2.modern buildings and structural materials
Many great buildings (that are) built in the earlier ages are still in existence and in use. Among them are the Pantheon and the Colosseum in Rome, Hagia Sophia in Istanbul; the Gothic churches of France and England, and the Renaissance cathedrals, with their great domes, like the Duomo in Florence and St. Peter's in Rome. They are massive structures with thick stone walls that counteract the thrust of their great weight. Thrust is the pressure exerted by each part of a structure on its other parts. 许多伟大的建筑很早就修建了现在仍然还存在并在使用中。在他们之中就包括希腊的帕提侬神庙和罗马的角斗场，伊斯坦布尔的圣索非亚教堂; 法国和英国哥特式教会, 和有着宏伟圆顶的文艺复兴时期大教堂,如佛罗伦萨的中央寺院和罗马的圣彼得大教堂。他们是使用厚重石头墙体来抵消他们巨大自重的巨型结构。轴向压力来自于结构的其它部份对其的传导。
These great buildings were not the product of knowledge of mathematics and physics. They were constructed instead on the basis of experience and observation, often as the result of trial and error. One of the reasons they have survived is because of the great strength that was built into them-strength greater than necessary in most cases. But the engineers of earlier times also had their failure. In Rome, for example, most of the people lived in insulae, great tenement blocks that were often ten stories high. Many of them were poorly constructed and sometimes collapsed with considerable loss or life.
这些伟大的建筑不是数学和物理知识的产物。而是根据经验和观察建造的,经常是不断摸索的产物。他们依然存在的原因之一是由于被修建得能承受很大的力量---在许多情况下大于他们需要的力量。但更早的时期的工程师也有过失败。例如,在罗马, 大多数人民,居住在公寓里, 大的经济公寓经常有十层楼高。许多修建得很拙劣且不时倒塌，造成了相当大的财产或生命损失。
Today, however, the engineer has the advantage not only of empirical information, but also of scientific data that permit him to make careful calculations in advance. When a modem engineer plans a structure, he takes into account the total weight of all its component materials. This is known as the dead load, which is the weight of the structure itself, He must also consider the live load, the weight of all the people, cars, furniture, machines, and so on that the structure will support when it is in use. In structures such as bridges that will handle fast automobile traffic, he must consider the impact, the force at which the live load will be exerted on the structure, He must also determine the safety factor, that is, an additional capability to make the structure stronger than the combination of the three other factors.
然而，今天工程师不仅有经验信息的优势，而且还有科学数据，允许他在事先进行仔细的计算。当现代工程师设计一个结构时，他要考虑所有组成材料的总重量。这就是所说的恒载，他是结构自身的重量，他还必须考虑活载，即所有人群、汽车, 家具, 机器, 等等结构在使用中需要承受的荷载。结构，比如桥梁将要承受高速的汽车通行（行使），它必须考虑冲力，这个力是活载作用在结构上的。他还必须确定安全因素，也即是，额外的承载能力以确保结构的强度大于这三个其他因素的组合作用。
The modern engineer must also understand the different stresses to which the
materials in a structure are subject. These include the forces of compression and tension. In compression the material is pressed or pushed together; in tension the material is pulled apart or stretched, like a rubber band. In the Fig.2.1 the top surface is concave,or bent inward,and the material in it is in compression. The bottom surface is convex ,or bent outward,and the material in it is in tension. When a saw cuts easily through a piece of wood,the wood is in tension,but when the saw begins to bind,the wood is in compression because the fibers in it are being pushed together.
现代工程师还要知道结构中材料承受的不同的力。这些力包括压力和拉力这对互反的力。受压时，材料受到挤压或是被推倒一起；受拉时，材料被撕开或是被拉长，就像一根橡皮圈。在图2.1中，上表面是凹面，或向里弯曲，它的材料是受压。下表面是凸的，或向外弯曲，它的材料受拉。当用锯毫不费力地锯开一块木头时，木头是受拉的，而当锯开始被夹住时，木头是受压的，因为木头的纤维被挤到一起了。
In additin to tension and compression, another force is at work, namely shear, which we defined as the tendency of a material to fracture along the lines of stress. The shear might occur in a vertical plane, but it also might run along the horizontal axis of the beam, the neutral plane, where there is neither tension nor compression.
除了拉力和压力以外，还有另一种力作用于结构上，也就是剪力，我们定义使材料趋于沿着力的作用线断裂的力为剪力。剪力可能在垂直面作用，但是也有可能沿着梁的水平轴，既不受拉也不受压的中性轴作用。
Altogether, three forces can act on a structure: vertical-those that act up or down; horizontal-those that act sideways; and those that act upon it with a rotating or turning motion. Forces that act at an angle are a combination of horizontal and vertical forces. Since the structures designed by civil engineers are intended to be stationary or stable, these forces must be kept in balance. The vertical forces, for example, must be equal to each other. If a beam supports a load above, the beam itself must have sufficient strength to counterbalance that weight. The horizontal forces must also equal each other so that there is not too much thrust either to the right or to the left. And forces that might pull the structure around must be countered with forces that pull in the opposite direction.
总之，这三个力会作用在结构上：竖直的--那些向上或向下作用的力；水平的—那些向一侧作用的力；以及那些使旋转或转动的作用力。以一个角度作用的力是水平力和竖向力的合力。因为由土木工程师设计的结构要固定或者稳定，这些力必须保持平衡。例如，竖向力必须相互平衡。如果一片梁要支撑作用在其上的荷载，梁自身必须要有足够的强度来平衡这个重量。水平力也必须相互平衡以使得没有向左或向右的太大推力。以及那些可能使结构转动的力必须和相反方向转动的力相平衡。
One of the most spectacular engineering failures of modern times, the collapse of the Tacoma Narrows Bridge in 1940, was the result of not considering the last of these factors carefully enough. When strong gust s of wind, up to sixty-five kilometers an hour, struck the bridge during a storm, they set up waves along the roadway of the bridge and also a lateral motion that caused the roadway to fall. Fortunately,
engineers learn from mistakes, so it is now common practice to test scale models of bridges in wind tunnels for aerodynamic resistance.
桥梁史上最有名的事故之一。美国华盛顿州Tacoma Narrows大桥倒塌的录像。这次事故令桥梁的设计规范重新修改，风力和共振分析成为桥梁设计必不可少的一部分。这次事故之所以有名，还在于大桥的倒塌过程被人录了下来。当时有一对新人正在桥头举行婚礼，摄影师本来是去拍喜庆场面的，不想却记录了一座大桥的末日。大桥在风中扭动的场面令人震撼，只使用了三个月。主跨853m。现代最为轰动一时的工程事故之一：1940 年的Tacoma Narrows大桥，就是因为对这些因素的最后一个因素：扭转考虑的不够仔细而导致的结果。当时在一次暴风雨中，时速达到65公里每小时的强阵风冲击着大桥，他们把大桥沿行车道吹成波浪形加上横向扭动导致大桥的行车道落水。幸运的是，工程师从错误中吸取教训，因此现在通常要在风洞试验室中进行桥梁的缩尺模型试验以确定其空气动力阻力。
The principal construction materials of earlier times were wood and masonry brick, stone, or tile, and similar materials. The courses or layers were bound together with mortar or bitumen, a tar-like substance or some other binding agent. The Greeks and Romans sometimes used iron rods or clamp s to strengthen their buildings. The columns of the Parthenon in Athens, for example, have holes drilled in them for iron bars that have now rusted away. 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石砖，石头或是瓦片以及类似这类的材料。用砂浆或是沥青，一种类似焦油的物质或是其他粘合剂将各层结合在一起。希腊人和罗马人有时使用铁杆或是夹子来加强他们的建筑。例如，希腊帕提农神庙的立柱，为了插铁棒在立柱中钻孔，这些铁棒现在已经锈蚀了。罗马人也使用称为火山灰的天然水泥，从火山灰中制成，当在水中时就会变得和石头一样硬。
Both steel and cement, the two most important construction materials of modern times, were introduced in the nineteenth century. Steel, basically an alloy of iron and a small amount of carbon, had been made up to that time by a laborious process that restricted it to such special uses as sword blades. After the invention of the Bessemer process 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 of tension, a force which, as we have seen, tends to 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.
钢材和水泥，这两种最为重要的现代建筑材料，是在十九世纪发明的。钢材，主要是铁合金和少量的碳，在那时要通过费力的过程才能提炼出来，这限制它只能用于一些特殊用途比如剑刃上。1856年在贝色麦法发明以后，才能以低价得到大量的钢材。钢材极大的优点是它的抗拉强度，也就是当在计算等级的拉力作用下，它不会丧失其强度，这个力，正如我们所见，可以撕开很多材料。新的合金可以增强钢材的强度并且消除它的一些问题，比如疲劳问题，钢材在应力持续不断的变化作用下所导致的强度减弱。
Modern cement, called Portland cement, was invented in 1824. It is a mixture of
limestone and clay, which is heated and then ground into a powder. It is mixed at or near the construction site with sand, aggregate (small stones, crushed rock, or gravel), and water to make concrete. Different proportions of the ingredients produce concrete with different strength and weight. Concrete is very versatile; it can be poured, pumped, or even sprayed into all kinds of shapes. And whereas steel has great tensile strength, concrete has great strength under compression. Thus, the two substances complement each other.
称为硅酸盐水泥的现代水泥，在1824年发明。它是石灰石和粘土的混合物，它们加热后碾(碎)成为粉末。它可以在施工场地或靠近施工场地的地方和沙子，骨料（小石子，碎石或砾石）以及水混合制成混凝土。成分的不同比例可以制造出不同强度和重量的混凝土。混凝土非常通用，它可以灌注，泵送甚至喷洒成各种形状。并且鉴于钢材有很大的抗拉强度，混凝土有很大的抗压强度。因而，这两种物质可以相互补足。
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 both compression and tension are factors. Steel rods are embedded in concrete to make reinforced concrete in concrete beams or structures where tension will develop. Concrete and steel also form such a strong bond-the force that unites them-that the steel cannot slip within the concrete. Still another advantage is that steel does not rust in concrete. Acid corrodes steel, whereas concrete has an alkaline chemical reaction, the opposite of acid.
他们还可以以另一种方式相互补充：他们有几乎相同的压缩和膨胀率。因此，他们可以在既受压又受拉的情况下共同工作。在混凝土梁或受拉的结构，钢筋嵌入混凝土中形成钢筋混凝土。混凝土和钢筋还能形成如此强的粘结-这个粘合他们的力—使得钢筋在混凝土中不会滑移。还有另外一个好处就是钢在混凝土中不会生锈。酸腐蚀钢材，而与酸相反，混凝土存在碱性化学反应。
Prestressed concrete is an improved form of reinforcement. Steel rods are bent into the shapes to give them the necessary degree of tensile strength. They are then used to prestress concrete, usually by pretensioning or posttensioning method. Prestressed concrete has made it possible to develop buildings with unusual shapes, like some of the modern sports arenas, with large spaces unbroken by any obstructing supports. The uses for this relatively new structural method are constantly being developed.
预应力混凝土是钢筋混凝土的改良形式。钢筋弯成一定形状（弯成形）给他们必要的抗拉强度等级。然后，通常通过先张法或者后张法，他们给混凝土施加预应力。预应力混凝土使得发展非常规形状的建筑成为可能，比如一些现代体育场，有很大的不被任何支柱阻断的空间。这种相对新的结构方法的使用在不断的发展。
The current tendency is to develop lighter materials. Aluminum, for example, weight much less than steel but has many of the same properties. Aluminum beams have already been uesed for bridge construction and for the framework of a few buildings. 目前的趋势是发展更轻的材料。例如，铝，比钢轻很多但是与钢有很多同样的性能。铝梁已经在桥梁结构以及一些框架建筑中使用。
Attempts are also being made to produce concrete with more strength and durability, and with a lighter weight. One system that helps cut concrete weight to some extent uses polymers, which are long chainlike compounds used in plastics, as part of the mixture.
人们还尝试制造出强度更高耐久性更好的混凝土同时重量更轻(高性能混凝土)。一个帮助在一定程度上减少混凝土重量系统是使用高分子材料作为混合料的一部分，这些材料具有用于塑料中的链状化合物。