A skyscraper is a tall, continuously habitable building of many storeys, usually designed for office and commercial use. There is no official definition or height above which a building may be classified as a skyscraper. One common feature of skyscrapers is having a steel framework from which curtain walls are suspended, rather than load-bearing walls of conventional construction. Some early skyscrapers have a steel frame that enables the construction of load-bearing walls taller than of those made of reinforced concrete. Modern skyscrapers' walls are not load-bearing, and most skyscrapers are characterized by large surface areas of windows made possible by the concept of steel frame and curtain walls. However, skyscrapers can have curtain walls that mimic conventional walls and a small surface area of windows.
Skyscrapers since 1960s utilize the tubular designs, innovated by Bangladeshi-American structural engineer Fazlur Rahman Khan. This engineering principle makes the buildings structurally more efficient and stronger. It reduces the usage of material (economically much more efficient), while simultaneously allows the buildings to reach greater heights. It allows fewer interior columns, and so creates more usable floor space. It further enables buildings to take on various shapes. There are several variations of the tubular design; these structural systems are fundamental to tall building design today. Other pioneers include Hal Iyengar, William LeMessurier, etc. Cities have experienced a huge surge in skyscraper construction.