Placing the membrane on top of the arches protects the arches and the suspension system from the weather outside. The free flow of wind over the membrane roof, unobstructed by the arches, creates uplift (suction) over nearly the entire surface, loading primarily the peripheral foundations, and not the arches. Therefore Spantheons can be designed to resist the strongest hurricanes. Having the membrane on top of the arches also allows the curvature of the membrane's ridge cable to be independent of the arch curvature. For instance, by raising the apex of the membrane, a peak can be created, that helps snow melting and sliding off the roof.
A hurricane-resistant Spantheon can be built over the skyscrapers of downtown Houston (the geodesic dome proposed by Discovery Channel can not). No air ships will be needed to erect the 1.2-mile diameter Spantheon. Its 20 arches will be raised from the ground up, with a technique utilizing Houston's rectangular grid of streets. The same technique can be used to build a dome over any city, which has at least three straight avenues meeting at the center.
Creating large communities with zero emission is one obvious way Spantheons will reduce global warming. Doing it through tourism is less obvious. But climate change is caused by greenhouse emissions, of which the fastest growing source is air traffic, with winter tourism being the worst polluter. Most winter tourists would probably prefer an express train, instead of a plane, to get to a not-too-distant tropical paradise of palm trees and warm beaches. In Europe, where all the beaches can have snow in the winter, the first giant, year-round tropical resort is being planned in Hungary, over rich geothermal water reserves, under a Spantheon.
A Spantheon might also solve two Olympic problems: how to prevent long delays by major storms, and what to do with useless stadia and arenas after the games. Conducting most events under a single Spantheon saves the cost of individual roofs, and keeps the games going during severe storms. After the games are over, the prefabricated steel bleachers can be dismantled and reused elsewhere (at the next Olympics?), and a city can be built in their place.
The savings realized from building cities under Spantheons will pay for the cost of the dome. These buildings have no wind or snow load, nor thermal insulation, the on-site prefabrication and assembly of their components can proceed non-stop, unaffected by the outside weather. Geothermal heat, simply by being retained inside a Spantheon, becomes very economical, even if extracted from great depths, or not hot enough to generate electricity. Space cooling can be provided through high altitude natural air exchange. Rich, evergreen vegetation, growing on all grounds and rooftops, even on "vertical farms", supplies food, produces oxygen, and absorbs CO2 emissions. Rainwater is collected along the dome's periphery, for regulated use. These and other unique features add up to produce an optimal human habitat.
A typical Spantheon membrane is assembled from individual "Spanels", made of transparent ETFE foil stretched inside an aluminum frame, which is firmly secured to the tensioned cable grid. The same foil is used at Eden Garden in Britain and at both Olympic stadia in Beijing. But unlike those air-inflated, cushion type ETFE panels, a Spanel's single layer of structural foil alone can support both the weight of snow and the uplift of wind, by its post-tensioned, anticlastic "saddle" surface. The Spanels' light aluminum frames, and their cable grid, are virtually invisible from the ground. Spanels keep out the weather, but let in the sky.
Inside the dome's outer, structural membrane there is a second skin, a giant "bubble", made of even thinner ETFE foil, and held up mainly by air temperature differential. The airspace between these two layers serves primarily as thermal insulation. The bubble's openings help to melt snow, or to separate fresh air intake from exhausted air, when both must occur at the top due to the height of pollution outside. Photovoltaic sheets, nanogel insulation, heat-reflective or shading surfaces may be added to the bubble. It may also be rotated seasonally.
It is a unique feature of Spantheons, that after the arches and cable grids are erected, but before the panels are installed, simply by tensioning the suspension cables, the entire structure can be load-tested, subjecting all of its members to their maximum design load. Such advance proof of the required load bearing capacity is unprecedented for giant building structures. It is also important to note, that due to their low mass Spantheons are resistant to earthquakes.
The need is urgent to prepare for erecting giant cities virtually overnight, in order to relocate entire societies escaping from severe air pollution, volcanic eruptions, rising ocean levels, earthquakes, floods, and other disasters. This preparation should start now.