土木工程類外文文獻(xiàn)翻譯 (2)

1、<p>　　畢業(yè)設(shè)計(論文)外文文獻(xiàn)翻譯</p><p><b>　　1 中文翻譯</b></p><p>　　摘要：為了研究連續(xù)型拓?fù)鋬?yōu)化理論在實際工程中的應(yīng)用，該文給出了一種多層鋼框架支撐體系連續(xù)型拓?fù)鋬?yōu)化設(shè)計方法?；陟`敏度分析，探討了連續(xù)體結(jié)構(gòu)在多工況荷載作用下、同時受應(yīng)力和多位移約束的拓?fù)鋬?yōu)化刪除準(zhǔn)則。為保證拓?fù)鋬?yōu)化結(jié)果的合理性，提出了設(shè)計區(qū)域平均

2、厚度的概念。在該文給出的優(yōu)化設(shè)計方法中，首先在不考慮位移約束的情況下對無支撐鋼框架進(jìn)行優(yōu)化設(shè)計，然后在有位移約束的條件下采用漸進(jìn)結(jié)構(gòu)優(yōu)化算法和刪除準(zhǔn)則對支撐體系進(jìn)行連續(xù)型拓?fù)鋬?yōu)化設(shè)計，并將獲得的支撐最優(yōu)拓?fù)錁?gòu)形轉(zhuǎn)化成相應(yīng)的桿件。通過一個3跨12層鋼框架支撐體系的拓?fù)鋬?yōu)化設(shè)計實例驗證了該文給出的鋼框架支撐體系連續(xù)型拓?fù)鋬?yōu)化設(shè)計方法的有效性。</p><p>　　關(guān)鍵詞：鋼框架；支撐體系；連續(xù)型；拓?fù)鋬?yōu)化；漸進(jìn)結(jié)構(gòu)優(yōu)

3、化</p><p><b>　　1.1鋼筋混凝土</b></p><p>　　素混凝土是由水泥、水、細(xì)骨料、粗骨料（碎石或；卵石）、空氣，通常還有其他外加劑等經(jīng)過凝固硬化而成。將可塑的混凝土拌合物注入到模板內(nèi)，并將其搗實，然后進(jìn)行養(yǎng)護(hù)，以加速水泥與水的水化反應(yīng)，最后獲得硬化的混凝土。其最終制成品具有較高的抗壓強度和較低的抗拉強度。其抗拉強度約為抗壓強度的十分之一。因此

4、，截面的受拉區(qū)必須配置抗拉鋼筋和抗剪鋼筋以增加鋼筋混凝土構(gòu)件中較弱的受拉區(qū)的強度。</p><p>　　由于鋼筋混凝土截面在均質(zhì)性上與標(biāo)準(zhǔn)的木材或鋼的截面存在著差異，因此，需要對結(jié)構(gòu)設(shè)計的基本原理進(jìn)行修改。將鋼筋混凝土這種非均質(zhì)截面的兩種組成部分按一定比例適當(dāng)布置，可以最好的利用這兩種材料。這一要求是可以達(dá)到的。因混凝土由配料攪拌成濕拌合物，經(jīng)過振搗并凝固硬化，可以做成任何一種需要的形狀。如果拌制混凝土的各種材料

5、配合比恰當(dāng)，則混凝土制成品的強度較高，經(jīng)久耐用，配置鋼筋后，可以作為任何結(jié)構(gòu)體系的主要構(gòu)件。</p><p>　　澆筑混凝土所需要的技術(shù)取決于即將澆筑的構(gòu)件類型，諸如：柱、梁、墻、板、基礎(chǔ)，大體積混凝土水壩或者繼續(xù)延長已澆筑完畢并且已經(jīng)凝固的混凝土等。對于梁、柱、墻等構(gòu)件，當(dāng)模板清理干凈后應(yīng)該在其上涂油，鋼筋表面的銹及其他有害物質(zhì)也應(yīng)該被清除干凈。澆筑基礎(chǔ)前，應(yīng)將坑底土夯實并用水浸濕6英寸，以免土壤從新澆的混凝土

6、中吸收水分。一般情況下，除使用混凝土泵澆筑外，混凝土都應(yīng)在水平方向分層澆筑，并使用插入式或表面式高頻電動振搗器搗實。必須記住，過分的振搗將導(dǎo)致骨料離析和混凝土泌漿等現(xiàn)象，因而是有害的。</p><p>　　水泥的水化作用發(fā)生在有水分存在，而且氣溫在50°F以上的條件下。為了保證水泥的水化作用得以進(jìn)行，必須具備上述條件。如果干燥過快則會出現(xiàn)表面裂縫，這將有損與混凝土的強度，同時也會影響到水泥水化作用的充分

7、進(jìn)行。</p><p>　　設(shè)計鋼筋混凝土構(gòu)件時顯然需要處理大量的參數(shù)，諸如寬度、高度等幾何尺寸，配筋的面積，鋼筋的應(yīng)變和混凝土的應(yīng)變，鋼筋的應(yīng)力等等。因此，在選擇混凝土截面時需要進(jìn)行試算并作調(diào)整，根據(jù)施工現(xiàn)場條件、混凝土原材料的供應(yīng)情況、業(yè)主提出的特殊要求、對建筑和凈空高度的要求、所用的設(shè)計規(guī)范以及建筑物周圍環(huán)境條件等最后確定截面。鋼筋混凝土通常是現(xiàn)場澆注的合成材料，它與在工廠中制造的標(biāo)準(zhǔn)的鋼結(jié)構(gòu)梁、柱等不同，

8、因此對于上面所提到的一系列因素必須予以考慮。</p><p>　　對結(jié)構(gòu)體系的各個部位均需選定試算截面并進(jìn)行驗算，以確定該截面的名義強度是否足以承受所作用的計算荷載。由于經(jīng)常需要進(jìn)行多次試算，才能求出所需的截面，因此設(shè)計時第一次采用的數(shù)值將導(dǎo)致一系列的試算與調(diào)整工作。</p><p>　　選擇混凝土截面時，采用試算與調(diào)整過程可以使復(fù)核與設(shè)計結(jié)合在一起。因此，當(dāng)試算截面選定后，每次設(shè)計都是對

9、截面進(jìn)行復(fù)核。手冊、圖表和微型計算機以及專用程序的使用，使這種設(shè)計方法更為簡捷有效，而傳統(tǒng)的方法則是把鋼筋混凝土的復(fù)核與單純的設(shè)計分別進(jìn)行處理。</p><p><b>　　1.2土方工程</b></p><p>　　由于和土木工程中任何其他工種的施工方法與費用相比較，土方挖運的施工方法與費用的變化都要快得多，因此對于有事業(yè)心的人來說，土方工程是一個可以大有作為的領(lǐng)域

10、。在1935年，目前采用的利用輪胎式機械設(shè)備進(jìn)行土方挖運的方法大多數(shù)還沒有出現(xiàn)。那是大部分土方是采用窄軌鐵路運輸，在這目前來說是很少采用的。當(dāng)時主要的開挖方式是使用正鏟、反鏟、拉鏟或抓斗等挖土機，盡管這些機械目前仍然在廣泛應(yīng)用，但是它們只不過是目前所采用的許多方法中的一小部分。因此，一個工程師為了使自己在土方挖運設(shè)備方面的知識跟得上時代的發(fā)展，他應(yīng)當(dāng)花費一些時間去研究現(xiàn)代的機械。一般說來，有關(guān)挖土機、裝載機和運輸機械的唯一可靠而又最新的

11、資料可以從制造廠商處獲得。</p><p>　　土方工程或土方挖運工程指的是把地表面過高處的土壤挖去（挖方），并把它傾卸到地表面過低的其他地方（填方）。為了降低土方工程費用，填方量應(yīng)該等于挖方量，而且挖方地點應(yīng)該盡可能靠近土方量相等的填方地點，以減少運輸量和填方的二次搬運。土方設(shè)計這項工作落到了從事道路設(shè)計的工程師的身上，因為土方工程的設(shè)計比其他任何工作更能決定工程造價是否低廉。根據(jù)現(xiàn)有的地圖和標(biāo)高，道路工程師應(yīng)

12、在設(shè)計繪圖室中的工作也并不是徒勞的。它將幫助他在最短的時間內(nèi)獲得最好的方案。</p><p>　　費用最低的運土方法是用同一臺機械直接挖方取土并且卸土作為填方。這并不是經(jīng)?？梢宰龅降?，但是如果能夠做到則是很理想的，因為這樣做既快捷又省錢。拉鏟挖土機。推土機和正鏟挖土機都能做到這點。拉鏟挖土機的工作半徑最大。推土機所推運的圖的數(shù)量最多，只是運輸距離很短。拉鏟挖土機的缺點是只能挖比它本身低的土，不能施加壓力挖入壓實的

13、土壤內(nèi)，不能在陡坡上挖土，而且挖。卸都不準(zhǔn)確。</p><p>　　正鏟挖土機介于推土機和拉鏟挖土機的之間，其作用半徑大于推土機，但小于拉鏟挖土機。正鏟挖土機能挖取豎直陡峭的工作面，這種方式對推土機司機來說是危險的，而對拉鏟挖土機則是不可能的。每種機械設(shè)備應(yīng)該進(jìn)行最適合它的性能的作業(yè)。正鏟挖土機不能挖比其停機平面低很多的土，而深挖堅實的土壤時，反鏟挖土機最適用，但其卸料半徑比起裝有正鏟的同一挖土機的卸料半徑則要小

14、很多。</p><p>　　在比較平坦的場地開挖，如果用拉鏟或正鏟挖土機運輸距離太遠(yuǎn)時，則裝有輪胎式的斗式鏟運機就是比不可少的。它能在比較平的地面上挖較深的土（但只能挖機械本身下面的土），需要時可以將土運至幾百米遠(yuǎn)，然后卸土并在卸土的過程中把土大致鏟平。在挖掘硬土?xí)r，人們發(fā)現(xiàn)在開挖場地經(jīng)常用一輛助推拖拉機（輪式或履帶式），對返回挖土的鏟運機進(jìn)行助推這種施工方法是經(jīng)濟(jì)的。一旦鏟運機裝滿，助推拖拉機就回到開挖的地點去

15、幫助下一臺鏟運機。</p><p>　　斗式鏟運機通常是功率非常大的機械，許多廠家制造的鏟運機鏟斗容量為8 m³，滿載時可達(dá)10 m³。最大的自行式鏟運機鏟斗容量為19立方米（滿載時為25 m³），由430馬力的牽引發(fā)動機驅(qū)動。</p><p>　　翻斗機可能是使用最為普遍的輪胎式運輸設(shè)備，因為它們還可以被用來送混凝土或者其他建筑材料。翻斗車的車斗位于大橡膠輪

16、胎車輪前軸的上方，盡管鉸接式翻斗車的卸料方向有很多種，但大多數(shù)車斗是向前翻轉(zhuǎn)的。最小的翻斗車的容量大約為0.5立方米，而最大的標(biāo)準(zhǔn)型翻斗車的容量大約為4.5m³。特殊型式的翻斗車包括容量為4 m³的自裝式翻斗車，和容量約為0.5 m³的鉸接式翻斗車。必須記住翻斗車與自卸卡車之間的區(qū)別。翻斗車車斗向前傾翻而司機坐在后方卸載，因此有時被稱為后卸卡車。</p><p><b>　

17、　1.3結(jié)構(gòu)的安全度</b></p><p>　　規(guī)范的主要目的是提供一般性的設(shè)計原理和計算方法，以便驗算結(jié)構(gòu)的安全度。就目前的趨勢而言，安全系數(shù)與所使用的材料性質(zhì)及其組織情況無關(guān)，通常把它定義為發(fā)生破壞的條件與結(jié)構(gòu)可預(yù)料的最不利的工作條件之比值。這個比值還與結(jié)構(gòu)的破壞概率（危險率）成反比。</p><p>　　破壞不僅僅指結(jié)構(gòu)的整體破壞，而且還指結(jié)構(gòu)不能正常的使用，或者，用更

18、為確切的話來說，把破壞看成是結(jié)構(gòu)已經(jīng)達(dá)到不能繼續(xù)承擔(dān)其設(shè)計荷載的“極限狀態(tài)”。通常有兩種類型的極限狀態(tài)，即：</p><p>　?。?）強度極限狀態(tài)，它相當(dāng)于結(jié)構(gòu)能夠達(dá)到的最大承載能力。其例子包括結(jié)構(gòu)的局部屈曲和整體不穩(wěn)定性；某此界面失效，隨后結(jié)構(gòu)轉(zhuǎn)變?yōu)闄C構(gòu)；疲勞破壞；引起結(jié)構(gòu)幾何形狀顯著變化的彈性變形或塑性變形或徐變；結(jié)構(gòu)對交變荷載、火災(zāi)和爆炸的敏感性。</p><p>　?。?）使用極

19、限狀態(tài)，它對應(yīng)著結(jié)構(gòu)的使用功能和耐久性。器例子包括結(jié)構(gòu)失穩(wěn)之前的過大變形和位移；早期開裂或過大的裂縫；較大的振動和腐蝕。</p><p>　　根據(jù)不同的安全度條件，可以把結(jié)構(gòu)驗算所采用的計算方法分成：</p><p>　?。?）確定性的方法，在這種方法中，把主要參數(shù)看作非隨機參數(shù)。</p><p>　?。?）概率方法，在這種方法中，主要參數(shù)被認(rèn)為是隨機參數(shù)。<

20、/p><p>　　此外，根據(jù)安全系數(shù)的不同用途，可以把結(jié)構(gòu)的計算方法分為：</p><p>　?。?）容許應(yīng)力法，在這種方法中，把結(jié)構(gòu)承受最大荷載時計算得到的應(yīng)力與經(jīng)過按規(guī)定的安全系數(shù)進(jìn)行折減后的材料強度作比較。</p><p>　?。?）極限狀態(tài)法，在這種方法中，結(jié)構(gòu)的工作狀態(tài)是以其最大強度為依據(jù)來衡量的。由理論分析確定的這一最大強度應(yīng)不小于結(jié)構(gòu)承受計算荷載所算得的強

21、度（極限狀態(tài)）。計算荷載等于分別乘以荷載系數(shù)的活載與恒載之和。</p><p>　　把對應(yīng)于不乘以荷載系數(shù)的活載和恒載的工作（使用）條件的應(yīng)力與規(guī)定值（使用極限狀態(tài)）相比較。根據(jù)前兩種方法和后兩種方法的四種可能組合，我們可以得到一些實用的計算方法。通常采用下面兩種計算方法：</p><p>　　確定性的方法，這種方法采用容許應(yīng)力。</p><p>　　概率方法，這種

22、方法采用極限狀態(tài)。</p><p>　　至少在理論上，概率法的主要優(yōu)點是可以科學(xué)的考慮所有隨機安全系數(shù)，然后將這些隨機安全系數(shù)組合成確定的安全系數(shù)。概率法取決于：</p><p>　?。?）制作和安裝過程中材料強度的隨機分布（整個結(jié)構(gòu)的力學(xué)性能數(shù)值的分散性）；</p><p>　?。?）截面和結(jié)構(gòu)幾何尺寸的不確定性（由結(jié)構(gòu)制作和安裝造成的誤差和缺陷而引起的）；<

23、;/p><p>　　對作用在結(jié)構(gòu)上的活載和恒載的預(yù)測的不確定性；</p><p>　　所采用的近似計算方法有關(guān)的不精確性（實際應(yīng)力與計算應(yīng)力的偏差）。</p><p>　　此外，概率理論意味著可以基于下面幾個因素來確定允許的危險率，例如：</p><p>　　建筑物的重要性和建筑物破壞造成的危害性；</p><p>　　（

24、2）由于建筑物破壞使生活受到威脅的人數(shù)；</p><p>　?。?）修復(fù)建筑的可能性；</p><p>　?。?）建筑物的預(yù)期壽命。</p><p>　　所有這些因素均與經(jīng)濟(jì)和社會條件有關(guān)，例如：</p><p>　　（1）建筑物的初始建設(shè)費；</p><p>　?。?）建筑物使用期限內(nèi)的折舊費；</p>

25、<p>　　（3）由于建筑物破壞而造成的物質(zhì)和材料損失費；</p><p>　?。?）在社會上造成的不良影響；</p><p>　?。?）精神和心理上的考慮。</p><p>　　就給定的安全系數(shù)而論，所有這些參數(shù)的確定都是以建筑物的最佳成本為依據(jù)的。但是，應(yīng)該考慮到進(jìn)行全概率分析的困難。對于這種分析來說，應(yīng)該了解活載及其所引起的盈利的分布規(guī)律、材料的力

26、學(xué)性能的分散性和截面的結(jié)構(gòu)幾何尺寸的分散性。此外，由于強度的分布規(guī)律和應(yīng)力的分布規(guī)律之間的相互關(guān)系是困難的。這些實際困難可以采用兩種方法來克服。第一種方法對材料和荷載采用不同的安全系數(shù)，而不需要采用概率準(zhǔn)則；第二種方法是引入一些而簡化假設(shè)的近似概率方法（半概率方法）。</p><p><b>　　2 外文翻譯</b></p><p>　　Abstract:To stu

27、dy the application of continuum structural topology optimization methods to real engineering structures,an optimization method for an optimal topology design of multistory steel frame bracing systems is presented.On a se

28、nsitivity analysis,an element removal criterion for continuum structures with stress and multi-displacement constraints under multiple lateral loading conditions is proposed.A concept of mean thickness of a design domain

29、 is provided to ensure the reasonableness of </p><p>　　Key words:steel frame;bracing system;continuum;topology optimization;evolutionary structural optimization</p><p>　　2.1 Reinforced Concrete&

30、lt;/p><p>　　Plain concrete is formed from a hardened mixture of cement ,water ,fine aggregate, coarse aggregate (crushed stone or gravel),air, and often other admixtures. The plastic mix is placed and consolida

31、ted in the formwork, then cured to facilitate the acceleration of the chemical hydration reaction lf the cement/water mix, resulting in hardened concrete. The finished product has high compressive strength, and low resis

32、tance to tension, such that its tensile strength is approximately one tenth lf its</p><p>　　It is this deviation in the composition of a reinforces concrete section from the homogeneity of standard wood or s

33、teel sections that requires a modified approach to the basic principles of structural design. The two components of the heterogeneous reinforced concrete section are to be so arranged and proportioned that optimal use is

34、 made of the materials involved. This is possible because concrete can easily be given any desired shape by placing and compacting the wet mixture of the constituent</p><p>　　The techniques necessary for pla

35、cing concrete depend on the type of member to be cast: that is, whether it is a column, a bean, a wall, a slab, a foundation. a mass columns, or an extension of previously placed and hardened concrete. For beams, columns

36、, and walls, the forms should be well oiled after cleaning them, and the reinforcement should be cleared of rust and other harmful materials. In foundations, the earth should be compacted and thoroughly moistened to abou

37、t 6 in. in depth to avoid ab</p><p>　　Hydration of the cement takes place in the presence of moisture at temperatures above 50°F. It is necessary to maintain such a condition in order that the chemical

38、hydration reaction can take place. If drying is too rapid, surface cracking takes place. This would result in reduction of concrete strength due to cracking as well as the failure to attain full chemical hydration.</p

39、><p>　　It is clear that a large number of parameters have to be dealt with in proportioning a reinforced concrete element, such as geometrical width, depth, area of reinforcement, steel strain, concrete strain,

40、 steel stress, and so on. Consequently, trial and adjustment is necessary in the choice of concrete sections, with assumptions based on conditions at site, availability of the constituent materials, particular demands of

41、 the owners, architectural and headroom requirements, the applicable codes, a</p><p>　　A trial section has to be chosen for each critical location in a structural system. The trial section has to be analyzed

42、 to determine if its nominal resisting strength is adequate to carry the applied factored load. Since more than one trial is often necessary to arrive at the required section, the first design input step generates into a

43、 series of trial-and-adjustment analyses.</p><p>　　The trial-and –adjustment procedures for the choice of a concrete section lead to the convergence of analysis and design. Hence every design is an analysis

44、once a trial section is chosen. The availability of handbooks, charts, and personal computers and programs supports this approach as a more efficient, compact, and speedy instructional method compared with the traditiona

45、l approach of treating the analysis of reinforced concrete separately from pure design.</p><p>　　2.2 Earthwork </p><p>　　Because earthmoving methods and costs change more quickly than those in a

46、ny other branch of civil engineering, this is a field where there are real opportunities for the enthusiast. In 1935 most of the methods now in use for carrying and excavating earth with rubber-tyred equipment did not ex

47、ist. Most earth was moved by narrow rail track, now relatively rare, and the main methods of excavation, with face shovel, backacter, or dragline or grab, though they are still widely used are only a few of </p>&

48、lt;p>　　Earthworks or earthmoving means cutting into ground where its surface is too high ( cuts ), and dumping the earth in other places where the surface is too low ( fills). Toreduce earthwork costs, the volume of t

49、he fills should be equal to the volume of the cuts and wherever possible the cuts should be placednear to fills of equal volume so as to reduce transport and double handlingof the fill. This work of earthwork design fall

50、s on the engineer who lays out the road since it is the layout of the </p><p>　　The cheapest way of moving earth is to take it directly out of the cut and drop it as fill with the same machine. This is not a

51、lways possible, but when it canbe done it is ideal, being both quick and cheap. Draglines, bulldozers and face shovels an do this. The largest radius is obtained with the dragline,and the largest tonnage of earth is move

52、d by the bulldozer, though only over short distances.The disadvantages of the dragline are that it must dig below itself, it cannot dig with force into c</p><p>　　Face shovels are between bulldozers and drag

53、lines, having a larger radius of action than bulldozers but less than draglines. They are anle to dig into a vertical cliff face in a way which would be dangerous tor a bulldozer operator and impossible for a dragline. E

54、ach piece of equipment should be level of their tracks and for deep digs in compact material a backacter is most useful, but its dumping radius is considerably less than that of the same escavator fitted with a face shov

55、el.</p><p>　　Rubber-tyred bowl scrapers are indispensable for fairly level digging where the distance of transport is too much tor a dragline or face shovel. They can dig the material deeply ( but only below

56、 themselves ) to a fairly flat surface, carry it hundreds of meters if need be, then drop it and level it roughly during the dumping. For hard digging it is often found economical to keep a pusher tractor ( wheeled or tr

57、acked ) on the digging site, to push each scraper as it returns to dig. As soon as the</p><p>　　Bowl scrapers are often extremely powerful machines;many makers build scrapers of 8 cubic meters struck capacit

58、y, which carry 10 m ³ heaped. The largest self-propelled scrapers are of 19 m ³ struck capacity ( 25 m ³ heaped )and they are driven by a tractor engine of 430 horse-powers.</p><p>　　Dumpers a

59、re probably the commonest rubber-tyred transport since they can also conveniently be used for carrying concrete or other building materials. Dumpers have the earth container over the front axle on large rubber-tyred whee

60、ls, and the container tips forwards on most types, though in articulated dumpers the direction of tip can be widely varied. The smallest dumpers have a capacity of about 0.5 m ³, and the largest standard types are o

61、f about 4.5 m ³. Special types include the self-loading </p><p>　　2.3 Safety of Structures</p><p>　　The principal scope of specifications is to provide general principles and computational

62、methods in order to verify safety of structures. The “ safety factor ”, which according to modern trends is independent of the nature and combination of the materials used, can usually be defined as the ratio between the

63、 conditions. This ratio is also proportional to the inverse of the probability ( risk ) of failure of the structure. </p><p>　　Failure has to be considered not only as overall collapse of the structure but a

64、lso as unserviceability or, according to a more precise. Common definition. As the reaching of a “ limit state ” which causes the construction not to accomplish the task it was designed for. There are two categories of l

65、imit state :</p><p>　　(1)Ultimate limit sate, which corresponds to the highest value of the load-bearing capacity. Examples include local buckling or global instability of the structure; failure of some sect

66、ions and subsequent transformation of the structure into a mechanism; failure by fatigue; elastic or plastic deformation or creep that cause a substantial change of the geometry of the structure; and sensitivity of the s

67、tructure to alternating loads, to fire and to explosions.</p><p>　　(2)Service limit states, which are functions of the use and durability of the structure. Examples include excessive deformations and displac

68、ements without instability; early or excessive cracks; large vibrations; and corrosion.</p><p>　　Computational methods used to verify structures with respect to the different safety conditions can be separat

69、ed into:</p><p>　　(1)Deterministic methods, in which the main parameters are considered as nonrandom parameters.</p><p>　　(2)Probabilistic methods, in which the main parameters are considered as

70、 random parameters.</p><p>　　Alternatively, with respect to the different use of factors of safety, computational methods can be separated into:</p><p>　　(1)Allowable stress method, in which the

71、 stresses computed under maximum loads are compared with the strength of the material reduced by given safety factors.</p><p>　　(2)Limit states method, in which the structure may be proportioned on the basis

72、 of its maximum strength. This strength, as determined by rational analysis, shall not be less than that required to support a factored load equal to the sum of the factored live load and dead load ( ultimate state ).<

73、;/p><p>　　The stresses corresponding to working ( service ) conditions with unfactored live and dead loads are compared with prescribed values ( service limit state ) . From the four possible combinations of th

74、e first two and second two methods, we can obtain some useful computational methods. Generally, two combinations prevail:</p><p>　　(1)deterministic methods, which make use of allowable stresses.</p>&

75、lt;p>　　(2)Probabilistic methods, which make use of limit states.</p><p>　　The main advantage of probabilistic approaches is that, at least in theory, it is possible to scientifically take into account all

76、 random factors of safety, which are then combined to define the safety factor. probabilistic approaches depend upon : </p><p>　　(1)Random distribution of strength of materials with respect to the condition

77、s of fabrication and erection ( scatter of the values of mechanical properties through out the structure );</p><p>　　(2)Uncertainty of the geometry of the cross-section sand of the structure ( faults and im

78、perfections due to fabrication and erection of the structure );</p><p>　　(3)Uncertainty of the predicted live loads and dead loads acting on the structure;</p><p>　　(4)Uncertainty related to th

79、e approximation of the computational method used ( deviation of the actual stresses from computed stresses ).</p><p>　　Furthermore, probabilistic theories mean that the allowable risk can be based on several

80、 factors, such as :</p><p>　　(1)Importance of the construction and gravity of the damage by its failure;</p><p>　　(2)Number of human lives which can be threatened by this failure;</p>&l

81、t;p>　　(3)Possibility and/or likelihood of repairing the structure;</p><p>　　(4)Predicted life of the structure.</p><p>　　All these factors are related to economic and social considerations s

82、uch as：</p><p>　　(1)Initial cost of the construction; </p><p>　　(2)Amortization funds for the duration of the construction; </p><p>　　(3)Cost of physical and material damage due

83、to the failure of the construction; </p><p>　　(4)Adverse impact on society; </p><p>　　(5)Moral and psychological views.</p><p>　　The definition of all these parameters, for a give

84、n safety factor, allows construction at the optimum cost. However, the difficulty of carrying out a complete probabilistic analysis has to be taken into account. For such an analysis the laws of the distribution of the l

85、ive load and its induced stresses, of the scatter of mechanical properties of materials, and of the geometry of the cross-sections and the structure have to be known. Furthermore, it is difficult to interpret the interac