High-rise buildings.

The high-rise building is generally defined as one that is taller than the maximum which people are willing to walk up; it thus requires mechanical vertical transportation. This includes a rather limited range of building uses, primarily residential apartments, hotels and office buildings, though occasionally including retail and educational facilities. A type that has appeared recently is the mixed-use building, which contains varying amounts of residential, office, hotel, or commercial space. Highrise buildings are among the largest buildings built, and their unit costs are relatively high; their commercial and office functions require a high degree of flexibility.

The foundations of high-rise buildings support very heavy loads, but the systems developed for low-rise buildings are used, though enlarged in scale. These include concrete caisson columns bearing on rock or building on exposed rock itself. Bearing piles and floating foundations are also used. The structural systems of tall buildings must carry vertical gravity loads, such as those due to wind and earthquakes, are also a major consideration. The effect of wind forces on tall buildings is twofold. A tall building may be thought of as a cantilever beam with its fixed end at the ground; the pressure of the wind on the building causes it to bend with the maximum deflection at the top. Thus under 26 wind forces there are several performance criteria that a high-rise structure must meet (stability, deflection, swaying motion).

Long-span buildings.

Long-span buildings create unobstructed, column-free spaces greater than 30 meters (100 feet) for a variety of functions. These include activities where visibility is important for large audiences (auditoriums and covered stadiums), where flexibility is important (exhibition halls and certain types of manufacturing facility), and where large movable objects are housed (aircraft hangars). In the late 20th century, durable upper limits of span have been established for these types: the largest covered stadium has a span of 204 meters (670 feet), the largest exhibition hall has a span of 216 meters (710 feet), and the largest commercial fixedwing aircraft has a wingspread of 66.7 meters and a length of 69.4 meters, requiring a 75-80 meter span hangar. In these buildings the structural system needed to achieve these spans is a major concern.