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Article Excerpt
Abstract: Tall building developments have been rapidly increasing worldwide. This paper reviews the evolution of tall building's structural systems and the technological driving force behind tall building developments. For the primary structural systems, a new classification--interior structures and exterior structures--is presented. While most representative structural systems for tall buildings are discussed, the emphasis in this review paper is on current trends such as outrigger systems and diagrid structures. Auxiliary damping systems controlling building motion are also discussed. Further, contemporary "out-of-the-box" architectural design trends, such as aerodynamic and twisted forms, which directly or indirectly affect the structural performance of tall buildings, are reviewed. Finally, the future of structural developments in tall buildings is envisioned briefly.
Keywords: Aerodynamics, Building forms, Damping systems, Diagrid structures, Exterior structures, Interior structures, Outrigger systems, Structural performance, Structural systems, Tall buildings
Introduction
Tall buildings emerged in the late nineteenth century in the United States of America. They constituted a so-called "American Building Type," meaning that most important tall buildings were built in the U.S.A. Today, however, they are a worldwide architectural phenomenon. Many tall buildings are built worldwide, especially in Asian countries, such as China, Korea, Japan, and Malaysia. Based on data published in the 1980s, about 49% of the world's tall buildings were located in North America (Table 1-1). The distribution of tall buildings has changed radically with Asia now having the largest share with 32%, and North America's at 24% (Table 1-2). This data demonstrates the rapid growth of tall building construction in Asian during this period while North American construction has slowed. In fact, eight of the top ten tall buildings are now in Asia and only two, the Sears Tower and the Empire State Building, are in North America.
Traditionally the function of tall buildings has been as commercial office buildings. Other usages, such as residential, mixed-use, and hotel tower developments have since rapidly increased as Figure 1 shows. There has been some skepticism regarding construction of tall buildings since September 11, 2001, however, they will continue to be built due to their significant economic benefits in dense urban land use.
Tall building development involves various complex factors such as economics, aesthetics, technology, municipal regulations, and politics. Among these, economics has been the primary governing factor. This new building type itself would not have been possible, however, without supporting technologies. A structural revolution--the steel skeletal structure--as well as consequent glass curtain wall systems, which occurred in Chicago, has led to the present state-of-the-art skyscraper. While this review paper encompasses the development spectrum of tall building's structural systems, there is emphasis on current trends. Speculations of future prospects of structural developments in tall buildings are based on this review.
Developments of Structural Systems
Structural development of tall buildings has been a continuously evolving process. There is a distinct structural history of tall buildings similar to the history of their architectural styles in terms of skyscraper ages (Ali & Armstrong, 1995; Huxtable, 1984). These stages range from the rigid frame, tube, core-outrigger to diagrid systems. A brief account of past developments in tall buildings is presented below.
Brief History
In the late nineteenth century, early tall building developments were based on economic equations--increasing rentable area by stacking office spaces vertically and maximizing the rents of these offices by introducing as much natural light as possible. In order to serve this economic driver, new technologies were pursued that improved upon the conventional load-bearing masonry walls that had relatively small punched openings. The result was the iron/steel frame structure which minimized the depth and width of the structural members at building perimeters. Consequently, the larger openings were filled with transparent glasses, while the iron/steel structures were clad with other solid materials such as brick or terra cotta. Different from traditional load-bearing masonry walls, these claddings did not carry any loads from buildings except their own weights and the lateral wind pressure. A new cladding concept--curtain walls--was developed with the emergence of the new structural systems.
The symbolic power of skyscrapers being recognized, a notable phenomenon occurred from the turn of the century. A skyscraper height race began, starting from the Park Row Building in New York, which had already reached 30 stories in 1899. This height race culminated with the completion of the 102-story tall Empire State Building in 1931. Even though the heights of skyscrapers were significantly increased during this period, contrary to intuition, there had not been much conspicuous technological evolution. In terms of structural systems, most tall buildings in the early twentieth century employed steel rigid frames with wind bracing. Among them are the renowned Woolworth Building of 1913, Chrysler Building of 1930 and Empire State Building of 1931 all in New York (Ali, 2005). Their enormous heights at that time were accomplished not through notable technological evolution, but through excessive use of structural materials. Due to the absence of advanced structural analysis techniques, they were quite over-designed.
[FIGURE 1 OMITTED]
In terms of architectural expression of tall buildings at this time period, as can be observed from many eclectic style tall buildings, architects returned to the traditional architecture for representational quality, after a short pursuit of a new style for a new building type based on new technologies mostly by Chicago architects in the late nineteenth century. However, the rebirth of the early Chicago spirit and the application of European modern movements to tall buildings were only a matter of time.
The mid-twentieth century, after the war, was the era of mass production based on the International Style defined already before the war, and the technology developed earlier. The major driving force of tall building developments was economy. Even the once-prevalent height race did not occur after World War II until the construction of the World Trade Center in New York and the Sears Tower in Chicago, completed in 1973 and 1974, respectively.
Structural systems for tall buildings have undergone dramatic changes since the demise of the conventional rigid frames in the 1960s as the predominant type of structural system for steel or concrete tall buildings. With the emergence of the tubular forms still conforming to the International Style, such changes in the structural form and organization of tall buildings were necessitated by the emerging architectural trends in design in conjunction with the economic demands and technological developments in the realms of rational structural analysis and design made possible by the advent of high-speed digital computers. Beginning in the 1980s, once-prevalent Miesian tall buildings were then largely replaced by the facade characteristics of postmodern, historical, diagrid and deconstructivist expressions. This was not undesirable because the new generation of tall buildings broke the monotony of the exterior tower form and gave rise to novel high-rise expressions. Innovative structural systems involving tubes, megaframes, core-and-outriggersystems, artificially damped structures, and mixed steel-concrete systems are some of the new developments since the 1960s.
Premium for Height
The primary structural skeleton of a tall building can be visualized as a vertical cantilever beam with its base fixed in the ground. The structure has to carry the vertical gravity loads and the lateral wind and earthquake loads. Gravity loads are caused by dead and live loads. Lateral loads tend to snap the building or topple it. The building must therefore have adequate shear and bending resistance and must not lose its vertical load-carrying capability.
Fazlur Khan realized for the first time that as buildings became taller, there is a "premium for height" due to lateral loads and the demand on the structural system dramatically increased, and as a result, the total structural material consumption increases drastically (Ali, 2001). If there would be no lateral forces on the building such as wind or earthquake, any high-rise building could be designed just for gravity loads. The floor framing system usually carries almost the same gravity loads at each floor, although the girders along the column lines need to be progressively heavier towards the base of the building to carry increasing lateral forces and to augment the building's stiffness. The column sizes increase progressively towards the base of the building due to the accumulated increase in the gravity loads transmitted from the floors above. Further to this, the columns need to be even heavier towards the base to resist lateral loads. The net result is that as the building becomes taller and the building's sway due to lateral forces becomes critical, there is a greater demand on the girders and columns that make up the rigid-frame system to carry lateral forces. The concept of premium for height is illustrated in Figure 2.
[FIGURE 2 OMITTED]
If we assume the same bay sizes, the material quantities required for floor framing is almost the same regardless of the number of stories. The material needed for floor framing depends upon the span of the framing elements, that is, column-to-column distance and not on the building height. The quantity of materials required for resisting lateral loads, on the other hand, is even more increased and would begin to exceed other structural costs if a rigid-frame system is used for very tall structures. This calls for a structural system that goes well beyond the simple rigid frame concept. Based on his investigations Khan argued that as the height increases beyond 10 stories, the lateral drift starts controlling the design, the stiffness rather than strength becomes the dominant factor, and the premium for height increases rapidly with the number of stories. Following this line of reasoning, Khan recognized that a hierarchy of structural systems could be categorized with respect to relative effectiveness in resisting lateral loads for buildings beyond the 20- to 30-story range (Khan, 1969).
Classification of Tall Building Structural Systems
In 1969 Fazlur Khan classified structural systems for tall buildings relating to their heights with considerations for efficiency in the form of "Heights for Structural Systems" diagrams (Khan, 1969). This marked the beginning of a new era of skyscraper revolution in terms of multiple structural systems. Later, he upgraded these diagrams by way of modifications (Khan, 1972, 1973). He developed these schemes for both steel and concrete as can be seen from Figure 3 (Ali, 2001; Ali & Armstrong, 1995; Schueller, 1986). Khan argued that the rigid frame that had dominated tall building design and construction so long was not the only system fitting for tall buildings. Because of a better understanding of the mechanics of...
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