Pneumatic Structures

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Introduction

• Membrane Structures that are stabilized by pressure of compressed air.

• Pressure difference between the enclosed space and the exterior are responsible for giving the building its shape and its stability.

• The pressure should be uniformly distributed for structural integrity.

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Pneumatic Structure

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Pneumatic Structures• Round in shape because it creates greatest volume

for least amount of material.

• The whole envelope has to be evenly pressurized for best stability.

• Pre stressing of membrane can be done either by applying external force or by internal pressurizing.

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Principle

• Use of relatively thin membrane supported by pressure difference.

• Dead weight increases by increasing the internal pressure and the membrane is stressed so that no asymmetrical loading occurs.

• Membrane can support both tension and compression and thus withstand bending moment.

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Types• Air Supported Structures• Air Inflated Structures

Air Supported Structures• They have air higher than the atmospheric pressure

supporting the envelope.

• Air locks or revolving doors help to maintain the internal pressure.

• Air must be constantly provided.• Life span of 20 – 25 years. 6

• They are either anchored to the ground or to a wall so that leakage is prevented.

• They have relative low cost and they can be installed easily.

Air Supported Structure

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Air Inflated Structures• Supporting frames consist of air under high pressure.

• Internal pressure of building remains at atmospheric pressure.

• There is no restrictions in no. and size of openings.

• They have potential to support an attached structure.

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History• The concept of pneumatic structures were developed

during the development of hot air balloons.

• A brazilian priest Gusmao conducted the first experiment in 1709.

• During second world war, after the invention of nylon, these structures were widely used in military operations, as shelters.

• These were later used for protecting radar from extreme weather conditions. 9

General CharacteristicsLight Weight• Weight compared to area is less.

• Low air pressure is required to balance it.Span• There is no theoretical maximum span.

• To span a distance of 36 km for a normal building is hard while such spans are quite possible for pneumatics.

Economy• Not expensive in case of temporary structures. 10

Safety• More safer but proper care should be taken.

• They are fire resistance structures.

Quick erection and dismantling• Suitable for temporary constructions.

• 1 km² area can be brought down in 6 hours and can be establish in less than 10 hours.

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Good Natural Light• If envelope is made up of transparent material good

natural light entre into the structure.

• Around 50% – 80% of sunlight can be obtained.Theft• They are very safe structures.

• If the air bag is cut with a knife or a pin a big bang is produced.

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System ComponentsEnvelope• They can be made up of different materials.

• Cannot be used as one continuous material.

• Material are seamed together by sealing, heat bonding or mechanical jointing.

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• The design of the envelope depends on an evenly pressurized environment.

Cable System• They act as the supporting system.

• They experience tension force due to the upward force of the air.

• Can be placed in one or two directions to create a network and for better stability.

• They do not fail since they are pulled tight enough to absorb the external loads.

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Pumping Equipment• It is used to supply and maintain internal pressure

inside the structure.

• Fans, blowers or compressors are used for constant supply of air.

• The amount of air required depends on the weight of the material and the wind pressure.

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Entrance Doors•Doors can be ordinary doors or airlocks.•Airlock minimize the chances of having an unevenly pressurized environment.

Foundation•Pneumatic structures are secured to ground using heavy weights, ground anchors or attached to a foundation.

•Weight of the material and the wind loads are used to determine the most appropriate anchoring system.

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• For bigger structures reinforcing cables or nets are used.

• For a dependent pneumatic structure (roof only air supported structure) the envelope is anchored to the main structure.

• When anchoring is done to soil, the cable is attached to the anchor directly inserted and frictional forces of the soil to hold it down.

• Soil anchoring systems include screw, disk, expanding duckbill and arrowhead anchors.

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Loading

• Wind and Snow loads are the primary loads that are acting on pneumatic structures.

• They are anchored very tight to the ground, so no horizontal forces are exerted to the envelope.

• As pneumatic structures are tensile, the envelope has the ability to gain stiffness in order to withstand the loads acting on them.

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• Wind loads produce a lateral force on the structures and snow load causes downward forces on envelope.

• Pneumatic structures are designed to withstand wind load of 120 mph and a snow load of 40 pounds/yard.

19Air Supported Structure

Air Inflated Structure

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Analysis• The analysis can be carried out either

mathematically, numerically or experimentally.

• In simple forms forces are resolved normal to the plane of the membrane.

• This leads to a formula relating the membrane tensions to the principle radii of curvature for a given internal pressure:

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• N₁ ,N₂ membrane tensions• r₁ , r₂ the principle radii of curvature• P internal pressure

RESPONSE OF HEMISPHERICAL STRUCTURE TO WIND

• Apart from rain & snow accumulations, the causes of failure are inadequate anchorage and the instability due to high wind speeds.

• The first part of this study, an aeroelastic model of a large-span, hemispherical, air-supported roof was designed and tested.

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• Objective is to examine the roof response and the internal pressure fluctuations caused by wind loading.

• In second part of study, procedure is based on the measurements of the external pressures using a rigid model.

Construction of Aeroelastic Model• Model was made using plexiglas mold to required

shape under pressure and elevated temperature.

• To make adjustments inside the model, a circular access door was made in the chamber wall.

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• A manometer was attached to the chamber to monitor the mean internal pressure and to detect leakage.

• Pressure transducer was connected inside the model to monitor the internal pressure fluctuations caused by the movement of the roof due to wind pressure.

• Aluminum foil targets were glued to the roof at the probe locations to provide electrical conductivity.

Free Vibration Tests of Aeroelastic Model• To find the natural frequencies, damping ratios, and

mode shapes of the structure.24

• 2 types of excitation• Random• Harmonic• The random excitation was done using noise with a

frequency range of 0-400 Hz.

• Harmonic excitation was then applied stepwise at each natural frequency.

• As the internal pressure increases, the natural frequency increases as the roof becomes stiffer.

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Wind Tunnel - Aeroelastic Tests•The models were tested in the high speed section that has dimensions of 2.5 m high, 3.4 m wide, and 39 m long.

•3 terrain exposures were used by varying the height: open country suburban urban exposures

Experimental Procedure•No. of locations were selected and the test was conducted for different rotation angle.

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• The outputs were connected either to the on-line computer or to the HP analyzer.

• The digital data acquisition system was used to obtain the maximum, minimum, mean, and RMS values of roof response and the internal pressure.

• The deflections and the internal pressure fluctuations were measured over a period.

Result• The aeroelastic model remained aerodynamically

stable for all of the internal pressures and wind speeds employed 27

.Effect of Wind Speed• The mean response is proportional to the square of

the wind speed, as in conventional structures.

• RMS deflections are small in comparison with the mean deflections of the roof and appear to decrease with turbulence intensity for the same wind speed.

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Effect of Exposure•The open-country exposure gives higher mean and smaller RMS deflections at roof center.

•Because for the open-country, the mean wind speed at the rooftop is higher than other exposures, yielding higher mean deflections.

Internal Pressure Effect•There is a tendency toward reduced mean and dynamic deflections with increasing internal pressure for a specific wind speed.

• The mean response decreases as the internal pressure of the model increases.

Wind-Tunnel Pressure Tests on Rigid Model• The terrain roughness affected the external

pressures.

• The mean external pressures for the open-country exposure were higher than those for other exposures.

• The urban exposure resulted in higher RMS pressure values than other exposures.

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Comparison of Analytical and Experimental Results•Internal pressure of the model increases, the mean and RMS deflections decrease.

•The maximum difference between the analytical and experimental results is about 14%.

Summary•The mean membrane deflections in strong winds are very large compared with conventional structures.

•The dynamic response is generally small compared with the mean deflections.

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THERMAL PERFORMANCE ANALYSIS• The major environmental factors governing the

comfort criteria are air temperature humidity air movement the mean radiant temperature• Air movement plays a major role in the process of

heat exchange in hot climates.

• The air inside the structure are periodic functions of time.

• Climatic data of Trinidad are used for the analysis.32

• The skin temperature of the structure can be calculated by using the energy-balance equation.

• Assumptions made are• the temperature gradient in the thin water film is

neglected• an arithmetic average value has been used for the

temperature of the water• the partial pressure of water vapour in atmospheric

air is assumed to be constant.

• The use of water spray and reflective coatings help to reduce the heat flux and the skin temperature of the structure. 33

• There is no significant reduction in the inside air temperature leading to comfort conditions.

VIBRATIONS OF PNEUMATIC STRUCTURES INTERACTING WITH AIR

• The effect of the air surrounding the structure on the structure’s natural frequencies is significant. For the higher frequencies the influence of the air is less pronounced.

• The effect of compressibility of the air varies with the frequency of the force applied. 34

Materials• Envelope Materials• Anchor Materials

Envelope Materials• They should be light weight.• Should have high tensile strength, tear

resistance etc.

Fiberglass• They high tensile strength, elastic behavior

and durability. 35

• Coated with Teflon or silicone to increase resistance to extreme temperatures and UV radiation.

Polyester• Most common envelope material for smaller

structures.• PVC-coated polyester is common for flexible,

smaller air-supported structures. 

• The PVC is applied to the polyester using a bonding or adhesive agent.

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ETFE• It is very energy efficient because of transparency,

insulation and UV resistance. 

• It is also light weight has an lifespan on 20 years and is recyclable.

Nylon• Vinyl-coated nylon has more strength, durability

and stretch than polyester.

• They have a higher cost.37

Anchor Materials• The anchor material depends on the application and

size of the pneumatic structure.

Steel Cables• Steel wires are twisted into strands which are then

twisted around a core to form the cable.

Ballasts• Materials for ballasts of smaller structures include

sand bags, concrete blocks or bricks.

• The ballasts must be placed around the perimeter of the structure to evenly distribute the load. 38

Advantages

• Light weight

• Covers large spans without internal supports

• Rapid assembly and have low initial and operating cost

• Portability

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Disadvantages• Need for continuous maintenance of excess pressure

in the envelope

• Relatively short service life

• Continuous operation of fans to maintain pressure

• Cannot reach the insulation values of hard-walled structures

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Construction Issues• Stability• Weather Condition• Continuous pressurization

Span Limitations : No limitation

Height Limitations: No limitation

Load Capacity: Internal pressure of 1psi for every 144 psf loading

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UsesSports and Recreation• Ability to span great distances without beams and columns. Eg. American Football or Baseball grounds

Military Structures• For storage, for emergency medical operations.

• To protect radar stations from weather conditions42

Hydro Engineering• Used within dams and flood prevention systems.

• It can be used in a relatively small river or stream.

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Military Radar Station Swimming pools

Case Study: Minnesota Metrodome• It requires 120 m³/s of air to keep it inflated.

• Air pressure is supplied by twenty 90-horsepower fans.

• The entire roof weighs roughly 580,000 pounds.

• It reaches 59 m or about 16 storey, at its highest point.

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Conclusion

• Pneumatic structures have found wide range of application.

• They are best suited for small and temporary construction.

• They can be quickly erected and dismantled.

• Provoke fascination among observers and bystanders.

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Reference• An Outline of the Evolution of Pneumatic Structures,

Jung Yun Chi and Ruy Marcelo de Oliveira Pauletti.

• Pneumatic Structures: A Review of Concepts, Applications and Analytical methods, C.G. Riches & P.D. Gosling.

 • Response of Hemispherical, Air supported structures

to wind by Magdy Kassem1 and Milos Novak,2 Fellow, ASCE.

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• Stability of Cylindrical Air supported structures by Robert Maaskant and John Roorda.

 • Stimpfle Bernd, "Structural Air - Pneumatic

Structures". • Thermal Performance Analysis of Pneumatic

Structures, P. Gandhidasan and K. N. Ramamurthy.

• Vibrations of Pneumatic Structures Interacting with air, R. Sygulski

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