Loop launch diagram
Some of the mooted projects are space towers in which orbiting stations suspend elevators to the surface and others include self suspending orbital rings, space fountains and maglev launching loops.
Current mega structures such as the Oresund bridge which connects Sweden to Denmark and is about 18 miles long, the Viaduc de Millau Bridge in the South of France 8000 ft and the Rion Antirion cable bridge in Greece is over 7000 ft long are dwarfed by the proposed structures. In the case of the orbital loop launcher, for optimal functioning the loop is to be 5000 km long and move at about 14 km/sec. It is hard to imaging that anything so immense could be planned and built but then there is also the tendency for whatever is possible to come to pass.
One of the hard to answer questions is why would any one bother? Two answers come to mind. Money and Prestige. If there is money in it the capitalists will do it. If there is prestige in it maybe as the century moves on some of the superpowers will see it as the war equivalent which will establish their premier place. Perhaps the first example of this is the plan announced by Shuichi Ono, chairman of the Japan Space Elevator Association in Sept 2008 to built an orbital tower.
More theoretically if humanity is tied to earth forever it is more or less certainly doomed by Asteroid impact or solar evolution and so if society is to survive and humanity is to persist such structures must sooner or later be built. There is also the fact that it would be such fun.
Japanese Space Tower base station