are tall, internally supported structures where the majority of load
bearing structure, outside of that providing structural support for dead load
is specifically designed to provide for large free spaces between supporting elements while still providing structural support and the building utilities required for the densely populated structure that results. The problems posed in skyscraper design are considered among the most complex encountered given the balances required between economics
, and construction
Basic design considerations
Good structural design is of importance in most building design, but especially among skyscrapers since even a small likelihood of catastrophic failure is unacceptable given the number of individuals served by skyscrapers and the resulting price of failure. 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. The only way to know of all modes of failure is to learn from previous failures. In this way, no engineer can be absolutely sure that a given structure will resist all loadings that could cause failure, but can only be sure, that given large enough margins of safety, that a sufficiently small percentage of the time will a failure ever occur. When buildings do fail, engineers question if the failure was due to some lack of foresight on their part or some unknowable factor that would have never been expected to have been designed for.
Loading and vibration
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, or borne out visually. The Empire State Building's setbacks are actually a result of the building code at the time, and were not 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 is also considerable. 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
A shear wall, in its simplest definition is a wall where the entire material of the wall is employed in the resistance of both horizontal and vertical loads. A typical example is a brick
wall, or a cinderblock
wall. Since the wall material is used to hold the weight, as the wall expands in size, it must hold considerably more weight. Due to the features of a shear wall, it is perfectly fine, and even ideal for small constructions such as suburban housing or a typical urban brownstone, because it requires low cost of material, low maintenance, and provides high reliability for small designs. In this way, shear walls typically in the form of either plywood
and framing, brick, or cinderblock, is used for these structures. For skyscrapers though, as the size of the structure increases, so does the size of the supporting wall. Previous large structures such as castles
could ignore these issues due to a large wall being advantageous (castles), or ingeniously designed around (cathedrals). Since skyscrapers seek to maximize the floor-space by consolidating structural support, shear walls tend to be used only in conjunction with other support systems.
The classic concept of a skyscraper is a large steel box with many small boxes inside it. The genius of the steel
frame is its simplicity. By eliminating the inefficient part of a shear wall, the central portion, and consolidating support members in a much stronger material, steel, a skyscraper could be built with both horizontal and vertical supports throughout. This method, though simple, has drawbacks. Chief among these is that as more material must be supported (as height increases), the distance between supporting members must decrease, which actually in turn, increases the amount of material that must be supported.
After 1965 a new structural system of framed tubes appeared. 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. Closely spaced interconnected exterior columns form the tube. Horizontal loads, for example 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-frame construction was used in the DeWitt-Chestnut apartment building, and in the building of the World Trade Center.
- Macaulay, David Unbuilding. Reprint, Houghton Mifflin/Walter Lorraine Books.
- Sabbagh, Karl Skyscraper: The Making of a Building. Reprint, Penguin (Non-Classics).
- Chew, Michael Y. L.; Michael Chew Yit Lin Construction Technology for Tall Buildings. 2 Sub, Singapore University Press.