2022年土木工程類外文文獻(xiàn)翻譯.d

1、.精品文本精品文本.精品文本1 中文翻譯摘要：為了研究連續(xù)型拓?fù)鋬?yōu)化理論在實(shí)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(jì)方法。基于靈敏度分析，探討了連續(xù)體結(jié)構(gòu)在多工況荷載作用下、同時(shí)受應(yīng)力和多位移約束的拓?fù)鋬?yōu)化刪除準(zhǔn)那么。為保證拓?fù)鋬?yōu)化結(jié)果的合理性，提出了設(shè)計(jì)區(qū)域平均厚度的概念。在該文給出的優(yōu)化設(shè)計(jì)方法中，首先在不考慮位移約束的情況下對(duì)無支撐鋼框架進(jìn)行優(yōu)化設(shè)計(jì)，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)那么對(duì)支撐體系進(jìn)行連續(xù)型拓?fù)鋬?yōu)化設(shè)計(jì)，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(gè)3跨12層鋼框架支撐體系的拓?fù)鋬?yōu)化設(shè)計(jì)實(shí)例驗(yàn)證了該文給出的鋼框架支撐體系

2、連續(xù)型拓?fù)鋬?yōu)化設(shè)計(jì)方法的有效性。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)化1.1鋼筋混凝土素混凝土是由水泥、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(shí)，然后進(jìn)行養(yǎng)護(hù)，以加速水泥與水的水化反響，最后獲得硬化的混凝土。其最終制成品具有較高的抗壓強(qiáng)度和較低的抗拉強(qiáng)度。其抗拉強(qiáng)度約為抗壓強(qiáng)度的十分之一。因此，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(qiáng)度。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(duì)結(jié)構(gòu)設(shè)計(jì)的根本原理進(jìn)行修改。將鋼筋混凝土這種非均質(zhì)截

3、面的兩種組成局部按一定比例適當(dāng)布置，可以最好的利用這兩種材料。這一要求是可以到達(dá)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配合比恰當(dāng)，那么混凝土制成品的強(qiáng)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。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、根底，大體積混凝土水壩或者繼續(xù)延長(zhǎng)已澆筑完畢并且已經(jīng)凝固的混凝土等。對(duì)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，鋼筋外表的銹及其他有害物質(zhì)也應(yīng)該被去除干凈。澆筑根底前，應(yīng)將坑底土夯實(shí)并用水浸濕6英寸，以免土壤從新澆的混凝土中吸收水分。一般情

4、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，并使用插入式或外表式高頻電動(dòng)振搗器搗實(shí)。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。水泥的水化作用發(fā)生在有水分存在，而且氣溫在50F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。如果枯燥過快那么會(huì)出現(xiàn)外表裂縫，這將有損與混凝土的強(qiáng)度，同時(shí)也會(huì)影響到水泥水化作用的充分進(jìn)行。設(shè)計(jì)鋼筋混凝土構(gòu)件時(shí)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(shí)需要進(jìn)行試算并作調(diào)整，根據(jù)施工現(xiàn)場(chǎng)條件、混凝土原材料的供應(yīng)情況、業(yè)主提出的特殊要

5、求、對(duì)建筑和凈空高度的要求、所用的設(shè)計(jì)標(biāo)準(zhǔn)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(chǎng)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，因此對(duì)于上面所提到的一系列因素必須予以考慮。對(duì)結(jié)構(gòu)體系的各個(gè)部位均需選定試算截面并進(jìn)行驗(yàn)算，以確定該截面的名義強(qiáng)度是否足以承受所作用的計(jì)算荷載。由于經(jīng)常需要進(jìn)行屢次試算，才能求出所需的截面，因此設(shè)計(jì)時(shí)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。選擇混凝土截面時(shí)，采用試算與調(diào)整過程可以使復(fù)核與設(shè)計(jì)結(jié)合在一起。因此，當(dāng)試算截面選定后，每次設(shè)計(jì)都是對(duì)截面進(jìn)行復(fù)核。手冊(cè)、圖表和微型計(jì)算機(jī)以及專用程序的使用，使這種設(shè)計(jì)方法更為簡(jiǎn)捷有效，而傳統(tǒng)的

6、方法那么是把鋼筋混凝土的復(fù)核與單純的設(shè)計(jì)分別進(jìn)行處理。1.2土方工程由于和土木工程中任何其他工種的施工方法與費(fèi)用相比較，土方挖運(yùn)的施工方法與費(fèi)用的變化都要快得多，因此對(duì)于有事業(yè)心的人來說，土方工程是一個(gè)可以大有作為的領(lǐng)域。在1935年，目前采用的利用輪胎式機(jī)械設(shè)備進(jìn)行土方挖運(yùn)的方法大多數(shù)還沒有出現(xiàn)。那是大局部土方是采用窄軌鐵路運(yùn)輸，在這目前來說是很少采用的。當(dāng)時(shí)主要的開挖方式是使用正鏟、反鏟、拉鏟或抓斗等挖土機(jī)，盡管這些機(jī)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局部。因此，一個(gè)工程師為了使自己在土方挖運(yùn)設(shè)備方面的知識(shí)跟得上時(shí)代的開展，他應(yīng)當(dāng)花費(fèi)一些時(shí)間去研究現(xiàn)代的機(jī)械

7、。一般說來，有關(guān)挖土機(jī)、裝載機(jī)和運(yùn)輸機(jī)械的唯一可靠而又最新的資料可以從制造廠商處獲得。土方工程或土方挖運(yùn)工程指的是把地外表過高處的土壤挖去挖方，并把它傾卸到地外表過低的其他地方填方。為了降低土方工程費(fèi)用，填方量應(yīng)該等于挖方量，而且挖方地點(diǎn)應(yīng)該盡可能靠近土方量相等的填方地點(diǎn)，以減少運(yùn)輸量和填方的二次搬運(yùn)。土方設(shè)計(jì)這項(xiàng)工作落到了從事道路設(shè)計(jì)的工程師的身上，因?yàn)橥练焦こ痰脑O(shè)計(jì)比其他任何工作更能決定工程造價(jià)是否低廉。根據(jù)現(xiàn)有的地圖和標(biāo)高，道路工程師應(yīng)在設(shè)計(jì)繪圖室中的工作也并不是徒勞的。它將幫助他在最短的時(shí)間內(nèi)獲得最好的方案。費(fèi)用最低的運(yùn)土方法是用同一臺(tái)機(jī)械直接挖方取土并且卸土作為填方。這并不是經(jīng)常可以

8、做到的，但是如果能夠做到那么是很理想的，因?yàn)檫@樣做既快捷又省錢。拉鏟挖土機(jī)。推土機(jī)和正鏟挖土機(jī)都能做到這點(diǎn)。拉鏟挖土機(jī)的工作半徑最大。推土機(jī)所推運(yùn)的圖的數(shù)量最多，只是運(yùn)輸距離很短。拉鏟挖土機(jī)的缺點(diǎn)是只能挖比它本身低的土，不能施加壓力挖入壓實(shí)的土壤內(nèi)，不能在陡坡上挖土，而且挖。卸都不準(zhǔn)確。正鏟挖土機(jī)介于推土機(jī)和拉鏟挖土機(jī)的之間，其作用半徑大于推土機(jī)，但小于拉鏟挖土機(jī)。正鏟挖土機(jī)能挖取豎直陡峭的工作面，這種方式對(duì)推土機(jī)司機(jī)來說是危險(xiǎn)的，而對(duì)拉鏟挖土機(jī)那么是不可能的。每種機(jī)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(jī)不能挖比其停機(jī)平面低很多的土，而深挖堅(jiān)實(shí)的土壤時(shí)，反鏟挖土機(jī)最適用，但其卸料半徑比

9、起裝有正鏟的同一挖土機(jī)的卸料半徑那么要小很多。在比較平坦的場(chǎng)地開挖，如果用拉鏟或正鏟挖土機(jī)運(yùn)輸距離太遠(yuǎn)時(shí)，那么裝有輪胎式的斗式鏟運(yùn)機(jī)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(jī)械本身下面的土，需要時(shí)可以將土運(yùn)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(chǎng)地經(jīng)常用一輛助推拖拉機(jī)輪式或履帶式，對(duì)返回挖土的鏟運(yùn)機(jī)進(jìn)行助推這種施工方法是經(jīng)濟(jì)的。一旦鏟運(yùn)機(jī)裝滿，助推拖拉機(jī)就回到開挖的地點(diǎn)去幫助下一臺(tái)鏟運(yùn)機(jī)。斗式鏟運(yùn)機(jī)通常是功率非常大的機(jī)械，許多廠家制造的鏟運(yùn)機(jī)鏟斗容量為8 m，滿載時(shí)可達(dá)10 m。最大的自行式鏟運(yùn)機(jī)鏟斗容量為19立方米滿載時(shí)為25 m，由43

10、0馬力的牽引發(fā)動(dòng)機(jī)驅(qū)動(dòng)。翻斗機(jī)可能是使用最為普遍的輪胎式運(yùn)輸設(shè)備，因?yàn)樗鼈冞€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4.5m。特殊型式的翻斗車包括容量為4 m的自裝式翻斗車，和容量約為0.5 m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(jī)坐在前方卸載，因此有時(shí)被稱為后卸卡車。1.3結(jié)構(gòu)的平安度標(biāo)準(zhǔn)的主要目的是提供一般性的設(shè)計(jì)原理和計(jì)算方法，以便驗(yàn)算結(jié)構(gòu)的平安度。就目前的趨勢(shì)而言，平安系數(shù)與所使用

11、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(gè)比值還與結(jié)構(gòu)的破壞概率危險(xiǎn)率成反比。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，用更為確切的話來說，把破壞看成是結(jié)構(gòu)已經(jīng)到達(dá)不能繼續(xù)承擔(dān)其設(shè)計(jì)荷載的“極限狀態(tài)。通常有兩種類型的極限狀態(tài)，即：1強(qiáng)度極限狀態(tài)，它相當(dāng)于結(jié)構(gòu)能夠到達(dá)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(jī)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(duì)交變荷載、火災(zāi)和爆炸的敏感性。2使用極限狀態(tài)，它對(duì)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

12、的過大變形和位移；早期開裂或過大的裂縫；較大的振動(dòng)和腐蝕。根據(jù)不同的平安度條件，可以把結(jié)構(gòu)驗(yàn)算所采用的計(jì)算方法分成：1確定性的方法，在這種方法中，把主要參數(shù)看作非隨機(jī)參數(shù)。2概率方法，在這種方法中，主要參數(shù)被認(rèn)為是隨機(jī)參數(shù)。此外，根據(jù)平安系數(shù)的不同用途，可以把結(jié)構(gòu)的計(jì)算方法分為：1容許應(yīng)力法，在這種方法中，把結(jié)構(gòu)承受最大荷載時(shí)計(jì)算得到的應(yīng)力與經(jīng)過按規(guī)定的平安系數(shù)進(jìn)行折減后的材料強(qiáng)度作比較。2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(qiáng)度為依據(jù)來衡量的。由理論分析確定的這一最大強(qiáng)度應(yīng)不小于結(jié)構(gòu)承受計(jì)算荷載所算得的強(qiáng)度極限狀態(tài)。計(jì)算荷載等于分別乘以荷載系數(shù)的活載與恒載之和。把對(duì)應(yīng)于不乘以荷

13、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(shí)用的計(jì)算方法。通常采用下面兩種計(jì)算方法：確定性的方法，這種方法采用容許應(yīng)力。概率方法，這種方法采用極限狀態(tài)。至少在理論上，概率法的主要優(yōu)點(diǎn)是可以科學(xué)的考慮所有隨機(jī)平安系數(shù)，然后將這些隨機(jī)平安系數(shù)組合成確定的平安系數(shù)。概率法取決于：1制作和安裝過程中材料強(qiáng)度的隨機(jī)分布整個(gè)結(jié)構(gòu)的力學(xué)性能數(shù)值的分散性；2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；對(duì)作用在結(jié)構(gòu)上的活載和恒載的預(yù)測(cè)的不確定性；所采用的近似計(jì)算方法有關(guān)的不精確性實(shí)際應(yīng)力與計(jì)算應(yīng)力的偏差。

14、此外，概率理論意味著可以基于下面幾個(gè)因素來確定允許的危險(xiǎn)率，例如：建筑物的重要性和建筑物破壞造成的危害性；2由于建筑物破壞使生活受到威脅的人數(shù)；3修復(fù)建筑的可能性；4建筑物的預(yù)期壽命。所有這些因素均與經(jīng)濟(jì)和社會(huì)條件有關(guān)，例如：1建筑物的初始建設(shè)費(fèi)；2建筑物使用期限內(nèi)的折舊費(fèi)；3由于建筑物破壞而造成的物質(zhì)和材料損失費(fèi)；4在社會(huì)上造成的不良影響；5精神和心理上的考慮。就給定的平安系數(shù)而論，所有這些參數(shù)確實(shí)定都是以建筑物的最正確本錢為依據(jù)的。但是，應(yīng)該考慮到進(jìn)行全概率分析的困難。對(duì)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(qiáng)

15、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(shí)際困難可以采用兩種方法來克服。第一種方法對(duì)材料和荷載采用不同的平安系數(shù)，而不需要采用概率準(zhǔn)那么；第二種方法是引入一些而簡(jiǎn)化假設(shè)的近似概率方法半概率方法。2 外文翻譯Abstract:To study the application of continuum structural topology optimization methods to real engineering structures,an optimization method for an optimal topology design of multistory st

16、eel frame bracing systems is presented.On a sensitivity 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 is provided to ensure the reasona

17、bleness of optimal results.In the proposed optimization method,the optimal design of an unbraced steel frame without displacement constraints is performed firstly,and then the optimal topology of a bracing system for the multistory steel frame considering displacement constraints is obtained by usin

18、g evolutionary structural optimization and the given removal criterion,and finally the optima layout of the bracing system is interpreted as bracing members.An example of 3-bay 12-story plane steel frame shows that it is effective for the given optimization method in the optimal design of bracing sy

19、stems for multistory steel frames. Key words:steel frame;bracing system;continuum;topology optimization;evolutionary structural optimization2.1 Reinforced ConcretePlain concrete is formed from a hardened mixture of cement ,water ,fine aggregate, coarse aggregate (crushed stone or gravel),air, and of

20、ten other admixtures. The plastic mix is placed and consolidated 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 resistance to tension, s

21、uch that its tensile strength is approximately one tenth lf its compressive strength. Consequently, tensile and shear reinforcement in the tensile regions of sections has to be provided to compensate for the weak tension regions in the reinforced concrete element.It is this deviation in the composit

22、ion of a reinforces concrete section from the homogeneity of standard wood or steel 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

23、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 ingredients are properly proportioned, the finished product becomes strong, durable, and, in combination with the reinforcing bars, ada

24、ptable for use as main members of any structural system.The techniques necessary for placing 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,

25、 columns, 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 about 6 in. in depth to avoid absorption of the moisture present in the wet

26、concrete. Concrete should always be placed in horizontal layers which are compacted by means of high frequency power-driven vibrators of either the immersion or external type, as the case requires, unless it is placed by pumping. It must be kept in mind, however, that over vibration can be harmful s

27、ince it could cause segregation of the aggregate and bleeding of the concrete.Hydration of the cement takes place in the presence of moisture at temperatures above 50F. It is necessary to maintain such a condition in order that the chemical hydration reaction can take place. If drying is too rapid,

28、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.It is clear that a large number of parameters have to be dealt with in proportioning a reinforced concrete element, such as geometrical width, de

29、pth, area of reinforcement, steel strain, concrete strain, 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 the owners, architect

30、ural and headroom requirements, the applicable codes, and environmental reinforced concrete is often a site-constructed composite, in contrast to the standard mill-fabricated beam and column sections in steel structures.A trial section has to be chosen for each critical location in a structural syst

31、em. The trial section has to be analyzed 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 series of trial-and-adjustment analyses

32、.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 once a trial section is chosen. The availability of handbooks, charts, and personal computers and programs supports this approach as a more effi

33、cient, compact, and speedy instructional method compared with the traditional approach of treating the analysis of reinforced concrete separately from pure design.2.2 Earthwork Because earthmoving methods and costs change more quickly than those in any other branch of civil engineering, this is a fi

34、eld 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 exist. Most earth was moved by narrow rail track, now relatively rare, and the main methods of excavation, with face shovel, backact

35、er, or dragline or grab, though they are still widely used are only a few of the many current methods. To keep his knowledge of earthmoving equipment up to date an engineer must therefore spend tine studying modern machines. Generally the only reliable up-to-date information on excavators, loaders a

36、nd transport is obtainable from the makers.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 the fills should be equal to the volume of the c

37、uts 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 falls on the engineer who lays out the road since it is the layout of the earthwork more than anything else which decides its cheap

38、ness. From the available maps ahd levels, the engineering must try to reach as many decisions as possible in the drawing office by drawing cross sections of the earthwork. On the site when further information becomes available he can make changes in jis sections and layout,but the drawing lffice wor

39、k will not have been lost. It will have helped him to reach the best solution in the shortest time.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 always possible, but when it canbe done it is ideal, being both quick and c

40、heap. Draglines, bulldozers and face shovels an do this. The largest radius is obtained with the dragline,and the largest tonnage of earth is moved 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 co

41、mpacted material, it cannot dig on steep slopws, and its dumping and digging are not accurate.Face shovels are between bulldozers and draglines, 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

42、 tor a bulldozer operator and impossible for a dragline. Each 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 shovel.Rubber-tyred bowl s

43、crapers 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 themselves ) to a fairly flat surface, carry it hundreds of meters if need be, then drop it and level it roughly during the

44、dumping. For hard digging it is often found economical to keep a pusher tractor ( wheeled or tracked ) on the digging site, to push each scraper as it returns to dig. As soon as the scraper is full,the pusher tractor returns to the beginning of the dig to heop to help the nest scraper.Bowl scrapers

45、are often extremely powerful machines;many makers build scrapers of 8 cubic meters struck capacity, which carry 10 m heaped. The largest self-propelled scrapers are of 19 m struck capacity ( 25 m heaped )and they are driven by a tractor engine of 430 horse-powers.Dumpers are probably the commonest r

46、ubber-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 wheels, and the container tips forwards on most types, though in articulated dumpers the direction of tip can b

47、e widely varied. The smallest dumpers have a capacity of about 0.5 m , and the largest standard types are of about 4.5 m . Special types include the self-loading dumper of up to 4 m and the articulated type of about 0.5 m . The distinction between dumpers and dump trucks must be remembered .dumpers

48、tip forwards and the driver sits behind the load. Dump trucks are heavy, strengthened tipping lorries, the driver travels in front lf the load and the load is dumped behind him, so they are sometimes called rear-dump trucks. 2.3 Safety of StructuresThe principal scope of specifications is to provide

49、 general principles and computational 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 conditions. This ratio is also proportiona

50、l to the inverse of the probability ( risk ) of failure of the structure. Failure has to be considered not only as overall collapse of the structure but also as unserviceability or, according to a more precise. Common definition. As the reaching of a “ limit state which causes the construction not t

51、o accomplish the task it was designed for. There are two categories of limit state :(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 sections and subsequent transformat

52、ion 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 structure to alternating loads, to fire and to explosions.(2)Service limit states, which are functions of the us

53、e and durability of the structure. Examples include excessive deformations and displacements without instability; early or excessive cracks; large vibrations; and corrosion.Computational methods used to verify structures with respect to the different safety conditions can be separated into:(1)Determ

54、inistic methods, in which the main parameters are considered as nonrandom parameters.(2)Probabilistic methods, in which the main parameters are considered as random parameters.Alternatively, with respect to the different use of factors of safety, computational methods can be separated into:(1)Allowa

55、ble stress method, in which the stresses computed under maximum loads are compared with the strength of the material reduced by given safety factors.(2)Limit states method, in which the structure may be proportioned on the basis of its maximum strength. This strength, as determined by rational analy

56、sis, 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 ).The stresses corresponding to working ( service ) conditions with unfactored live and dead loads are compared with prescribed values ( service limit state

57、) . From the four possible combinations of the first two and second two methods, we can obtain some useful computational methods. Generally, two combinations prevail:(1)deterministic methods, which make use of allowable stresses.(2)Probabilistic methods, which make use of limit states.The main advan

58、tage of probabilistic approaches is that, at least in theory, it is possible to scientifically take into account all random factors of safety, which are then combined to define the safety factor. probabilistic approaches depend upon : (1)Random distribution of strength of materials with respect to the conditions of fabrication and erection ( scatter of the values of mechanical properties through out the structure );(2)Uncertainty of the geometry of the cross-section sand of the structure ( fa