Form Building

Inflatable Forming System

There have been many kinds of inflatable systems for creating buildings, but the system which I have developed is simple and profound. It does not use large inflatable balloons, but combines the advantages of both rigid frame and inflatable technologies. Since it is adjustable, it requires little to no precision and could be made from cheap bamboo and polyethylene if need be. The inflated volume is drastically reduced, allowing quick inflation by cheap blowers or even a hand operated bellows, with check valves to prevent deflation.

First the shape of the frame is defined using lightweight rigid tubing. For my structures I am using 1/2 inch EMT, electrical conduit. I think this lightweight tubing would be suitable for rooms up to 30 feet in diameter, perhaps more. EMT conduit comes in larger sizes also, if the need for greater rigidity is needed. The tubular frame elements can be 3 or 4 sided and either curved or straight to define virtually any shape. Conduit can be bent to shape with a bender designed for the purpose or by constructing a homemade bender out of any curved surface. I like to think in terms of creating the perimeter shape of a single gore of an inflatable balloon. However, the balloon or dome shape is not the most desirable since the flat apex accumulates snow and retards runoff. A more pointed shape at the apex is more to my liking. I am using a circular arch 30% larger than the diameter to create the base to apex elements. The arch shapes at the perimeter could be catenary or gothic and only need to be 8 foot at the apex, regardless of room size, to allow for doorways and windows. I prefer catenary for these arches in which case a hanging rope or chain can be used as a pattern.

The tubular frame is braced with straight adjustable elements created from two tubes which slide inside each other. I use emt/pvc which just slide into each other. An adjustable stop sets the length of the brace allowing the elements of the tubular frame to be adjusted relative to each other. I cut the ends off an EMT connector to make the stop. It slides on the EMT for easy setting and sets with a screw. The braces hang onto screws or eyelets in the arches. An elastic band inside each braces pulls the arches together holding the frame gently together.

The frame is wrapped with two layers of polyethylene or other plastic film, heat sealed at the perimeter. Oiled fabric may also work if sewn tightly at the edges. The double-film wrapper is oversized to accommodated multiple settings of the dimensions of the frame. For instance the frame could be set for 3, 4, 5, 6, 7, or 8 sided rooms, each requiring a different angle at the apex. The length of the base to apex arches could be adjustable as well. The oversized wrapper should fit the largest intended size. It simply wraps over the frame with the extra material wrapping around underneath the frame. Smaller gore sizes simply have more material wrapped underneath. It may be rolled up at the ends also, if need be. It is pulled taut from the back side and held taut with specially designed clamps (grommets will pull out).

The space between the two films is inflated with low pressure air, using a $10 rotary blower to provide 2 water column inches of pressure. This provides a taut surface for applying casting materials such as fibrous cement. This overcomes the central obstacle to ferrocement which has always been the expensive and tedious nature of creating one-time-use rebar/mesh surfaces to define a structure. These inflatable forms can be reused many times. Easy removal is accommodated by placing knockouts at the base, allowing the frame to drop a few inches for easy separation and repositioning. For economy, only two opposing gore shapes need be constructed with a domed bridge at the apex. The pair is coated with fiber cement. After hardening, each opposing pair is rotated to complete a thin, lightweight, structure-defining shell in steps. This method allows easy access to the apex with ladders or temporary scaffolds along the sides of the frame, eliminating complex methods to reach the apex for application of fiber cement or other coatings, as the structure is created.

Overhanging arches of lightweight plywood (or inflated forms shaped appropriately using the same methods described) are placed around the perimeter of the forming system as indicated (see photo).

Not much pressure is needed -- less than 2 column inches of water column pressure (about 1/14th of a psi). Every column inch of water column pressure (1/27th of psi) can support 5 pounds of cement matrix per square foot!! Far more than necessary. You can experiment with black poly garbage bags to get a feel for how just a little pressure can support substantial weight. A bathroom exhaust fan (rotary vane type) may be all that is actually needed but a more robust fan could be faster. Even a auto heater fan may suffice. Inexpensive sump pump or other flexible corrogated hose (clothes washer exhaust) connects the fan output to the dome using PVC plumbing connections taped to the hose. Try some of your own experiments with models using 3 mil poly. It's quite fun to blow up inflatable forms.

Various systems are being tried to prevent the sprayed layer of fibrous cement from sliding. One system is simply to wrap the inflated form with a sheet or other tight weave fabric or texture mat that won't bond with the first layer of fibrous cement -- allowing it to be reused. Or use strips of kraft paper which are less slippery than polyethylene. For minimal integrated reinforcment, use loose-weave cloth strips (such as burlap) or other open weave reinforcement such as gauze or cheesecloth. All have been used successfully, but they become integrated into the structure. Overlap the cloth to create a continuous shell which fits all the way over the tops of the forms. When the first layer hardens, add more layers to increase strength. Do not try to climb on it until sufficient thickness and hardening is achieved. Use scaffolding at the perimeter. Also the spray-sprinkle-spray method can be used with discrete fibers, even steel fibers, or with small patches of mesh or netting. A thin layer is sprayed, then sprinkled with fibers or patches of mesh to a density comparable to 2 or more layers of chickenwire, then sprayed again in one continuous process.

Virtually any shape can be constructed. There is no neccesity for symetry or straight lines. Curves, even compound curves, can exist for either for the base outline or the arch shapes. Almost anything you can imagine can be created. These variably shaped rooms can be connected to create housing as varied as your imagination.

As with all construction, spraying cementious materials on vertical or undercut surfaces requires something for the material to grab to such as a stretched mesh or netting.

Any strength dome can be constructed with this method. It has the added advantage of not requiring high tech equipment. Add an insulation layer of honeycombed perlite-cement, then another structural layer to complete the inner and outer shells. The inner layer of insulation may contain a variety of hollow forms, to create a honeycomb, for greater insulation. Vertical walls can be constructed similarly using cavity forms placed into the arches or tilt-up shell-walls can be made with simple forms (perhaps the archs laying flat on the ground). Smaller forms create the shapes needed for windows and doors. Plaster the walls inside and out using the same fibrous cement techniques.

We will be providing designs for each component of these domes in a forthcoming CD.

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