The main Space Fountain (and related concepts) stumbling block that hasn’t been addressed is that whatever reduction in weight is experienced by the structure will be experienced as a downwards force on whatever turns the pellets around. Unless the pellets will be turned around over a much greater distance than the building is tall, the same limitations on material properties will be encountered. However if the building is to be tall enough to make low orbit cheap and easy, a vertical underground shaft deep enough to effect a manageable rate of change in momentum would proabably penetrate into the earth’s mantle.
