Air-supported structure

An air-supported structure is any structure that derives its structural integrity from the use of internal pressurized air to inflate a pliable material (i.e. structural fabric) envelope, so that air is the main support of the structure. It is usually dome-shaped, since this shape creates the strongest structure for the least amount of material. To maintain structural integrity, the structure must be pressurized such that the internal pressure is equal to or exceeds any external pressure being applied to the structure (i.e wind pressure). The structure does not have to be airtight to retain structural integrity. As long as the pressurization system that supplies internal pressure replaces any air leakage, the structure will remain stable. All access to the structure interior must be equipped with 2 sets of doors or revolving door (airlock). Air-supported structures are secured by heavy weights on the ground, ground anchors, attached to a foundation, or a combination of these.

Among its many uses are: sports and recreation facilities, warehousing, temporary shelters, radomes. The structure can be either wholly, partial, or roof-only air supported. A fully air-supported structure can be intended to be a temporary or semi-temporary facility or permanent, whereas structures with only an air-supported roof can be built as permanent buildings.

Design

hape

The shape of an air-supported structure is limited by the need to have the whole envelope surface evenly pressurized. If this is not the case, the structure will be unevenly supported, creating wrinkles and stress points in the pliable envelope which in turn may cause it to fail.

In practice, any inflated surface involves a double curvature. Therefore the most common shapes for air-supported structures are hemispheres, ovals, and half cylinders.

tructure

The main loads acting on the air-supported envelope are the internal air pressure, wind, and snow loads. In order to cope with the varying loads of wind and snow, the inflation of the structure must be adjusted accordingly. Modern structures have computer controlled mechanical systems that can sense the dynamic loads and compensate the inflation for it. The highest quality ones are able to withstand winds up to 120 mph (190 km/h), and snowloads up to 40 pounds per square yard .

Of course, the air pressure on the envelope is equal to the air pressure exerted on the inside ground, pushing the whole structure up. Therefore it needs to be securely anchored to the ground (or substructure in the case of roof-only). For wide span structures, cables are required for anchoring and stabilization. All forms of anchoring require some form of ballast. Earlier designs used to use sand bags, concrete blocks, bricks, or the like, placed all around the perimeter on the seal skirt. Nowadays most manufactures have proprietary anchoring systems.

Danger of sudden collapse is nearly negligible, since the structure will deform or sag in case a heavy load (snow or wind) is exerted on it. Only if these warning signs are ignored or not noticed, then the build-up of an extreme load may rupture the envelope, leading to a sudden deflation and collapse.

Material

The materials used for air-supported structures are similar to those used in tensile structures, namely synthetic fabrics such as fibreglass and polyester. In order to prevent deterioration from moisture and ultraviolet radiation, these materials are coated with vinyls such as PVC and Teflon.

Depending on use and location, the structure may have inner linings made of lighter materials for insulation or acoustics.

Air pressure

The interior air pressure required for air-supported structures is not as much as most people expect and certainly not discernible when inside. The amount of pressure required is a function of the weight of the material - and the building systems suspended on it (lighting, ventilation, etc.) - and wind pressure. Yet it only amounts to a small fraction of atmospheric pressure.

Advantages and disadvantages

There are some advantages and disadvantages as compared to conventional buildings of similar size and application.

Advantages:
*Considerably lower initial cost than conventional buildings
*Lower operating costs due to simplicity of design (wholly air-supported structures only)
*Easy and quick to set up, dismantle, and relocate (wholly air-supported structures only)
*Unobstructed open interior space, since there is no need for columns
*Able to cover almost any project
*Custom fabric colors and sizes, including translucent fabric, allowing natural sunlight in

Disadvantages:
*Continuous operation of fans to maintain pressure, often requiring redundancy or emergency power supply.
*Dome collapses when pressure lost or fabric compromised
*Cannot reach the insulation values of hard-walled structures, increasing heating/cooling costs
*Limited load-carrying capacity
*Conventional buildings have longer lifespan

Air-supported domes in operation

*St. Louis Science Center Exploradome, Saint Louis, Missouri, United States
*Carrier Dome, Syracuse, New York, United States
*Hubert H. Humphrey Metrodome, Minneapolis, Minnesota, United States
*Tokyo Dome, Tokyo, Japan
*BC Place Stadium, Vancouver, British Columbia, Canada (to be removed in 2010, replaced with a retractable roof)
*Burswood Dome, Perth, Western Australia
*Dedmon Center, Radford University, Radford, Virginia, United States (removal began on April 30th, 2008)
*Generations Sports Complex Dome, Muncy, Pennsylvania, United States
*Bennett Indoor Complex, Toms River, New Jersey, United States
*Dalplex (athletics complex), Halifax, Nova Scotia, Canada

External links

* NRC Canadian Building Digest CBD-137 - 1971 [http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd137_e.html Online version]
* [http://www.arcaro.org/tension/ Tension Structures]