【正文】 roduced steel construction in France. His designs for the Galerie des Machines and the Tower for the Paris Exposition of 1889 expressed the lightness of the steel framework. The Eiffel Tower, 984 feet (300 meters) high, was the tallest structure built by man and was not surpassed until 40 years later by a series of American skyscrapers. Elisha Otis installed the first elevator in a department store in New York in 1889, Eiffel installed the first elevators on a grand scale in the Eiffel Tower, whose hydraulic elevators could transport 2,350 passengers to the summit every hour. 2. LoadCarrying Frame Until the late 19th century, the exterior walls of a building were used as bearing walls to support the floors. This construction is essentially a post and lintel type, and it is still used in frame construction for houses. Bearingwall construction limited the height of building because of the enormous wall thickness required； for instance, the 16story Monadnock Building built in the 1880’s in Chicago had walls 5 feet ( meters) thick at the lower floors. In 1883, William Le Baron Jenney (18321907) supported floors on castiron columns to form a cagelike construction. Skeleton construction, consisting of steel beams and columns, was first used in 1889. As a consequence of skeleton construction, the enclosing walls bee a “curtain wall” rather than serving a supporting function. Masonry was the curtain wall material until the 1930’s, when light metal and glass curtain walls were used. After the introduction of buildings continued to increase rapidly. All tall buildings were built with a skeleton of steel until World War Ⅱ . After the war, the shortage of steel and the improved quality of concrete led to tall building being built of reinforced concrete. Marina Tower (1962) in Chicago is the tallest concrete building in the Unit ed States； its height—588 feet (179 meters)—is exceeded by the 650foot (198meter) Post Office Tower in London and by other towers. 2 A change in attitude about skyscraper construction has brought a return to the use of the bearing wall. In New York City, the Columbia Broadcasting System Building, designed by Eero Saarinen in 1962,has a perimeter wall consisting of 5foot () wide concrete columns spaced 10 feet (3 meters) from column center to center. This perimeter wall, in effect, constitutes a bea ring wall. One reason for this trend is that stiffness against the action of wind can be economically obtained by using the walls of the building as a tube； the World Trade Center building is another example of this tube approach. In contrast, rigid frames or vertical trusses are usually provided to give lateral stability. 3. Skin The skin of a building consists of both transparent elements (windows) and opaque elements (walls). Windows are traditionally glass, although plastics are being used, especially in schools where breakage creates a maintenance problem. The wall elements, which are used to cover the structure and are supported by it, are built of a variety of materials: brick, precast concrete, stone, opaque glass, plastics, steel, and aluminum. Wood is used mainly in house construction； it is not generally used for mercial, industrial, or public building because of the fire hazard. 4. Floors The construction of the floors in a building depends on the basic structural frame that is used. In steel skeleton construction, floors are either slabs of conc