Practical Design ofConcrete Shells

Arnold Wilson-~

Figure 1.9 - Dome of tI Home in Pensacola Beach, Florida survived a direct hit byhurricanes Ivan, Dennis and Katrina. Storm surge forced water through the ground floorgarage and tore away the breakaway staircase. The living quarters were u,ndamaged.

top of the dome; it forced the upper dome back into the bottom part of thedome. The result was that no material was deposited outside the dome butwas contained within, saving much property damage as well as injury orloss of life. This safety valve technique may have other useful applicationsfor other farm product storage, munitions storage and manufacturing pro­cesses for explosives or fireworks.

Housing for people is still an untouched area which deserves much moreattention in the future. Several years ago, two severe hurricanes, Hugo andAndrew, caused extensive damage along the east coast of the United States.Thousands of houses were destroyed and billions of dollars lost as a result ofthis damage. But if you lived in a reinforced concrete thin shell dome in thecenter of the worst damage, your house would have remained in good condi­tion (Figure 1.7,1.8).

On February 23 and 24, 1998, tornadoes ravaged central Florida, destroy­ing more than 2000 houses. Here too, concrete thin shells would have savedlives and property. For the most part, the dome can withstand the extremeviolence of even a tornado.

On September 15, 2004, Hurricane Ivan with 130 mph (209 km/h) windshit the coast in Pensacola Beach, Florida. Ivan was one of the fiercest anddeadliest storms ever to hit this area, causing great destruction. A concrete

13

1.3 Domes: Where have we been and where are we going?

home in the shape of a dome, named Dome of a Home, and its owners Markand Valerie Sigler, stood firm and remained relatively dry. The concretedome withstood the hurricane as neighboring homes were virtually washedout to sea. The impact of the storm was witnessed first hand as Mark Sigler,the owner, accompanied by Craig White, an NBC cameraman, Chuck Stew­art, an NBC sound engineer and several other people stayed in the domethrough the storm (Figure 1.9).

Throughout the world, simple domes for houses could provide stablehousing by utilizing local materials such as concrete. Because of their shape,domes require less construction material than most other structures.,

Since about 1975, the ability to create a dome ecqnpmi<;:allytriggeredworld-wide interest. Now, twenty-five years later, the indu~try is still in its ,infancy. Only a few people are involved. More architects, engineers andbuilders are needed to promote concrete thin shells, particularly those utiliz­ing economical forming systems.

Domes in sizes as large as 260 feet in diameter and 130 feet high havebeen built successfully, but the future now holds promise for much largerdomes. Wouldn't it be exciting if we could build a dome structure as a largestadium for professional football and basketball by using concrete with itsmany advantages and at a price competitive with any other system? We can.Research at Brigham Young University demonstrated that very large domescan now be built using inflated forms restrained by a cable system (Jacobs1996;South 1997) (Figure 1.10).

Until recently the size of the dome was limited to the strength of thematerial used to make its inflatable form. Force in the fabric is created bythe magnitude of air pressure times the radius of curvature of the fabric. Asdomes get larger and/or of lower profile, the radius of curvature increases,while the pressure required for construction remains nearly constant.

When the safe capacity of the material is reached, the size and/or flat­ness of the dome is also reached. The invention of the cable restrained sys­tem demonstrated that with exterior cables attached to the foundation and

secured over the inflated form an increase in air pressure is possible withoutendangering the safety of the inflated form. Since a certain air pressure isnecessary to meet construction requirements, it then becomes possible toincrease the dome's radius of curvature.

A unique phenomenon takes place as the air pressure is increased. Thefabric stretches between the cable restraints. The modulus of elasticity of thefabric is very low compared to the modulus of the steel cables; thus the fabricstretches more than the cables and creates small domes by puffing out be­tween the cables. These small domes now have a smaller radius of curvature

and hence a reductioll ill fflbric force. This means that a very large structurecan now be consII'lH;14·d wilh n very large radius of curvature because the

15