The construction of tall buildings is the result of urbanization as seen in America since the late nineteenth century when the first skyscrapers were built. The industrialized society attracts more people to the cities, requiring more space for offices as well as for habitation.?Tall buildings, however, require two basic technical ingredients.?First, economic method of building a tall building must be found and second, a reliable and economic method of transporting people vertically through the building must be available. Even though the Otis elevator in the late 19th century provided the logical answer to vertical transportation, the structure still remained a very significant deterrent to building very tall buildings. Buildings built up to 1920s were mostly below 30 stories. A heroic step was taken in 1930 when the Empire State Building was built with more or less conventional method of construction. After World War II the socioeconomic needs in the U. S. opened the way for more taller office and apartment buildings.

Within the last few years, research on building materials such as reinforced concrete and structural steel have made great strides and opened horizons for more efficient use of these materials. The structural engineers and architects also have met the challenge to find efficient and economical new structural systems for various ranges and heights of buildings going all the way to well over 100 stories.?Consequently, the process of selecting a structural system for a tall building has become more complicated than it ever was.

The process of choosing the structural system for a tall building depends on many criteria which are not always structural. The following is a brief discussion of these design criteria.

(1) Environmental Planning Consideration?- In any environment the addition of a tall building will certainly influence the operation of the traffic system, as well as the flow of people in the entire neighborhood. Therefore, a tall building project must be resolved in terms of pedestrian, auto and other traffic.

(2) The Overall Proportions of the Tower - The relationship of the building to the surrounding environment and other existing buildings and plazas may dictate the proportions of the tower, itself.?The increase in height-to-width ratio will generally mean increase in premium for height caused by additional material required to reduce lateral sway, as well as to increase resistance to overturning.

(3) Permissible Floor Area Ratio -The construction of?buildings in the city area is generally controlled by zoning. Zoning, in most cases, unfortunately, is the product of political and economic consideration. Zoning would normally allow a maximum number of square feet that can be built at any given site. In urban centers this becomes a critical consideration, particularly because in terms of high land cost the more area that can be built on a given piece of land, the more the potential for economic return.

(4) Inner Space Criteria - Only a few years back, column spacing of 20 ft. was accepted as a structural limitation that could not be overcome. Newer structural systems have given the architect and developers the choice to create larger column free spaces in the office, as well as residential buildings. In the office buildings for instance, 35 ft. clear spacing is now normally considered the minimum and most developers would not mind if they can get a 60 ft. clear span between?the core and the exterior walls.

(5) Climatic Considerations-Climatic considerations sometimes play a strong part in choosing the structural system. For extremely cold winter climate the general tendency is to provide larger, clear windows, perhaps because there is a greater need for the occupant to look out and enjoy the nature as much as possible. On the other hand, in hot tropical climate it is more economical to have less glass and more solid masonry or concrete surface.

(6) Structural Material Considerations - The selection of a system depends strictly on the local relative economies between the various structural materials. It is because of this reason that while in one area a concrete structural system may be economical, in another area of the country a structural steel system may become more economical. Sometimes, of course, a combination of both materials in the same building may result in the optimum design.

(7) Foundation Considerations - Tall buildings require a stable foundation. Where rock is directly available near the ground level, the choice of structural system or material is not affected. However, in many areas where, for instance, floating foundation is the only way to support a building, the total weight of the structure becomes very significant because it controls the total depth of excavation. In such cases, light weight construction is the obvious structural choice. This could lead to either a steel structural framing, or an all lightweight concrete construction, or a composite system depending on the local material economies.