Skyscrapers

        A skyscraper is a tall, continuously habitable building of over 40 floors, mostly designed for office, commercial and residential uses. A skyscraper can also be called a high-rise, but the term skyscraper is often used for buildings higher than 150 m (492 ft). For buildings above a height of 300 m (984 ft), the term Supertall can be used, while skyscrapers reaching beyond 600 m (1,969 ft) are classified as Megatall.

           A relatively big building may be considered a skyscraper if it protrudes well above its built environment and changes the overall skyline. The maximum height of structures has progressed historically with building methods and technologies and thus what is today considered a skyscraper is taller than before. The Burj Khalifa is currently the tallest building in the world.

        One common feature of skyscrapers is having a steel framework that supports curtain walls. These curtain walls either bear on the framework below or are suspended from the framework above, 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. Modern skyscrapers often have a tubular structure, and are designed to act like a hollow cylinder to resist lateral loads (wind, seismic, etc.). To appear more slender, allow less wind exposure, and in order to transmit more daylight to the ground, many skyscrapers have a design with setbacks; sometimes they are also structurally required.