广西左江崇左港多用途码头工程-桥吊码头结构设计译文

广西左江崇左港多用途码头工程-桥吊码头结构译文 2007届港海专业毕业设计?译文 A、Concrete a、Plain Concrete Concrete is a stone-like material obtained by permitting a carefully proportioned mixture of cement, sand and gravel or other aggregate, and water to harden in forms of the shape and dimensions of the desired structure. The bulk of the material consists of fine and coarse aggregate. Cement and water interact chemically to bind the aggregate particles into a solid mass. Additional water, over and above that needed for this chemical reaction, is necessary to give the mixture the workability that enables it to fill the forms prior to hardening. Concretes in a wide range of strength properties can be obtained by appropriate adjustment of the proportions of the constituent materials. Special cements (such as high-early-strength cements), special aggregates (such as various lightweight or heavyweight aggregates), and special curing methods (such as cream-curing) permit an even wider variety of properties to be obtained. These properties depend to a very substantial degree on the proportions of the mix, on the thoroughness with which the various constituents are intermixed, and on the conditions of humidity and temperature in which the mix is maintained from the moment it is placed in the forms until it is fully hardened. The process of controlling these conditions is known as curing. To protect against the unintentional production of substandard concrete, a high degree of skillful control and supervision is necessary throughout the process, from the proportioning by weight of the individual components, through mixing and placing, until the completion of curing. The factors which make concrete a universal building material are so pronounced that it has been used, in more primitive kinds and ways than at present, for thousands of years, probably beginning in Egyptian antiquity. The facility with which, while plastic, it can be deposited and made to fill forms or molds of almost any practical shape is one of these factors. Its high fire and weather resistance are evident advantages. Most of the constituent materials, with the possible exception of cement, are usually available at low cost locally or at small distances from the construction site. Its compressive strength, like that of natural stones, is high, which makes it suitable for members primarily subject to compression, such as columns and arches. On the other hand, again as in natural stones, it is a relatively brittle material whose tensile strength is small compared with its compressive strength. Hence plain concrete 第 1 页 2007届港海专业毕业设计?译文 is used only for footings and concrete slabs laid on the ground, and for massive structures such as retaining walls; even then reinforcement is frequently employed. b、Properties of Concrete and reinforced Concrete Concrete is a man-made conglomerate stone composed of essentially four ingredients: Portland cement, water, sand, and coarse aggregate. The cement and water combine to make a paste that binds the sand and stones together. Ideally, the aggregates are graded so that the volume of paste is at a minimum, merely surrounding every piece with a thin layer. Most structural concrete Is stone concrete, but structural lightweight concrete (roughly two-thirds the density of stone concrete) is becoming increasingly popular. Concrete is essentially a compressive material having almost no tensile strength, so concrete s weakness in tension also causes it to be weak in shear. These deficiencies are overcome by using steel bars for reinforcement at the places where tensile and shearing stresses are generated. Under load, reinforced concrete beams actually have numerous minute cracks which run at right angles to the direction of major tensile stresses. The tensile forces at such locations are being taken completely by the steel “re-bars.” The compressive strength of a given concrete is a function of the quality and proportions of its constituents and the manner in which the fresh concrete is cured. (Curing is the process of hardening during which time the concrete must be prevented from “drying out”, as the presence of water is necessary for the chemical action to progress.) Coarse aggregate that is hard and well graded is particularly essential for quality concrete. The most important factor governing the strength, however, is the percentage of water used in the mix. S minimum amount of water is needed for proper hydration of the cement. Additional water is needed for handling and placing the concrete, but excess amounts cause the strength to drop markedly. These and other topics are fully covered in the booklet, “Design and Control of Concrete Mixtures,” published by the Portland Cement Association. This is an excellent reference, treating both concrete mix design and proper construction practices. The American Concrete Institute publishes a widely adopted code specifying the structural requirements for reinforced concrete. Concrete is known as the “formable” or “moldable” structural material. Compared to other materials, it is easy to make curvilinear members and surfaces with concrete. It has no inherent texture but adopts the texture of the forming material, so it can range widely in 第 2 页 2007届港海专业毕业设计?译文 surface appearance. It is relatively inexpensive to make, both in terms of raw materials and labor, and the basic ingredients of Portland cement are available the world over. (It should be noted, however, that the necessary reinforcing bars for concrete may not be readily available in lessdeveloped countries.) Concrete is naturally fireproof and needs no separate protection system. Because of its mass, it can also serve as an effective barrier to sound transmission. In viewing the negative aspects, concrete is unfortunately quite heavy and it is often noted that a concrete structure expends a large portion of its capacity merely carrying itself. Attempts to make concrete less dense, while maintaining high quality levels, have generally resulted in increased costs. Nevertheless, use of lightweight concrete can sometimes result in overall economies. Concrete requires more quality control than most other building materials. Modern transit-mixed concrete suppliers ate available to all U. S. urban areas and the mix is usually of a uniformly high quality. Field-or-job-mixed concrete requires knowledgeable supervision, however. In any type of concrete work, missing or mislocated reinforcing bars can result in elements with reduced load capacities. Poor handling and/or curing conditions can seriously weaken any concrete. For these and other reasons, most building codes require independent field inspections at various stages of construction. Proper concrete placement is also somewhat dependent upon the ambient weather conditions. Extremely high temperatures and, more important, those below (or near) freezing can make concrete work very difficult. Concrete derives its strength from the fact that pulverized portland cement，when mixed with water，hardens by a process called hydration(In an ideal mixture，concrete consists of about three fourths sand and gravel(aggregate)by volume and one fourth cement paste(The physical properties of concrete are highly sensitive to variations in a mixture of the components，so a particular combination of these ingredients must be custom-designed to achieve specified results in terms of strength or shrinkage(When concrete is poured into a mold or form，it contains free water，not required for hydration，which evaporates(As the concrete hardens，it releases this excess water over a period of time and shrinks(As a result of this shrinkage，fine cracks often develop(In order to minimize these shrinkage cracks， concrete must be hardened by keeping it moist for at least 5 days(The strength of concrete 第 3 页 2007届港海专业毕业设计?译文 increases in time because the hydration process continues for years;as a practical matter，the strength at 28 days is considered standard( Concrete deforms under load in an elastic manner(Although its elastic modulus is one tenth that of steel，similar deformations will result since its strength is also is one tenth that of Concrete is basically a compressive material and has negligible tensile strength( steel( c、Advantages and Disadvantages of Concrete and Its Water-Cement Ratio Concrete is a mixture of Portland cement, water, sand, and crushed gravel or stone. The water and cement form a cement paste in which the sand and stone or gravel are mixed. The sand and stone or gravel together make up the aggregate of a concrete mixture. The aggregate serves no structural function. It is merely a filler that adds low-cost bulk to the cement paste; it usually makes up about 75 percent of a given mass of concrete, by volume, although a poor aggregate can reduce the4 strength of a batch of concrete considerably, good aggregate adds only slightly to the strength of the cement. The two principal advantages of concrete as a construction material are its relative cheapness and the ease with which it can be handled and placed while it is in the plastic state. The principal structural advantages of concrete are its great compressive strength and its durability. Concrete can withstand very high compressive loads. This is what makes concrete so suitable for the foundations, walls, and columns of buildings, and for driveways and walks as well. The principal structural disadvantage of concrete is its poor tensile strength. That is, it cannot withstand pulling or bending loads without cracking or breaking. For this reason, steel rods, or reinforcement steel, are often embedded in concrete, the reinforcement steel providing the tensile strength the concrete lacks. Concrete with reinforcement steel embedded in it is reinforced concrete. In addition to its poor tensile strength, concrete, like most construction materials, expands in hot weather and when wet and contracts in cold weather and as it dries out. Unless these movements are allowed for during construction, the concrete will crack. And, contrary to common belief, solid concrete is not impervious to water. Some moisture will migrate into the best-made concrete. But if the concrete should be excessively porous, which can happen if too much water has been used in mixing it, moisture can easily enter the concrete after it has cured. If this moisture should be present within the concrete 第 4 页 2007届港海专业毕业设计?译文 when cold weather comes, the moisture may freeze, which may result in serious frost damage to the structure. Despite these limitations, concrete is an inherently strong and durable construction material. If the proportions of water, cement, and aggregate are carefully calculated and if the concrete is placed and allowed to cure according to simple but definite rules, it is possible to obtain from the concrete all the strength and durability that is inherent in it. The ratio of water to cement in a batch of concrete is the principal determinant of the concrete ， s final strength. As one time the instructions for preparing a batch of concrete would have contained proportions such as 1:2:4, indicating that 1 part of Portland cement to 2 parts of sand to 4 parts of gravel by volume were to be mixed together, after which sufficient water was to be added to obtain a workable mixture. This procedure ignored entirely the importance of the water-cement ratio. It also resulted very often in the preparation of a very weak concrete, since the natural tendency is to add enough water to make placement of the concrete as easy as possible-the sloppier the better, as far as the workmen are concerned. This manner of specifying the proportions of concrete is obsolete and should never be followed. In theory, it takes only 3 gal of water to hydrate completely 1 cu ft of cement. (A sack of cement contains 1 cu ft exactly, and the sack weighs 94 1b). But this water-cement ratio produces a mixture that is too stiff to be worked. In practice, therefore, additional water, between 4 and 8 gal per sack of cement, is used to obtain a workable mixture. But the greater the proportion of water in a water-cement ratio, the weaker the final concrete will be. The additional water that is necessary to achieve a workable batch will only evaporate from the concrete as the concrete sets, and it will leave behind in the concrete innumerable voids. This is the reason there will always be some porosity in the concrete. When an excessive amount of water has been used, there will be an excessive number of voids, which may cause the concrete to leak badly. If these voids should be filled with moisture when cold weather comes, they will cause the frost damage alluded to above. As a general rule, therefore, 6 gal of water per sack of cement should be the maximum amount used for making concrete; and the less the amount of water that is used, the stronger the concrete will be. Also included in the 6 gal is whatever surface moisture is contained is the sand that is part of the aggregate. 第 5 页 2007届港海专业毕业设计?译文 B、Reinforced Concrete To offset the limitation of plain concrete, it has been found possible, in the second half of the nineteenth century, to use steel with high tensile strength to reinforce concrete, chiefly in those places where its small tensile strength would limit the carrying capacity of the member. The reinforcement, usually round steel rods with appropriate surface deformations to provide interlocking, is placed in the forms in advance of the concrete. When completely surrounded by the hardened concrete mass, it forms an integral part of the member. The resulting combination of two materials, known as reinforced concrete, combines many of the advantages of each: the relatively low cost, good weather and fire resistance, good compressive strength, and excellent formability of concrete and the high tensile strength and much greater ductility and toughness of steel. It is this combination which allows the almost unlimited range of uses and possibilities of reinforced concrete in the construction of buildings, bridges, dams, tanks, reservoirs, and a host of other structure. The steel bars in concrete take the tensile component of the bending moment. But they do not prevent the concrete from cracking (Fig. 6-1) The presence of fine cracks in reinforced concrete is inevitable. The stress in the lowest-grade reinforcing steel under the normal working loads is of the order of 140 Mpa. Taking the modulus of elasticity of steel as 200 Gpa, this amounts to an elastic strain of -47×10, which is more than the ultimate tensile strain of concrete. Cracks are thus produced in the concrete by the mere process of the reinforcing steel being stressed under the normal working loads. It is perhaps fortunate that this was not understood when reinforced concrete was first employed more than a century ago, otherwise building authorities with a reasonable concern for public safety would probably have forbidden its use. Because if the cracks are kept very small and are bridged by tension steel they have no adverse effect on the safety or durability of the structure, the safety of reinforced concrete structures depends on the width of the cracks being kept below a permissible minimum and this has become a more serious problem in recent years because the use of higher steel stresses also increases the strain of the concrete. Cracks would not only be unsightly but would expose the steel bars to corrosion by moisture and other chemical action. Thus crack control is a more serous matter in reinforced concrete design as compared with, say, twenty years ago. 第 6 页 2007届港海专业毕业设计?译文 Reinforced concrete has steel bars that are placed in a concrete member to carry tensile forces(These reinforcing bars，which range in diameter from 0.25 inch (0.64cm)to 2.25 inches(5.7cm)，have wrinkles on the surfaces to ensure a bond with the concrete(Although reinforced concrete was developed in many countries，its discovery usually is attributed to who used a wire network to reinforce concrete tubes in Joseph Monnier，a French gardener， 1868(This process is workable because steel and concrete expand and contract when the temperature changes(If this were not case，the bond between the steel and concrete would be broken by a change in temperature since the two materials would respond differently(Reinforced concrete can be molded into innumerable shapes，such as beams， columns，slabs，and arches，and is therefore easily adapted to a particular form of building(Reinforced concrete with ultimate tensile strengths in excess of 10,000 psi(700 kg/sqcm)is possible，although most commercial concrete is produced with strengths under 6,000 psi(420 kg/sqcm)( The best structural use of reinforced concrete, in terms of the characteristics of the material, is in those structures requiring continuity and or rigidity. It has a monolithic quality which automatically makes fixed or continuous connections. These moment-resistant joints are such that many low-rise concrete buildings do not require a secondary bracing system for lateral loads. In essence, a concrete beam joins a concrete column very differently from the way steel and wood pieces join, and the sensitive designer will not ignore this difference. (These remarks do not apply to precast structural elements, which are usually not joined in a continuous manner.) C、Prestressing Concrete Methods of inducing compression in concrete member before it is loaded is known as prestressing. The construction which uses steels and concrete of very high strength in combination is known as prestressed concrete (Fig. 6-2). The steels, mostly in the shape of wires or strands but sometimes as bars, is embedded in the concrete under high tension which is held in equibrium by compression stresses in the surrounding concrete after hardening. Because of this compression, the concrete in a flexural member will crack on the tension side at much larger load than when not so pre-compressed. This reduces radically both the deflections and the tensile cracks at service loads in such structures and thereby enables these high-strength materials to be used effectively. 第 7 页 2007届港海专业毕业设计?译文 Prestressed concrete is particularly suited to prefabrication on a mass-production basis. Its introduction has extended, to a very significant degree, the range of structural uses of the combination of these two materials. Prestressed is being successfully used in the manufacture of concrete sleepers. Ordinary reinforced concrete sleepers quickly fail in service due to the rapid opening and closing of cracks when loads are applied and removed in rapid succession. It has been found that some prestressed sleepers do not crack, and disintegration due to the opening and closing of cracks is therefore avoided. Hollow beams for bridge and concrete piles constructed of precast blocks are examples of the use of prestressed members. It is not possible to predict the length required for a pile, and it is a great convenience to be able to extended a pile during driving by adding blocks; the handing of long piles is avoided and, provided the lateral support of the ground is adequate, the prestressing cables can be removed upon completion of driving and used again. Another advantage of prestressed concrete is that the concrete and the steel are severely tested during the prestressing operation, and a lower factor of safety is justified. The permissible working stress in the concrete is generally one-third of the compressive strength, thus allowing a margin to covet the risk of poor concrete occurring at a critical section.The risk is reduced by prestressing, because the stress induced the stress induced in the concrete during the prestressing operation may be 50% to 70% of its compressive strength. Reference 《prestressed concrete analysis and design》New York;mcgraw-Hill book 《The design and placing of high quality concrete》 London,spon 第 8 页 2007届港海专业毕业设计?译文 A混凝土 a素混凝土简介 混凝土是一种类似石头的材料，是将水泥、砂、砾石或其他集料和水以适当比例混合，硬化后得到所需形状和大小的结构。混凝土主要由粗、细集料形成骨架，而水泥和水发生化学反应，将集料颗粒黏结成固态整体。拌和水比水化反应所需的水要多，以使混合料具有和易性，这使得混合料能够在硬化之前注满，适当调整组成材料的比例可以获得不同强度等级的混凝土。用特殊的水泥(比如早强水泥)、特殊的集料(比如各种轻集料或重集料)以及特殊的养护方法(比如蒸汽养护)还可以获得具有更广泛特性的混凝土。 混凝土的这些特性在相当大的程度上取决于混合物的比例、组成材料拌和的均匀性以及从浇注后到完全硬化之前的整个过程中混合物所处环境的温度和湿度。控制这些条件的过程称为养护。为了避免不小心生产出不合格的混凝土，从组成材料的质量配比到拌和、浇注、养护等整个过程中必须在技术上严格控制和监督。 使混凝土成为通用建筑材料的因素是非常明显的，以至于可能从几千年前的古代埃及以来，混凝土就一直以比现在更原始的种类和形式得以应用。拌和物的可塑性使其便于浇注并填满几乎任何形状的模板就是其中一个因素。混凝土较好的耐火性和耐候性也是其明显的优势。除了水泥以外，混凝土的大多数组成材料通常都可以以低价在当地或建筑工地附近购得。像石头一样，混凝土的抗压强度很高，因此，它适用于做诸如柱、杆等受压构件。另一方面，混凝土是一种脆性材料，与抗压强度相比，它的抗拉强度较低，这一点和石头也有些类似。因此，素混凝土仅用做基础或铺置在地面上的混凝土板，以及像挡土墙这样的大体积结构，即使如此也常常掺加钢筋增强。 b混凝土的特性 混凝土是一种人造石材，主要由四种成分组成:波特兰水泥、水、砂和粗骨料。水泥和水混合成水泥浆，将砂子和石子黏结在一起。理想的骨料级配应该使泥浆的体积最小，仅将每块骨料包上薄薄的一层即可。大多数结构混凝土是碎石混凝土，但是轻质结构混凝土(大约是碎石混凝土密度的2/3)正日益得到普及。 混凝土基本上是一种几乎没有抗拉强度的抗压缩材料。所以，混凝土抗拉强度低也导致混凝土抗剪强度低。在产生拉伸应力和剪应力的部位加入钢筋克服了这些缺陷。在何在作用下，钢筋混凝土梁实际上会有许多沿垂直于主拉应力方向的微裂缝。这些部位的拉力完全被钢筋所承担。 给定混凝土的抗压强度是其组成成分的质量和比例以及新浇混凝土的养护方法的 第 9 页 2007届港海专业毕业设计?译文 数。(养护是硬化过程，在此期间，必须防止混凝土过干，因为水分的存在是进行化学反应所必须的。)坚硬并具有良好级配的粗骨料对于优质混凝土是至关重要的。然而，决定其强度的最重要因素是其配料中的含水率。水泥适当水化需要最小量的水分。为了操作和浇注混凝土需要多加一些水，但是过量的水分会导致强度明显降低。 由波特兰水泥协会出版的《混凝土混合物的设计与控制》一书中全部包含了这类以及其他一些论题。这是一本优秀的参考书，它涉及混凝土配料设计及适合的施工实践。美国混凝土协会出版了广泛采用的专用于钢筋混凝土结构要求的。 混凝土被认为是做“可成型的”或“可模性的”结构材料。与其他材料相比，混凝土易制成曲线型构件和各种曲面。混凝土没有固有的纹理，但它呈现其成模材料的纹理，因此混凝土可呈现各种各样的外观。制造混凝土无论是原材料还是人工费都相当便宜，普通水泥基本材料在世界各地都可以得到。(然而，应注意的是混凝土所必须的钢筋在不发达国家可能不易得到。) 混凝土本身是耐火的，不需要单独的保护系统。由于它的质量，混凝土也可用作为有效的隔声材料。 从缺点方面考虑，遗憾的是混凝土相当重，经常可见有混凝土结构仅支撑它本身重量就要消耗它大部分的承载能力。制造低密度而保持其高性能水准的混凝土通常会导致成本提高。不过，使用轻质混凝土有时能产生综合经济效益。 混凝土比大多数其他建筑材料需要更多的质量监督。现代化的输送搅拌混凝土的供应厂商遍布美国各城市，其混合物通常具有均匀的高质量。然而，现场或临时搅拌的混凝土需要技术性的监督。无论在何种混凝土的施工中漏加钢筋或放错钢筋位置都会导致构件承载能力降低。无论是操作或养护条件不好或二者兼而有之都能严重削弱混凝土的强度。由于种种原因，大多数建筑规范都要求在施工的各个阶段进行独立的现场检查。 适当的混凝土浇注也多少取决于周围气候条件。更重要的是，过高的温度以及低于(或接近于)冰点的温度都能使混凝土施工非常困难。 混凝土的强度来源于被粉碎的水泥与水混合凝固硬化后的强度，这个凝固硬化的过程称为水化。混凝土的理想配合是用3/4体积的砂和砾石与1/4体积的水泥浆混合而成。混凝土的物理特性变化在其成分发生变化时是很敏感的，所以混凝土中各成分的组合必须满足设计标准以求在强度和收缩率方面达到特定要求。当混凝土被放入模型中后，它含有的在水化时没有用掉的自由水就会被蒸发掉。在混凝土坚固硬化的过程中，它会释放出大量的水分并且会收缩，收缩的结果是会形成裂缝。混凝土必须在保持潮湿的条件下坚固硬化至少5天，混凝土的强度也因为坚固硬化过程会一直连续存在数年而一直增 第 10 页 2007届港海专业毕业设计?译文 加。实际上，混凝土制成28天后的强度被规定为标准强度。 在荷载作用下，混凝土的变形是弹性变形。虽然混凝土的弹性模量只有钢筋的1/10，但它们的变形却是相同的。因为混凝土的强度也正好是钢筋强度的1/10。混凝土基本上一种抗压材料，它的抗拉强度通常可以忽略。 c混凝土的优缺点及其水灰比 混凝土是波特兰水泥、水和砂子与卵石或碎石的拌和物。水与水泥调制成水泥浆，在水泥浆中把砂子和碎石或卵石混合在一起。砂子和碎石或卵石共同构成混凝土拌和物的骨料。骨料不起结构作用，它仅仅是加入到水泥浆中来降低成本的一种填充物。骨料 。劣质骨料可以极大的降低混凝土的强度，可是优质通常占一块特定混凝土体积的3/4 骨料对于增强水泥的强度却没有多大影响。 作为建筑材料，混凝土头两大主要优点:一是比较便宜;二是当它处于塑性状态时容易操作和浇注。混凝土的主要结构优点是抗压强度高、耐久性好。它能承受很高的压力荷载。这使它非常适用于建造基础、墙壁、楼房和支柱，也适用于修筑公路及街道。 混凝土的主要结构缺点是它抗拉强度低。就是说，它承受拉伸或弯曲荷载时容易断裂。因此，常常将钢筋埋入混凝土中。加固钢筋给混凝土提供了它所缺乏的抗拉强度。置入钢筋的混凝土叫做钢筋混凝土。 除了抗拉强度差之外，像大多数建筑材料一样，混凝土还会热胀冷缩，湿胀干缩。在施工中，如果这些胀缩运动超出允许限度，混凝土就会开裂。 与常识相反，硬化后的混凝土并不是不透水的。一些湿气会迁移到加工最好的混凝土中。在拌和中加入了太多的水会使混凝土内出现过多的气孔。万一这种现象出现，混凝土养护完以后，湿气会很容易的进入其内。当冷天到来时，万一水分还存在于混凝土内，便会冻结，这会给建筑物带来严重的损伤。 尽管混凝土有这些缺点，它仍不失为一种天生具有坚固与耐久性的建筑材料。如果水、水泥、和骨料的比例经过认真计算，如果混凝土的浇注与养护按照简明的规章来进行，完全可能获得夫人他固有的全部强度与耐久性。 混凝土的水灰比是它最终强度的决定因素。曾经有一段时间，混凝土的配料比例规定为1:2:4，它示从体积上一份波特兰水泥、二份砂子和四份卵石拌和在一起，之后加入足够的水来获得和易性良好的拌和物。这种作业过程完全忽视了水灰比的重要性。其结果是经常调配出强度非常低的混凝土。因为就工人而言日，自然倾向于多加水，足以使混凝土的浇注尽可能容易，越稀滑越好。这种规定混凝土成分比例的做法现已淘汰，不应该遵循。 第 11 页 2007届港海专业毕业设计?译文 从理论上讲，仅有3加仑水便可以水化1立方英尺水泥(一袋水泥正好1立方英尺，重94磅)。可是按照这种水灰比加工出来的拌和物太干不能操作。因此，在实践中需要再多加些水来获得和易性良好的拌和料，每袋水泥加水在4,8加仑之间为宜。 但是在水灰比中，水的比例越大，最终制成的混凝土的强度越低。为了得到和易性好的混凝土而需要多加入的水将随着混凝土的硬化从混凝土中蒸发掉，这将在混凝土内留下无数的气孔。这便是为什么在混凝土内总存在一些孔隙的原因。当超量的水使用后，就会产生超量的孔隙，这会使混凝土严重渗漏。万一当冷天到来，这些孔隙充满了水分，它们会引起上述的结冻损伤。 因此，作为一般规定，每袋水泥加入6加仑水应该被视为为配制混凝土的最大加水量，并且所用的水量越少，混凝土强度越高。作为骨料组成部分的砂子所含表面水分也应包括在6加仑加水量之内。 B钢筋混凝土 为了弥补素混凝土的局限性，在19世纪后半叶，人们发现可以使用具有高抗拉强度的钢筋来增强混凝土，主要是用于那些由于混凝土较低的抗拉强度限制了构件的承受能力的地方。在浇注混凝土之前，把表面适当变形的圆形钢筋放入模板，钢筋表面变形是为了增加黏结力。当钢筋被硬化的混凝土包裹后，钢筋就成为构件的一部分了。两种材料组成的复合材料称为钢筋混凝土，它具有两种材料的许多优点:混凝土的廉价、较高的耐火性和耐候性、较高的抗压强度、良好的可塑性以及钢筋较高的抗拉强度、更好的延展性和韧性。正是由于这些优点的结合使钢筋混凝土几乎能不受限制地广泛应用于修建房屋、桥梁、水坝、蓄水池、水库和其他许多建筑结构中。 当钢筋混凝土受弯矩作用时，混凝土内的钢筋承受拉力，但它们并不能阻止混凝土的开裂。 钢筋混凝土中出现细小的裂缝是不可避免的。在正常的使用荷载作用下，最低等 5级钢筋的应力在140Mpa之内。假设钢筋的弹性模量为2×10Mpa，则钢筋的弹性应变为 -47×10，超过了混凝土最大应变。混凝土中的钢筋仅在正常使用荷载下受拉，混凝土中就产生了裂缝。也许值得庆幸的是一个世纪前钢筋混凝土刚刚被使用时没有人知道这一点，否则，建筑管理部门可能会基于公众安全的考虑而禁止使用钢筋混凝土。 如果裂缝很小而且被拉筋连接，这些裂缝对建筑结构的安全和耐久性没有副作用，因此钢筋混凝土结构的安全性就取决于低于允许最小值的裂缝的宽度，这已经成为近几年来比较重要的一个问题，因为使用较大应力的钢筋也就增加了混凝土的应变，裂缝不仅仅难看，而且暴露出来的钢筋受到空气中的湿气或其他化学反应的腐蚀。因此，与以 第 12 页 2007届港海专业毕业设计?译文 前，比如说二十年前相比，控制裂缝大小在钢筋混凝土设计中成为更重要的问题。 钢筋混凝土是将钢筋插入到混凝土中以承受拉力。这类钢筋的直径的范围为0.25,2.25英寸(0.64,5.7厘米)，钢筋表面有褶皱以确保混凝土和钢筋的紧密胶结。虽然钢筋混凝土在许多国家都有所发展，但它最早的发现者却是一位法国的园艺家Joseph Monnier，他在1868年用一种金属网结构来加固混凝土管道。这个过程是符合常理的，因为钢筋和混凝土在温度变化时的膨胀和收缩情况是相同的。如果不是这样，钢筋和混凝土之间的胶结就会因温度变化时两种材料变形不同的影响而被破坏。钢筋混凝土可以被烧铸成各种形状，例如:梁、柱、板、拱，因此它很适用于建筑的特殊结构。 /平方英寸(420千克/平方厘米)生产的，虽然大多数的商用混凝土是按极限强度6000磅 但钢筋混凝土的极限抗拉强度却可能会超过10000磅/平方英寸(700千克/平方厘米)。 就这种材料的特性而言，钢筋混凝土最佳的结构用途是在那些要求连续性和刚性的建筑结构中。钢筋混凝土具有整体性，可自动实现固定连接或连续连接。这些抗弯结点使得许多低层混凝土建筑物在横向荷载作用下不需要辅助斜支撑系统。实质上混凝土梁与混凝土柱的连接与钢和木构件的连接不太一样，聪明的设计人员是不会忽视这种差异的。(这些要点不适合于预制构件，这些预制构件通常不以连续方式连接。) C预应力混凝土 在混凝土构件承载前使混凝土产生预加压应力的方法被称为预应力。结合使用了预应力钢筋和高强度混凝土的结构被称为预应力混凝土结构。 钢筋，通常是钢丝或钢索，有时也用钢条，被埋入混凝土中并用高强度的张力拉伸，在混凝土硬化后对周围混凝土产生相等的压应力。由于这种压应力，受弯构件受拉一侧的混凝土开裂时的荷载比不采用预压应力的荷载大得多。这从根本上减小了这些结构在使用荷载作用下的变形和拉伸裂缝，从而就能够更有效地利用这些高强度材料。 预应力混凝土特别适合大批量工厂预制。预应力的引入在很大程度上扩展了这两种材料在结构上的应用范围。 预应力技术正被成功地应用于制造混凝土轨枕。当荷载快速反复的施加，随即卸载，裂缝也快速的开合，普通钢筋混凝土轨枕在荷载的作用下很快就会破坏。人们已经发现一些采用了预应力技术的轨枕并不开裂，由于裂缝的开合而引起的轨枕损毁也就避免了。 桥梁上的预制钢筋混凝土空心梁和混凝土桩就是使用预应力构件的例子。打桩前预测桩所需的长度是很困难的，如果能在打桩时通过增添混凝土桩段来延长桩的长度，一切就变得很方便，这样就可避免处理长桩。假设地面的侧向支撑足够大,预应力筋在 第 13 页 2007届港海专业毕业设计?译文 施工完成后还可撤掉以再次使用。 预应力混凝土的另一优点就是钢筋和混凝土在预应力操作时都经过严格的测试，较低的安全系数也是可取的。通常，混凝土允许的工作应力为抗压强度的1/3，这就留有余地以避免出现在关键部位的劣质混凝土所造成的危险。采用预应力技术可以有效地减 ,到70,。 低这种危险，因为施加预应力时混凝土中的应力可能是其抗压强度的50 文献选自:《prestressed concrete analysis and design》New York;mcgraw-Hill book 《The design and placing of high quality concrete》 London,spon 第 14 页