【正文】 ion and flow effects, buildings tend settle. Uneven settlements, exemplified by the leaning towers in Pisa and Bologna, can have damaging effects—the building may lean, walls and partitions may crack, windows and doors may bee inoperative, and, in the extreme, a building may collapse. Uniform settlements are not so serious, although extreme conditions, such as those in Mexico City, can have serious consequences. Over the past 100 years, a change in the groundwater level there has caused some buildings to settle more than 10 feet (3 meters). Because such movements can occur during and after construction, careful analysis of the behavior of soils under a building is vital. The great variability of soils has led to a variety of solutions to the foundation problem. Where 4 firm soil exists close to the surface, the simplest solution is to rest columns on a small slab of concrete(spread footing). Where the soil is softer, it is necessary to spread the column load over a greater area；in this case, a continuous slab of concrete(raft or mat) under the whole building is cases where the soil near the surface is unable to support the weight of the building, piles of wood, steel, or concrete are driven down to firm soil. The construction of a building proceeds naturally from the foundation up to the superstructure. The design process, however, proceeds from the roof down to the foundation (in the direction of gravity). In the past, the foundation was not subject to systematic investigation. A scientific approach to the design of foundations has been developed in the 20th century. Karl Terzaghi of the United States pioneered studies that made it possible to make accurate predictions of the behavior of foundations, using the science of soil mechanics coupled with exploration and testing procedures. Foundation failures of the past, such as the classical example of the leaning tower in Pisa, have bee almost nonexistent. Foundations still are a hidden but costly part of many buildings. Although there have been many advancements in building construction technology in general, spectacular achievements have been made in the design and construction of ultrahighrise buildings. The early development of highrise buildings began with structural steel framing. Reinforced concrete and stressedskin tube systems have since been economically and petitively used in a number of structures for both residential and mercial purposes. The highrise buildings ranging from 50 to 110 stories that are being built all over the United States are the result of innovations and development of new structural systems. Greater height entails increased column and beam sizes to make buildings more rigid so that under wind load they will not sway beyond an acceptable limit. Excessive lateral sway may cause serious recurring damage to partitions, ceilings, and other architectural details. In addition, excessive sway may cause disfort to the occupants of the building because of their perception of such motion. Structural systems of reinforced concrete, as well as steel, take full advantage of the inherent potential stiffness of the total building and therefore do not require additional stiffening to limit the sway. In a steel structure, for example, the economy can be defined in terms of the total average quantity of steel per squar