第7章土木工程专业文献选读3.pptVIP

Lesson 3 Prestressed Concrete 预应力混凝土; 1.Concrete is strong in compression, but weak in tension: its tensile strength varies from(不同) 8 to 14 percent of its compressive strength. Due to such a low tensile capacity(受拉承载能力), flexural cracks(挠曲裂缝) develop at early stages of loading. In order to reduce or prevent such cracks from developing, a concentric(adj.同中心的，轴心的) or eccentric force (偏心力)is imposed in the longitudinal direction (轴向)of the structural element. 混凝土的抗压性能强而抗拉性能弱:它的抗拉强度仅仅是它抗压强度的8%-14%不等.由于它如此低的抗拉承载力,在荷载作用的初期挠曲裂缝就会出现。为了减小或阻止这种裂缝的开展,在结构杆件纵向施加一个轴心或偏心的压力.;*;*; 2.Such an imposed longitudinal force(纵向力) is called a prestressing force, i.e.(也就是), a compressive force that prestresses (vt.给…预加应力)the sections along the span of the structural element prior to (adv.在前, 居先)the application of the transverse(adj.横向的) gravity dead and live loads or transient(adj.短暂的, 瞬时的) horizontal live loads. 这样施加的一个纵向力叫做预应力,也就是说,在横(竖)向重力恒载和活载或短暂的水平活载(风,地震)作用之前, 沿结构杆件跨度方向的截面预加的压缩力.;*;3.Circular prestressing(环形预应力), used in liquid containment(容纳，容积) tanks(箱，槽 ), pipes(管), and pressure reactor vessels(反应堆容器), essentially (adv.本质上)follows(遵循，追随, 跟随) the same basic principles(基本原理) as does linear prestressing. The circumferential(adj.圆周的) hoop(箍筋), or “hugging” (n.拥抱)stress on the cylindrical(圆柱的) or spherical(adj.球的, 球形的 ) structure, neutralizes(抵消；压制，中和) the tensile stresses at the outer fibers (外部纤维)of the curvilinear(adj.曲线的, 由曲线而成的 ) surface caused by the internal contained(包含) pressure. 用于液体容器箱、管道和压力反应堆容器的环向预应力结构,本质上遵循着和长线型预应力结构相同的基本原理。环向(预应力)箍筋,或者圆柱或球形结构的”环向”应力,在曲线表面外部纤维下，抵消由内部包含物质(产生的)压力所引起的拉应力. ;4. From the preceding (adj.在前的, 前述的)discussion, it is plain(adj. 清晰的,简单的, 明白的, 普通的, 朴素的) that permanent stresses in the prestressed(adj.预应力的（混凝土）) structural member are created before the full dead and live loads are applied in order to eliminate or considerably reduce the net tensile stresses(净拉应力) caused by these loads. With reinforced concrete, it is assumed that the ten