专业英语桥梁17、22.pptVIP

Bridges Bridges are great symbols of mankind’s conquest of space. The sight of the crimson（深红色的） tracery (装饰的图案、花纹)of the Golden Gate Bridge against a setting sun in the Pacific Ocean, or the arch of the Gambit Viaduct（高架桥） soaring（高飞、翱翔） triumphantly （成功的、获胜的）above the deep gorge(深谷), fills one’s heart with wonder and admiration for the art of their builders. They are the enduring （持久的）expressions （表达、表示）of mankind’s determination to remove all barriers （障碍物）in its pursuit（追求、追逐） of a better and freer world. Their design and building schemes（安排、设计） are conceived in dream-like visions（视力、想象力）. But vision and determination are not enough. All the physical forces of nature and gravity must be understood with mathematical precision （精确、正确）and such forces have to be resisted by manipulating（熟练地使用） the right materials in the right pattern. This requires both the inspiration（灵感） of an artist and the skill of an artisan（工匠）. Scientific knowledge about materials and structural behaviour（结构性能） has expanded(扩大；展开) tremendously（极大的、巨大的）, and computing techniques are now widely available to manipulate （熟练地使用） complex theories in innumerable（无数的） ways very quickly. Engineers have virtually revolutionized bridge design and construction methods in the past decade. The advances apply to short-（小跨径桥梁）, medium, and long-span bridges. For permanent（永久的） bridges, the most commonly used materials are steel and concrete. Bridges of many different types are built with these materials, used singly or in combination. Timber（木材） may be used for temporary above-water construction, for the elements of a structure that lie below the waterline (particularly timber piles桩), or for short-span bridges located on secondary roads(次要道路). A few short-span aluminum（铝） bridges have been built in the United States on an experimental basis. The principal （主要的、首要的）portions of a bridge may be said to be the “substructure” （下部结构）and “superstructure”（上部结构）. This division is used here simply for