The ultimate goal of “green building” is to create sustainable habitats, with pollution-free and oxygen-rich air, evergreen vegetation, constant summer climate, and reduced energy needs using renewable sources on-site. In most of the industrialized world these green habitats can only be created by atmospheric separation. They have to be build under giant, transparent domes, which protect these ultimate human habitats like the ozon layer protects the Earth.
It was 43 years ago, that Buckminster Fuller, the inventor of the geodesic dome, has proposed the idea of covering mid-Manhattan with a giant dome, clear-span and transparent, to create a livable city with clean air. “The dome would also save at least 90% of heating and cooling energy”, he wrote, because the thermal exchange would be through the dome’s surface, which is less than 10% of the total building surfaces below. Since then, a great deal of research has been done to build dome structures with at least 500 meter clear-span, and sufficient height to rise well over the top layer of urban smog. But all the known technologies are limited to spans only about half that long. Since 1992 the 256 meter clear-span of the Georgia Dome in Atlanta has been the longest in the world. What causes this clear-span limit?
Most of today’s large, clear-span domes have prestressed membrane surfaces. Snow and wind loads cause tension in the membrane. This is transmitted to the supporting frames or arches, which resist it with compression and bending stresses. Our ability to build very long spans is limited primarily by the bending stresses, because, as a structure’s span becomes longer, the increases in compression are linear, however, bending moments increase exponentially. This means, that doubling the clear span doubles compression, but quadruples bending. Those very large cross sections, which are required for increased bending, make the structure too heavy.
An arch could be free of bending moments only if its loads were symmetrical and uniform. But nature’s loads on any roof membrane are always asymmetrical: wind blows from left to right, and snow melts off the South side first. The only solution would be to somehow tell nature not to do this... not to subject the supporting arches of a giant dome to asymmetrical loads. Sheer fantasy? Not quite. Because the recently invented Spantheon structural system does exactly that: it uses nature’s laws to provide simple, but unique support means, which convert the asymmetrical, random membrane loads into symmetrical, uniform arch loads.
