07-07-2011, 02:43 AM
The design of the towers from the PANYNJ was driven by economic concerns. They wanted enormous flexible rental area on a relatively small site. The only way to achieve this was to build up. NYC was asked to give all sorts of special zoning and building code waivers to accommodate the towers.
Building them tall with conventional materials and methods would be too heavy, too slow and way too expensive. Every additional floor added to the height meant that all the columns below it had to be carry that weight. So the design load for the floors was reduced from 100 psf to 58 psf lowering the weight of the floor system by over 40%.
The next problem was getting the people up into the towers since the normal use for elevators would consume most of the rentable floor space. This meant the innovative express and local elevator system which was essentially 3 - 32 floor towers stacked one atop the other with 3 lobbies. The ground floor served the other two lobbies at 44 and 78 plus all the local floors up to 41 and the public spaces at floors 106 and 107 - restaurant and observation decks in the two towers.
The whole tower was built like an erector set and the floor assemblies were only of 3 types long span, long span with a corner and short span. They assembles were so light weight that the metal floor pans were on .03" thick and the "concrete" had no stone aggregate and used light weight fly ash and entrained air to lighten them as much as possible. Each floor weighed about 1000 tons and was designed to support about 900 tons of superimposed dead and live loads. Floor 93 was identical in design as floor 33! The only "different" floors were the 8 mechanical floors which carried heavy HVAC equipment, huge tanks of water, and electrical substations. They were separating the 3 stacked towers with one set of mechanical floors above the lobby and another below the roof.
No buildings have been built like that before or since.
Building them tall with conventional materials and methods would be too heavy, too slow and way too expensive. Every additional floor added to the height meant that all the columns below it had to be carry that weight. So the design load for the floors was reduced from 100 psf to 58 psf lowering the weight of the floor system by over 40%.
The next problem was getting the people up into the towers since the normal use for elevators would consume most of the rentable floor space. This meant the innovative express and local elevator system which was essentially 3 - 32 floor towers stacked one atop the other with 3 lobbies. The ground floor served the other two lobbies at 44 and 78 plus all the local floors up to 41 and the public spaces at floors 106 and 107 - restaurant and observation decks in the two towers.
The whole tower was built like an erector set and the floor assemblies were only of 3 types long span, long span with a corner and short span. They assembles were so light weight that the metal floor pans were on .03" thick and the "concrete" had no stone aggregate and used light weight fly ash and entrained air to lighten them as much as possible. Each floor weighed about 1000 tons and was designed to support about 900 tons of superimposed dead and live loads. Floor 93 was identical in design as floor 33! The only "different" floors were the 8 mechanical floors which carried heavy HVAC equipment, huge tanks of water, and electrical substations. They were separating the 3 stacked towers with one set of mechanical floors above the lobby and another below the roof.
No buildings have been built like that before or since.