Like Spandomes, the new Spantheon system is most efficient with diagonal arches, although it can eliminate bending moments from all other types of arch-supported membrane structures as well. In fact, the invention was conceived during the erection of a single-arch membrane structure, the Bank of America Concert Pavilion, built in Boston Harbor by Span Systems.
|Bank of America Concert Pavilion, Boston Harbor|
The membrane of this structure is suspended below the arch at 16 points. Watching the erection, as the membrane was being lifted by 16 individual hoists, (each one remote-controlled by a computer, to assure their uniform load), it suddenly seemed logical to the inventor of the future Spantheon system, that by securing at each lift point two pulleys to the arch above and one pulley to the membrane below, a single suspension cable, taken through these pulleys, could lift up the membrane, by tensioning only the two ends of the cable. The block-and-tackle physical law guarantees, that all lifting forces will be identical. Furthermore, by leaving this suspension system in place after the lift is completed, it will keep converting the random, asymmetrical membrane loads into identical and symmetrical arch loads, thereby eliminating all bending moments from the arch. Thus the Spantheon system was invented.
The invention provides two, alternative positions for the membrane, relative to the supporting arches: the membrane can be positioned either below or above the arches. The first alternative places the membrane below the arches; it secures the upper pulleys to the bottom of the arch, and the lower pulley to the membrane's ridge cable. Tensioning the suspension cable pulls up the lower pulleys and the ridge cable, and thereby pre-stresses the roof membrane.
The second alternative places the membrane above the arches, and uses support posts, or "floating masts" placed inside the arch truss, to push up the membrane's ridge cable. The upper end of each mast is secured to the ridge cable, and its lower end is secured to the lower pulley of the suspension system, which is secured inside the top of the arch. Tensioning the suspension cable raises the lower pulleys; their attached masts are pushing up the ridge cable, and thereby pre-stressing the roof membrane.