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Chad Carreras | Nick Babiac | Jeff Yates | Jean Boda

Assignment #3 Pneumatic Structures

SYSTEM DESCRIPTION

A pneumatic structure is a non-conventional approach to providing shelter from theelements. Instead of a system supported by a series of members, joints and pins undercompression and tension; pneumatic systems use air to support the loads. This isaccomplished by forcing air into the a contained membrane, which in turn causes the internalpressure to rise. Think of how hard it was as a child to pop a balloon by squeezing it uniformly.However, when squeezing the balloon with a rotation it was much easier to pop. Later thisreport shall discuss some of the drawbacks that parallel this rotation.

Unfortunately, Pneumatic structures only perform well when the pressure is maintained.If the membrane were to tear air would rush out, allowing the external pressure to increase adthe internal pressure decrease. It is for this reason that some pneumatic structures userevolving doors as an attempt to seal the air inside. As children we saw this first hand whenplaying the parachute game.

When the children raise the parachute in the air they are drawing external pressure intothe system, like a pump. As the children rasp the handles and lower the parachute to theground the trap the air and the pressure, allowing the parachute to appear like a dome.Eventually the dome sinks as air leaks out. The image to the right shows the modernequivalent with the yellow box serving as the pump/fan device. The cut away shows theinternal pressure pushing out on the surface to maintain the desired form. The modern systemis built to account for lateral wind loads (when properly braced to the ground) and compressivesnow loads upon the structure. The internal pressure inside the membrane applies a tensile

force to account for all of these. Later we shall discuss alternatives in anchoring systems andmaterial choice to satisfy various design criteria. Unlike balloons, a failure would not typicallyresult in a sudden explosion. A tear would most likely cause the structure to droop down anddeflate slowly. However there are some more advanced tensile stringers in place to limit thedegree of sag in the advent of a failure.


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COMPONENTS AND SUBSYSTEMS

The diagram to the left below shows the blower, which is responsible for providing the

forced air into the system. The envelope is responsible for containing the air and ensuring thatthe internal pressure is greater than the external pressure. The airlock prevents the escape ofair to ward against a system failure. The diagram to the left is one one type of pneumaticsystem, which can be somewhat uncomfortable, because it forces the occupants under thesame pressure constraints.

The diagram to the right above shows more innovative approaches, which allowpneumatic structures to allow open environment. The pressure constraints are dependent uponthe structure, and are not reliant upon a good seal with the foundation. The system used tobrace down the systems on the right do not require a seal squirt along the foundation.However the structure would require a more substantial anchoring component apposed toseamed catenary cable. The structural systems for anchoring will be elaborated upon later,

but for now know that there will be systems in place to prevent the structure from driftingaway when lateral wind are applied.


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APPLICATIONS

Air-supported buildings had a first breakthrough as shelters for radar devices after World War II. Theshelter needed to be lightweight, reusable, mobile and deployable in short time and without any metallicparts, ideal requirements for pneumatic structures. Walter Bird proposed using pneumatic domes domes tohouse military radar equipment as a means of weatherproofing the equipment. These radomesspreadthroughout the united states and eventually over the world. This housing was also practical because themembrane had no affect on the waves that were transmitted and received.

Today, air supported buildings are erected for an array of purposes, which ranges from sports andrecreational purposes facilities. Many professional sports teams use them for their stadium roofs as acheaper alternative to shielding patrons from the elements. Syracuse university's Carrier dome (named forthe HVAC manufacturer yet ironically doesn't require air conditioning) utilizes this system also to cover largespans because a sports arena with a column going down home plate would hinder game play. Pneumaticstructures also serve the as field hospitals due to their mobility, and low level maintenance. They can also

serve as containment structures for hazardous waste materials and agricultural crops.

Many pavilions use pneumatic structures to provide a consist venue no matter the location. Therehas been a rise in the use of pneumatic structures for trade shows and artistic performances. However thisstructures are of course limited to single story use. If a person were to walk along a structure with apneumatic base, the base would displace around the person, making it very difficult to stay afloat. Forsnow loads, the forced in air is heated to avoid the buildup and to prevent a collapse.


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LIMITATIONS

Although there are some great applications where pneumatic structures prove to be thebest alternative, there are some limitations to be considered in the initial planning phase ofdesign.

Since the system requires a constant internal pressure, there is a demand forsophisticated detection systems and fans that can constantly applied the required pressure tomaintain the form. This can be translated to increased operating costs as the price of electricityincreases over the years.

Snow loads can pose a threat is the air is not heated, the Carrier dome at Syracuse hasducts that allow for the melted stormwater to flow safely off the structure into collectionstructures. Snow has been found to cause wear in the membrane as well, and could contributeto a tearing failure. If possible, it is best to use pneumatic systems for temporary structures that

function only during specific times of the year due to some HVAC complications with increasingpressures as a result of temperature changes.

Although the structures are anchored down, they could have trouble in areas with highwinds such as the Meadowlands of East Rutherford NJ.

They are not recommended for musical concerts because many materials have beenknown to cause loud echoing of the sound. This is because there is far less material to absorbthe sound, leaving the waves to bounce around in a convoluted mess of distortion.

Fire protection is also a concern due to the nature of the materials used.

MATERIALS AND CONSTRUCTION RESTRAINTS

Materials used in pneumatic structures are not typical in construction. Thesecomponents, unlike various other building elements, are selected based on their lightweightcharacteristics rather than their tough and rigid qualities. The materials used, can beseparated into two common categories: isotropic and anisotropic. They must also meet inaccordance by the following standards: Air structures and Design Manual ASI, ASCE, CSA

Vancouver Dome collapse


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CAN-S109, NFPA 701, UBC and BOCA.

a. IsotropicIsotropic stems from the word isotropy, meaning uniform in all directions. Materialssuch as plastics, membranes, foils and fabrics are considered isotropic because of their

molecular structure and uniform design characteristics.

The following are common isotropic materials used in the construction of pneumaticstructures:

i. Metal Foil: Metal foil presents a non-rigid material with a high tensile strength quality making it ideafor use pneumatic structures. With a high strength to weight ratio, it is used primarily in the outerlayers of the structure because of its reflective qualities.

ii. Plastics: Plastics and plastic films such as polyethylene, polyester, and PVC are used in pneumaticstructures primarily because of their strength-to-weight ratio and versatility. Plastics can be foundanywhere from the very outer layers of a pneumatic structure to the inner braces and supports

(particularly smaller structures).

iii. Fabrics: Primarily consisting of finely woven synthetic fibers and glasses, fabrics are utilized in theoutermost layers of a pneumatic structure. Often coated with a synthetic weathering solution, fabricsplay a crucial role in preventing air leakage as well as over 2/3s the building materials.

iv. Rubber Membranes: Often used for their durability and flexibility, much like the rubbermembrane in a standard building, a rubber membrane is used in the building envelope in theoutermost layers to prevent the elements from entering the structure.

b. Anisotropic

Meaning directionally dependent, anisotropicrefers the fabric of a pneumaticstructure. A combination of fine and course fabrics.

The following are common anisotropic materials used in the construction ofpneumatic structures:

i. Finley Woven Fabrics: Used primarily in the outer layers of the structure to retain air and preventleakage. These fabrics are typically weaved synthetic and organic materials ranging from metals topolyester.

ii. Coarsely Woven Fabrics: Fibers spaced where between 2 mm to 20 mm apart, coarselywoven fabrics are used similarly to finely woven fabrics or often in conjunction with.

c. Anchorage

i. There are two techniques used in anchoring a pneumatic structure to the ground.One consists of a series cables attached to anchoring points secured firmly to theground. The other utilizes sandbags, concrete blocks, or any densely packedcontainer to weigh down the structure secure in position. Depending upon theenvironment of the structure, one method may work better than the other. Forinstance, in the desert, it may be more difficult to anchor the structure to the groundusing anchor rods hammered in the ground. In this case, the second method usingsand bags, poured concrete, or bricks would be utilized because of the coarse sandyground.


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ii. Cables range from woven metal to elastic bands. Depending upon the purpose of thestructure and load from the compressor, the bands are selected based on both thesize and anchor configuration. It is not uncommon in larger, more permanent

structures for the cables to hold more rigid characteristics in case of mechanicalfailure or envelope failure. In the event of failure, the outer envelope is supported buythe rigid cables to ensure safety for those using the structure.

iii. The connections used in anchoring consists a series of nuts, bolds, and joints oftenconstructed out of steel or combination of alloys.


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NUMERIC PARAMETERS

In Thomas Herzog's Pneumatic Structures, there are specifications useful in

recommending a design. These include but are not limited to data on the various types of materialsand how they will deform, while relating them to resistances. For example PVC with a high thicknessand weight will have a longer durability due to a higher tearing resistance. However, due to theweight, it will require a more powerful blower to prevent sagging. All of these result in a moreexpensive product and operating costs. It is impossible for engineers to satisfy the Cost, durability,and speed of installation without allowing one to falter.

HERTZOG, THOMAS, PNEUMATIC STRUCTURES, NY: OXFORD UNIVERSITYPRESS, 1976.COURTESYOF: HTTP://WWW.PAGES.DREXEL.EDU/~CJR35/AE390/A3/INDEX.HTM

HERTZOG, THOMAS, PNEUMATIC STRUCTURES, NY: OXFORD UNIVERSITYPRESS, 1976.Courtesy of: http://www.pages.drexel.edu/~cjr35/AE390/A3/Index.htm
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EXTRA CREDIT

In general, the pneumatic system performs greatly in single story structures and as a

large roof for minimal construction and design costs. However, unlike some of the otherstructures discussed they require a continuous input in order to remain a structure. This canget costly overtime, especially when realizing that pneumatic structure is basically temporary.Unlike steel or concrete, it will not last 20+ years and must be continuously monitored.However this system is more fluid and can allow for daring curves and shapes, that undertraditional methods can become costly. For example a traditional dome requires a lot moretime, money, and effort to construct. However, wood has been used in several temporarystructures like at the burning man festival. Possible one day engineers will think of a way tocreate a multi-story pneumatic structures, but in the mean time, this can only be achieved witha combination of some other other systems covered in this course.

As a further exploration we would recommend requiring groups to provide a model ordemonstration during their presentation. We believe that it will assist in enhancing the learningexperience for many students.
http://scotthaefner.com/photos/place/Nevada/1949/http://scotthaefner.com/photos/place/Nevada/1949/

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