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           The adoption of structural steel and reinforced concrete caused major changes in traditional construction practices£¨Ê©¹¤×÷Òµ£©. It was no longer necessary to use thick walls of stone or brick for multistory buildings, and it became much simpler to build fire-resistant floors£¨·À»ðµØÃ棩. Both these changes served to£¨ÓÐÀûÓÚ£© reduce the cost of construction. It also became possible to erect£¨½¨Ô죩buildings with greater heights and longer spans.
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          Since the weight of modern structures is carried£¨³ÐÊÜ£© by the steel or concrete frame, the walls do not support the building. They have become curtain walls, which keep out the weather and let in light. In the earlier steel or concrete frame building, the curtain walls were generally made of masonry; they had the solid look of bearing walls£¨³ÐÖØǽ£©. Today, however, curtain walls are often made of lightweight materials such as glass, aluminum, or plastic, in various combinations.
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          Another advance in steel construction£¨½á¹¹£© is the method of fastening together£¨Á¬ÔÚÒ»Æ𣩠the beams. For many years the standard method was riveting. A rivet is a bolt with a head that looks like a blunt screw£¨Ô²Í·ÂÝË¿¶¤£© without threads£¨ÂÝÎÆ£©. It is heated, placed in holes through the pieces of steel£¨¸Ö¹¹¼þ£©, and a second head is formed at the other end by hammering£¨´¸»÷£©it to hold it in place£¨¹Ì¶¨¾Í룩. Riveting has now largely been replaced by welding, the joining together of pieces of steel by melting£¨ÈÛ»¯£© a steel material between them under high heat.
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             Prestressed concrete is an improved form of reinforcement£¨¼ÓÇ¿·½·¨£©. Steel rods are bent into the shapes to give them the necessary degree of tensile strength. They are then used to prestress £¨¶Ô..Ô¤¼ÓÓ¦Á¦£©concrete, usually by one of two different methods. The first is to leave channels in a concrete beam that correspond to£¨ÏàÓ¦ÓÚ£© the shapes of the steel rods. When the rods are run through the channels, they are then bonded to the concrete by filling the channels with grout, a thin mortar or binding agent. In the other  (and more common) method, the prestressed steel rods are placed in the lower part of a form£¨Ä£°å£© that corresponds to the shape of the finished structure£¨³ÉÆ·½á¹¹£©, and the concrete is poured around them. Prestressed concrete uses less steel and less concrete. Because it is so economical, it is a highly desirable£¨·Ç³£ÀíÏ룩 material.
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            Prestressed concrete has made it possible to develop£¨½¨Ô죩 buildings with unusual shapes, like some of the modern sports arenas, with large space unbroken by any obstructing supports£¨×è°­µÄÖ§³ÅÎ. The uses for this relatively new structural method are constantly being developed£¨²»¶ÏµØÀ©´ó£©.
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