Low-Profile Air Structures in the USA

by David Geiger
Building Research and Practice, March / April 1975, p. 80-7.
Since the design of the United States Pavilion at Expo '70 in Osaka, Japan, research in the United States has concentrated on 'permanent' incombustible fabrics permitting the classification of low-profile air structures and tension structures as permanent buildings within existing building codes. Five low-profile air structures built to date or under construction are discussed in detail by Dr. Geiger, of Geiger Berger Associates, New York.

At Expo 70 in Osaka, Japan, the United States built as its pavilion a low-profile cable-restrained air structure weighing 59.7 N/m2 (1.25 lb psf) and spanning 18.6 m x 138.0 m in plan. The design, fabrication, construction and experimental testing of this structure has been fully reported in the literature (ref. 1,2,3). The low cost, the aesthetic quality of the clear-span translucent space and the confirmation of the design theory through the experimental test results have led to the extension of this structural system to other applications and to spans up to 2400 m (8000 ft) in diameter. (ref. 4)

In extending this system beyond the US Pavilion design at Expo 70, it was necessary to develop a fabric that was permanent and incombustible so as to permit construction within existing United States building codes. The resultant product was also self-cleaning, thus enabling translucency to be maintained.

The material was developed under the sponsorship of the Educational Facilities Laboratories. The final product consisted of Fiberglas Fabric coated with Teflon Fluorocarbon Resin. The coating is a formulated Teflon TFE and or FEP dispersion, which is applied after heat cleaning and a first coat of silicone. The silicone coat prevents wicking of water, which would cause the glass to deteriorate. To the Teflon dispersion is added ten micronglass beads, which both lowers the fabric cost and increases abrasion resistance. The strengths [1] of fabric presently available vary from 200 lbs/in to 1000 lbs/in. For a 600 lbs/in fabric, the translucency can be made to vary from nearly opaque to 18% translucent.

Under accelerated weathering conditions, where 300 hours in a weatherometer is approximately equal to one year of real time weathering, the fabric exhibits the following properties:

    After 6000 hours of accelerated weathering (carbon arc) the subject fabric showed no loss in strength.

    Seamed samples of a 800 lb/in fabric have the ability to withstand 400 lb/in stress after 6000 hours accelerated weathering.
    Trapezoidal Tear Strength showed a slight reduction after 2000 hours accelerated weathering. (Tests were discontinued after 2000 hours due to a limited sample.)

Fine testing

Tests are continuing. Fire tests on this material were performed for the Owens-Corning Fiberglas Corporation. The test results are summarized as follows:

1 ASTM E-84 tunnel test and 'Life Safety Code' NFPA-101 surface burning characteristics:

Flame spread -5
Smoke developed -5
Fuel contributed -5

2 Standard Method of Test of Non-combustibility of Elementary Materials, ASTM E-136-65:

3 ASTM Specification E-108 and Underwriters' Laboratory Standard UL-790:

The straightness of the fibers in the warp direction accounts for the linear stress strain curve which mirrors the physical properties of the straight yarn. On the other hand, the fill fibers are kinked around these and must straighten and consequently kink the warp fiber before exhibiting the linear stress-strain characteristics of the base yarn. For bi-axial loading, the stress strain characteristics are non-linear,non-isotropic and vary according to previous stress history.

For fabric structures which are not cable reinforced it is necessary to introduce the elastic properties of the fabric in the structural analysis; while for cable-reinforced structures, the stiffness of the fabric is negligible compared to that of the cables, and the cable stiffness alone is used. The fabricator then accounts for the elastic properties of the fabric in patterning and fabricating the individual panels.

Interior of the U.S. Pavilion

Fabricated in rolls

The brittle characteristics of the glass fabric must be accounted for in the basic design of the structure and the fabrication, packaging and erection of the individual panels. It is preferred to deal with fabricated material shipped to and handled at the site in rolls rather than folded sheets. This is easily accomplished for the low-profile air structures discussed herein. The fabric comes to the site in rolls from 9 m to 12 m (30 to 40 ft) wide and as long as 60 m (200 ft). The fabric is clamped along its edges to the cables of the cable net. In those cases where panels must be folded, it must be done by the fabricator with care and handled in the field so that fracturing of the glass fibers will not occur. The fabric used to date is made from beta yarn, a 3.8 micrometre diameter filament, which is the smallest diameter and consequently the most flexible fiber commercially available.

Illustrations show the various low-profile air structures that have either been constructed or are under construction and which have been designed by my firm. Shaded areas represent those where there exist two layers of fabric for thermal and/or acoustic reasons. Cables, represented by dashed lines, indicate those along which fabric is not clamped. The principle of skewed symmetry is embodied in United States Patents. This principle requires that the cable directions be parallel to the diagonals of the superscribed rectangle, so that the ring segments to which the cable ends are anchored may, for a given roof load, be designed for zero moment or for a minimum moment. Once the horizontal components have been established from the conditions of minimum moment, the roof ordinates are established for the given load case by solving a system of simultaneous equations generated from conditions of vertical equilibrium at each cable intersection. Under subsequent load cases, the cable net distorts and the forces delivered to the compression ring introduce larger moments which dictate the ring design.

Figures 4a-4e show the ring cross sections corresponding to the roofs shown. The ring shown in 4d also served as a mechanical plenum and, for snow melting, hot air is introduced between the fabric membranes. (The table summarizes information as to structural and mechanical design and costs.)

Location United States Pavilion
Osaka, Japan
Milligan College
Johnson City, Tennessee
Santa Clara University
Santa Clara, California
Univ. of North Iowa
Cedar Falls, Iowa
Pontiac Stadium
Pontiac, Michigan
Size (m) 138.6 x 78.0 dia. 64.6 90.5 x 59.4 129.2 x 129.2 220.0 x 168.3
Roof rise (m) 7.0 4.8 7.2 14.6 15.2
Cable spacing (m) 6.0 9.0 12.0 12.9 12.6
Cable thickness (dia) (mm) 38.1 to 57.3 38.1 and two 27.0 47.6 73.0 79.4
Fabric material vinyl glass single layer teflon glass double, insulated; also single teflon glass single teflon glass part double teflon glass single layer
Wind suction (in lbs/sq.ft.) 30.0 15.0 12.0 12.9 12.6
Wind suction (in N/sq. m.) 1,464.6 732.3 585.9 732.3 732.3
Snow load deflated (lbs/sq.ft) 12.0 20.0 12.0 30.0 12.0
Snow load deflated (N/sq.m) 585.9 976.5 585.9 1,464.6 585.9
Roof weight (in lbs/sq.ft) 1.25 1.0 0.9 1.0 1.0
Roof weight (in N/sq.m) 61.0 48.8 43.9 48.8 48.8
Blower capacity (cu.ft/min) 120,000 130,000 400,000 370,000 3,500,000
Blower capacity (cu.m/min) 3,398 3,681 11,326 10,477 99,109
Maximum no. of people 3,398 3,681 11,326 10,477 99,109
Roof translucency 5,000 (6%) 1,800 (0% and 14%) 5,000 (14%) 18,000 (11% average) 80,000 (8% average)
Building costs
As bid (in US $/sq.ft) January 1969 March 1973 October 1973 July 1974 February 1974
Ring cost 50 1.77 1.20 2.89 4.00
Roof cost 4.00 6.60 6.47 9.07 7.27
Total 4.50 8.37 7.67 11.96 11.27
HIVAC system 5 4.25 7.27 2.92 3.19
When inflated November 1969 October 1974 February 1975 July 1975 November 1975

For each of these structures the normal internal pressure is 4.5 lbs/f2 (215.1 N/m2). Under high winds only, suction is created on the roof surface. Consequently, the internal pressure need not be increased for dynamic stability. The maximum design wind velocity, to date, was 150 mph (240 kph) in Osaka, Japan. The low mass of the roof permits construction in the most severe seismic zones without modification of the design.

Snow loads up to 12 lb/ft2 (574 N/m2) may be carried by increasing the internal pressure. Beyond this load snow must be melted since greater internal pressures are impractical when one considers the design of doors and exit wind velocities. In the event of failure of the snow melting or pressurization system, the roof slowly deflates and hangs free in the deflated positions.

The solar transmission properties of the roof fabric are summarized as follows: (Table)

Because of the internal pressure, special care is used to reduce the exfiltration and maintain airtightness. The horizontal expanse of earth and structure under the nearly horizontal roof and the large volume of air within the structure tend to moderate the swing of the day-night cycle. Computer programs which model the above thermal properties have enabled us to design mechanical systems with a lower first cost and lower operating cost than conventional structures enclosing the same space.

It is our belief that future progress will be in the area of design, where further developments in the membrane material and techniques for changing the thermal properties will permit a considerable reduction in the energy required for controlling the environment within the encapsulated space.

[1]Editor's Footnote: Presumably tensile strength according to standard textile industry tests on 2 inch strip of material. A near equivalent commonly used internationally is kilogram force per 5 cm (50 mm), expressed as 5 cm/kg. Thus 200lb/in = 180 kg 5 cm approx. Back to text


1. D.H. Geiger, "US Pavilion at Expo '70 Features Air-Supported Cable Roof," Civil Engineering ASCE, March, 1970.
2. Y. Kida, N. Yamashita, and M. Takagi, "Experimental Reports of Air Structure (As the Pavilion of USA for the 1970 World Exposition). IASS Pacific Symposium - Part II on Tension Structures and Space Frames, October 17-23, 1971.
3. Y. Isono and Y. Nakahara, "Fabrication and Construction of US Pavilion Air Structure". IASS Pacific Symposium - Part II on Tension Structures and Space Frames, October 17-23, 1971. 4. "Air Fare" Progressive Architecture, August, 1972.
5. D.H. Geiger, Pneumatic Structures, Progressive Architecture, August,1972.

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