Skyscraper design and construction

Good structural design is important in most building designs, but particularly for skyscrapers since even a small chance of catastrophic failure is unacceptable given the high prices of construction. This presents a paradox to civil engineers: the only way to assure a lack of failure is to test for all modes of failure, in both the laboratory and the real world. But the only way to know of all modes of failure is to learn from previous failures. Thus, no engineer can be absolutely sure that a given structure will resist all loadings that could cause failure, but can only have large enough margins of safety such that a failure is acceptably unlikely. When buildings do fail, engineers question whether the failure was due to some lack of foresight or due to some unknowable factor.

One of the many things that make skyscrapers special is their substructure. For example, the depth of the pit that holds the substructure has to reach all the way to bedrock. If bedrock lies close to the surface, the soil on top of the bedrock is removed, and enough of the bedrock surface is removed to form a smooth platform on which to construct the building's foundation.[1]

Taipei 101 endures a typhoon (2005)

The load a skyscraper experiences is largely from the force of the building material itself. In most building designs, the weight of the structure is much larger than the weight of the material that it will support beyond its own weight. In technical terms, the dead load, the load of the structure, is larger than the live load, the weight of things in the structure (people, furniture, vehicles, etc.). As such, the amount of structural material required within the lower levels of a skyscraper will be much larger than the material required within higher levels. This is not always visually apparent. The Empire State Building's setbacks are actually a result of the building code at the time, and were not structurally required. On the other hand, John Hancock Center's shape is uniquely the result of how it supports loads. Vertical supports can come in several types, among which the most common for skyscrapers can be categorized as steel frames, concrete cores, tube within tube design, and shear walls.

The wind loading on a skyscraper should also be considered. In fact, the lateral wind load imposed on super-tall structures is generally the governing factor in the structural design. Wind pressure increases with height, so for very tall buildings, the loads associated with wind are larger than dead or live loads.

Other vertical and horizontal loading factors come from varied, unpredictable sources, such as earthquakes.

Tube frame

Main article: Tube (structure)

The Willis Tower showing the bundled tube frame design

A new structural system using framed tubes was developed in the early 1960s. Fazlur Khan and J. Rankine defined the framed tube structure as "a three dimensional space structure composed of three, four, or possibly more frames, braced frames, or shear walls, joined at or near their edges to form a vertical tube-like structural system capable of resisting lateral forces in any direction by cantilevering from the foundation."[4] Closely spaced interconnected exterior columns form the tube. Horizontal loads (primarily wind) are supported by the structure as a whole. About half the exterior surface is available for windows. Framed tubes allow fewer interior columns, and so create more usable floor space. Where larger openings like garage doors are required, the tube frame must be interrupted, with transfer girders used to maintain structural integrity. Tube structures cut down costs, at the same time allow buildings to reach greater heights. Tube-frame construction was first used in the DeWitt-Chestnut Apartment Building, designed by Khan and completed in Chicago in 1963.[5] It was used soon after for the John Hancock Center and in the construction of the World Trade Center.

A variation on the tube frame is the bundled tube, which uses several interconnected tube frames. The Willis Tower in Chicago used this design, employing nine tubes of varying height to achieve its distinct appearance. The bundle tube design was not only highly efficient in economic terms, but it was also "innovative in its potential for versatile formulation of architectural space. Efficient towers no longer had to be box-like; the tube-units could take on various shapes and could be bundled together in different sorts of groupings."[6] The bundled tube structure meant that "buildings no longer need be boxlike in appearance: they could become sculpture."[7] Cities have experienced a huge surge in skyscraper construction, thanks to Khan's innovations allowing economic skyscrapers.

The tubular systems are fundamental to tall building design. Most buildings over 40-stories constructed since the 1960s now use a tube design derived from Khan's structural engineering principles,[2][8] examples including the construction of the World Trade Center, Aon Center, Petronas Towers, Jin Mao Building, and most other supertall skyscrapers since the 1960s.[9] The strong influence of tube structure design is also evident in the construction of the current tallest skyscraper, the Burj Khalifa.[7]

Elevators in the Empire State Building

The invention of the elevator was a precondition for the invention of skyscrapers, given that most people would not (or could not) climb more than a few flights of stairs at a time. The elevators in a skyscraper are not simply a necessary utility like running water and electricity, but are in fact closely related to the design of the whole structure. A taller building requires more elevators to service the additional floors, but the elevator shafts consume valuable floor space. If the service core (which contains the elevator shafts) becomes too big, it can reduce the profitability of the building. Architects must therefore balance the value gained by adding height against the value lost to the expanding service core.[10] Many tall buildings use elevators in a non-standard configuration to reduce their footprint. Buildings such as the former World Trade Center Towers and Chicago's John Hancock Center use sky lobbies, where express elevators take passengers to upper floors which serve as the base for local elevators. This allows architects and engineers to place elevator shafts on top of each other, saving space. Sky lobbies and express elevators take up a significant amount of space and add to the amount of time spent commuting between floors. Other buildings such as the Petronas Towers use double-deck elevators allowing more people to fit in a single elevator and reaching two floors at every stop. It is possible to use even more than two levels on an elevator although this has yet to be tried. The main problem with double-deck elevators is that they cause all elevator occupants to stop when only people on one level need to get off at a given floor.

Another solution, employed by the Shanghai Tower and the under-construction (2019) Jeddah Tower is for buildings to be created for mixed-use, putting floorspace-hogging office suites towards the bottom, but multistory penthouses and atria - which require little cross-sectional floor space - towards the top.[11]

Building skyscrapers can be difficult due to factors other than complexity and cost. For example, in European cities like Paris, the difference between the appearance of old architecture and modern skyscrapers can make it hard to get approval from local authorities to construct new skyscrapers. Building skyscrapers in an old and famous town can drastically alter the image of the city. In cities like London, Edinburgh, Portland, and San Francisco there is a legal requirement called protected view, which limits the height of new buildings within or adjacent to the sightline between the two places involved.[12] This rule also makes it harder to find suitable sites for new tall buildings.

• List of architects of supertall buildings
• List of cities with most skyscrapers
• Plyscraper
• Seascraper

• Barr, Jason M. (2016-06-09). Building the Skyline: The Birth and Growth of Manhattan's Skyscrapers. Oxford University Press. p. 456. ISBN 978-0199344369.

• Macaulay, David (1987-10-26). Unbuilding (Reprint ed.). Houghton Mifflin/Walter Lorraine Books. p. 80. ISBN 0-395-45425-5.

• Sabbagh, Karl (1991-07-01). Skyscraper: The Making of a Building (Reprint ed.). Penguin (Non-Classics). p. 400. ISBN 0-14-015284-9.

• Chew, Michael Y. L.; Michael Chew Yit Lin (2001-02-15). Construction Technology for Tall Buildings (2 Sub ed.). Singapore University Press. p. 436. ISBN 981-02-4338-3.

• How Stuff Works explanation of Skyscrapers
• "Skyscrapers will continue to be built, author of book on tall buildings says". Archived from the original on 2006-04-25. Retrieved 2007-05-14.