1、本課程介紹在學(xué)習(xí)普通英語(yǔ)（包括常用詞匯和語(yǔ)法結(jié)構(gòu)）的基礎(chǔ)上，接合前幾個(gè)學(xué)期已經(jīng)掌握的關(guān)于土木工程的專業(yè)知識(shí)，本課程節(jié)選了土木工程專業(yè)英語(yǔ)上冊(cè)中的內(nèi)容，涉及建筑材料、材料力學(xué)、結(jié)構(gòu)力學(xué)、鋼筋混凝土結(jié)構(gòu)、鋼結(jié)構(gòu)、測(cè)量、土力學(xué)、招投標(biāo)、建筑施工等九個(gè)方面的專業(yè)英語(yǔ)知識(shí)，從而使大家對(duì)土木工程領(lǐng)域內(nèi)的專業(yè)詞匯以及科技類文獻(xiàn)中的常用句型有個(gè)初步的、基本的學(xué)習(xí)和了解，為以后查閱專業(yè)文獻(xiàn)和參與國(guó)際交流打好基礎(chǔ)。第三單元The principal construction materials of earlier times were wood and masonry-brick, stone, or tile

2、, and similar materials. The courses or layers（磚層）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（雅典的帕臺(tái)農(nóng)神廟）, for example, have h

3、oles drilled（鉆孔） in them for iron bars that have now rusted away（銹蝕殆盡）. 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體，如磚、石、瓦以及類似的材料。磚層之間通過(guò)砂漿、瀝青（一種焦油狀的物質(zhì)）或其他一些粘合劑粘合在一起。希臘人和羅馬人有時(shí)用鐵條或夾子來(lái)加固他們的房屋。例如，雅典的帕臺(tái)農(nóng)神廟柱子中曾鉆孔

4、以便加入鐵條，如今都已銹蝕殆盡。羅馬人也用稱作白榴火山灰的天然水泥，它用火山灰制作，在水中會(huì)變得與石頭一樣堅(jiān)硬。Both steel and cement, the two most important construction materials of modern times, were introduced（推廣） in the nineteenth century. Steel, basically an alloy of iron （鐵合金）and a small amount of carbon, had been made up to that time（到那個(gè)時(shí)候） by a

5、laborious（繁復(fù)的） process that restricted it to such special uses as sword blades（刀刃）. After the invention of the Bessemer process （貝塞麥煉鋼法）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

6、 it is under a calculated degree (適當(dāng)?shù)? 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 chang

7、es in stress（連續(xù)的應(yīng)力變化）.作為現(xiàn)代兩種最重要的建筑材料，鋼材與水泥在十九世紀(jì)得到了推廣。直到那個(gè)時(shí)候，鋼材才通過(guò)繁復(fù)的過(guò)程制造出來(lái)，基本上是鐵合金，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年發(fā)明了貝塞麥煉鋼法后，鋼材才得以大量低價(jià)獲得。鋼材巨大的優(yōu)勢(shì)即是它的抗拉強(qiáng)度，也就是當(dāng)它在適當(dāng)?shù)睦ο虏粫?huì)失去強(qiáng)度，正如我們所看到的，該力往往能夠?qū)⒑芏嗖牧侠_(kāi)。新的合金進(jìn)一步提高了鋼材的強(qiáng)度，并消除了一些缺點(diǎn)，如疲勞，即在連續(xù)的應(yīng)力變化下導(dǎo)致強(qiáng)度減弱的趨勢(shì)。Modern cement, called Portland cement, was invented in 1

8、824. 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 （施工現(xiàn)場(chǎng)）with sand, aggregate (small stones, crushed rock, or gravel), and water to make concrete. Different proportions of the ingredients （配料）produce conc

9、rete with different strength and weight. Concrete is very versatile; it can be poured, pumped, or even sprayed into （噴射成）all kinds of shapes. And whereas steel has great tensile strength, concrete has great strength under compression. Thus, the two substances complement each other（互補(bǔ)）. 現(xiàn)代水泥發(fā)明于1824年，

10、稱為波特蘭水泥。它是石灰石和粘土的混合物，加熱后磨成粉末。在或靠近施工現(xiàn)場(chǎng)，將水泥與砂、骨料（小石頭、壓碎的巖石或礫石）、水混合而制成混凝土。不同比例的配料會(huì)制造出不同強(qiáng)度和重量的混凝土?；炷恋挠猛竞芏?，可以澆筑、泵送甚至噴射成各種形狀?；炷辆哂泻艽蟮目箟簭?qiáng)度，而鋼材具有很大的抗拉強(qiáng)度。這樣，兩種材料可以互補(bǔ)。They also complement each other in another way: they have almost the same rate of contraction and expansion. They therefore can work together

11、in situations where（在情況下） both compression and tension are factors（主要因素）. Steel rods（鋼筋） are embedded in（埋入）concrete to make reinforced concrete in concrete beams or structures where tension will develop（出現(xiàn)）. Concrete and steel also form such a strong bond - the force that unites（粘合） them - that the

12、 steel cannot slip（滑移） with the concrete. Still（還有） 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. 它們也以另外一種方式互補(bǔ)：它們幾乎有相同的收縮率和膨脹率。因此，它們?cè)诶簽橹饕蛩貢r(shí)能共同工作。在出現(xiàn)拉力的混凝土梁或結(jié)構(gòu)中，將鋼筋埋入混凝土而成鋼筋混凝土?；炷僚c鋼筋形成如此

13、強(qiáng)大的結(jié)合力這個(gè)力將它們粘合在一起以致于鋼筋在混凝土中不會(huì)滑移。還有另一個(gè)優(yōu)勢(shì)是鋼筋在混凝土中不會(huì)銹蝕。酸能腐蝕鋼筋，而混凝土?xí)l(fā)生堿性的化學(xué)反應(yīng)，與酸相反。The adoption of structural steel and reinforced concrete caused major changes in traditional construction practices（施工作業(yè)）. It was no longer necessary to use thick walls of stone or brick for multistory buildings, and it b

14、ecame 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. 結(jié)構(gòu)鋼與鋼筋混凝土的采用使傳統(tǒng)的施工作業(yè)發(fā)生了明顯的變化。對(duì)多層建筑，再也沒(méi)必要采用厚的石墻或磚墻，且施工防火地面變?yōu)槿菀椎枚?。這些變化有利于降低建筑的成本。它也使建造高度更高和跨度

15、更大的建筑物成為可能。Since the weight of modern structures is carried（承受） 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 o

16、f masonry; they had the solid look of bearing walls（承重墻）. Today, however, curtain walls are often made of lightweight materials such as glass, aluminum, or plastic, in various combinations. 由于現(xiàn)代結(jié)構(gòu)的重量由鋼或混凝土框架承受，墻體不再支承建筑物。它們成為幕墻，將日曬風(fēng)吹雨打阻擋在外，而讓光線進(jìn)入。在較早的鋼或混凝土框架建筑中，幕墻一般由砌體構(gòu)成；它們具有承重墻的結(jié)實(shí)外觀。但是今天，幕墻通常由輕質(zhì)材料組成

17、，如玻璃、鋁或塑料，并形成不同的組合。Another advance in steel construction（結(jié)構(gòu)） is the method of fastening together（連在一起） the beams. For many years the standard method was riveting. A rivet is a bolt with a head that looks like a blunt screw（圓頭螺絲釘） without threads（螺紋）. It is heated, placed in holes through the pieces

18、of steel（鋼構(gòu)件）, and a second head is formed at the other end by hammering（錘擊）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. 鋼結(jié)構(gòu)中的另一個(gè)進(jìn)步是梁的連接方式。在很多年里，連接的標(biāo)準(zhǔn)方式是鉚接。鉚釘是個(gè)有頭

19、的螺栓，看上去象個(gè)沒(méi)有螺紋的圓頭螺絲釘。鉚釘加熱后穿過(guò)鋼構(gòu)件之間的孔洞，并通過(guò)錘擊另一端而形成第二個(gè)鉚釘頭，從而將其固定就位。如今鉚接已大量地被焊接所替代，鋼構(gòu)件間的連接通過(guò)在高熱下熔化它們之間的鋼材料（即焊條）進(jìn)行。Prestressed concrete is an improved form of reinforcement（加強(qiáng)方法）. Steel rods are bent into the shapes to give them the necessary degree of tensile strength. They are then used to prestress （對(duì).

20、預(yù)加應(yīng)力）concrete, usually by one of two different methods. The first is to leave channels in a concrete beam that correspond to（相應(yīng)于） 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 bindin

21、g agent. In the other (and more common) method, the prestressed steel rods are placed in the lower part of a form（模板） that corresponds to the shape of the finished structure（成品結(jié)構(gòu)）, and the concrete is poured around them. Prestressed concrete uses less steel and less concrete. Because it is so econom

22、ical, it is a highly desirable（非常理想） material.預(yù)應(yīng)力混凝土是加強(qiáng)法的改進(jìn)形式。將鋼筋彎成一定的形狀以使它們具有必要的抗拉強(qiáng)度，然后用該鋼筋對(duì)混凝土施加預(yù)應(yīng)力，通?？刹捎脙煞N不同方法中的任何一種。第一種方法是在混凝土梁中按鋼筋的形狀留下孔道，當(dāng)鋼筋穿過(guò)孔道后，通過(guò)在孔道內(nèi)灌注薄砂漿（一種稀薄的砂漿或粘合劑）將鋼筋與混凝土粘結(jié)在一起。另一種（更常用的）方法是將預(yù)應(yīng)力鋼筋置于按成品結(jié)構(gòu)的形狀設(shè)置的模板的較低部位，然后將混凝土倒入（模板）而包圍著鋼筋。預(yù)應(yīng)力混凝土使用了較少的鋼筋和混凝土，由于它是如此的經(jīng)濟(jì)，因此是一種非常理想的材料。Prestressed

23、 concrete has made it possible to develop（建造） buildings with unusual shapes, like some of the modern sports arenas, with large space unbroken by any obstructing supports（阻礙的支撐物）. The uses for this relatively new structural method are constantly being developed（不斷地?cái)U(kuò)大）. 預(yù)應(yīng)力混凝土使建造獨(dú)特形狀的建筑物成為可能，象一些現(xiàn)代的運(yùn)動(dòng)場(chǎng)

24、，它具有不受任何支撐物阻擋視線的大空間。這種較新的結(jié)構(gòu)方法的使用正在不斷地被擴(kuò)大。The current tendency is to develop（采用） lighter materials, aluminum, for example, weighs much less than steel but has many of the same properties. Aluminum beams have already been used for bridge construction and for the framework of a few buildings. 目前的趨勢(shì)是采用較

25、輕的材料。例如，鋁的重量比鋼輕得多，但具有很多相同的性能。鋁材梁已經(jīng)用于橋梁建筑和一些建筑的框架。Lightweight concretes, another example, are now rapidly developing（發(fā)展） throughout the world. They are used for their thermal insulation(絕熱性). The three types are illustrated below（舉例說(shuō)明如下）: (a) Concretes made with lightweight aggregates; (b) Aerated co

26、ncretes (US gas concretes) foamed（起泡） by whisking（攪拌）or by some chemical process during casting; (c) No-fines concretes. 另一個(gè)例子是輕質(zhì)混凝土，如今已在全世界快速地發(fā)展，因它們的絕熱性而被采用，其三種類型舉例說(shuō)明如下：(a)輕質(zhì)骨料制成的混凝土；(b)通過(guò)澆筑時(shí)攪拌或一些化學(xué)方法起泡而成的加氣混凝土（US加氣混凝土）；(c)無(wú)細(xì)骨料混凝土。All three types are used for their insulating properties（絕熱性）, mainl

27、y in housing, where they give high（非常） comfort in cold climates and a low cost of cooling（降溫成本）in hot climates. In housing, the relative weakness of lightweight concrete walls is unimportant, but it matters（有重大關(guān)系） in roof slabs, floor slabs and beams. 這三種類型的混凝土都是由于它們的絕熱性而被使用，主要用于房屋，使其在寒冷的氣候中非常舒服，在炎熱

28、的氣候中降溫的成本不高。在房屋中，墻采用較薄弱的輕質(zhì)混凝土不重要，但是屋面板、樓面板和梁（采用輕質(zhì)混凝土）則有重大關(guān)系。In some locations, some lightweight aggregates cost little more than（幾乎等于） the best dense（致密） aggregates and a large number of （大量） floor slabs have therefore been built of lightweight aggregate concrete purely for its weight saving, with n

29、o thought of（沒(méi)考慮） its insulation value. 在某些地區(qū)，一些輕質(zhì)骨料的費(fèi)用幾乎等于最致密的骨料，因此大量的樓面板采用輕骨料混凝土制作純粹是節(jié)約重量，而沒(méi)考慮它的絕熱價(jià)值。The lightweight aggregate reduces the floor dead load（恒載） by about 20 per cent resulting in（導(dǎo)致）considerable savings in the floor（樓蓋結(jié)構(gòu)） steel in every floor and the roof, as well as in the column st

30、eel and (less) in the foundations. One London contractor（承包商）prefers to use lightweight aggregate because it gives him the same weight reduction in the floor slab as the use of hollow tiles, with simpler organization and therefore higher speed and profit. The insulation value of the lightweight aggr

31、egate is only important in the roof insulation, which is greatly improved（改進(jìn)）. 輕質(zhì)骨料使樓面的恒載減少了約20%，因而大量的節(jié)約了每層樓面以及屋面的樓蓋結(jié)構(gòu)中的鋼材和柱子與基礎(chǔ)中（較少）的鋼材使用量。一位倫敦的承包商寧愿使用輕質(zhì)骨料，因?yàn)檫@使樓面板上減少的重量與用空心磚相同，且組織更簡(jiǎn)單，因而速度和利潤(rùn)更高。輕質(zhì)骨料的絕熱價(jià)值只在屋面絕熱時(shí)顯得重要，它已被大大地改進(jìn)了。作業(yè)練習(xí)通過(guò)兩篇 Reading Materials 的學(xué)習(xí)，進(jìn)一步了解建筑材料中最常用的混凝土材料的一些特點(diǎn)、種類和性能等，從而更多地掌握一些專業(yè)

32、詞匯和句法。教學(xué)目標(biāo)了解構(gòu)件（主要為梁）的設(shè)計(jì)過(guò)程了解單軸應(yīng)力與多軸應(yīng)力對(duì)失效理論的影響熟悉材料力學(xué)中涉及的專業(yè)詞匯熟悉科技類文獻(xiàn)常用句型熟悉in general、usually、frequently的不同含義與 be referred to as、be known as、that is、be defined as、in other words 等的用法Introduction 介紹Mechanics of Materials deals with（研究）the response of various bodies, usually called members（構(gòu)件）, to applied

33、 forces（施加力）. In Mechanics of Engineering Materials the members have shapes that either exist in actual structures or are being considered for their suitability（根據(jù)其需要）as parts of proposed（擬建的）engineering structures. The materials in the members have properties that are characteristic of commonly use

34、d（常用的）engineering materials such as steel, aluminum, concrete, and wood. 材料力學(xué)用以研究不同物體（通常稱為構(gòu)件）對(duì)施加力的響應(yīng)。在工程材料力學(xué)中，構(gòu)件的形狀可以是實(shí)際結(jié)構(gòu)中存在的，也可以根據(jù)其需要而進(jìn)行考慮（設(shè)計(jì)），作為擬建工程結(jié)構(gòu)的部件。構(gòu)件中材料的性能即是常用的工程材料如鋼材、鋁材、混凝土和木材的特性。As you can see already from the variety of materials, forces, and shapes mentioned, Mechanics of Engineering

35、Materials is of interest to（對(duì).有價(jià)值）all fields of engineering. The engineer uses the principles of Mechanics of Materials to determine if the material properties and the dimensions of a member are adequate to（足以）ensure that it can carry its loads safely and without excessive distortion. In general（通常）

36、, then, we are interested in both the safe load that a member can carry and the associated（相關(guān)的）deformation. Engineering design would be a simple process if the designer could take into consideration（考慮）the loads and the mechanical properties of the materials, manipulate（利用）an equation, and arrive at

37、（得到）suitable dimensions. Design is seldom that simple. Usually（通常）, on the basis of（根據(jù)）experience, the designer selects a trial（試算） member and then does an analysis to see if that member meets the specified requirements. Frequently（常常）, it does not and then a new trial member is selected and the ana

38、lysis repeated. This design cycle（設(shè)計(jì)周期） continues until a satisfactory solution is obtained. The number of cycles（循環(huán)次數(shù)） required to find an acceptable design diminishes as the designer gains experience.正如你已經(jīng)從提到的各種各樣的材料、力和形狀所看到的，工程材料力學(xué)對(duì)所有的工程領(lǐng)域都有價(jià)值。工程師利用材料力學(xué)的原理來(lái)確定是否該材料的性能和構(gòu)件尺寸足以保證它能安全地承受荷載且沒(méi)有過(guò)多的變形。通常，

39、我們關(guān)心的是構(gòu)件能承受的安全荷載及其相應(yīng)的變形。如果設(shè)計(jì)者能通過(guò)考慮荷載和材料的力學(xué)性能，并利用公式得到合適的構(gòu)件尺寸，那么工程設(shè)計(jì)將是一個(gè)簡(jiǎn)單的過(guò)程。然而設(shè)計(jì)很少那么簡(jiǎn)單。通常，根據(jù)經(jīng)驗(yàn)，設(shè)計(jì)者選擇一個(gè)試算構(gòu)件，然后進(jìn)行分析，看它是否滿足規(guī)定的要求。它常常不會(huì)滿足要求，則再選擇一個(gè)新的試算構(gòu)件，再進(jìn)行分析。這樣的設(shè)計(jì)不斷重復(fù)，直至得到一個(gè)滿意的結(jié)果。當(dāng)設(shè)計(jì)師擁有一定的經(jīng)驗(yàn)后，為得到一個(gè)可接受的設(shè)計(jì)所需要的循環(huán)次數(shù)會(huì)減少。Design of Axially Loaded Members 軸向力構(gòu)件的設(shè)計(jì) To give you some insight into （使.有一些了解）the de

40、sign cycle, an extremely simple member will be dealt with first. That member is a prismatic bar with a force, P, acting along its longitudinal axis in the direction（縱軸向）such that it tends to elongate the bar. Such a force is referred to as（稱為）an axial tensile load（軸向拉力）, and we can readily imagine i

41、t trying to（努力.）pull the fibers apart and to cause failure on a transverse plane（橫向平面）. It is safe to assume that all fibers of the bar, in regions remote from（遠(yuǎn)離）the point of application of the load, are being pulled apart with the same load intensity（荷載強(qiáng)度）. With this assumption, the load intensity

42、 or stress is uniform on a transverse plane and is given by when P is in（以.為單位）Newtons and A is in square metres, stress,s ,is in Newtons per square metre (N/m2), which is by definition（根據(jù)定義）Pascals (Pa).為了使你對(duì)設(shè)計(jì)周期有一些了解，首先研究一個(gè)非常簡(jiǎn)單的構(gòu)件。構(gòu)件是個(gè)棱形的桿件，其上沿著它的縱軸向作用一個(gè)力P，這樣往往使桿件在該方向上伸長(zhǎng)。這樣的力稱為軸向拉力，我們能容易地想象它在努力地將纖

43、維拉開(kāi)，導(dǎo)致橫向平面的破壞。安全地假定桿件的所有纖維在遠(yuǎn)離荷載施加點(diǎn)的區(qū)域以相同的荷載強(qiáng)度被拉開(kāi)。在此假定下，荷載強(qiáng)度或應(yīng)力在橫向平面上是均勻的，為 當(dāng)P的單位為牛頓、A的單位為平方米時(shí)，應(yīng)力的單位為牛頓每平方米（N/m2），根據(jù)定義為帕斯卡（Pa）。For a given axial load and given dimensions, the stress can be calculated from (4-1) and compared with（與.相比）the stress that can be safely carried by the material. The safe st

44、ress, known as（稱為）the design stress or allowable stress（許用應(yīng)力）, is determined by tests performed on material made to（按照） the same specifications as the part being considered. A safety factor（安全系數(shù)）, frequently imposed by a legally established code（法規(guī)）, is applied to the strength, as determined by test

45、s, to give the allowable stress. The allowable stress, sa , is given bywhere sf is the stress at which the material fails (failure to be defined later) and n is the safety factor.對(duì)已知的軸向力和（構(gòu)件）尺寸，可根據(jù)公式（4-1）計(jì)算出應(yīng)力，并與材料能安全承受的應(yīng)力作比較。安全應(yīng)力，稱為設(shè)計(jì)應(yīng)力或許用應(yīng)力，它是通過(guò)對(duì)材料的試驗(yàn)來(lái)確定的，該（試驗(yàn)）材料按照與所考慮（驗(yàn)算）的桿件相同的規(guī)范制作。根據(jù)法規(guī)規(guī)定，通常對(duì)試驗(yàn)所確

46、定的強(qiáng)度考慮安全系數(shù)后得到許用應(yīng)力。許用應(yīng)力 sa 為這里，sf 為材料失效（失效在下文有定義）時(shí)的應(yīng)力，而n為安全系數(shù)。Before approving（核準(zhǔn)）trial dimensions, the designer makes certain（確信）that the design is safe by determining that the inequality（不等式）is satisfied. The inequality is usually more convenient in the form不等式常常以更合適的形式出現(xiàn)，即在核準(zhǔn)試算的尺寸之前，設(shè)計(jì)者通過(guò)確定不等式成立而確

47、信設(shè)計(jì)的安全，即It might at first（起先）seem that the designer would always dimension（選定.的尺寸）the cross section（橫截面） so that the stress would exactly equal the allowable stress. However, it may be very costly to produce parts that have nonstandard sizes, so it is usually more economical to waste some material b

48、y selecting the next（接近的）larger standard size above that required by the allowable stress. Departure from（背離）standard sizes is justified（合理的） in cases where the penalty（不利后果）for excess weight is very severe, as in aircraft（航天器）or space-ship（宇宙飛船）design. 起先似乎設(shè)計(jì)者總是在選定橫截面的尺寸，以使應(yīng)力恰好等于許用應(yīng)力。但是，生產(chǎn)非標(biāo)準(zhǔn)尺寸部件的成

49、本可能很高，因此，通常人們會(huì)選擇比按許用應(yīng)力要求的尺寸大一些的標(biāo)準(zhǔn)尺寸部件，這樣盡管浪費(fèi)了一些材料，但總體上更經(jīng)濟(jì)。但不選擇標(biāo)準(zhǔn)尺寸的做法在諸如航天器和宇宙飛船的設(shè)計(jì)中證明是合理的，因?yàn)槎嘤嘀亓慨a(chǎn)生的不利后果是很嚴(yán)重的。Design of Beams 梁的設(shè)計(jì)Up to this point（至此）we have looked at（考慮）the beam problem as a problem in analysis; that is（即）, for a given set of loads, span, and cross section we have been calculating

50、 the stress. The more commonly encountered problem is to select a standard section, or design a member, for a given span and loads without exceeding a certain allowable stress. Under some conditions the allowable stress may be dependent upon the dimensions and shape of the cross section, in which ca

51、se the selection of the member becomes more difficult. For the present（暫時(shí)）we will take the allowable stress as though（似乎）it depends only on the strength of the material and the safety factor.至此，我們已經(jīng)考慮了梁的問(wèn)題而進(jìn)行了（問(wèn)題）分析，即對(duì)給定的一組荷載、跨度和橫截面，我們已經(jīng)計(jì)算了應(yīng)力。更常遇到的問(wèn)題是在不超過(guò)某個(gè)許用應(yīng)力下對(duì)一個(gè)給定的跨度和荷載選擇一個(gè)（構(gòu)件的）標(biāo)準(zhǔn)截面，或設(shè)計(jì)一個(gè)構(gòu)件。在某些條件

52、下，許用應(yīng)力可能依賴于橫截面的尺寸和形狀，這種情況下的構(gòu)件選擇會(huì)變得比較困難。暫時(shí)我們將采用許用應(yīng)力法，似乎它只取決于材料的強(qiáng)度和安全系數(shù)。A trial member will be acceptable（合格）when the stress is equal to, or less than, the allowable stress, that is, ifFor design purposes this inequality is more useful in the formIn the usual design process the maximum bending moment

53、is taken from（取自于）the bending moment diagram（彎矩圖） and the allowable stress is determined (quite frequently in accordance with（根據(jù)）the rules of some legally constituted code) from standard strength tests in combination with（與.結(jié)合）a safety factor. The right-hand side of (4-6) is then known, and it remai

54、ns（仍然是） to select or design a member that will satisfy the inequality. When a standard section is to be used, the tables（表格）could be searched until a member is found such that the combination of I and c satisfies (4-6). This takes more time than is really necessary, since the tables also provide the

55、 value of I/c for each member under the heading（標(biāo)題）S, the section modulus（截面模量）. 當(dāng)試算構(gòu)件的應(yīng)力等于或小于許用應(yīng)力時(shí)，也就是說(shuō)，如果在通常的設(shè)計(jì)過(guò)程中，最大的彎距從彎距圖上取得，而許用應(yīng)力通過(guò)標(biāo)準(zhǔn)強(qiáng)度試驗(yàn)并考慮安全系數(shù)后確定（往往是根據(jù)一些法規(guī)的規(guī)則）。這樣，已知式（4-6）右手邊的值，則仍然是選擇或設(shè)計(jì)構(gòu)件以滿足該不等式。當(dāng)使用標(biāo)準(zhǔn)截面時(shí)可以查找表格，直至找到的構(gòu)件其I和c值的組合能滿足式（4-6）。這樣花費(fèi)的時(shí)間比實(shí)際需要的多，因?yàn)楸砀裰性诮孛婺Ａ縎的標(biāo)題下也提供了每一個(gè)構(gòu)件的I/c的值。試算構(gòu)件即為合格。根

56、據(jù)設(shè)計(jì)的需要，（上述）不等式以下列形式出現(xiàn)更有用，即That is, the section modulus is defined as（定義為）To select a member, the S column（列） is consulted（查閱） and any member that satisfies (4-8) could be used. The members with very high values of S will obviously be understressed（應(yīng)力不足的）and wasteful of material. The best design, if

57、there are no other constraints, will be that which satisfies (4-8) with the minimum amount of material. With tabulated values of S available it is much more convenient to use (4-6) in the formThe smallest acceptable S does not necessarily coincide with（符合）the most economical member. To select the li

58、ghtest and most economical standard section, the listed values of mass should be examined to find the lightest member with an acceptable S. The problem becomes much more complex if built-up（組合）member is being designed because its cost will depend upon the combined costs of web plate, angles and cove

59、r plates as well as fabrication（裝配）costs so that the lightest member is not necessarily the most economical.截面模量定義為為選擇構(gòu)件而查閱S這一列，則任何滿足式（4-8）的構(gòu)件都可采用。顯而易見(jiàn)，對(duì)S值很高的構(gòu)件，其應(yīng)力是不足的，并浪費(fèi)了材料。如果沒(méi)有其他的限制，最好的設(shè)計(jì)將是以最少的材料滿足式（4-8）。能接受的最小的S值不必是最經(jīng)濟(jì)的構(gòu)件。為了選擇最輕和最經(jīng)濟(jì)的標(biāo)準(zhǔn)截面，應(yīng)檢查列出的質(zhì)量值，以找到能接受的S值下的最輕構(gòu)件。如果在設(shè)計(jì)一個(gè)組合構(gòu)件時(shí)，則問(wèn)題變得復(fù)雜得多，因?yàn)樗馁M(fèi)用將依賴于腹板、角鋼和蓋板