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This type of Pneumatic Structures use internal air pressure to support the roof membrane. The air pressure differential between the inside and the outside of the structure is only of the order of 1/500 atmospheric pressure. It is hence not noticeable to the occupants of the building, but sufficient to hold up the covering membrane. An air lock has to be provided to contain the air pressure, and continuously operating fans or blowers are necessary to maintain it. The membrane used is a though, light, plastic cloth membrane that can be wire reinforced for additional strength and durability. The two basic types of air supported structures are high profile and low profile. Profile refers to the height of the structure relative to its span. High profile structures are typically used for temporary or storage facilities and are often free standing, which means they have no foundation upon which they rest. Low profile structures are used to span long distances such as sports stadiums, also they tend to be placed upon a building rather then the ground itself, thus being used as roofs. Geometric plans used for this type of Pneumatic Structures are shown below:
There are some disadvantages for this type of system. An uninterrupted air supply must be available, people and equipment must enter and leave the building through air - locks, and the life of available skin materials is generally shorter than that of more convetional buildings.
Air-supported structures need only small pressures to hold them up, approximately 1 inch of water static pressure or 0.036 psi. To visualize how low a pressure this represents, consider that when one puffs ones cheeks with air, about 30 inches of water static pressure can be created inside the mouth, or that an automobile tire carries about 800 inches of water static pressure. Only when opening the door can anyone entering an air-supported structure know that air pressure is involved. Based on extensive wind tunnel and full-scale tests, the minimum inflation pressure for air-supported structures recommended by the ASMSA Standards is 50 per cent of the dynamic wind pressure. In the United States air-supported structures are usually designed to withstand a minimum wind velocity of 60 or 70 mph. It is recommended that blowers should be capable of providing at least twice the calculated airflow. The National Building Code requires that structures classified, assembly occupancy, should be furnished with two blowers, each with sufficient capacity to supply the required air. The standby unit is set to turn on automatically in the event of mechanical or electrical failure in the operating unit. An automatic starting power generator is normally provided for this purpose.
The most widely used type of door for high or continuous traffic applications is the revolving door. Its advantage for air-supported structures is that resistance to rotation does not noticeably increase with increasing internal pressure and pressure difference can be negotiated easily. Air-locks also are used to provide access for people, goods and vehicles. Every air-lock requires a two-door system in which only one door can be opened at a time so that there is no loss of pressure. It is possible, by providing excess blower capacity, to have fairly large openings in air-supported structures without air-locks to permit the direct entry and exit of trucks. Additional blower capacity is not, however, a practical solution since to maintain pressure at 1 inch of water pressure requires blower capacity of about 48,000 cfm (10-hp fan) for even a 3- by 7-foot door opening.
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