From HowStuffWorks:[0] > The design of the Twin Towers is often called a "tube within a tube," referring to the fact that all of the weight of the building was

From HowStuffWorks:[0] > The design of the Twin Towers is often called a "tube within a tube," referring to the fact that all of the weight of the building was supported by the external walls and an internal column. Previously, the exterior walls of a skyscraper were called curtain walls -- they weren't relied upon for strength, so it wasn't imperative that super-sturdy materials were used for them. And from 9-11 Research:[1] > The system of design of the World Trade Center Towers is called tubular framing, since the perimeter frames of the building are designed to act as a cantilevered tube in resisting lateral forces. This design concept (the so-called tube within a tube architecture) has been employed in the construction of many of the world's tallest buildings. These include the John Hancock Center (1105 ft), the Standard Oil of Indiana Building (1125 ft) and the Sears Tower (1450 ft). I wouldn't rely on much else in that site, though. 0) https://science.howstuffworks.com/engineering/structural/world-trade-center-construction-unique2.htm https://science.howstuffworks.com/engineering/structural/wor... 1) https://www.911research.wtc7.net/mirrors/guardian2/wtc/nova.htm https://www.911research.wtc7.net/mirrors/guardian2/wtc/nova....