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土建学院

土木1201

李明月

1206090105

The first skyscrapers .Meantime ,in the United states an other important development was taking place .In 1884-85Maj .William Le baron Jenney ,a Chicago engineer ,had designed the Home Insurance Building ,ten stories high ,with a metal skeleton .Jenney's ,beams were of Bessemer steel ,though h is columns were cast iron .Cast iron lintels supporting mas onry over window opening were ,in turn ,supported on the ca st iron columns Solid masonry court and party walls provide d lateral support against wind loading .Within a decade the same type of construction had been used in more than 30 of fice buildings in Chicago and New York .Steel played a larg er and larger role in there ,with revered connections for b eams and columns sometimes strengthened for wind bracing by overlaying gusset plates at the junction of vertical and h orizontal members .Light masonry curtain walls ,supported a t each floor level replaced the old heavy masonry .

Though the new construction form was to remain centred almost entirely in America for several decades, its impact on the steel industry was worldwide. By the last years of the 19th century the basic structural shapes-I beams up to 20 in.(0.508m) in depth and Z and T shapes of lesser proportions-were readily available ,to combine with plates of several widths and

thicknesses to make efficient members of any required size and strength .In 1885 the heaviest structural shape produced through hot-rolling weighed less than 100 pounds (45 kilograms) per foot; decade by decade this figure rose until in the 1960s,it exceeded 700 pounds(320 kilograms) per foot.

Coincident with the introduction of structural steel came the introduction of the Otis electric elevator in 1889.The demonstration of a safe passenger elevator, together with that of a safe and economical steel construction method, sent building heights soaring .In New York the 286-ft(87.2-m) Flatiron Building of 1902 was surpassed in 1904 by the 375-ft(115-m) Times Building (renamed the Allied Chemical Building), the 468-ft(143-m) City Investing Company Building in Wall Street ,the 612-ft(187-m) Singer Building (1908), the 700-ft(214-m) Metropolitan Tower (1909) and ,in 1913 ,the 780-ft(232-m) Woolworth Building.

The rapid increase in height and the height-to-width ratio brought problem. To limit street congestion , building setback design was prescribed .On the technical side , the problem of lateral support was studied .A diagonal bracing system ,such as that used in the Eiffel Tower ,was not architecturally desirable in offices relying on sunlight for illumination .The