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1、Lesson PlanReviewNew Words and PhrasesReading and TranslatingUnit SixClassroom Exercises,Review tension steels bend steels stirrup steels longitudinal steelsresist the tension forces inclined (transverse) steelsresist the shear forces provide a high degree of interlocking of the two materials therma
2、l expansion coefficients the corrosion resistance of bare steel is poor A moderate amount of concrete cover provides sufficient thermal insulation for the embedded reinforcement. In prestressed concrete structures, large tension forces are applied to the reinforcement prior to letting it act jointly
3、 with the concrete in resisting external loads.,Unit Five Prestressed Concrete 单 元 5 预应力混凝土,Modern structural engineering tends to progress toward more economic structures through gradually improved methods of design and the use of higher strength materials. This results in a reduction of cross-sect
4、ional dimensions and consequent weight savings.,Modern structural engineering tends to progress toward more economic structures through gradually improved methods of design and the use of higher strength materials. This results in a reduction of cross-sectional dimensions and consequent weight savin
5、gs. 逐渐改进设计方法和采用高强材料，使现代结构工程向更经济的结构形式发展，构件的截面尺寸减小、重量减轻。,Such developments are particularly important in the field of reinforced concrete, where the dead load represents a substantial part of the total design load.,Such developments are particularly important in the field of reinforced concrete, where
6、 the dead load represents a substantial part of the total design load.在钢筋混凝土结构中，恒荷载在总设计荷载中占据相当大的比例，这种发展趋势就显得尤为重要。,Also, in multistory buildings, any saving in depth of members, multiplied by the number of stories, can represent a substantial saving in total height, load on foundations, length of hea
7、ting, electrical ducts, plumbing risers, and wall and partition surfaces.,Also, in multistory buildings, any saving in depth of members, multiplied by the number of stories, can represent a substantial saving in total height, load on foundations, length of heating, electrical ducts, plumbing risers,
8、 and wall and partition surfaces. 同样，在多层建筑中，构件截面高度减小，并逐层累积，会大大降低结构的总高度，减小作用在基础上的荷载，减少供热、供电和排水立管的长度以及减少墙和隔断的面积。,Significant savings can be achieved by the use of high strength concrete and steel in conjunction with present-day design methods, which permit an accurate appraisal of member strength. How
9、ever, there are limitations to this development, due mainly to the interrelated problems of cracking and deflection at service loads.,Significant savings can be achieved by the use of high strength concrete and steel in conjunction with present-day design methods, which permit an accurate appraisal
10、of member strength. However, there are limitations to this development, due mainly to the interrelated problems of cracking and deflection at service loads. 采用高强混凝土和高强钢筋，并结合能准确预测构件强度的先进设计方法，可取得显著的节省效果。但是，主要由于在使用荷载作用下与开裂和挠度有关的问题，这种发展受到了限制。,The efficient use of high strength steel is limited by the fa
11、ct that the amount of cracking (width and number of cracks) is proportional to the strain, and therefore the stress, in the steel.,The efficient use of high strength steel is limited by the fact that the amount of cracking (width and number of cracks) is proportional to the strain, and therefore the
12、 stress, in the steel. 由于裂缝（裂缝宽度和数量）与钢筋的应变、应力成正比，因而限制了有效地利用高强钢筋。,Although a moderate amount of cracking is normally not objectionable in structural concrete, excessive cracking is undesirable in that it exposes the reinforcement to corrosion, it may be visually offensive, and it may trigger a premat
13、ure failure by diagonal tension.,Although a moderate amount of cracking is normally not objectionable in structural concrete, excessive cracking is undesirable in that it exposes the reinforcement to corrosion, it may be visually offensive, and it may trigger a premature failure by diagonal tension.
14、 尽管在混凝土结构中通常允许适度开裂，但却不希望发生过宽的裂缝，因为这会暴露钢筋使之锈蚀，并在感观上使人难以接受，还会导致因斜向受拉而过早破坏。,The use of high strength materials is further limited by deflection considerations, particularly when refined analysis is used. The slender members which result may permit deflections which are functionally or visually unaccept
15、able. This is further aggravated by cracking, which reduces the flexural stiffness of members.,The use of high strength materials is further limited by deflection considerations, particularly when refined analysis is used. The slender members which result may permit deflections which are functionall
16、y or visually unacceptable. This is further aggravated by cracking, which reduces the flexural stiffness of members. 挠度问题也会进一步限制高强材料的应用，采用精确的分析方法时更是如此。这样得到的细长构件，可能产生不符合功能要求和视觉要求的挠度。开裂降低了构件的弯曲刚度，这会进一步使挠度增大。,These undesirable characteristics of ordinary reinforced concrete have been largely overcome by the development of prestressed concrete. A prestressed concrete member can be defined as one in which there have been introduced internal stresses of such magnitude and distribution that the stresses resulting from the given external loading are counteracted to a desired degree.,
