【正文】 it and level it roughly during the 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 are often extremely powerful machines。 heaped )and they are driven by a tractor engine of 430 horsepowers. Dumpers are probably the monest rubbertyred 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 rubbertyred wheels, 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 m 179。. The distinction between dumpers and dump trucks must be remembered .dumpers 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 reardump trucks. Safety of Structures The principal scope of specifications is to provide general principles and putational methods in order to verify safety of structures. The “ safety factor ”, which according to modern trends is independent of the nature and bination of the materials used, can usually be defined as the ratio between the conditions. This ratio is also proportional 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 to acplish 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 loadbearing capacity. Examples include local buckling or global instability 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 use and durability of the structure. Examples include excessive deformations and displacements without instability。 (2) Uncertainty of the geometry of the crosssection sand of the structure ( faults and imperfections due to fabrication and erection of the structure )。 華北水利水電學院畢業(yè)設計 土木工程專業(yè) 7 (3)Possibility and/or likelihood of repairing the structure。 (4) Adverse impact on society。其最終制成品具有較高的抗壓強度和較低的抗拉強度。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布置，可以最好的利用這兩種材料。 澆筑混凝土所需要的技術取決于即將澆筑的構(gòu)件類型，諸如：柱、梁、墻、板、基礎，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。必須記住，過分的振搗將導致骨料離析和混凝土泌漿等現(xiàn)象，因而是有害的。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進行。 對結(jié)構(gòu)體系的各個部位均需選定試算截面并進行驗算，以確定該截面的名義強華北水利水電學院畢業(yè)設計 土木工程專業(yè) 9 度是否足以承受所作 用的計算荷載。手冊、圖表和微型計算機以及專用程序的使用，使這種設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核與單純的設計分別進行處理。當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。為了降低土方工程費用，填方量應該等于挖方量，而且挖方地點應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。 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。拉鏟挖土機的工作半徑最大。 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。 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時，則裝有輪胎式的斗式鏟運機就是比不可少的。 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 m179。 翻斗機可能是使用最為普遍的輪胎式運輸設備，因為它們還可以被用來送混凝土 或者其他建筑材料。的自裝式翻斗車，和容量約為 m179。 規(guī)范的主要目的是提供一般性的設計原理和計算方法，以便驗算結(jié)構(gòu)的安全度。通常有兩種類型的極限狀態(tài)，即： （ 1）強度極限狀態(tài)，它相當于結(jié)構(gòu)能夠達到的最大承載能力。 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： （ 1）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)。由理論分析確定的這一最大強度應不小于結(jié)構(gòu)承受計算荷載所算得的強度（極限狀態(tài)）。通常采用下面兩種計算方法： 確定性的方法，這種方法采用容許應力。 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： 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）修復建筑的可能性； （ 4）建筑物的預期壽命。對于這種分析來說，應該了解活載及其所引起的盈利的分布規(guī)律、材料的力學性能的分散性和截面的結(jié)構(gòu)幾何尺寸的華北水利水電學院畢業(yè)設計 土木工程專業(yè) 12 分散性。