet with 2/3 of that clear of any posts.
We are told that the design was modeled in a computer simulation program and tested there in up to 120 MPH winds without failure. Since first installed in 2000, it has probably not seen winds half that high, but it’s reassuring nonetheless. At those times, it seems somewhat like being in the eye of a hurricane, outside — a roaring “white out”, while inside — simply dust and fluttering.