Unit 3
          The principal construction materials of earlier times were wood and masonry-brick, stone, or tile, and similar materials. The courses or layers（砖层）were bound together with mortar or bitumen, a tarlike substance, or some other binding agent. The Greeks and Romans sometimes used iron rods or clamps to strengthen their building. The columns of the Parthenon in Athens（雅典的帕台农神庙）, for example, have holes drilled（钻孔） in them for iron bars that have now rusted away（锈蚀殆尽）. The Romans also used a natural cement called pozzolana, made from volcanic ash, that became as hard as stone under water.
            早期主要的建筑材料是木材和砌体，如砖、石、瓦以及类似的材料。砖层之间通过砂浆、沥青（一种焦油状的物质）或其他一些粘合剂粘合在一起。希腊人和罗马人有时用铁条或夹子来加固他们的房屋。例如，雅典的帕台农神庙柱子中曾钻孔以便加入铁条，如今都已锈蚀殆尽。罗马人也用称作白榴火山灰的天然水泥，它用火山灰制作，在水中会变得与石头一样坚硬。
            Both steel and cement, the two most important construction materials of modern times, were introduced（推广） in the nineteenth century. Steel, basically an alloy of iron （铁合金）and a small amount of carbon, had been made up to that time（到那个时候） by a  laborious（繁复的） process that restricted it to such special uses as sword blades（刀刃）. After the invention of the Bessemer process （贝塞麦炼钢法）in 1856, steel was available in large quantities at low prices. The enormous advantage of steel is its tensile strength; that is, it does not lose its strength when it is under a calculated degree (适当的) of  tension, a force which, as we have seen, tends to （往往）pull apart many materials. New alloys have further increased the strength of steel and eliminated some of its problems, such as fatigue, which is a tendency for it to weaken as a result of continual changes in stress（连续的应力变化）.
            作为现代两种最重要的建筑材料，钢材与水泥在十九世纪得到了推广。直到那个时候，钢材才通过繁复的过程制造出来，基本上是铁合金，并含有少量的碳，因而被限制在一些特殊的用途如刀刃。在1856年发明了贝塞麦炼钢法后，钢材才得以大量低价获得。钢
材巨大的优势即是它的抗拉强度，也就是当它在适当的拉力下不会失去强度，正如我们所看到的，该力往往能够将很多材料拉开。新的合金进一步提高了钢材的强度，并消除了一些缺点，如疲劳，即在连续的应力变化下导致强度减弱的趋势。
          Modern cement, called Portland cement, was invented in 1824. It is a mixture of limestone（石灰石） and clay, which is heated and then ground into a powder（磨成粉末）. It is mixed at or near the construction site （施工现场）with sand, aggregate (small stones, crushed  rock, or gravel), and water to make concrete. Different proportions of the ingredients （配料）produce concrete with different strength and weight. Concrete is very versatile; it can be poured, pumped, or even sprayed into （喷射成）all kinds of shapes. And whereas steel has great tensile strength, concrete has great strength under compression. Thus, the two substances complement each other（互补）.
        现代水泥发明于1824年，称为波特兰水泥。它是石灰石和粘土的混合物，加热后磨成粉末。在或靠近施工现场，将水泥与砂、骨料（小石头、压碎的岩石或砾石）、水混合而
制成混凝土。不同比例的配料会制造出不同强度和重量的混凝土。混凝土的用途很多，可以浇筑、泵送甚至喷射成各种形状。混凝土具有很大的抗压强度，而钢材具有很大的抗拉强度。这样，两种材料可以互补。
        They also complement each other in another way: they have almost the same rate of contraction and expansion. They therefore can work together in situations where（在…情况下） both compression and tension are factors（主要因素）. Steel rods（钢筋） are embedded in（埋入）concrete to make reinforced concrete in concrete beams or structures where tension will develop（出现）. Concrete and steel also form such a strong bond - the force that unites（粘合） them - that the steel cannot slip（滑移） with the concrete. Still（还有） another advantage is that steel does not rust in concrete. Acid（酸） corrodes steel, whereas concrete has an alkaline chemical reaction, the opposite of acid.
        它们也以另外一种方式互补：它们几乎有相同的收缩率和膨胀率。因此，它们在拉、压为主要因素时能共同工作。在出现拉力的混凝土梁或结构中，将钢筋埋入混凝土而成钢
筋混凝土。混凝土与钢筋形成如此强大的结合力——这个力将它们粘合在一起——以致于钢筋在混凝土中不会滑移。还有另一个优势是钢筋在混凝土中不会锈蚀。酸能腐蚀钢筋，而混凝土会发生碱性的化学反应，与酸相反。
          The adoption of structural steel and reinforced concrete caused major changes in traditional construction practices（施工作业）. 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（防火地面）. Both these changes served to（有利于） reduce the cost of construction. It also became possible to erect（建造）buildings with greater heights and longer spans.
        结构钢与钢筋混凝土的采用使传统的施工作业发生了明显的变化。对多层建筑，再也没必要采用厚的石墙或砖墙，且施工防火地面变为容易得多。这些变化有利于降低建筑的成本。它也使建造高度更高和跨度更大的建筑物成为可能。
          Since the weight of modern structures is carrieddesign翻译（承受） by the steel or concrete frame, the walls do not support the building. They have become curtain w
alls, 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（承重墙）. Today, however, curtain walls are often made of lightweight materials such as glass, aluminum, or plastic, in various combinations.
        由于现代结构的重量由钢或混凝土框架承受，墙体不再支承建筑物。它们成为幕墙，将日晒风吹雨打阻挡在外，而让光线进入。在较早的钢或混凝土框架建筑中，幕墙一般由砌体构成；它们具有承重墙的结实外观。但是今天，幕墙通常由轻质材料组成，如玻璃、铝或塑料，并形成不同的组合。
          Another advance in steel construction（结构） is the method of fastening together（连在一起） the beams. For many years the standard method was riveting. A rivet is a bolt with a head that looks like a blunt screw（圆头螺丝钉） without threads（螺纹）. It is heated, placed in holes through the pieces of steel（钢构件）, and a second head is formed at the other end by hammering（锤击）it to hold it in
place（固定就位）. Riveting has now largely been replaced by welding, the joining together of pieces of steel by melting（熔化） a steel material between them under high heat.
        钢结构中的另一个进步是梁的连接方式。在很多年里，连接的标准方式是铆接。铆钉是个有头的螺栓，看上去象个没有螺纹的圆头螺丝钉。铆钉加热后穿过钢构件之间的孔洞，并通过锤击另一端而形成第二个铆钉头，从而将其固定就位。如今铆接已大量地被焊接所替代，钢构件间的连接通过在高热下熔化它们之间的钢材料（即焊条）进行。