Concrete is strong in compression, but  weak in tension: its tensile strength varies from 8 to 14 percent of its  compressive strength. Due to such a low tensile capacity, flexural cracks  develop at early stages of loading. In order to reduce or prevent such cracks  from developing, a concentric or eccentric force is imposed in the  longitudinal direction of the structural element. This force prevents the  cracks from developing by eliminating or considerably reducing the tensile  stresses at the critical midspan and support sections at service load,  thereby raising the bending, shear, and torsional capacities of the sections.  The sections are then able to behave elastically, and almost the full  capacity of the concrete in compression can be efficiently utilized across  the entire depth of the concrete sections when all loads act on the  structure.

Such an imposed longitudinal force is  called a prestressing force, i.e., a compressive force that prestresses the  sections along the span of the structural element prior to the application of  the transverse gravity dead and live loads or transient horizontal live  loads. The type of prestressing force involved, together with its magnitude,  are determined mainly on the basis of the type of system to be constructed  and the span length and slenderness desired. Since the prestressing force is  applied longitudinally along or parallel to the axis of the member, the  prestressing principle involved is commonly known as linear prestressing.

Circular prestressing, Used in liquid  containment tanks, pipes, and pressure reactor vessels, essentially follows  the same basic principles as does linear prestressing. The circumferential  hoop, or "hugging" stress on the cylindrical or spherical structure, neutralizes the tensile  stresses at the outer fibers of the curvilinear surface caused by the  internal contained pressure.

From the preceding discussion, it is  plain that permanent stresses in the prestressed structural member are  created before the full dead and live loads are applied in order to eliminate  or considerably reduce the net tensile stresses caused by these loads. With  reinforced concrete, it is assumed that the tensile strength of the concrete is  negligible and disregarded. This is because the tensile forces resulting from  the bending moments are resisted by the bond created in the reinforcement  process. Cracking and deflection are therefore essentially irrecoverable in  reinforced concrete once the member has reached its limit state at service  load.

The reinforcement in the reinforced  concrete member does not exert any force of its own on the member, contrary  to the action of prestressing steel. The steel required to produce the  prestressing force in the prestressed member actively preloads the member,  permitting a relatively high controlled recovery of cracking and deflection.  Once the flexural tensile strength of the concrete is exceeded, the  prestressed member starts to act like a reinforced concrete element.

Prestressed members are shallower in  depth than their reinforced concrete counterparts for the same span and  loading conditions. In general, the depth of a prestressed concrete member is  usually about 65 to 80 percent of the depth of the equivalent reinforced  concrete memher. Hence, the prestressed mem ber requires less concrete, and  about 20 to 35 percent of the amount of reinforcement. Unfortunately, this  saving in material weight is balanced by the higher cost of the higher  quality materials needed in prestressing. Also, regardless of the system  used, prestressing operations themselves result in an added cost: formwork is  more complex, since the geometry of prestressed sections is usually composed  of flanged sections with thin webs.

In spite of  these additional costs, if a large enough number of precast units are  manufactured, the difference between at least the initial costs of  prestressed and reinforced concrete systems is usually not very large. And  the indirect long-term savings are quite substantial, because less  maintenance is needed, a longer working life is possible due to better  quality control of the concrete, and lighter foundations are achieved due to  the smaller cumulative weight of the superstructure.

Once the beam span of reinforced  concrete exceeds 70 to 90 feet (21.3 to 27.4m), the dead weight of the beam  becomes excessive, resulting in heavier members and, consequently, greater  long-term deflection and cracking. Thus, for larger spans, prestressed  concrete becomes mandatory since arches are expensive to construct and do not  perform as well due to the severe long term shrinkage and creep they undergo.  Very large spans such as segmental bridges or cable-stayed bridges can only  be constructed through the use of prestressing.

Prestressed concrete is not a new  concept, dating back to 1872, when P.H. Jackson, an engineer from California,  patented a prestressing system that used a tie rod to construct beams or  arches from individual blocks. After a long lapse of time during which little  progress was made known because of the unavailability of high-strength steel  to overcome prestress losses, R. E. Dill of Alexandria, Nebraska, recognized  the effect of the shrinkage and creep (transverse material flow) of concrete  on the loss of prestress. He subsequently developed the idea that successive  post-tensioning of unbonded rods would compensate for the time-dependent loss  of stress in the rods due to the decrease in the length of the member because  of creep and shrinkage. In the early 1920s, W. H. Hewett of Minneapolis  developed the principles of circular prestressing. He hoop-stressed  horizontal reinforcement around walls of concrete tanks through the use of  turnbuckles to prevent cracking due to internal liquid pressure, thereby  achieving watertightness. Thereafter; prestressing of tanks and pipes  developed at an accelerated pace in the United States, with thousands of  tanks for water, liquid, and gas storage built and much mileage of  prestressed pressure pipe laid in the two to three decades that followed

Linear prestressing continued to develop  in Europe and in France, in particular through the ingenuity of Eugene  Freyssinet, who proposed in 1926-28 methods to overcome prestress losses  through the use of high-strength and high-ductility steels. In 1940, he  introduced the now well-known and well-accepted Freyssinet system.

P. W. Abeles of  England introduced and developed the concept of partial prestressing between  the 1930s and 1960s. F. Leonhardt of Germany, V. Mikhailov of Russia, and T.  Y. Lin of the United States also contributed a great deal to the art and  science of the design of prestressed concrete. Lin's load-balancing method  deserves particular mention in this regard, as it considerably simplified the  design process, particularly in continuous structures. These  twentieth-century developments have led to the extensive use of prestressing  throughout the world, and in the United States in particular.

Today, prestressed concrete is used in  buildings, underground structures, TV towers, floating storage and offshore  structures, power stations, nuclear reactor vessels, and numerous types of  bridge systems including segmental bridge and cable-stayed bridges. They  demonstrate the versatility of the prestressing concept and its  all-encompassing application. The success in the development and construction  of all these structures has been due in no small measures to the advances in  the technology of materials, particularly prestressing steel, and the  accumulated knowledge in estimating the short-and long-term losses in the  prestressing forces.

林同炎首创的“荷载平衡法”设计理论，成为预应力混凝土设计三大基础理论之一，被尊为现代建筑的一代宗师。 （弹性应力法和极限强度法）

tensile  capacity 受拉承载能力 

flexural  cracks 挠曲裂缝 

concentric  adj.同中心的，轴心的 

eccentric  force 偏心力

longitudinal  direction轴向 

torsional  capacity  扭转承载力 

elastically  adv.弹性地

longitudinal  force 纵向力   

transverse  adj.横向的 

prestressing  force预应力 

circular  prestressing 环形预应力 

tank 箱，槽 

reactor  vessel反应堆容器 

circumferential  adj.圆周的 

hoop 箍筋   

cylindrical  圆柱的 

spherical  adj.球的, 球形的

neutralize  抵消；压制，中和 

curvilinear  adj.曲线的, 由曲线而成的 

net  tensile stresse 净拉应力 

bending  moment 弯矩

irrecoverable  不可恢复的 

limit  state 极限状态 

deflection挠度 

flexural  strength抗弯强度,挠曲强度

span 跨径 

formwork  模板 

flange翼缘 

web腹板,梁腹 

precast  unit 预制混凝土构件

quality  control  n.质量管理, 质量控制　 

superstructure  上部结构

arche 拱 

shrinkage  收缩   

creep 徐变 

segmental  bridge 分段拼装式桥 

cable-stayed  bridge  斜拉桥

date back  to从...时就有, 回溯到, 远在...(年代) 

tie rod 拉杆 

unavailability  n.无效,不能利用 

successive  adj. 连续的

post  tensioning 后张拉 

hoop  stress 环向应力, 箍应力 

turnbuckle   n.螺丝扣, 套筒螺母 

watertightness  水密性, 不透水性 

mileage  n.英里数, 英里里程 

in this  regard adv.在这点上 

design  process 设计程序 

underground  structures 地下构造物 

offshore  structures 海上结构物 

power  station  n.发电站 

nuclear  reactor vessel 核反应堆容器 

segmental  bridge 分段拼装式桥