Complex Monolithic Dome designs push Airform technology to the limit. Airforms are very strong and function well, but in some cases, aesthetics present a problem.
A simple, smooth dome is relatively easy to achieve. For homes and small domes, we generally utilize a nylon base fabric with significant stretch. Patterns are adjusted to allow for this stretch. With reasonable stretch in the fabric, air pressure pulls out most wrinkles and an almost wrinkle-free Airform is achieved.
The simplest shape for an Airform is spherical. Any portion of a sphere can be used, but a hemisphere or a spherical segment is most common.
Another shape is the ellipsoid. It is a favorite for most homes. The walls are more vertical and the roofs are somewhat flatter. This saves serious money. We would be happy to explain the profile of the ellipsoid. (Technically, the maximum degree of elipticity is limited to a major, minor axis ratio of 1.45:1.0 or less. When skin stress is equal to or less than zero, the fabric buckles.)
Cylinders make valid forms, but, due to their singly curved surface, they present structural strength limitations. With cylinder-type forms, both ends must be closed either by bulkheads or by using fabric closures. Cylinder ends can also be closed by joining to another shape, such as a sphere or ellipsoid.
Combinations of shapes joined into a single Airform can produce complex structures. An endless variety of shapes can be built by adding isolating elements such as cables, bucks, stiffeners and tensioning devices. Airform design is a trade-off between cost and aesthetics. Irregular shapes complicate design and fabrication. Airform prices reflect these added costs.
Augmentations and Intersections
When augmentations and intersections are added to the Airform, especially intersections of various sizes and shapes, a new set of dynamics is created in the Airform itself that is not always predictable.
For instance, when a small dome is attached to a large dome, the laws of physics dictate more tension, hence more stretch in the larger of the two domes. The large dome tends to overpower and pull the small dome. Generally, this pulling and tugging does not cause a big problem in the finished product. After all, it is an Airform structure and certain anomalies must be expected.
Augmentations are a problem in and of themselves. Varying radiuses of curvature throughout the augmentation create varying tensions within each augmentation. Since the augmentation is part of a larger Airform, tension and stretch occur. Every attempt is made to be as accurate as possible in patterning, but it is still an Airform.
Air that inflates the Airform will follow natural laws and seek to inflate round – always round. Shapes can be modified by putting frames in the openings and pulling the openings. Nevertheless, we are trying to make an Airform do something that it does not want to do. Thus far, we have not been able to make a truly wrinkle-free Airform. To some buyers that presents a problem; for others, it does not.
The Airform is, in reality, a temporary cover. The 22-ounce nylon material will last, in most cases, up to 10 years with no outside help. However, Airforms should be coated somewhere between the fifth and the seventh year before the “life” goes out of the fabric, before the surface is burned by the sun and before the foam is exposed. An Airform’s life depends greatly on environmental conditions.
Fabrics for Airforms must be selected to meet requirements for strength, elongation, fabrication, ruggedness, durability and desired surface characteristics.
Strength requirements are dictated by inflation pressures and the shape and size of the structure.
Another consideration in material selection is durability. The Airform must be able to withstand handling during spreading and fastening to a foundation and must resist degradation from ultraviolet rays and weather exposure.
PVC coated nylon or polyester fabrics are the usual choices for fabricating forms. These materials are available in several styles, colors, finishes and weights. Damaged Airforms can usually be repaired on the jobsite. Special adhesives and repair procedures are required. Care must be exercised to prevent disturbing shape and elongation characteristics.
The stress-strain characteristics of each fabric must be determined to properly design form patterns. A form may stretch as much as ten percent at normal inflation pressures. Inflation systems must be carefully designed and pressures must be accurately controlled. A change in inflation pressure during construction affects the stress and the geometry of the Airform. If the resulting stress is excessive the form may be destroyed.
June 17, 2009