Low-Profile Air Structures in the USA

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.

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:

• The material was rated 'non-combustible'. The recorded temperatures of the interior and surface thermocouples did not at any time during the test rise above the 750 degree C starting temperature of the furnace.

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

• This test contained three parts: A, Intermittent Flame Exposure Test; B, Spread of Flame Test; and C, Burning Brand Test. The test deviated from the standard procedure since the roof covering was supported by a positive air pressure rather than the wooden decks described in the ASTM Standard E-108 or UL-790. Pressure varied from 25.4 mm (1 in) of water at the beginning of the test and, because of leaks, it reduced at times to 6.35 mm (.25 in) of water. Temperatures reached were from 760 degrees C for Test A and B to 1204 degrees C for Test C. The results indicated that this Teflon Coated Fiberglas air-supported roof covering will satisfactorily withstand the three methods of test in ASTM E-108 and UL-790 when tested for a Class B rating.

• For a non air-supported structures these identical tests were run for a composite construction consisting of Teflon coated glass cloth and 50.8 mm (2 in) of Fiberglas flexible insulation with a white vinyl-reinforced-foil vapor barrier facing. These test results also established a Class B rating.

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

References

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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