• What is so unique about the Metrodome?
• How does it remain standing?
• Was it easier and more economical to build than a standard stadium?
• What other similar structures could an architect design?
• Are there other stadiums like the Metrodome?

If you have ever bounced around on a "moon jump" at the fair or slept on an air mattress, then you know that fabric filled with air can support hundreds of kilograms of weight. This same principle is used at the Hubert H. Humphrey Stadium, also called the Metrodome, in Minneapolis, Minnesota.

The stadium is an air-supported structure. The structure is covered with 10 acres of teflon-coated fabric. This flame-retardant coating on the outside is strong enough to protect the fabric from ultraviolet radiation from the Sun, air pollution, acid rain, and other sources of physical damage.

The stadium has 20 electric air fans to maintain a positive air pressure inside the structure, although only three usually operate at one time. A computerized pneumatic system continually monitors the air pressure and engages extra 100-horsepower fans when the air pressure drops. These fans can blow air into the structure at about 100,000 cubic feet per minute. While positive air pressure keeps the Metrodome up, 26 cables are connected to the fabric to keep the entire structure in place.

Full-sized, air-supported sports stadiums are no longer being built in the United States. Too many problems with wet snow conditions and inadequate heating systems have had architects reconsider the design of large fabric-enclosed structures. Cable-supported or tensile structures are currently the preferred design for stadiums.

But designers have found other extremely practical uses for air-supported structures. They are used for temporary buildings and small sports facilities. The most exciting design is for hazardous waste clean-up sites. The air-supported covers prevent the sites from emitting toxic material into the environment.

Things to Talk About

1. What kind of structures would lend themselves to inflated support?
   
2. How would you make an air-supported structure?
   
3. Have you visited an inflated structure? Are there any where you live?
   

Pneumatic--Moved or worked by air pressure. Adapted for holding or inflated with compressed air.

Tensile Strength--The greatest longitudinal stress a substance can tolerate without separating.

Teflon--A modern polymer or plastic containing resins used especially for molding articles and for coatings to prevent sticking.

Ultraviolet Radiation--Electromagnetic radiation situated beyond the visible spectrum (light) at its violet end; abbreviated as UV radiation, it can damage materials exposed to direct sunlight for extended periods.

Resources

• Berry, Donna A. A Potpourri of Physics Teaching Ideas. College Park, Maryland: American Association of Physics Teachers, 1987.

Additional Sources of Information

• Air Supported Structure Committee, IFAI, 345 Cedar Street, Suite 800, St.Paul, MN 55101 (612) 222-2508
• Foundation for Architecture, Attn: Rolaine Copeland, One Penn Center at SuburbanS,tation Philadelphia, PA 19103, (215) 569-3187
• Metrodome Stadium, Attn: Steve Macki, 900 South 5th Street, Minneapolis, MN, 55415 (612) 332-0386
• Thermo-Flex, Hazardous Waste Structures, Attn: Jim Roesner, P.O. Box 1184, Salinas, KS 67402 (913) 827-7201

Activity Page

Blow It Up!

Find out how moving air can support weight in different conditions.

Main Activity

You can observe the effect that moving air has on inflatable materials. Discover how important changing conditions are on an inflated structure and how moving air can support it.

Materials

1. Small fan with high and low settings and a protective covering
2. 5 five-gallon plastic trash bags
3. Masking tape
4. cloth tape measure
5. Thumb tacks
6. Pencil
7. Paper

1. Tape the open end of the trash bag around the front of the fan.

2. Turn the fan on low. After the bag inflates, measure its widest part. Record your observations.

3. Turn the fan on high and measure the bag again. Record the new measurements.

4. Now turn the fan off and remove the bag. Using the thumb tack, poke a number of holes in the bag. Record the number of holes you made. Retape the bag to the fan in the same manner as before.

5. Turn the fan on low and measure the bag again. Turn the fan on high and measure once again. Record both observations.

6. Try adding more holes or larger holes to the bag.

ALWAYS REMEMBER TO TURN THE FAN OFF COMPLETELY EACH TIME YOU REMOVE AND REPLACE THE BAG. DO NOT PUT YOUR FACE OR HEAD IN THE PLASTIC BAGS!

Questions

1. How did the bag feel when it was inflated at the lowest setting? When the fan was on high?

2. How did the holes change the way the bag inflated?

3. Which holes affected the bag the most, the large ones or the small ones?

Draw a line on a balloon and then blow it up as much as you can. See what happens to the balloon if you change its environment. Try putting it in the freezer for an hour and then in the Sun or another warm place for an hour. How do these different conditions affect the balloon? Besides temperature, what other conditions might have an effect?

Contact a local car dealership and ask them about air bags. (Or watch "Newton's Apple's" show #913!) How do the same principles apply? Investigate the effectiveness of both air bags and seat belts. Would you want air bags in your car?

Conduct a "rip test" on a variety of everyday materials. Try tearing nylon, mylar, cardboard, and paper towels. Which ones are easiest to tear? Which ones are the hardest to tear? How are each of these materials used? Why would they need to be tear-resistant?

Have some fun with your hair dryer. See if you can keep different kinds of balls floating in the air! Try using cotton balls, Nerf balls and super balls. Or make your own light-weight balls out of tissue paper and aluminum foil. Which materials float the best? Why? Are the balls difficult to control? What other objects could you use with the hair dryer?