【正文】 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： 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)點是可以科學(xué)的考慮所有隨機(jī)安全系數(shù)，然后將這些隨機(jī)安全系數(shù)組合成確定的安全系數(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）； 對作用在結(jié)構(gòu)上的活載和恒載的預(yù)測的不確定性； 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）。 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(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)和社會條件有關(guān)，例如： （ 1） 建筑物的初始建設(shè)費； （ 2） 建筑物使用期限內(nèi)的折舊費； （ 3） 由于建筑物破壞而造成的物質(zhì)和材料損失費； （ 4） 在社會上造成的不良影響； （ 5） 精神和心理上的考慮。 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)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畢 業(yè) 設(shè) 計 6 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）。 畢 業(yè) 設(shè) 計 7 2 外文翻譯 Reinforced Concrete 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 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 pressive strength, and low resistance to tension, such that its tensile strength is approximately one tenth lf its pressive strength. Consequently, tensile and shear reinforcement in the tensile regions of sections has to be provided to pensate for the weak tension regions in the reinforced concrete element. It is this deviation in the position 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 ponents of the heterogeneous reinforced concrete section are to be so arranged and proportioned that optimal use is made of the materials involved. This is possible because concrete can easily be given any desired shape by placing and pacting the wet mixture of the constituent ingredients are properly proportioned, the finished product bees strong, durable, and, in bination with the reinforcing bars, adaptable 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, 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 pacted and thoroughly moistened to about 6 in. in depth to avoid absorption of the moisture present in the wet concrete. Concrete should always be placed in horizontal layers which are pacted by means of high frequency powerdriven 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 since it could cause segregation of the aggregate and bleeding of the concrete. 畢 業(yè) 設(shè) 計 8 Hydration of the cement takes place in the presence of moisture at temperatures above 50176。 F. It is necessary to maintain such a condition in order that the chemical 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. 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,